Symposium S.NM11—Topological and Quantum Phenomena in Oxides and Oxide Heterostructures

Many physical phenomena in condensed matter arise from strong correlation and spin-orbit coupling (SOC), and their proper control is important for creating novel quantum materials (QMs), such as Mott insulators, quantum spin liquids, topological insulators, Weyl semimetals, etc. to name a few. As the importance of quantum information science emerges, there is an increased attention to QMs to address the need for new materials for addressing challenges in quantum computing and sensing. Among many classes of QMs, complex oxides offer a rich playground to exploit both correlations of Coulombic interactions between electrons often found in 3d transition metal oxides (TMOs) and SOC from heavy metal based 5d TMOs, such as SrIrO₃ and Na₂IrO₃, leading to many intriguing physical properties. Hybrid materials with a proper balancing of the correlation and SOC offer unique opportunities to discover new properties, and heterostructuring or combining dissimilar materials is a great approach to harnessing the complex interplay between quantum wavefunctions and various control parameters, such as proximity, dimensionality, topology, and symmetry. In this talk, I will review recent progresses in topological quantum materials and present our recent work on topological Hall effect from chiral magnetism and its control via interfacial symmetry control. Furthermore, an effort to combine topology and correlation to develop correlated topological materials and an approach to develop novel Berry phase engineered heterostructures will be presented.

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

Complex topological configurations are a fertile area to explore novel emergent phenomena and exotic phases in Condensed-Matter Physics. The discovery of magnetic skyrmions one decade ago [1] have triggered a flurry of research on these particle-like nanometer-sized topologically protected spin textures, mostly motivated by their potential use in spintronic devices such as race track memories [2]. Since then, researchers have long wondered whether ferroelectrics may present topological textures akin to magnetic skyrmions and chiral bubbles. In this quest, the side by side advances in experimental growth and characterization techniques, and the development of new modelling tools are of paramount importance. In this respect, the emerging field of second-principles simulations [3-4], where an effective model is constructed by a judicious choice of the essential physics, and the parameters of the model are extracted by fitting to Density Functional Theory, are proving increasingly valuable in the study of large systems, overlapping in size with those that can be currently grown by atomic layer deposition methods. As a result of this continuous feedback between theory and experiment, the recent discovery of polarization vortices [5] and their associated complex-phase coexistence and response under applied fields in superlattices of (PbTiO₃)_n/(SrTiO₃)_n [6] suggests the presence of a complex, multi-dimensional system capable of emergent physical responses, such as a chirality [7]. In this talk, I shall describe the latest developments to create, tune, and characterize topological textures in ferroelectrics [8,9]. Polar structures with a well-defined topological charge and chirality will be presented. The stabilization of such non-uniform polarization topology results in highly enhanced susceptibilities, and also provides a pathway for engineering novel functionalities previously inaccessible, such as regions with negative capacitance [10].
Financial support from the Spanish Ministry of Economy and Competitiveness through Grant PGC2018-096955-B-C41

References
[1] N. Nagaosa and Y. Tokura, Nature Nanotechnology, 8, 899-911 (2013), and references therein.
[2] S.S. Parkin, M. Hayashi, and L. Thomas, Science, 320, 190-194 (2008)
[3] J. Wojdel et al., J. Phys.: Condens. Matter, 25, 305401 (2013)
[4] P. García-Fernández et al., Physical Review B, 93, 195137 (2016)
[5] A.K. Yadav et al., Nature, 530, 198-201 (2016)
[6] A.R. Damodaran et al., Nature Materials, 16, 1003-1009 (2017)
[7] P. Shafer et al., Proceedings of the National Academy of Sciences of the United States of America, 115, 915-920 (2018)
[8] M. A. Pereira Gonçalves et al., Science Advances, 5, eaau7023 (2019)
[9] S. Das et al., Nature 568, 368-372 (2019)
[10] A.K. Yadav et al., Nature, 565, 468-471 (2019)

Photoinduced topotactic reduction of colloidal iron oxide nanocrystals
Magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) nanocrystals have been extensively studied due to their superparamagnetic behavior that potentiates their application in magnetic storage devices, catalysis, and medical diagnosis and treatment. As the ratio of Fe3+/Fe2+ determines the phase and magnetic properties of iron oxide nanocrystals, controlling the oxidation state of Fe is critical for such applications. Oxidation of Fe₃O₄ to γ-Fe₂O₃ has been thoroughly studied and is known to occur slowly in air or more rapidly with heat or additional oxidants. Reduction of γ-Fe₂O₃ to Fe₃O₄, however, is less well understood. We report reduction of colloidal γ-Fe₂O₃ nanocrystals by UV irradiation in the presence of a sacrificial reductant. Specifically, anaerobic irradiation in the presence of ethanol reduces Fe³⁺ to Fe²⁺, yielding a topotactic reduction of γ-Fe₂O₃ to Fe₃O₄. The properties of the nanocrystals upon photoreduction are monitored by UV/visible/near-IR electronic absorption spectroscopy, superconducting quantum interference device magnetometry and powder X-ray diffraction. Factors influencing the photoreduction, such as the type of hole quencher and nanocrystal size will be discussed. Our findings demonstrate a new route to control the oxidation states of iron species in iron oxides.

Controlling topological properties by varying external parameters is the quintessential step for developing next-generation devices with topological materials. Correlated topological phases (CTPs) can be induced by the interplay of electron correlation and spin-orbit coupling. They are the promising candidates whose band topology can be engineered by breaking the intricate balance between different energy scales. Pyrochlore iridate films belong to a family of cubic 5d transition metal oxides. They are representative systems where various CTPs can be studied by manipulating magnetism with lattice strain. However, their in-situ epitaxial growth has been known to be notoriously difficult. We recently succeeded in developing a new technique to grow in-situ fully strained Nd2Ir2O7 thin films. A fully strained film with antiferromagnetism shows an anomalous Hall conductivity which is an order of magnitude larger than that in the bulk or the relaxed film. Moreover, we observed the large spontaneous Hall conductivity which comes from the strain-induced magnetic T1-octupole. Our work demonstrates that topological properties can be engineered by manipulating magnetism with strain in oxide heterostructures.

Non-coplanar spin textures with scalar spin chirality can gen- erate an effective magnetic field that deflects the motion of charge carriers, resulting in a topological Hall effect (THE) [1–3]. However, spin chirality fluctuations in two-dimensional ferromagnets with perpendicular magnetic anisotropy have not been considered so far. In this talk, I will present evidence of spin chirality fluctuations by probing the THE above the Curie temperature in two very different ferromagnetic ultra-thin films, SrRuO₃ and V-doped Sb₂Te₃ [4]. The temperature, magnetic field, thickness and carrier-type dependence of the THE signal, along with Monte Carlo simulations, suggest that spin chirality fluctuations are a common phenomenon in two-dimensional ferromagnets with perpendicular magnetic anisotropy. Our results open a path for exploring spin chirality with topological Hall transport in two-dimensional magnets and beyond [5–8].

References:
[1] Wen, X. G., Wilczek, F. & Zee, A., Chiral spin states and superconductivity. Phys. Rev. B 39, 11413–11423 (1989).
[2] Taguchi, Y., Oohara, Y., Yoshizawa, H., Nagaosa, N. & Tokura, Y., Spin chirality, Berry phase, and anomalous Hall effect in a frustrated ferromagnet. Science 291, 2573–2576 (2001).
[3] Neubauer, A. et al. Topological Hall effect in the A phase of MnSi. Phys. Rev. Lett. 102, 186602 (2009).
[4] Wang W. et al, “Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular magnetic anisotropy”, Nature Mater., 18, 1054–1059 (2019).
[5] Bonilla, M. et al., Strong room-temperature ferromagnetism in VSe₂ monolayers on van der Waals substrates. Nat. Nanotechnol. 13, 289–293 (2018).
[6] Fei, Z. et al. Two-dimensional itinerant ferromagnetism in atomically thin Fe₃GeTe₂. Nat. Mater. 17, 778–782 (2018).
[7] Deng, Y. J. et al., Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2. Nature 563, 94–99 (2018).
[8] Machida, Y., Nakatsuji, S., Onoda, S., Tayama, T. & Sakakibara, T., Time reversal symmetry breaking and spontaneous Hall effect without magnetic dipole order. Nature 463, 210–213 (2010).

Domain walls naturally arise whenever a symmetry is spontaneously broken. They interconnect regions with different realizations of the broken symmetry, promoting structure formation from cosmological length scales to the atomic level. In my talk, I will present topological 1D spin textures and domain walls with intriguing physical properties, which emerge in spin-spiral multiferroics and chiral magnets and which hold great promise for nanoelectronics and spintronics applications. In particular, I will discuss that a wide variety of new domain walls occurs in the presence of spatially modulated domain states. In contrast to domain walls in conventional ferroics, such domain walls exhibit a well-defined inner structure, which—analogous to cholesteric liquid crystals—consists of topological disclination and dislocation defects. Similar to the magnetic skyrmions, the domain walls can carry a finite topological charge, permitting an efficient coupling to spin currents and contributions to a topological Hall effect. Our studies establish domain walls in chiral magnets as functional nano-objects with non-trivial topology, opening the door to innovative device concepts in information and communication nanotechnology.

SrMnO₃ (SMO) has recently aroused enormous interest because theoretical studies predicted the paraelectric-antiferromagnetic system in its bulk state can be transformed into a new multiferroic system via strain-engineering. However, experimental verification is still inadequate due to high-leakage current density (180 A/cm²) and surface cracks, which are derived from oxygen deficiency of strained SMO thin films. Hence, we devised a selective oxygen annealing (SOA) method effectively filling the oxygen vacancies while protecting the surface of the SMO. By applying this method, clean surface and insulating behavior (current density: 3.8ｘ10^-6 A/cm² and dielectric loss <0.1) are successfully achieved in a 1.7% tensile-strained SMO thin film. This SOA method not only allows direct investigation of the intrinsic multiferroicity of strained SMO but also device application without leakage issues. Based on the obtained insulating behaviors, the SMO thin film shows the room-temperature ferroelectric polarization loop (P_s: 1 μC/cm²) and the dielectric constant (ε_r: 138.1). Moreover, a significant reduction of the dielectric constant from 138.1 to 6.7 is observed by systematically releasing the epitaxial strain. These findings can be important experimental evidence of phase transition from paraelectric to ferroelectric phase via strain-engineering as theoretically predicted. Furthermore, since SMO can be optionally a low-k or high-k material, it can be applied to a wide range of capacitors or memory devices. We also believe that our SOA method will be an important key to open the next-step of multiferroic SMO by enabling us to directly investigate its intrinsic characteristics.

The tunable resistivity of materials undergoing a metal-insulator transitions (MIT) holds great promise for resistive switching applications, such as adaptable electronics and cognitive computing. However, a complete understanding of the metallic phase in these strongly correlated electron systems is still one of the central open problems in condensed matter physics.
Here we will present a detailed analysis of the electrical resistivity exponent of thin films of NdNiO₃ as a function of epitaxial strain. Strain-free thin-films show a linear dependence of the resistivity versus temperature, consistent with a classical Fermi gas ruled by electron-phonon interactions. In addition, the apparent temperature exponent, n, can be tuned with the epitaxial strain between n= 1 and n= 3. We discuss the critical role played by quenched random disorder in the value of n. Our work shows that the assignment of Fermi/Non-Fermi liquid behaviour based on experimentally obtained resistivity exponents requires an in-depth analysis of the degree of disorder in the material.
This work is being considered for publication as Qikai Guo, Saeedeh Farokhipoor, Cesar Magen, Francisco Rivadulla and Beatriz Noheda, “Tunable resistivity exponents in the metallic phase of epitaxial nickelates” and it can be found at
https://nature-research-under-consideration.nature.com/users/37265-nature-communications/posts/53928-tunable-resistivity-exponents-in-the-metallic-phase-of-epitaxial-nickelates,
and at https://arxiv.org/abs/1909.06256

Majorana zero mode (MZM) can be used in fault-tolerant quantum computation relying on their non-Abelian braiding statistics, therefore, lots of efforts have been made to find them. Signatures of the MZMs have been reported as zero energy modes in various systems. As predicted, MZM in the vortex of topological superconductor appears as a zero energy mode with a cone like spatial distribution. Also, MZM can induce spin selective Andreev reflection (SSAR), a novel magnetic property which can be used to detect the MZMs. Here, I will show you that the Bi₂Te₃/NbSe₂ hetero-structure is an artificial topological superconductor and all the three features are observed for the MZMs inside the vortices on the Bi₂Te₃/NbSe₂. Especially, by using spin-polarized scanning tunneling microscopy/spectroscopy (STM/STS), we observed the spin dependent tunneling effect, which is a direct evidence for the SSAR from MZMs, and fully supported by theoretical analyses. More importantly, all evidences are self-consistent. Our work provides definitive evidences of MFs and will stimulate the MZMs research on their novel physical properties, hence a step towards their statistics and application in quantum computing. Finally, the possible application in topological quantum computing is discussed.

References
[1] Mei-Xiao Wang, et al., Science 336, 52-55 (2012)
[2] J.P. Xu, et al., Phys. Rev. Lett. 112, 217001 (2014)
[3] J.P. Xu, et al., Phys. Rev. Lett. 114, 017001 (2015)
[4] H.H. Sun, et al., Phys. Rev. Lett. 116, 257003 (2016)
[5] H.H. Sun, Jin-Feng Jia*, NPJ Quan. Mater. 2, 34 (2017)

Topological materials host exotic properties such as dissipationless spin and charge currents or Majorana fermions that could find numerous technological applications, for example in low-power electronics and spintronics or in quantum computers. However, most topological properties are currently restricted to extremely low temperatures, precluding applications.

In this talk, we will describe our first principles quantum mechanical calculations to understand the microscopic origin of the interplay between temperature and topological order in materials. Our work reveals that this interplay is dominated by a combination of thermal expansion and electron-phonon coupling, and that increasing temperature should destroy topological order in most materials, in agreement with all available experimental reports. Interestingly, our results also suggest avenues for designing materials with the opposite behaviour, and we present a polymorph of PbO₂ as the first example of a material in which increasing temperature promotes rather than destroys topological order.

The Mott insulator/ band insulator interface of LaTiO₃/ SrTiO₃ was first shown to exhibit metallicity due to electronic reconstruction by Ohtomo and Hwang in 2002. This work led to significant research activity exploring emergent metallicity at complex oxide interfaces – most notably at the LaAlO₃/SrTiO₃ interface. However there has been comparably less study of the LaTiO₃/ SrTiO₃ interface versus the LaAlO₃/SrTiO₃ interface. We have recently found that the metallic behavior at the LaTiO₃/ SrTiO₃ interface is accompanied by giant Rashba spin-orbit coupling. Evidence for this giant Rashba spin splitting is manifest in two sets of Shubnikov-de Haas oscillations associated with an inner and outer Fermi surface, Berry phase of π, substantial anisotropic magnetoresistance and a weak anti-localization correction to the magnetoconductivity. Together these results indicate a giant Rashba coupling which is an order of magnitude larger than other complex oxide interfaces such as LaAlO₃/SrTiO₃. By gating the samples with an electric field, we observe (i) quantized resistance peaks which are independent of sample dimensions in a conventional 6-terminal Hall bar geometry, (ii) a strong negative magneto-conductance near the quantized peaks in resistance, and (iii) nonlocal transport in a 4-terminal geometry. All of these observations are consistent with edge state conduction and the quantum spin Hall effect.

^†This work was funded by the Vannevar Bush Fellowship of the Department of Defense under Contract No. N00014-15-1-0045.

The unconventional, iron-based superconductor FeSe has a low T_c in bulk form at 8 K but sees a massive enhancement up to as high as ∼109 K when monolayers are interfaced with the complex oxide SrTiO₃. The origin of this T_c enhancement is due to both electron transfer from SrTiO₃ to FeSe as well as interfacial electron-phonon interactions. Since FeSe monolayers are highly unstable in air, protective layers must be grown on top to enable ex-situ study or application. These capping layers limit the T_c enhancement to ∼30 K. In order to preserve the high T_c as well as environmentally stabilize the FeSe layers, we have explored the growth of novel SrTiO₃/FeSe superlattice heterostructures by pulsed laser deposition (PLD). Starting with TiO₂ terminated SrTiO₃ single crystals as the growth substrate, alternating FeSe and SrTiO₃ layers are grown in a reducing, high vacuum environment. Typically, SrTiO₃ growth takes place in an oxygen background, but here we have achieved low to zero defect SrTiO₃ layers by controlling the kinetic energy of the laser-generated plasma plume by monitoring the plasma properties in-situ with planar and cylindrical Langmuir probes. The kinetic energy cut-off for both the SrTiO₃ and FeSe plume is kept below ~30 eV by balancing the laser energy and spot size on the ablation targets. Adjusting the kinetic energy allows control over the oxygen defects, which dope electrons into the FeSe, while the superlattice structure enables double-sided interfacial electron-phonon interactions between SrTiO₃ and FeSe. We report on both in-situ and separate, extensive plasma measurements during PLD conditions used for superlattice growth. The plasma density can be fine-tuned over orders of magnitude between 1×10¹⁸-1×10²⁰ m^-3 by changing the spot size on the target while utilizing the same laser fluence, which changes the growth rate. The kinetic energy distribution of the plasma plume constituents is also highly tunable below the plasma absorption threshold. Ions and neutrals can be easily generated with kinetic energies as high as 150 eV if large laser spot sizes greater than ∼6 mm² are used. However, it is by decreasing the spot size down to 3.5 mm² or less, and thereby reducing the high-energy tail of the kinetic energy distribution, that layers with significantly fewer defects are produced. We also report on x-ray diffraction and x-ray photoelectron characterization of the superlattices.