Symposium QN07—Emergent Phenomena in Oxide Quantum Materials

Oxide materials offer a playground to study rich correlated phenomena such as high temperature superconductivity and other broken symmetry phases. Recently the effects of spin-orbit coupling have gained increasing attentions, because different topological phases have been found in various quantum materials due to spin-orbit coupling. In particular, heavy transition metal oxides such as Iridium oxides generate a new platform to investigate the interplay between strong electron-electron interactions and spin-orbit coupling. I will explain recent theoretical and experimental developments of correlated iridium oxides, and discuss how topological phases including quantum spin liquids with fractionalized excitations emerge in these materials and their experimental signatures.

Spintronic devices in which pure spin currents propagate without dissipative charge flow are viewed as essential for realizing energy-efficient nanomagnetic devices for low power computing. In complex oxides, the subtle interplay of lattice, charge, orbital, and spin degrees of freedom provide unique opportunities of engineering and tailoring the materials characteristics for such applications. Mechanisms such as spin-polarized current injection, the spin Hall effect, and precessional spin pumping have been used to generate spin currents, while spin current may be detected by the inverse spin Hall effect or via spin transfer torque exerted upon a ferromagnet.

We have recently demonstrated that X-ray detected ferromagnetic resonance (XFMR) provides unique capabilities to probe the generation and propagation of spin currents in multilayered spintronics structures directly [1]. Taking advantage of magnetic contrast and element specificity of X-ray magnetic circular dichroism (XMCD), we showed that in a Ni₈₁Fe₁₉/Cu/Cu₇₅Mn₂₅/Cu/Co multilayer, spin current excited in the Ni₈₁Fe₁₉ layer propagates across the Cu spacer layer and excites a precession in the Co layer.

In magnetic thin films, the amplitude of the magnetization precession is typically not isotropic, i.e. the magnetization vector does not precess on a symmetrical cone but rather on an elliptical trajectory with an amplitude of the precession varying along different directions perpendicular to the static applied field. The asymmetry reflects the magnetic anisotropy of the system. Making use of the fact that XMCD probes the magnetization component collinear with the direction of the incoming X-ray beam, we are able to measure the precession amplitude along orthogonal directions by choosing suitable experimental geometries. This allows us to determine the exact shape of the precession cone with element-selectivity, valence state specificity and sensitivity to the symmetry of the absorber site in complex oxides such as Ni_0.65Zn_0.35Al_0.8Fe_1.2O₄ (NZAFO) [2].

Moreover, utilizing the sensitivity of X-ray magnetic linear dichroism (XMLD) to antiferromagnetic order, allows for expanding the capabilities of X-ray detected magnetic resonance to antiferromagnetic systems. We will present our current efforts to probe the propagation of spin currents in ferrimagnetic materials as well as ferromagnetic/antiferromagnetic multilayer structures.

References
[1] J. Li et al., Phys. Rev. Lett. 117, 076602 (2016).
[2] S. Emori et al., Adv. Mater. 29, 1701130 (2017).

Frustrated magnetic materials and strongly correlated electron systems are a forefront of research in modern condensed matter physics and materials science. Despite almost three decades of investigations, the theoretical understanding of these fascinating systems remains incomplete. The most prominent theoretical frameworks used to tackle these systems take the form of an emergent gauge theory akin to the gauge theory that describes conventional electromagnetism.

Spin ice is an unusual substance in which the magnetic moments of individual atoms behave very similarly to the protons in conventional water ice — hence the name spin ice — failing to align even at very low temperatures and displaying the same residual entropy that Linus Pauling calculated for water ice and which is measured experimentally. Spin ices, which belong to the broad class of compounds called magnetic pyrochlores, actually have something in common with electromagnetic fields; both can be described by a gauge theory. Many aspects of conventional electromagnetism are sensitive to constraints from enclosure boundaries, such as total internal reflection used in communication with optical fibers. It is then reasonable to wonder if spin ices have similar sensitivities to boundary effects and confinement. Motivated by the recent experimental realizations of spin ice and other magnetic pyrochlore thin films, I will discuss in this talk some of the exotic physical phenomena that arise when considering spin ice thin films such as, for example, a novel magnetic charge crystallization on the film surface while the bulk remains thermally disordered [1]. From a broader context, magnetic pyrochlore thin films offer a natural platform to study the confinement of emergent gauge fields describing strongly correlated systems and the evolution of nontrivial magnetic correlations as one moves from three to two dimensional spin textures [2]. Finally, I will discuss the consequences of open surfaces on the mechanism of order by disorder in thin films of the XY pyrochlore antiferromagnet. We find that a complex competition between multiple orders take place, as a function of temperature and film thickness. A gradient of ordering spreads over long length scale inside the film while the nature of the phase transitions is blurred between two and three dimensional critical phenomena [3]. Beyond the physics of films, this work also pertains to near-surface effects in single crystals of rare-earth pyrochlore oxides.

[1] L. D. C. Jaubert, T. Lin, T. S. Opel, P. C. W. Holdsworth and M. J. P. Gingras; Phys. Rev. Lett. 118, 207206 (2017).
[2] Étienne Lantagne-Hurtubise, Jeffrey G. Rau and Michel J. P. Gingras; Phys. Rev. X 8, 021053 (2018).
[3] L.D.C. Jaubert, J.G. Rau, P. C. W. Holdsworth and M. J. P. Gingras; unpublished.

The ABO₂ family of delafossite oxide metals can be considered as natural superlattice structures comprising high-conductivity metal layers separated by insulating transition-metal oxide building blocks. I will present our angle-resolved photoemission (ARPES) measurements from several members of this series. In PdCrO₂, the oxide layer is an antiferromagnetically-ordered Mott insulator, where we find that a coupling between the metallic and insulating subsystems renders photoemission sensitive to the spin susceptibility of the Mott layer [1]. In PdCoO₂ and PtCoO₂, the oxide layer is a band insulator, but this becomes hole-doped at the surface due to an electronic reconstruction driven by a polar surface charge. We demonstrate how the resulting CoO₂ surface supports metallic states which host a surprisingly-large Rashba-like spin-orbit splitting, resulting from a structural configuration which ensures a large energy scale associated with inversion symmetry breaking at this surface [2]. For the Pd-terminated surface, we find how an electron self-doping, again driven by the surface polarity, mediates a Stoner transition to itinerant ferromagnetism [3], in contrast to the non-magnetic nearly-free electron-like character of the Pd-derived states in the bulk [4]. Together, these results indicate the wide range of materials properties that can be stabilized in delafossites, as well as at their surfaces and interfaces.

Key collaborators on this work include Veronika Sunko (St Andrews and Max-Planck Institute for Chemical Physics of Solids, Dresden), Federico Mazzola (StA), Helge Rosner, Seunghyum Khim, Pallavi Kushwaha, and Andy Mackenzie (MPI-CPFS), Sota Kitamura and Takashi Oka (MPI-PKS), and Leonid Pourovskii and Antoine Georges (College de France).

[1] Sunko et al., arXiv:1809.08972 (2018)
[2] Sunko et al., Nature 549 (2017) 492
[3] Mazzola et al., arXiv:1710.05392 (2017)
[4] Kushwaha, Sunko et al., Science Advances 1 (2015) e1500692

The ability to create and manipulate materials in two-dimensional (2D) form has repeatedly had transformative impact on science and technology. In parallel with the exfoliation and stacking of intrinsically layered crystals, the atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with novel functionality and emergent phenomena, as seen in perovskite oxides. We present a general method to create freestanding complex oxide membranes and heterostructures with millimeter-scale lateral dimensions and nanometer-scale thickness. This facilitates many new opportunities we are beginning to explore, including the topological melting transition of 2D crystalline order, the application of extreme strain states, and integration with other materials families.

Distortive modulation of the transition metal-oxygen (MO_x) polyhedra provides an efficient strategy for designing the functional transition metal oxides. Ferroelectricity occurs as the collective distortion of the MO_x polyhedral network breaks a spatial inversion symmetry. For conventional perovskite oxides, ferroelectricity occurs owing to the ionic displacements within a 3D network of MO₆ octahedra. Meanwhile, the polyhedral tilting and rotation within some 2D networks of MO_x (x = 5 or 6) polyhedra can also lead to centrosymmetry breaking through the trilinear coupling with the spontaneous polarization. This gives rise to the geometric ferroelectricity even in the oxides including magnetic ions, i.e., multiferroic.
Considering the fundamentally different principle of 2D network for ferroelectricity from that of 3D network, one can further suggest the 1D chain network to explore unprecedented nature of ferro-orderings. Here we exhibit the combined polar distortion (CPD) induced ferroelectricity and coupled ferromagnetism in 1D chain network by investigating the brownmillerite SrFeO_2.5 epitaxial thin film of FeO₄ tetrahedra. The constrained distortion of low symmetry of FeO₄ tetrahedra leads to the simultaneous displacement of ions and rotation of the tetrahedra. The CPD further play a role in inducing canted ferromagnetism coupled with electric polarization via Dzaloshinskii-Moriya interaction. Our result provides a new paradigm for designing 1D MO_x networks, expected to benefic the emergent ferro-ordering materials including room-temperature ferroelectric ferromagnets.

Tuning the dimensionality of a system offers a useful tool for realizing new quantum phenomena associated with critical phase transitions and topological properties. However, only a few naturally occurring complex oxides with tunable, intrinsic 1D and 2D structures are available for experimental studies. In this talk, I will present a new approach of synthesizing 1D – 2D quantum stripe systems by creating dimensionally-confined superlattices from in-plane oriented layered materials. For example, we have demonstrated this method to synthesize 1D – 2D IrO₂ stripes using a-axis oriented superlattices of Sr₂IrO₄ and insulating (La,Sr)GaO₄, both are of the K₂NiF₄ symmetry [1]. The dimensional confinement of the superlattices has been confirmed by structural characterizations. Optical spectroscopy shows clear anisotropic characteristics and dimensional electronic confinement of the spin-orbit coupled J_eff = 1/2 band. Spin and orbital excitations observed in resonant inelastic x-ray scattering spectra suggest larger exchange interactions and more confined orbital excitations in the 1D IrO₂ stripes as compared to its 2D counterpart. The observed electronic confinement and localized spin-structure are quite consistent with density functional theory calculations. This method of tuning the dimensionality between 1D and 2D via stripe-superlattices is a viable technique for obtaining dimensionality-induced quantum phase transitions, in which exotic excitations such as the fractional quantizations of quasi particles can emerge.

[1] J. H. Gruenewald et al., Adv. Mater. 29, 163798 (2017).

Sr₂Mn₂O₅ (SMO) in the Ca₂Mn₂O₅-type structure is one structure among A₂B₂O₅ compounds derived from ABO₃ perovskites with ordered oxygen vacancies (OOVs). The ordered arrangements of vacancies enable unique functional properties in such compounds, because the ligand-field modified orbital structure reconstructs as the octahedral BO₆ units are transformed into different BO_6-x polyhedra arising from the OOVs. The magnetic order of transition metal oxides is governed by interaction of these orbitals and occupancy of electrons. For example, SMO exhibits a unique E-type antiferromagnetic (AFM-E) ordering because of the d⁴ electronic configuration of Mn³⁺ in the square pyramidal network. Hydrostatic pressure and biaxial strain provide additional routes to tune the magnetic properties of materials through lattice strain-induced changes to the orbital structure. Here, we investigate the relative stabilities of different magnetic orders realized through hydrostatic pressure and strain induced non-equilibrium local structures using density functional theory. We show that SMO hosts multiple magnetic transitions from AFM-C, AFM-E*, AFM-E, AFM-E*, and FM upon application of biaxial strain from compressive to tensile strain in (001) thin film geometries. In contrast, the AFM-E state is robust under hydrostatic pressure. As epitaxial strain produces anisotropic stresses, we observe different magnetic transitions depending on the relative orientation of the vacancy order with respect to the biaxial epitaxial strain. We conclude by summarizing the different changes in local structures and its impact on magnetic orders.

The majority of digital information is stored magnetically today. Nanoscale magnetic domains encode “1”s and “0”s on common commercial hard drives and newer magnetic data storage concepts rely on domain walls or parallel and antiparallel magnetization in magnetic multilayers. To develop novel materials and concepts for data storage applications, we employ X-ray spectroscopy as well as X-ray magnetic dichroism to determine spin-states in a diverse range materials. With the same tools, we are exploring new materials for data processing, as indications increase the the potential for further miniaturization of Silicon chips is soon to be exhausted.

The control of ferro- and antiferromagnetic spin orientation is a crucial element of today’s and tomorrows information technology applications, for instance needed for hard drive read heads or magnetic random access memory (MRAM). Novel device concepts using antiferromagnetic spintronics or antiferromagnetic skyrmions promise revolutionary innovations in information science. A detailed understanding of the mechanism which antiferromagnetic spin orientation is the prerequisite for pursuing theses ideas.

With soft x-ray linear dichroism we follow the thickness dependent spin re-orientation in a NiO/CoO bilayer with varying CoO and NiO layer thicknesses. The oxide-oxide interface in this simple oxide system can be regarded as a model for the a supercell of complex oxide superlattices which have been used to demonstrate a host of new and exciting properties, such as interfacial ferromagnetism or novel electronic states.

In this simple system with negligible charge transfer or structural distortions we first investigate how the purely thickness dependent coupling between CoO and NiO forces the system from an antiferromagnetic in-plane spin orientation for larger CoO layer thickness to an out-of-plane spin orientation for thicker NiO. To this end we use a double wedge sample geometry which we locally probe with x-ray spectroscopy to access different thickness combinations ranging from 0-2.5nm for CoO and 0-14nm for NiO. We find the spin reorientation to be a consequence of the balance between interfacial coupling and bulk anisotropies of NiO and CoO.

Furthermore, we augment this system with a ferromagnetic Permalloy (Py) layer and show how an in-plane antiferromagnetic NiO spin configuration enhances the coercivity of the ferromagnetic permalloy layer substantially to ca. 400Oe while in the in-plane configuration the coercivity remains comparable to the coercivity of pristine Py [1].

[1] M. Yang, Q. Li, A. T. N’Diaye, et al.: J. Magn. Magn. Mater. Vol. 460, 18 (2018)

Recent interest in antiferromagnetic (AFM) spintronics has surged due to the possibility of obtaining ultrafast magnetization dynamics with no stray field,¹ and complex oxide AFMs offer possibilities to engineer AFM spin textures via their many degrees of freedom and sensitivity to epitaxial strain. However, this lack of stray field makes imaging of the AFM domain structure difficult though conventional magnetic imaging techniques. This talk will discuss the use of x-ray photoemission electron microscopy (X-PEEM) with both x-ray linear and circular dichroism to spatially resolve the ferroic order in the AFM and ferromagnetic (FM) layers of a model system of alternating FM La_0.7Sr_0.3MnO₃ and G-type AFM La_1-xSr_xFeO₃ (x=0 or 0.3). In the limit of few unit-cell thick layers for (001)-oriented samples, the AFM spin axis lies in the plane of the film in contrast to thicker layers where the spin axis cants out of plane.² In contrast, (111)-oriented superlattices show markedly different behavior with two populations of spin orientations, a subset with spin axes canted along low-index <110> directions and which exhibit spin flop coupling to the FM layer, and a subset whose spin axis lies within the (111) plane.³ The coupling at the AFM/FM interface for both orientations allows for control of the AFM domain structure through magnetization rotation of the FM layer, and recent work has shown shape anisotropy offers an additional degree of freedom to control complex oxide AFM domain orientation.⁴ X-PEEM with variable temperature and x-ray polarization offers a powerful tool to determine the AFM spin axis on the scale of single AFM domains, and such a local probe is necessary for a full understanding of how the similar energy scales of shape anisotropy and interfacial exchange can be tailored in order to design spin textures in these oxide heterostructures.

1. Baltz et al, Rev. Mod. Phys. 90, 015005 (2018).
2. Takamura et al, Phys. Rev. B 80, 180417 (2009).
3. Jia, Chopdekar et al, Phys. Rev. B. 92, 094407 (2015); Jia, Chopdekar et al, Phys. Rev. B. 96, 214411 (2017).
4. Lee et al, Phys. Rev. Mater. 1 014402 (2017).

Competing antiferromagnetic interactions in triangular motifs of magnetic Fe³⁺ in the noncentrosymmetric compound Fe₃PO₄O₃ (spacegroup R3m) give rise to magnetic frustration. This leads to an unusual helical magnetic state below T_N=163K, in which needle-like domains exhibit long range antiferromagnetic order along the c axis, but have a correlation length restricted to ζ=7nm in the ab plane. Through single crystal neutron diffraction, we report a degenerate manifold of ordering wavevectors in the ab plane, which smear the incommensurate Bragg peaks into a continuous ring. We show this result to be consistent with the competition between J₁ (nearest neighbor) and J₂ (next nearest neighbor) interactions in a Heisenberg model, which produces a quasi-degenerate manifold of ordering wavevectors. When doped with non-magnetic Ga³⁺, polycrystalline samples show helical winding length and correlation length in the ab plane to be equal. The restriction to short range order in the ab plane, along with the intimate connection of helical winding length and correlation length, imply the presence of a high density of non-trivial topological defects which are tied to the magnetic structure.

Physics of magnetic domains of conventional magnetic materials can be well described by minimization Landau-Lifshitz free energy. However, for magnetic oxides, competition between various types of exchange interactions has often led to complex magnetic domain structures that are far from being understood. One of the most typical example is the domain structure in colossal magnetoresistive manganites, which is featured by spatial coexistence of ferromagnetic, antiferromagnetic and even spin glass domains. These domains are not only in different magnetic states, but are also in different conducting states. By studying the effect of spatial ordering of the chemical dopants, we conclude that the dopants-induced disorder is the key reason for the formation of the complex domain structures in magnetic oxides. Based on the understanding of the physical origin of the complex magnetic domains in oxides, we have developed various methods to control the domain patterns in oxides and fabricated multi-bit memory device that can also carry out logic operations.

Silicon photonics is now widely accepted as a key technology in a variety of systems. But owing to material limitations, now it is challenging to greatly improve the performance after decades of development. Here, we show a high-performance broadband photodetector with significantly enhanced sensitivity and responsivity operating over a wide wavelength range of lights from near-ultraviolet to near-infrared at low power consumption. The uniquely designed textured top ceiling electrode works effectively as an antireflection layer to greatly improve the absorption of near-infrared light, thereby overcoming the absorption limitation of near-infrared light. Instead of the conventional p-n junction and p-intrinsic-n junction, we introduce a ~15 nm thick alumina insulator layer introduced between a p-type Si substrate and n-type ZnO nanowire (NW) arrays, which are found that the charge carriers separation and collection efficiency have significantly enhanced. The photo-sensing responsivity and sensitivity are found to be nearly one order of magnitude higher than that of a reference device of p-Si/n-ZnO NW arrays, significantly higher than the commercial silicon photodiodes as well. The light-induced charge carriers flow across the appropriate thickness of insulator layer via the quantum mechanical Fowler-Nordheim tunneling mechanism. By virtue of the piezo-phototronic effect, the charge density at the interfaces can be tuned to alter the energy bands and the potential barrier distance for tunneling. Additionally, along with the use of incident light of different wavelengths, the influence of the insulator layer on the transport of electrons and holes separately is further investigated. The demonstrated concepts and study would lead to sensitivity improvement, quality enhancement of data transfer, decrease of power consumption and cost-deduction of silicon photonics.

REBa₂Cu₃O_7-x (REBCO, RE=rare earth) coated conductors (CCs) have the huge market potential for large scale of commercial applications due to their excellent superconducting properties. Up to now, the high cost is still a bottleneck for practical applications. The most effective way to reduce the cost is to increase critical current density J_c of thick REBCO films. However, J_c of REBCO films will decrease as the increasing of REBCO film thickness. In this work, we report the enhancement of T_c and J_c of REBCO films achieved through the surface/interface quantum modulation. REBCO multilayers were fabricated on CeO₂/IBAD-MgO buffered metal tapes by pulsed laser deposition (PLD). At first, Y_0.5Gd_0.5Ba₂Cu₃O_7-δ (YGBCO) films were grown on CeO₂/IBAD-MgO buffered metal tapes. Then, metal nanoparticles such as Ag and In were grown on YGBCO films. Comparing with the pristine REBCO films, higher T_c and I_c were observed from the YGBCO films with metal nanoparticles. The T_c and I_c values were increased by 1K and 40A, respectively. Furthermore, the YGBCO/In doped CeO₂ (CeO₂+In)/YGBCO hetero-structures were fabricated. It was found that the I_c value of the YGBCO/ CeO₂+In/YGBCO trilayers was almost doubled. Interaction at the interface comprises a variety of effects, such as epitaxial strain, interface defects, elemental inter-diffusion, interface charge layers, electrical charge transfer between layers, and others, which can improve the superconducting properties of REBCO films. The results provide the important insights into the superconducting property engineering of REBCO CCs by artificial microstructure control.

Focus helium ion beam (FHIB) materials modification is an emerging technique with the capability to make high-quality thin-film YBa₂Cu₃O_7-δ (YBCO) superconducting Josephson junctions [1]. In this method, ion irradiation converts the film from a superconductor to an insulator by disordering the crystalline lattice. No material is removed or etched, as a result nanoscale insulating features are obtainable for both Josephson junctions[2] and superconducting electrodes for devices such as superconducting quantum interference devices (SQUIDs) [3].
We investigate YBCO nano-SQUIDs to push the scaling limits. YBCO SQUIDs with loop dimension from 1.2 μm down to 10 nm have been fabricated. Current-voltage characteristics revealed high-quality of Josephson tunnel junctions with insulating barriers. Voltage modulation as a function of magnetic field was measured for the nano-SQUIDs by both an on-chip control line and external coil in a wide temperature range from 4.2 K up to ~50 K. The highest modulation voltage was ~0.8 mV for a single nano-SQUID, and the smallest magnetic flux effective area achievable was ~0.25 μm². The flux noise characteristics was measured open loop using a small signal method, which showed a white noise of ~0.6 µΦ₀/Hz^-1/2 for the Nano-SQUID with loop dimension 400 nm.

Acknowledgments: This work supported by AFOSR, NIH, NSF, and UCOP.

Reference
[1] Shane A. Cybart, E. Y. Cho, T. J. Wong, Björn H. Wehlin, Meng K. Ma, Chuong Huynh, and R. C. Dynes. "Nano Josephson superconducting tunnel junctions in YBa₂Cu₃O_7-δ directly patterned with a focused helium ion beam." Nature Nanotechnology 10.7 (2015): 598.
[2] Ethan Y. Cho, Y. W. Zhou, J. Y. Cho and S. A. Cybart. "Superconducting nano Josephson junctions patterned with a focused helium ion beam." Applied Physics Letters 113.2 (2018): 022604.
[3] Ethan Y. Cho, H. Li, J. C. LeFebvre, Y. W. Zhou, R. C. Dynes, and S. A. Cybart. "Direct-coupled micro-magnetometer with Y-Ba-Cu-O nano-slit SQUID fabricated with a focused helium ion beam." Applied Physics Letters 113.16 (2018): 162602.

Oxygen vacancies are ubiquitous in oxide systems, which can play important roles to cause interesting physical phenomena such as metal-insulator transition, magnetic and ferroelectric orders, and high-T_c superconductivity. Oxygen vacancy concentration has been a key control parameter to modulate physical properties [1]. In this work, we study the electronic structure and transport properties of Ca-substituted BiFeO₃ (BCFO) thin films with controlling the oxygen vacancy concentration [2]. BCFO films can spontaneously produce oxygen vacancies owing to the stable oxidation number of Fe³⁺ ions. The as-grown state can be transformed into an electrically-formed state through electro-migration of oxygen vacancies, thereby creating a spatially uniform stoichiometric hole doping region [3]. We measure conduction band and valence band spectra by using X-ray photoemission spectroscopy and find that a non-rigid polaronic band emerges in the hole doping system. In addition, we investigate temperature dependence of electronic transport leading to a disorder-driven formation of a Coulomb-glass state.

[1] C.-H. Yang, et al., Nature Materials 8, 485 (2009).
[2] J. S. Lim, et al., Physical Review B 94, 035123 (2016).
[3] J. S. Lim et al., NPG Asia Materials 10, 943 (2018).

EuO_1-x combines several interesting properties: large magneto-optical effects, colossal magneto-resistance, and enhanced Curie temperature due to bound magnetic polarons. EuO_1-x is also one of a very few ferromagnetic insulators and, as an ultra thin film, very sensitive to interface effects. Utilizing pulsed laser deposition, we deposited a film of EuO_1-x onto a Si substrate in order to create a ferromagnetic (FM)/anti-ferromagnetic (AFM)/Semiconducting (SC) interface,EuO_1-x/EuSi₂/Si, for applications in spintronics (spin filters). X-ray diffraction indicates the films crystallize into a preferred EuO (111) orientation, it also indicates a clear presence of EuSi₂ which suggests Si diffuses across MgO buffer layer into the EuO_1-x. Density functional theory calculations are employed to investigate the molecular orbital contributions to the conduction and valence bands from Eu and O and compared to x-ray absorption spectroscopy data. A magnetic signature, revealed by x-ray magnetic circular dichroism measurements, suggests the presence of a magnetic order above the Curie temperature. Strong, temperature dependent exchange bias coupling, due to the AFM/FM interface of EuO_1-x/EuSi_2, is observed in magneto optical kerr effect spectroscopy hysteresis loops. Additionally the temperature dependent oscillatory nature of the exchange bias suggests the AFM layer undergoes a magnetic moment flip. This study demonstrates the realization of a ferromagnetic/antiferromagnetic/semiconductor interface and will be beneficial to future spintronic applications.

Various forms of titanium dioxide (TiO₂) have been applied in several fields - including electronics, energy, as well as environmental engineering - all which capitalize on the semiconducting behavior of this inexpensive, abundant and chemically stable material. Both common crystal structures of TiO₂, rutile and anatase, have large band gap (3.0 and 3.2 eV, respectively) limiting the principal optical absorption to the short wavelength UV photons only. To further enhance the photocatalytic activity, and thus make TiO₂ suitable for solar applications, a number of strategies have been worked out in the last decade. While anatase is known to have much higher photocatalytic activity than that of rutile, their mixed-phases have been known to have superior activity and have been utilized as commercial catalyst for decades. The band alignment between anatase and rutile mixed phases plays the decisive role in carrier separation after photogeneration. Electrochemical impedance analysis has confirmed that the flat band potential of anatase is about 0.2 eV lower than that of rutile. At the interface of the two phases, the photoexcited electrons tend to transfer from the anatase to the rutile, which has been corroborated by many researchers. However, Scanlon et al. proposed a reversed model of mixed-phase TiO₂ in which the conduction band of rutile lies above that of anatase. In their work, density functional theory simulations and X-ray photoemission measurements indicated a staggered band alignment of ∼0.4 eV between anatase and rutile in which anatase had the higher electron affinity. Therefore, engineering the junction between two phases is crucial to properly address photocatalytic behavior in anatase-rutile systems. In this study, two types of core@shell heterojunction TiO₂ nanofibers were synthesized by sequential hydrothermal, calcination, and impregnation processes. Rutile TiO₂ NFs core with anatase TiO₂ NPs shell is denoted as R@A TiO₂ NFs, and the reverse structure is denoted as A@R TiO₂ NFs. In our study, the photodegradation of organic dyes and Kelvin Probe Force Microscopy (KPFM) analysis were applied to shed light on the mechanism of the excited electron-hole pair separation. The results of photodegradation showed that the A@R TiO₂ NFs had the highest activity under UV-B and UV-A irradiation, being nearly 3-fold higher as compared to AEROXIDE^® TiO₂ P25. The results in conjunction with KPFM measurements indicated that in the heterojunction structure, electron-hole pairs were efficiently separated and the excited electrons stayed in the anatase phase, and holes were injected to rutile phase. When the A@R TiO₂ NFs heterostructures are decorated with Pt nanoparticles, the nanocomposite is particularly active in photocatalytic hydrogen evolution from ethanol-water mixtures with a production rate of ~8,500 μmol/h●g. Our study not only explains the role of anatase-rutile junctions in photocarrier separation, but also projects the development of other efficient photocatalytic heterostructures for green energy production and conversion.

Two-dimensional electron gases (2DEGs) at oxide interfaces provide unique playgrounds for the exploration for emergent phenomena which motivate not only new concepts but also applied research. The perovskite SrTiO₃ is composed of 3d electrons, and as well established, oxygen vacancies and the magnetic proximity effect can be utilized to introduce magnetic moments (1µ_B per Ti³⁺) [1], generating well spin-polarized 2DEGs [2]. This offers us valuable opportunities to build up magnetic 2DEGs, which is centrally important for spintronics. While most of the previous works focused on the 2DEGs at LaAlO₃/SrTiO₃ interfaces, here we report on two new kinds of 2DEGs systems: anatase TiO₂-based 2DEGs induced by low-energy argon ion irradiation and the 2DEGs formed between a magnetic insulator EuO and a high-k perovskite KTaO₃. In anatase TiO₂-based 2DEGs, by irradiating the surface layer of the anatase TiO₂ films with argon ion beams, we gained the 2DEGs via oxygen vacancies with a thickness of 4 nm. Unique transport behaviours of a T^-1/3 or ln (1/T) sheet resistance dependence below 100 K are observed depending on irradiation time. With the increase in carrier density, the magnetoresistance changes from positive to negative can be ascribed to the depression of magnetic scattering, very similar to those of the diluted magnetic semiconductor [3]. Another result is 2DEGs at EuO/KTaO₃ interface. As Eu atoms uptake oxygen from the surface layer of KTaO₃ to form EuO phase, it will generate oxygen vacancies in KTO [4-6]. When the content of oxygen vacancies is high enough a metallic 2DEG residing in the interfacial layer of KTO will appear. The 2DEGs are not only highly conducting, with a maximal Hall mobility of 111.6 cm²/Vs at 2 K, but also well spin polarized, showing strongly hysteretic magnetoresistance up to 25 K and well-defined anomalous Hall effect up to 70 K. Unambiguous correspondences between the hysteretic behaviors of 2DEGs and the EuO layer are captured, suggesting the proximity effect of the latter on the former.

BaTiO₃ (BTO) is an intensively studied ferroelectric material with several ground-state polar structures: tetragonal, orthorhombic and rhombohedra varying from room temperature to low temperatures.[1] BTO is known that tetragonal phase of which polarization is [001]-direction is the most stable at room temperature. BTO films were grown epitaxially on (111)-oriented SrTiO₃ substrates with an intermediate SrRuO₃ electrode by pulsed laser deposition in order to see the results of the competition between six stable polarization each other at room temperature. Here, we use spherical aberration-corrected scanning transmission electron microscope (STEM) to identify domain structures and phases. We demonstrate that the ferroelectric polarization domains of (111)-BTO at room temperature contain both tetragonal and orthorhombic phases, by measuring pm-scale atomic displacements of Ti-cations directly from the images. Furthermore, the domain sizes range down to few nanometers. Such a peculiar domain configuration is in good agreement with our phase field simulation, which shows that the orthorhombic domain is bridging different 90° tetragonal domains as a way to relieve the elastic strain of domain walls. We believe that the phase coexistence along with miniaturized domains have given rise to relaxor-like behaviors in the (111)-BTO film as we observed: slanted polarization-electric field hysteresis loop and frequency dispersion of the temperature-dependent dielectric response. In addition, phase field simulation reveals that the (111)-BTO film undergoes a phase transition at low temperatures because of reduced thermal vibration. In-situ cryo-atomic resolution HAADF STEM analysis is employed to compare the nm-scale polarization domain structure at low temperature to that at room temperature.

Reference
[1] G. H. Kwei et al., J. Phys. Chem., 97, 2368-2377 (1993)

Mott insulating states featured in layered irridates, such as Sr₂IrO₄ have recently attracted substantial interest due to their unexpected novel properties caused by the coexistence of strong spin-orbit interactions and electron correlations. Recent studies on Sr₂IrO₄ have revealed the possibilities of novel phases such as potential high-T_c superconductivity with d-wave gap.¹ However, even though much experimental work using different tuning parameters, such as lattice strain, pressure and chemical doping, has been done, superconductivity has yet to be observed in this system. Moreover, it is also puzzling that electron-doped Sr₂IrO₄ bulk crystals are metallic while Sr₂IrO₄ thin films with as much as 15 % electron-doping are insulating.^2,3 This discrepancy may be due to strain-induced defects which are common for thin-film systems.

We have deposited Sr₂IrO₄ thin-films on SrTiO₃ substrates using Sr₃Al₂O₆ buffer layer.⁴ The water soluble Sr₃Al₂O₆ has been shown to generate strain-free thin-films.⁴ High quality samples were obtained by depositing Sr₂IrO₄/Sr₃Al₂O₆ heterostructures on SrTiO₃ (100) substrates using pulsed laser deposition. X-Ray diffraction and reciprocal space mapping shows that the Sr₃Al₂O₆ buffer layer is relaxed and the Sr₂IrO₄ is partially relaxed with respect to the SrTiO₃ substrate. The optical transmission spectra show that the peak energies (both α and β) are blue shifted in the Sr₂IrO₄/Sr₃Al₂O₆ heterostructure as compared to the Sr₂IrO₄ thin-film on SrTiO₃. Our results demonstrate that the structural and optical properties of our Sr₂IrO₄ thin-films can be modified using Sr₃Al₂O₆ buffer layers and we will further discuss the electronic properties of freestanding Sr₂IrO₄ thin-films in comparison to Sr₂IrO₄ single crystal and Sr₂IrO₄ thin-films on different substrates.

[1] Fa Wang and T. Senthil, Phys. Rev. Lett. 106, 136402 (2011).
[2] O. B. Korneta, et al. Phys. Rev. B 82, 115117 (2010).
[3] J Ravichandran, et al. J. Phys.: Condens. Matter 28, 505304 (2016).
[4] Di Lu. et al. Nat. Mat. 15, 1255 (2016).


Preferred Type of Presentation: Poster

Although SrTiO₃ was the first oxide superconductor to be discovered, the nature of its superconducting state has been a longstanding subject of debate in the literature, reflecting in many ways the elusiveness of other families of superconductors, such as the cuprates. A striking feature is that superconductivity already appears at very low carrier densities, when the Fermi temperature is lower than the Debye temperature, which is at odds with BCS theory. Bulk, undoped SrTiO₃ is an incipient ferroelectric for which quantum fluctuations suppress a low-temperature transition to a ferroelectric ground state. Several recent proposals have suggested that there could be a connection between ferroelectricity and superconductivity of SrTiO₃. In this presentation, we will report on experiments that tune the superconducting states of SrTiO₃ thin films grown by molecular beam epitaxy. We will discuss the effects of proxmity to ferroelectricity as well as other ground states on the superconducting transition temperature and critical fields.

Large unsaturated magnetoresistance has been recently reported in numerous semi-metals [1].
Many of them have a topologically non-trivial band dispersion, such as Weyl nodes or lines.
In the first part of my presentation I will show that elemental antimony displays the largest high-field
magnetoresistance among all known semi-metals. I will present a detailed study of the angledependent
magnetoresistance and use a semi-classical framework invoking an anisotropic
mobility tensor to fit the data [2]. A slight deviation from perfect compensation and a modest
variation with magnetic field of the components of the mobility tensor are required to attain perfect
fits at arbitrary strength and orientation of magnetic field in the entire temperature window of study.
In the second part I will discuss the case of lightly doped SrTiO3-x [3]. At low temperature we find
that SrTiO3-x displays a large transverse (j⊥B) but also longitudinal (j//B) magnetoresistance which
can be captured, like in the case of the semi-metals, by a semi-classical framework including a
field dependent mobility carriers.
Our results demonstrate that large orbital magnetoresistance is an unavoidable consequence of
low carrier concentration and the sub-quadratic magnetoresistance seen in many semi-metals and
doped semi-conductors can be attributed to field-dependent mobility, expected whenever the
disorder length-scale exceeds the Fermi wavelength [4].
[1] M. N. Ali and al., Nature, 514, 205–208 (2014).
[2] B. Fauqué, Phys. Rev Mat. (2018), in press
[3] C. Collignon and al., unpublished
[4] J.C Song and al., Phys. Rev. B, 92, 1-5 (2015)

The non-trivial spin topology protects the spin texture against disorder and fluctuations, leading to substantial potential to miniaturize, store, and transport information with excellent stability and efficiency. Significant efforts have been focused on harnessing the Dzyaloshinskii-Moriya (DM) interaction, which results from strong spin-orbit coupling combined with broken inversion symmetry to generate magnetization rotations with fixed chirality and is a highly effective mechanism to generate skyrmions. To date, the most promising results have been achieved in multi-layer systems, which benefit from cooperative DM interactions from multiple interfaces and possible additional stabilization from inter-layer magnetic coupling of skyrmion columns. Furthermore, recent studies have identified the important roles of interfacial oxygen, hybridization, and charge transfer in determining the sign and strength of DM interactions. Transition metal oxides, in particular those containing heavy 5d elements, also possess the requisite materials parameters necessary for skyrmion formation. However, the majority of the studies focused on non-oxide-based systems. In this talk, we will present our observations on charge transfer induced magnetism, anomalous Hall effect, topological Hall effect in interface engineered 3d-5d transition metal oxide heterostructures. The critical role of the DMI and its interfacial control will be discussed together with the potential of 5d transition metal oxides for developing novel quantum materials.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

In the magnetic insulating oxides_, it is known that the cubic crystal symmetry is favorable for higher Curie temperatures (T_C) [1], since the network morphology hosting the mechanisms of exchange interactions is subject to the crystal symmetry. So far, these oxides are typically 3d transition metal oxides where spin-orbit coupling (SOC) is tiny. An enhancement of the SOC in 5d systems is a promising way to boost the long range ferri/ferromagnetic (FM) order. For example, it was shown for a pyrochlore osmate (Cd₂Os₂O₇) that the SOC is responsible for its long range order [2]. Here we show that a highly B-site ordered cubic double-perovskite Sr₃OsO₆, which satisfies above criteria, surpasses the long standing T_C (943 K for spinel ferrite LiFe₅O₈ [3]) record in all insulators and oxides by more than 100 K.
We synthesized the magnetic insulator Sr₃OsO₆ by molecular beam epitaxy (MBE) on (001) SrTiO₃ substrates (300-nm thick). Atomic-resolution scanning transmission electron microscopy (STEM) along with diffraction (TED) measurements have revealed the cubic structure and excellent Sr/Os ordering on the B’/B-site. For double perovskites, it is well known that the crystal symmetry is dominating the exchange interactions, e.g., Sr₂FeMoO₆ (T_C = 415 K, cubic) [4] and Sr₂FeWO₆ (antiferromagnetic, T_N = 37 K, monoclinic) [5]. Thus, the cubic symmetry of Sr₃OsO₆ plays a crucial role for the emergence of high T_C whereas an electronically isostructural compound, Ca₃OsO₆, having a monoclinic structure, exhibits T_N = 50 K. Owing to an optical band gap of ~2.65 eV in Sr₃OsO₆, the electronic charge carriers move by hopping between localized electronic states, and this is supported by the temperature dependence of resistivity (ρ); ln(ρ) ∝ T^-1/4 (variable range hopping model), along with the high resistivity value (ρ(300 K) = 75 Ωcm).
Using density functional theory calculations based on a Parder-Burke-Ernzerhof (PBE) functional, we reproduced the ferromagnetic ground state of Sr₃OsO₆. We show that the large SOC of the Os⁶⁺ 5d² orbitals indeed drive the system toward an insulating state with J_eff = 3/2 [6]. Our calculations reveal that the t_2g↑ states are split into effective total angular momenta of J_eff = 3/2 (doublet) and J_eff = 1/2 (singlet) states. The J_eff = 3/2 states are fully occupied with two 5d electrons per Os⁶⁺, resulting in an insulating state.
Our experimental results for Sr₃OsO₆ show that in 5d systems extraordinary high T_C is possible in spite of the long distance (5.81 Å) between 5d ions. The J_eff = 3/2 insulating state with T_C above 1000 K is thus consequences of the cubic rock-salt type environment of Os⁶⁺ ions and the enhanced SOC found in 5d systems. Further investigations, e.g., XMCD, are required to analyze the hitherto unidentified exchange interactions.
[1] G. Koster, et al., Rev. Mod. Phys. 84, 253 (2012).
[2] S. Calder. et al., Nat. Commun. 7, 1 (2016).
[3] Y. K. Wakabayashi, et al., arXiv: 1806.09308 (2018).
[4] K. Kobayashi, et al., Nature 395, 677 (1998).
[5] F. P. De la Cruz, et al., Solid State Commun. 127, 703 (2003).
[6] S. S. Erickson, et al., Phys. Rev. Lett. 99, 016404 (2007).

Recently, great interest has been shown in exploring quantum states in effective spin-½ systems. Many of the quantum phenomena observed with “bare” spin ½ - such as continuum excitations, quantum tetramers, and spinon excitations from 1D spin chains – have been observed with effective spin-½ magnets. However, a notable absence in this lineup is the quantum dimer magnet (QDM) with a field-induced Bose-Einstein condensate (BEC). The QDM state consists of entangled sets of spins (dimers) that have quasiparticle excitations called “triplons”. When exchange is U(1) symmetric these triplons can undergo BEC by applying a magnetic field, this BEC is analogous to the famous BEC observed in ultracold gases.

Numerous compounds based on 3d magnetic cations with “bare” spins have been found that exhibit a quantum dimer state with a BEC phase [1]. Here we have found a new realization of a quantum dimer magnet based on the strongly spin-orbit coupled ion Yb³⁺ in the distorted honeycomb-lattice material Yb₂Si₂O₇. Yb³⁺’s high spin-orbit coupling, combined with crystal field effects, results in an effective spin-½ angular momentum that often leads to anisotropic exchange interactions between quantum spins. Our single crystal neutron scattering, specific heat, and ultrasound velocity measurements show the expected field-induced transition to a BEC-like magnetically ordered phase, that is bounded by significantly lower critical fields than previously studied compounds (~0.4 T to 1.3 T) owing to weak superexchange interactions between 4f moments. However, the high-field part of the BEC-like dome is interrupted by an unusual regime. The aforementioned low critical fields allow for previously inaccessible measurement techniques, such as inelastic neutron scattering, to be brought to bear on the entire phase diagram. Our results on Yb₂Si₂O₇ provide the opportunity to study how anisotropic exchange in strongly spin orbit coupled materials modifies the field induced phases of the canonical quantum dimer magnet system.
[1] V. Zapf, M. Jaime, and C. D. Batista, Rev. Mod. Phys. 86, 563 (2014).

An emerging direction in oxide research aims at discovering novel electronic phases at interfaces between two oxide materials. A well-known example is the LaAlO3/SrTiO3 system: while both LaAlO3 (LAO) and SrTiO3 (STO) are wide bandgap insulators, a high-mobility two-dimensional electron system (2DES) forms at their interface [1]. Interestingly, LAO/STO possesses several remarkable extra functionalities, including a gate-tunable Rashba effect [2], which makes it particularly appealing for spintronics.
Spin currents, which are at the heart of spintronics, allows the electrical control of magnetic (and resistance) states in magnetic nanostructures. In this context, there is a huge interest in finding new techniques for the efficient generation and manipulation of spin currents, especially using the spin-orbit interactions in non-magnetic materials. This has been firstly achieved using the Spin Hall Effect (SHE), before to be found possible by taking advantage of several interfacial effects [3].
Indeed, structures with inversion asymmetry, made of ferromagnetic (FM) material in contact with a non-magnetic material with large spin orbit coupling, such as Pt or Ta, can produce efficient spin orbit torques [4]. Beyond the sole field of spinorbitronics, spin-to-charge conversion effects and spin orbit torques are already ubiquitous in several branches of spintronics, as in spincaloritronics and magnonics.
The Rashba effect is a manifestation of spin-orbit interaction (SOI) in solids, where the spin degeneracy associated with the spatial inversion symmetry is lifted due to a symmetry-breaking electric field normal to the heterointerface [5]. Eldelstein has realized that in a Rashba two-dimensional electron system, the flow of a charge current is accompanied by a non-zero spin accumulation [6] coming from uncompensated spin textured Fermi surfaces. The reverse effect, corresponding to a spin-to-charge conversion through SOI (inverse Edelstein effect), was first demonstrated at Ag/Bi(111) interfaces [7].
In this presentation we will report the observation of a giant inverse Edelstein effect in NiFe/LaAlO3/SrTiO3 heterostructures, with spin-to-charge conversion one order of magnitude more efficient than in previous systems [8]. Moreover the large dielectric constant of the STO substrate makes possible the use of a back-gate voltage Vg to modulate the 2DES carrier density and electronic properties. By this way, we found that the amplitude of the converted current can be modulated over one order of magnitude, and even changes sign. This can be interpreted in terms of a crossover between the occupancy of one to several bands with different orbital characters and different spin–orbit textures. Our results suggest that oxide interfaces have a strong potential for spintronics, both for the generation or detection of spin currents through direct or inverse Edelstein effects, and for their electrical modulation. More generally, our observation of a very large spin-to-charge conversion efficiency at an interface with a moderate Rashba splitting highlights the importance of a long scattering time, and calls for the design of novel Rashba interfaces in which confinement and electrical insulation from metallic layers are carefully engineered. We will compare the conversion efficiency of STO/LAO interface with that of the Spin Hall effects of metals, other Rashba interfaces and surface of Topological Insulators. We will also show that this record value can even be much enhanced by tuning carefully the heterostructure.

[1] A. Ohtomo et al. Nature 427, 423 (2004).
[2] S. Thiel et al. Science 313, 1942 (2006).
[3] A. Soumyanarayanan et al. Nature 539, 509 (2016).
[4] M.I. Miron et al. Nature 476, 189 (2011).
[5] Y.A. Bychkov & E.I. Rashba, J. Phys. C 17, 6039 (1984).
[6] V.M. Edelstein, Solid State Commun. 73, 233 (1990).
[7] J.C. Rojas Sánchez et al. Nat. Commun. 4, 2944 (2013).
[8] E. Lesne et al. Nat. Materials. 15. 1261 (2016)

The emergent discovery of two-dimensional electron gas (2DEG) in LaAlO₃/SrTiO₃ (LAO/STO) heterostructure has attracted considerable attention over the past decade. Recently, unlike strongly localized 2DEG in LAO/STO (001) heterostructure, a wide n-type 2DEG density distribution is reported in the LAO/STO (111) interface (K. Song et al., Nat. Nanotechnol. 13, 198 (2018)). According to the polar catastrophe scenario, the LAO/STO (111) interface consisting of [Ti]⁴⁺ and [LaO₃]^3- layers is expected to exhibit the highly localized p-type characteristics. Despite these controversies, the origin of n-type 2DEG at the LAO/STO (111) interface has not been clearly revealed yet. Here, we carry out first-principles density functional theory calculations to reveal the origin of n-type 2DEG widely distributed through the the interface of LAO/STO (111) heterostructure. We find that the n-type 2DEG originates from extra electrons created by the oxygen vacancy at the [[LaO₃]^3- terminated LAO (111) surface, and these excess oxygen vacancies significantly modulate the relative displacements between La and oxygen atoms and octahedral rotations in LAO layers. The presence of oxygen vacancy, atomic displacement modulation and octaheral rotation in the vicinity of LAO (111) surface are confirmed by annular dark-field scanning transmission electron microscope image at the atomic scale.

Advances in thin film synthesis enable unique opportunities to enhance and control the physical properties of interfaces by controlling the interactions in complex oxides. It is well known that interface layers provide opportunities to create or control functional properties of oxide heterostructures. However, the role of such an interfacial layer in controlling functionalities has not been fully explored. In this talk, the influence of buried interfaces in oxide heterostructures on their magnetic properties will be discussed. A shift of the magnetization hysteresis along the applied field axis was observed. We show the loop shift is an exchange bias (as opposed to the shift of a minor loop) and is present in single phase manganite thin films. Interestingly, the sign of exchange bias is controlled by the cooling field strength. When the cooling field is small, negative exchange bias is observed while positive exchange bias was observed for cooling fields exceeding a threshold. The origin of such an exchange coupling is related to the pinned interfacial layer, confirmed by polarized neutron reflectometry. Our results shed new light on using oxide interfaces to design functional devices.

Interface functionality in oxide heterostructures can be controlled by strain engineering, due to the nearly degenerate ground states of competing order in these systems. A central question is which role structural reconstructions can be used to establish and control new magnetic spin textures. In general probing antiferromagnetic spin structures are difficult, however important in new spintronic devices. To address this question (111)-oriented epitaxial heterostructures of antiferromagnetic (AF) LaFeO₃(LFO) and ferromagnetic La_0.7Sr_0.3MnO₃(LSMO)is used as model system, and we present data on the interplay between AF spin axis of LFO and the occurrence of magnetic reconstructions at the (111)-oriented LSMO/LFO interface. To probe the spin texture of the different layers, we rely on a combination of soft x-ray spectroscopy, x-ray photoemission electron microscopy and neutron reflectometry. The AF LFO is spin-flopped coupled (perpendicular) to the FM LSMO, however at the interface a canting of the AF spins is induced, resulting in a net moment in the LFO and a spiral spin structure. We show that the AF spin axis in single layers of LFO can be tuned by thickness, crystallographic orientation and strain. Hence, we can by using different substrates induce different types of spiral structures at the interface. We use soft x-ray resonant reflectivity with linear polarized light to directly probe the depth dependence on the AF spin axis in these systems, which is corroborated with circular light and neutrons for the depth dependence of the ferromagnetic moment. In addition, we can use magnetic fields to change the spiral structure. Due to the magnetic exchange coupling at the interface, the AF spin axis turns/rotates when an applied field aligns the FM spins. Hence, creating systems with a competition between exchange coupling and magneto crystalline anisotropy enables us to control the spin canting in the AF layer.

Mobile electrons coupled to a noncoplanar spin texture give rise to a Hall effect, called chirality-induced Hall effect or the topological Hall effect (THE). This effect has been studied so far for the strong- and weak-coupling regimes separately; the former in terms of the Berry phase and the latter by perturbation theory. In this work, we present a unified treatment that bridges the strong- and weak-coupling regimes [1]. This is done by using the spin gauge field, considering not only the adiabatic (Berry phase) component of the gauge field but also the nonadiabatic component. While only the adiabatic contribution is important in the strong-coupling regime, it is completely canceled by the nonadiabatic contribution in the weak-coupling regime, and the THE in the weak-coupling regime is governed by the remaining nonadiabatic contribution. We found a new weak-coupling region that cannot be accessed by a simple perturbation theory, where the Hall conductivity is linearly proportional to the exchange splitting of the electron spectrum and also to the electron (effective) mass. This formula seems to offer an important clue about the origin of the giant THE observed in Ce-doped CaMnO₃ thin films [2]. Our recent efforts in this direction will also be presented, which may include the consideration of (canted) antiferromagnetism and strong electron correlation.

These works have been done in collaboration with K. Nakazawa (Osaka U.), J. Nakane (Nagoya U.) and M. Bibes (CNRS).

References

[1] K. Nakazawa, M. Bibes, and H. Kohno, J. Phys. Soc. Jpn. 87, 033705 (2018).
[2] L. Vistoli, W. Wang, A. Sander, Q. Zhu, B. Casals, R. Cichelero, A. Barthélémy, S. Fusil, G. Herranz, S. Valencia, R. Abrudan, E. Weschke, K. Nakazawa, H. Kohno, J. Santamaria, W. Wu, V. Garcia, and M. Bibes, Nature Physics (2018).

Spin-orbit coupling plays a central role in the anisotropic magnetoresistance (AMR) and anomalous Hall effect (AHE) of itinerant ferromagnets and in materials with topologically non-trivial electronic states. Recently, both AMR and AHE have also attracted significant attention in antiferromagnetic metals and semiconductors. For example, significant AHEs in non-collinear antiferromagnets have been discovered. Another phenomenon of recent interest is the potential coupling between the orientation of the Néel vector and the topology of the electronic states in antiferromagnets with symmetry-protected Dirac and/or Weyl. Both AHE and AMR can serve as signatures of such interactions. While an elegant theoretical framework exists for the intrinsic AHE and its connection to the Berry curvature of the electronic bands, AMR remains comparatively less well understood. The degenerately doped antiferromagnetic semiconductor EuTiO₃ is a unique testbed for these ideas for several reasons. Despite a small net magnetization, it exhibits an intrinsic AHE that changes sign as a function of the carrier density. We report on the symmetry of the anisotropic magnetoresistance (AMR) in doped EuTiO₃ films as a function of the applied magnetic field. Multiple transitions in the AMR symmetry are observed and are attributed to magnetic field induced changes in the band topology. In particular, at high fields a transition from positive to negative magnetoresistance is observed, which coincides with change from four-fold to two-fold symmetry in the AMR. This indicates a non-trivial phase transition in the electronic structure. We discuss the results in the context of Weyl points that form in the band structure of the EuTiO₃ as a function of magnetic fields.

Quantum properties in oxides have been well explored in correlated electron systems with rich interplay among charge-spin-orbital degrees of freedom in electrons, now extending the playground to a variety of topics such as spin-orbit, Weyl, and edge states. Research on quantum transport in high electron mobility oxides started from such conventional semiconductors as ZnO and SrTiO₃ (STO). To extend the research further, EuTiO₃ (ETO) is an ideal system by adding a knob of magnetic control (local 4f⁷ moment on Eu²⁺). We employ a metalorganic gas source molecular beam epitaxy (MOMBE) at very high substrate temperature that enabled us to observe quantum Hall effect in STO quantum well [1]. The mobility in ETO films has reached to 300 and 3,000 cm²V^-1s^-1 at 2 K, respectively, for strained and strain-free states. Those films show unique quantum properties due to crystal-field and Zeeman splittings, respectively.
In the case of strained films, we observed additional terms in the anomalous Hall effect during the magnetization process, which is not proportional to the magnetization, caused by the type II Weyl nodes in the conduction band [2]. For the strain-free films, Shubnikov–de Haas (SdH) oscillations turned out to be clearly observable at the ferromagnetic state. Our band calculation suggests that the oscillating bands, originally derived from Ti 3d orbitals, are fully spin polarized and modified by the f-d coupling between Eu 4f and Ti 3d orbitals. These new findings strongly suggest that the electron-doped ETO film with high mobility is an ideal magnetic semiconductor to explore novel magneto ‘quantum’ transport phenomena.

[1] Y. Matsubara, K. S. Takahashi et al., Nat. Commun. 7 11631 (2016).
[2] K. S. Takahashi et al., Sci. Adv. 4 eaar7880 (2018).

Ionic substitution during the growth forms an essential pathway to manipulate the carrier density, leading to a rich spectrum of electronic states in strongly correlated systems. To obtain a further control of the material functionalities after the growth, the electrostatic charge modulation methods through dielectrics, ferroelectrics and ionic liquids have been widely employed, demonstrating great reversible tunability. However, an intrinsic limitation of these electrostatic approaches is that they are only effective for materials with thicknesses of a few nanometers, due to the short screening length of the charge carriers. Recently the electric-field induced ionic evolution demonstrates a great tunability of bulk compounds. Among the studies, the hydrogen ion (proton) attracts particular attention due to its comparatively small radius as well as easyaccessibility. In this talk, we will demonstrate an efficient and reversible control of the carrier density in a series of strongly correlated oxide systems through the ionic liquid gating induced protonation. The insertion of protons electron-dopes the materials, leading to an exotic magnetic phase transition along with the increase of proton concentration. We envision that electric-field controlled protonation opens a new avenueto systematically control the electronic and magnetic phase diagram in strongly correlated complex oxide systems.

Oxygen defects can have a profound effect on the physical properties of transition metal oxides. In complex oxides where magnetic, ferroelectric and superconducting phases emerge from strong correlations between localized transition metal electrons, oxygen vacancies can radically alter a plurality of quantum phenomena via valance changes and structural phase transitions. The ability to control the concentration and profile of oxygen vacancies in oxide nanostructures would thus open up comprehensive prospects for new functional ionic devices.

Here, we use in situ transmission electron microscopy (TEM) to demonstrate reversible switching between uniform structural phases in epitaxial La_2/3Sr_1/3MnO₃ films. In our experiments, we employ a piezo-controlled probing holder to apply short voltage pulses and local strain. Simultaneous high-resolution imaging and resistance probing under zero strain reveals reproducible voltage-induced transformations between a low-resistance perovskite phase, a high-resistance La_2/3Sr_1/3MnO_2.5 brownmillerite structure, and an intermediate-resistance perovskite-like phase [1]. Reversible horizontal migration of oxygen vacancies within the manganite film, driven by combined effects of Joule heating and bias voltage, predominantly triggers the structural and resistive phase transitions. Concurrent application of perpendicular strain and voltage pulses produces an entirely new structural phase whereby oxygen vacancies order in regular 3D rather than 2D patterns.

[1] L. Yao, S. Inkinen, and S. van Dijken, Nature Commun. 8, 14544 (2017)

Charge ordering and the localization of electrons in periodic wells is an intrinsic property of extended solids. Synthetic approaches that allow for precise control over this property are greatly desirable; however, systematically modulating periodic electron localization with some measure of tunability of the electron migration barriers represents a difficult challenge. The wider energy dispersion of bands when directly compared to discrete molecular orbitals in single molecules typically favors far greater delocalization of electrons and the multiplicity of sites implies that dimensional confinement of carriers can be established only for low-dimensional crystallographic motifs. One promising approach to modifying carrier density is by varying the electronic coupling across adjacent metal sites, but such attempts often result in phase transitions to entirely different crystal structures. Given the complexity of the problem, the chosen chemical system should exhibit electronic behavior spanning extremes between highly correlated and itinerant. Vanadium oxides represent such a system due to availability of multiple accessible redox states and an unequaled variety of structural motifs that can accommodate the intercalation of ions spanning the breadth of periodic table to create a diverse set of ‘bronzes’ with the stoichiometry M_xV₂O₅. Finally, vanadium has narrow V 3d-bands and the resulting oxides tend to crystalize in low dimensional motifs which surprisingly have the ability to avoid collapse upon topochemical transformations. Successfully achieving precise control over the strength of electron correlation in this system has significant implications for the design of materials in disparate fields. Two such examples include the design of multivalent ion cathode materials and the design of materials that exhibit controlled and reversible electronic instabilities.
As a first example, charge localization in battery cathode materials represents a significant obstacle. In fact, coupling of a highly localized electron to a phonon mode hinders diffusion of the donated electrons through the vanadium oxide framework and must be addressed through chemical modification of the cathode material. Although moving beyond Li⁺ to Mg²⁺ and other divalent species represents the holy grail of sustainable battery chemistry, charge localization for the doubly-polarizing divalent ions becomes a significant problem, with only a small number of materials capable of this difficult feat. An alternative approach to mitigate the self-trapping of polarons is to utilize metastable phase space to design vanadium oxide frameworks that mitigate charge localization in α-V₂O₅. We have recently shown that metastable phases (ζ-V₂O₅ and γ’-V₂O₅) introduce frustrated coordination environments which facilitate cation diffusion and mitigate charge localization, enabling the reversible intercalation of Mg²⁺ and Ca²⁺, respectively.
As a second example, one solution to the breakdown in Dennard scaling between transistor size and power density is to replace the traditional metal-oxide-semiconductor field-effect transistors (MOSFET) with novel computing architectures that encode complexity through highly parallelized operations. Highly correlated materials which teeter at the precipice of an electronic transition are of interest because they can switch internal resistance values (often from metallic to insulating) rapidly upon external perturbation of the system; however, tailoring the temperature or gate-voltage threshold of these electronic transitions is critical. Modulation of this threshold and its magnitude requires precise control over charge localization and electron diffusion barrier to promote electronic transitions. We have recently reported examples of method for tuning electron correlation in M_xV₂O₅ bronzes with electronic instabilities through dimensional reduction, interlayer separation, and stoichiometry.

Sr₂RuO₄ is an unconventional superconductor with potentially a spin-triplet, odd-parity superconducting ground state. There are many reports of high purity single crystals of Sr₂RuO₄ with a T_c of up to 1.5 K. Furthermore, recent studies on Sr₂RuO₄ single crystals have shown that the T_c can be further increased up to 3.5 K using uniaxial strain. To date, however, there are only three published reports of superconducting Sr₂RuO₄ thin films and the T_cs achieved are significantly below 1.5 K. This relative paucity of superconducting thin films is likely due to the extreme sensitivity of the odd-parity superconducting ground state in Sr₂RuO₄ to disorder. According to recent theoretical predictions biaxially strained epitaxial thin films with isotropic in-plane strain can potentially maintain the topologically nontrivial p_x ± ip_y superconducting ground state while simultaneously enhancing T_c by tuning the Fermi level towards a van Hove singularity. Thin films also provide a pathway for scalability, which is critical for potential practical applications of spin-triplet superconductors such as qubits for ground-state quantum computing. Here, we outline and demonstrate a thermodynamic growth window to achieve repeatable growth of superconducting Sr₂RuO₄ with higher T_c than all prior thin films using molecular-beam epitaxy. We will also present some preliminary evidence of epitaxial strain-induced tuning of T_c.

We will present a study of Josephson junctions created with direct-write focused helium ion irradiation. Josephson devices were created by irradiating a narrow channel across a superconducting electrode with a focused helium ion beam. We studied how the irradiation dose affected the electrical transport properties such as critical current and voltage state resistance. For lower doses of irradiation, the critical current rapidly increased with decreasing temperature consistent with a metallic barrier junction. Whereas for higher doses the critical current approaches a maximum value like that seen in conventional insulating barrier Josephson junctions. Furthermore, we observed a continuous transition from metallic to an insulating behavior in the voltage state resistance with increasing dose. This transition can be modeled with a single parameter related to barrier strength which is directly proportional to the irradiation dose.

In the quest for analogs to high-T_c cuprates, nickelates stand out as a promising candidate and have received considerable research interest, due to their electronic structure potentially being in proximity to that of the cuprates. The strategy is therefore to engineer the electronic structure of nickelates to resemble the key ingredients of cuprates, such as a half-filled single band near the Fermi level, antiferromagnetic correlations in the undoped parent compound (S= ½), and hybridization between d_x²_-y² (e_g) orbitals and oxygen ligands. Here, we present the synthesis of an infinite-layer nickelate (LaNiO₂) thin film, where the Ni ions have square planar coordination with O in quasi-2D NiO₂ planes [1], using a low temperature metal hydride reduction process [2]. We study its electronic structure, in comparison with that of an isostructural cuprate (SrCuO₂) thin film, by using x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) at the O K- and Ni L-edges.

[1] V. I. Anisimov, D. Bukhvalov, and T. M. Rice, Phys. Rev. B 59, 7901 (1999).
[2] M. Kawai, S. Inoue, M. Mizumaki, N. Kawamura, N. Ichikawa, and Y. Shimakawa, Appl. Phys. Lett. 94, 082102 (2009).

Two dimensional electron systems (2DES) formed at the interface between insulating transition metal oxides have demonstrated an extraordinary range of properties. The coexistence among these properties can be studied vie electric field effect, making oxide 2DES an ideal test bench for the investigation of novel quantum phenomena.
A notable example is the coexistence between superconductivity and Rashba spin-orbit coupling in the 2DES at the interface between LaAlO₃ and SrTiO₃ (LAO/STO). We will review the recent remarkable progresses in realization of complex LAO/STO superconducting nanodevices [1], which can shed some light on the nature of superconductivity in LAO/STO, and focus on indications of the presence of an unconventional superconducting order parameter obtained in LAO/STO based Josephson junctions [2].
The large and electric field tunable Rashba spin-orbit coupling shown by the LAO/STO 2DES is also of interest for possible spintronic applications [3]. A viable route for electric field control of the spin transport in spintronic devices requires, however, the creation of a spin polarized current. We will present the transport properties of a spin polarized oxide 2DES realized using a thin layer of delta doping EuTiO₃ (ETO) intercalated between LAO and STO [4] and demonstrate how ferromagnetism and Rashba spin-orbit coupling in this heterostructure can be controlled vie electric field effect and temperature [5].


[1] G.Cheng et al., Nature 521, 196 (2015); L. Kuerten et al., Phys. Rev. B 96, 014513 (2017); G.E.D.K Prawiroatmodjo et al., Nat. Comm. 8, 395 (2017), H, Thierschmann et al., Nat. Comm. 9, 2276 (2018)
[2] D. Stornaiuolo et al., Physical Review B, 95, 140502(R) (2017)
[3] E. Lesne et al., Nature materials (2016)
[4] D. Stornaiuolo et al., Nature Materials 15, 278-283 (2016).
[5] D. Stornaiuolo et al., Physical Review B 98 (7), 075409 (2018)

Geometric phases in condensed matter play a central role in topological transport phenomena such as the quantum, spin and anomalous Hall effect (AHE). In contrast to the quantum Hall effect - which is characterized by a topological invariant and robust against perturbations - the AHE depends on the Berry curvature of occupied bands at the Fermi level and is therefore highly sensitive to subtle changes in the band structure. A unique platform for its manipulation is provided by transition metal oxide heterostructures, where engineering of emergent electrodynamics becomes possible at atomically sharp interfaces. We demonstrate that the Berry curvature and its corresponding vector potential can be manipulated by interface engineering of the correlated itinerant ferromagnet SrRuO3 (SRO). Measurements of the AHE reveal the presence of two interface-tunable spin-polarized conduction channels. Using theoretical calculations, we show that the tunability of the AHE at SRO interfaces arises from the competition between two topologically non-trivial bands. Our results demonstrate how reconstructions at oxide interfaces can be used to control emergent electrodynamics on a nanometer-scale, opening new routes towards spintronics and topological electronics.

Magnetic property of perovskite SrRuO₃ (SRO, a ferromagnetic metal) epitaxial thin films is strongly affected by the Ru-O bonding geometry, electronic structure, and dimensionality. Therefore, atomic-scale precision oxide superlattice composed of SRO and SrTiO₃ (STO, a nonmagnetic insulator) is an attractive model system to modulate and study the complex magnetic interaction. Various magnetic phenomena such as interlayer exchange coupling, quantum confinement, and spin-lattice coupling near the oxide interfaces can be investigated. In this presentation, we precisely and systematically control the number of atomic unit cell layers of SRO and STO in atomically designed SRO/STO superlattices. By controlling both the thicknesses of SRO and STO layers using pulsed laser epitaxy, we first demonstrate the structure-dependent magnetization of the artificial crystal. A strong correlation between the lattice symmetry (or Ru-O bonding geometry) and ferromagnetic ordering could be realized. In addition, we observed exotic magnetic interlayer exchange coupling between the ferromagnetic SRO layers across the insulating STO layer. Possible mechanisms for the unprecedented magnetic ground state behavior are suggested and discussed.

Emergent topologies in ferroelectric heterostructures—the polar analogs of magnetic vortices and skyrmions—have become a recent topic of interest for their potential to host exotic functionalities (e.g., emergent chirality and negative capacitance). These topologies can be stabilized in low-dimensional ferroelectrics. Superlattices of PbTiO₃/SrTiO₃ have proven to be particularly fruitful in this area; for example, the existence of polar vortex structures in such superlattices grown on DyScO₃ substrates has been demonstrated. Furthermore, polar skyrmion structures have been observed in PbTiO₃/SrTiO₃ superlattices grown on SrTiO₃ substrates using a combination of scanning transmission electron microscopy, X-ray diffraction, and second-principles calculations. Using resonant soft X-ray diffraction, we study the chirality of these polar skyrmions and show that the skyrmions have a preferred handedness. The origin of the circular dichroism is shown to be a chiral configuration of the titanium orbitals in the skyrmions.

The multiferroic BiFeO₃ (BFO) is an interesting material to explore correlated electronic conduction. Here, we substitute divalent Ca ions into the parent BFO and apply an external electric field at elevated temperatures to spatially redistribute spontaneously created oxygen vacancies, thereby generating hole carriers in regions of less dense oxygen vacancy concentrations. X-ray diffraction and photoemission spectroscopic measurement are employed to quantify a large variation of local oxygen vacancy concentration as much as ~10²¹ cm^-3 and explore the consequent evolution of electronic band structure. We find that a non-rigid polaronic band is created by hole doping as a result of a strong electron-lattice coupling. We also show strong evidence for the disorder-driven formation of a Coulomb glass state through electronic transport measurement on a quantitative level. These spectroscopic and transport results can be combined and understood in the framework of intrinsic spatial inhomogeneity of polaronic charge density. Dynamical properties of oxygen vacancy in motion will be also discussed in a spatial-resolved way. The finding not only provides a promising material for low-temperature oxygen conduction, but also offers an alternative pathway for visualization and quantification of defects dynamics.

Understanding and manipulating oxygen octahedral rotations have been subjects of much recent interest, and particular octahedral rotations are now parameters used in the design of new perovskite materials and heterostructures. As examples, modifications to the Ni-O-Ni bond angle in the nickelates have a profound effect on the temperature of metal–insulator transition, and changes to the octahedral rotation angles in the manganites promote different types of antiferromagnetic order. Although ferroelectricity is not conventionally related to oxygen octahedra, it has been predicted that geometrically modulated octahedral rotations can induce ferroelectric behavior, which is promising for the design of cross-coupled multiferroics. Control over the oxygen octahedral framework is widely recognized as key to the design of functional properties in perovskite oxide heterostructures. Although the oxygen octahedral framework can be manipulated during synthesis, the as-grown oxygen octahedra generally remain fixed, preventing the development of adaptive behavior in electronic and ionotronic systems. Theoretical calculations have shown that the coupling between oxygen octahedra and an external electric field is typically very weak if present at all, due to the geometrical centricity of octahedra. Therefore, it is considered extremely challenging to dynamically tune octahedral rotation by electric field.

In this talk, we will demonstrate that oxygen octahedral rotations can be dynamically and reversibly manipulated by an electric field when in the presence of defects, leading to significant changes in the electronic properties of a perovskite oxide during ionic liquid gating. Employing in situ synchrotron X-ray techniques to investigate heterostructures of the simple perovskite WO3, we find that while application of a negative voltage leads to only subtle changes in structure and electronic properties, positive voltages have a dramatic effect on the oxygen octahedral rotation due to defect injection. The process is fully reversible, with the material returning to its original state after the gate voltage is removed. The results of density functional theory show that some octahedral rotation angles depend more strongly on the oxygen vacancy concentration than others and that both rotations and defects alter the electronic structure. Ionic liquid gating not only allows switchable defect structure and behavior by an electric field, but also provides a means of attaining the dynamic manipulation of octahedral rotations and the associated correlated properties. Such tunable properties can be used to establish nonequilibrium rotation patterns not available by static methods and facilitate the development of low-power electronics. Our results illustrate a highly effective approach for dynamically tuning the oxygen octahedral rotation in perovskite heterostructures for applications in oxide electronics and ionotronics.

Vanadium dioxide (VO₂) can be switched from an insulating to a metallic state with either ultrafast laser pulses [1] or heating above T_MIT=340K [2,3]. In both the photoexcited and thermal transitions, the insulator-to-metal transition (IMT) is accompanied by a structural change from a monoclinic (M1) to a rutile (R) structure, and numerous prior efforts have focused on elucidating the evolution of both the electronic and lattice degrees of freedom. The photoexcited transition occurs in a time scale of hundred femtoseconds, and while ultrafast x-ray diffraction has provided tremendous insight into the atomic dynamics during such ultrafast transformations, diffraction peaks alone probe only an average over multiple unit cells and are less sensitive to deviations from the average atomic path connecting the initial and final state. To overcome this limitation, we use femtosecond total x-ray scattering (diffuse and Bragg) from the LCLS x-ray free-electron laser to study the dynamics of the structural transition of bulk VO₂ at all length-scales [1]. We observe that the structural transition proceeds by uncorrelated disordering of the vanadium ions from their initial dimerized distribution [1], rather than the previously proposed synchronized motion along an optical phonon mode. After photoexcitation, the system explores a large volume of the available phase-space in a timescale comparable with a single phonon oscillation [1,2]. Our ab-initio molecular dynamics simulation quantitatively match our ultrafast x-ray scattering measurements, and show an unusual highly anharmonic, flat potential energy surface for the quasi-rutile structure in the photoexcited state, developing on femtosecond timescales and disrupting the vanadium dimers of the M1 phase by populating a continuum of modes. The rapid evolution after photoexcitation is enabled by the large phase space of phonon modes with low-frequency V-vibrations, which was also noted in [2] to yield a large phonon entropy gain stabilizing the rutile phase, and the strong damping of phonons in the rutile phase. Our current observations thus reveal an interesting parallel between the ultrafast and the thermal transitions. These results overturn our current understanding of an archetypal ultrafast phase transition and provide new microscopic insights into rapid evolution toward equilibrium in photoexcited matter.

[1] S. Wall*†, S. Yang,* L. Vidas, M. Chollet, M. Glownia, M. Kozina, T. Katayama, T. Henighan, M. Jiang, T. A. Miller, D. A. Reis, L. Boatner, O. Delaire† and Mariano Trigo†, “Ultrafast disordering of vanadium dimers in photoexcited VO₂”, Science (2018). DOI:10.1126/science.aau3873

[2] J. D. Budai*, J. Hong*, M. E. Manley, E. D. Specht, C. W. Li, J. Z. Tischler, D. L. Abernathy, A. H. Said, B. M. Leu, L. A. Boatner, R. J. McQueeney, and O. Delaire, “Metallization of vanadium dioxide driven by large phonon entropy”, Nature 515, 535–539 (2014).

[3] S. Lee, K. Hippalgaonkar, F. Yang, J. Hong, C. Ko, J. Suh, K. Liu, K. Wang, J. J. Urban, X. Zhang, C. Dames, S. A. Hartnoll, O. Delaire†, J. Wu†, “Anomalously low electronic thermal conductivity in metallic vanadium dioxide”, Science, 355 (6323): 371 (2017)

The development of intense ultrashort mid and far infrared laser sources has created an opportunity for functional materials based on the direct excitation of infrared active phonons. Strong excitation of infrared active phonons can produce sizable unidirectional distortions of crystal structure through non-linear coupling to other phonons. Complex oxide ceramics provide an important test-ground for this experimental approach due to their diversity, strong coupling to optical fields, and demonstrated connection between subtle structural changes and functional properties.

Our recent theoretical efforts in perovskite oxides explore selective control of functional properties that exploit nonlinear lattice dynamics induced by the excitation of infrared phonons. Using first-principles techniques we predict that ultrafast optical control of magnetism is experimentally accessible in rare-earth titanates and show that, when combined with strain, this optical technique exposes a magnetic phase inaccessible in the equilibrium phase diagram.

Er₂Sn₂O₇ has been proposed as an example of the dipolar XY antiferromagnetic (AFM) pyrochlore. This system is theorized to undergo a first order phase transition to an ordered phase known as the Palmer-Chalker (PC) state. The behavior of Er₂Sn₂O₇ has been found to be somewhat enigmatic, however, with neutron scattering measurements revealing the emergence of the PC state at a lower temperature than predicted, via an apparently second order transition. In addition, slow dynamics have been observed throughout the low temperature regime, including within the PC state. These deviations from theoretical expectations have been attributed to quantum fluctuations resulting from a competition between classical ordered phases. We report on field dependent specific heat measurements on Er₂Sn₂O₇. In zero field, we find a slightly higher phase transition temperature than previously reported. In finite field, we find a reentrant phase diagram that compares well quantitatively with classical Monte Carlo simulations only in the high field region of the phase diagram. This provides further evidence of strong quantum fluctuations in Er₂Sn₂O₇ in low fields, suggesting its proximity to a quantum disordered regime.