There have recently been numerous demonstrations of the formation or melting of magnetic or electronic order in transition metal oxides using coherent intense THz light pulses. Here we use first-principles calculations to understand the origin and dynamics of the structural and electronic processes induced by excitation of an optical phonon in the high-temperature superconductor, YBa₂Cu₃O_6+x. First we introduce the three lowest order anharmonic phonon couplings -- bilinear, cubic and quartic --that are accessible in such experiments, and calculate their strengths using density functional theory. We then solve the equations of motion with our calculated couplings, and compute the structural modulations caused by excitation of a polar phonon. We find that, for the usually accessible range of excitation strength, cubic coupling dominates, and gives rise to a quasi-static structural modulation that is large enough to significantly affect the electronic properties. For increased excitation strength, such as might be accessible at a THZ free electron laser source, we find that quartic coupling becomes relevant and adds an additional dynamic shear strain to the structural distortion. At both levels, we find changes in the magnetic exchange interactions caused by the modulations, which might correlate with the reported persistence of superconductivity above the usual transition temperature on THz excitation of YBa₂Cu₃O_6+x. Finally, we discuss the application of our approach to dynamically driven magnetic and electronic phase transitions in other complex oxides.
Mankowsky, R. et al. Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa₂Cu₃O_6.5. Nature 516, 71-73 (2014).
Hu, W. et al. Optically enhanced coherent transport in YBa₂Cu₃O_6.5 by ultrafast redistribution of interlayer coupling. Nature Mat. 13, 705-711 (2014).

Ferroelectric materials comprise non-centrosymmetric unit cells with permanent electric dipole moments switchable by electric fields, exhibiting strong coupling between polarization, strain, and electronic degrees of freedom. The dynamics of the ferroelectric polarization underlies their functionality but these processes, and the speed limits determining how fast the polarization can change, remain largely unknown. I will describe here two recent experiments using strong terahertz fields as an all-optical bias to manipulate the polarization in ferroelectric thin films. First, using nonlinear optical probing techniques sensitive to the non-centrosymmetry of the unit cell, I will describe how these fields lead to giant modulations in the nonlinear optical response of Sm-BiFeO₃ thin films near the morphotropic phase boundary between the ferroelectric and paraelectric phase, corresponding to on-off ratios of 220x gateable on femtosecond time-scales. These effects are further increased using metallic electrode structures to enhance the applied fields while probing the dynamics within the electrode structure. Second, I will describe measurements carried out at the Linac Coherent Light Source using femtosecond x-rays to probe the atomic-scale response of BaTiO₃ (001) thin films following terahertz excitation. These measurements show ultrafast modulations in the unit-cell structure factor associated with out-of-plane modulations in the ferroelectric polarization, developing on few picosecond time-scales, and long-lived transient shifts of the rocking curve peak, driven by vibrational excitation.

Many unusual phenomena are supported by non-equilibrium phases of materials. An intriguing recent example is the occurrence of ferroelectricity in some of the metastable phases of hafnia. Commonly known structural phases of hafnia are centrosymmetric, and thus nonpolar. Hence recent observations of ferroelectric behavior of hafnia (when doped with Si, Zr, Y, Al, or Gd) are rather surprising as ferroelectricity requires the presence of switchable spontaneous electrical polarization. It has been hypothesized that certain non-equilibrium phases of hafnia are stabilized by dopants, and these could lead to ferroelectricity. If the origins of this unexpected ferroelectricity can be understood and appropriately harnessed, materials based on hafnia (and other simple oxides like zirconia and titania) may find applications in nonvolatile memories and ferroelectric field effect transistors. In this talk, we will outline a 2-step strategy, based on first principles computations that may be used to systematically identify ferroelectric phases, which can either be equilibrium or non-equilibrium, of a given material. First, a computation-based structure search method is used to identify low-energy phases. Then, group theoretical symmetry reduction principles are used to single out ferroelectric phases. This strategy has been used to understand the curious ferroelectric behavior of doped hafnia, zirconia and titania.
References
[1] T. D. Huan, V. Sharma, G. A. Rossetti, Jr., and R. Ramprasad, Phys. Rev. B 90, 064111 (2014)

Discovery of new nanomaterial functionalities lie at the core of technological advancement, driving ample efforts in the fundamental and applied research areas. A particular topic of interest is to obtain novel transient material states, which are not available or understood yet in steady state. Here, this approach is applied to a rich class of materials: the case of complex oxides with ferroic properties. Our experimental studies uncover novel paths in light-induced manipulation of coherently strained, ferrolectric superlattices, which are used to tune their ultrafast ferroic responses at the nanoscale.
In order to explore the ultrafast transient properties of high quality ferroelectric PbTiO3/SrTiO3 superlattices, we have used near bandgap femtosecond optical excitation for the PbTiO3 nanolayers, while maintaining the optical excitation energy below the absorption edge of the SrTiO3 nanolayers. This approach is very useful for enhancing the contrast in the local excitation spectrum, while offering opportunities to monitor individual relaxation and interactions of the superlattice constituents.
The dynamical ferroelectric response of these superlattice samples are interrogated using time-resolved second harmonic generation combined with time-resolved synchrotron X-ray scattering. With this scheme, we observed excitation dependent enhancement and suppression of ferroelectricity spanning from fs to ns timescales, in contrast to the behavior of bulk or single layer PbTiO3 samples. We have observed that that local excitation of PbTiO3 nanolayers from the superlattice, has a high degree of localization, with a characteristic relaxation time of ~ 20 ns, while in surprising contrast, the SrTiO3 layers remain unexcited during the same temporal interval. Moreover, the anisotropic, short-range and long interaction ultrafast responses will presented, including time-dependent SHG polarimetry, Bragg X-ray diffraction and nanodomain X-ray scattering. In addition, we will discuss other selective excitation mechanisms such as the resonant infrared optical mode absorption.
VS, JF, HW, and VG acknowledges the department of energy grant DE-SC0012375 for ultrafast optical measurements ay Penn State and ultrafast X-ray measurements performed at Argonne National Labs. The use of Advanced Photon Source was supported by the U.S Department of Energy, Office of Science, Office of Basic Energy Sciences, DE-AC02-06CH11357.

Guided by theory, unparalleled properties in oxide heterostructures—those of hidden ground states—are being unleashed in the static realm by exploiting epitaxial engineering. One method is strain engineering, where the misfit strain imposed by an underlying substrate makes it is possible to strain oxide thin films to percent levels—far beyond where they would crack or plastically deform in bulk. Other static methods we have been utilizing are epitaxial stabilization, dimensional confinement, and polarization doping. Three examples utilizing a mix of these static techniques of controlling structure will be shown in hopes of stimulating a discussion of related opportunities utilizing dynamic optical methods. These examples are (1) transmuting a material (EuTiO₃) that is not ferroelectric or ferromagnetic in its unstrained state, into a ground state that is both at the same time; (2) utilizing a combination of strain and dimensional confinement to create a tunable dielectric (Sr₇Ti₆O₁₉) with world record figure of merit at room temperature; and (3) epitaxial stabilization of rumpling in combination with polarization doping to create a strong magnetically-ordered ferroelectric at nearly room temperature in (LuFeO₃)₉/(LuFe₂O₄)₁ superlattices.
(1) J.H. Lee, L. Fang, E. Vlahos, X. Ke, Y.W. Jung, L.F. Kourkoutis, J-W. Kim, P.J. Ryan, T. Heeg, M. Roeckerath, V. Goian, M. Bernhagen, R. Uecker, P.C. Hammel, K.M. Rabe, S. Kamba, J. Schubert, J.W. Freeland, D.A. Muller, C.J. Fennie, P. Schiffer, V. Gopalan, E. Johnston-Halperin, and D.G. Schlom, Nature466 (2010) 954-958.
(2) C.H. Lee, N.D. Orloff, T. Birol, Y. Zhu, V. Goian, E. Rocas, R. Haislmaier, E. Vlahos, J.A. Mundy, L.F. Kourkoutis, Y. Nie, M.D. Biegalski, J. Zhang, M. Bernhagen, N.A. Benedek, Y. Kim, J.D. Brock, R. Uecker, X.X. Xi, V. Gopalan, D. Nuzhnyy, S. Kamba, D.A. Muller, I. Takeuchi, J.C. Booth, C.J. Fennie, and D.G. Schlom, Nature502 (2013) 532-536.
(3) J.A. Mundy, C.M. Brooks, M.E. Holtz, J.A. Moyer, A.F. Rebola, H. Das, S.M. Disseler, J.T. Heron, R. Held, R. Hovden, E. Padgett, Q. Mao, H. Paik, R. Misra, J.A. Borchers, W.D. Ratcliff, L.F. Kourkoutis, C.J. Fennie, P. Schiffer, D.A. Muller, and D.G. Schlom (unpublished).

The study of complex materials and heterostructures composed of them requires a detailed knowledge of the composition, atomic structure, electronic structure and magnetic order of both bulk samples and interfaces or buried layers. E.g., it is clear that interfaces in complex oxide heterostructures often represent emergent materials that possess surprising properties not associated with the parent oxides, such as two-dimensional electron gases (2DEGs), superconductivity, and magnetism. Photoemission (PS) and angle-resolved photoemission (ARPES) represent techniques of choice for such studies, but as often carried out have certain limitations in being too surface sensitive and in not being able to measure truly bulk properties or to focus specifically on buried interfaces or layers. In this talk, I will discuss combining two newer elements of PS/ARPES to deal with this challenge:
bull; The use of soft x-rays in the ca. few hundred-to-2000 eV regime (SXPS) [1], as well as into the true hard x-ray regime >2000 eV (HXPS or HAXPES) [2], to probe more deeply into the structure,
bull; Tailoring of the x-ray intensity profile, either by generating a strong standing wave (SW) through reflection from a multilayer heterostructure [3], or Bragg planes in a crystal [4],[5], or by going into the near total reflection (NTR) regime of incidence angles [6], so as to provide much enhanced depth resolution. The relative advantages of soft/hard x-ray PS and ARPES and their complementarity to conventional VUV ARPES in the ca. 5-150 eV regime will be discussed [1]. As few illustrative examples are: Hard x-ray ARPES (HARPES) has been used to study the bulk electronic structure of the dilute magnetic semiconductor (Ga,Mn)As [2(b)]. Combining SW-PS and SW-ARPES, it has been possible to measure for the first time the detailed concentration profiles and momentum-resolved electronic structure at the SrTiO₃/La_0.67Sr_0.33MnO3 interface [3] and to directly measure the depth profile of the 2DEG at the SrTiO₃/GdTiO₃ interface [5]. Using NTR HXPS, the thickness of the 2DEG at the interface between BiFeO₃ and Ce-doped CaMnO₃ has also been determined [6]. Future directions for such measurements will also be discussed.
References:
[1] C.S. Fadley, Synchrotron Radiation News 25, 26 (2012).
[2] (a) A. X. Gray et al., Nature Materials 10, 759 (2011); (b) A.X. Gray et al., Nature Materials 11, 957 (2012).
[3] A.X. Gray et al., Phys. Rev. B 82, 205116 (2010) and Europhys. Letters 104, 17004 (2013).
[4] S. Thiess,T.-L. Lee, F. Botti, J. Zegenhagen, Solid State Comm. 150, 553 (2010)
[5] S. Nemsak et al., to be published.
[6] M. Marinova, J. E. Rault, et al. Nano Letters 15, 2533 (2015).

We present atomic scale determination of the interfacial reconstruction at complex oxide interfaces using quantitative scanning transmission electron microscopy (STEM). Many exciting new properties at oxide heterostructures are often linked to the atomic scale details of the interfacial structure, such as the epitaxial strain, connectivity of the atoms, and lattice distortion, such as octahedral tilts. When the interface is formed between two oxides with a polarity conflict, the interface may exhibit structural reconstruction that is confined within a single unit-cell. Understanding such reconstructions is therefore crucial to identifying the structural origins of the important phenomena at interfaces. However, examining such spatially confined structures requires a characterization technique that uses an atomically small probe. In this work, we determined the atomic reconstruction at the KTaO₃/GdScO₃ system using quantitative STEM. Quantitative STEM measures the scattering from individual atomic columns on an absolute scale. This enables a direct comparison between the experimental and simulated STEM images, which allows for the interpretation of images in terms of the atomic species and number of atoms in the column. From one single quantitative STEM image, we identify several important aspects of the interfacial reconstruction, including the sequential ordering of different atomic species along the depth direction (parallel to the electron beam). High depth precision is achieved by using an aberration-corrected STEM equipped with a high-brightness electron source. The depth information is then combined with other information from the image, including the cation displacement, lattice distortion, and oxygen octahedral tilt, to generate a comprehensive 3D picture of the reconstruction at the oxide interface. This atomic reconstruction pattern can then be directly related to the properties of the film.

Discovery and improvement of fast oxygen ion conductors are of significantly importance to many technologically important devices, such as oxygen pumps, oxygen separation membranes, and solid oxide fuel cells (SOFC). In this work, we show that the oxygen ion transport properties of SrCrO_3-d can be effectively modified through interface mechano-chemical coupling. Specifically, in-plane tensile strain applied by an SrTiO₃ substrate at the heteroepitaxial interface can be employed to stabilize the nanostructure of a rhombohedral SrCrO_2.8 (R-SCO) phase, which would otherwise (e.g., R-SCO on LaAlO₃(001)) go through a phase transition into perovskite SrCrO₃ (P-SCO) in an oxidizing environment at temperatures above 250^oC. Fast oxygen ion transport is achieved through the preserved ordered oxygen vacancy channels (OVC) in R-SCO. The oxygen transport pathways, kinetics, and mechanisms are investigated by means of (environmental) transmission electron microscopy, isotope labeled secondary ion mass spectrometry, and density functional theory calculations. We show the potential to combine high oxygen reduction, fast oxygen ion transport, and low electronic conductivity in one material. This work highlights the importance of interface engineering and post-synthesis processing in designing new perovskite related materials with enhanced ion transport properties while maintaining high structural stability.

This talk provides an overview of theoretical issues relevant to the new generation of ultrafast experiments in which an initial laser pulse excites a high density of carriers above the gap of a correlated insulator. A physical picture of the post-pump time evolution is presented. The key roles played by inter-orbital, intra-orbital and across band-gap relaxation processes are explicated. Different regimes of carrier excitation (laser fluence) are delineated. Time scales are estimated and compared to experiment. In certain regimes, energy barriers suppressing relaxation to the equilibrium state are documented and a first-order energy landscape is found, suggesting the possibility of a long-lived metastable conducting phase. The theoretical and computational challenges associated with performing a full nonequilibrium dynamical mean field calculation are summarized. Progress, based on memory function and bold-line quantum Monte Carlo techniques, towards meeting the computational challenges is outlined. Open issues are discussed.
This work was performed in collaboration with G. Cohen, E. Gull, Z. He and D. Reichman and was supported by the US Department of Energy under grant DE-SC00123175

One of the great drivers for understanding the spectacular, yet complex properties of strongly correlated electron materials is the desire to ultimately transcend the use of semiconductor electronics. The need to establish speed limits for electronic switching is clearly important for technology. However, it also serves fundamental science: it enables us to disentangle competing interactions in the time domain. In this talk I will focus on the insulator-to-metal transitions in magnetite (Fe₃O₄) and vanadium dioxide (VO₂). A combination of fs optical spectroscopy and fs x-ray diffraction at the Linac Coherent Light Source can be used to unravel their mechanism by separating electron and lattice dynamics. I will also show how intense THz pulses will ultimately allow us to address the feasibility of future oxide based electronics devices.

The macroscopic response of numerous correlated transition metal oxides is strongly influenced by mesoscopic phenomena including phase coexistence. This manifests in the dynamic response of these materials that can be probed - with the looming grand challenge of control - using ultrafast optical pulses. The insulator to metal transition (IMT) is a particularly rich example, with numerous interesting questions being experimentally addressed in materials that include manganites, vanadates, and nickelates. For example, what are the characteristic timescales of the IMT? Can a phase transition be driven along unique pathways, and can long-lived nonequilibrium states be accessed? I will present the results of some of our recent investigations of IMT dynamics utilizing optical-pump terahertz-probe spectroscopy to measure conductivity changes with sub-picosecond resolution. For example, we have investigated LCMO thin films that have been strain-engineered to quench the thermal IMT. This yields, at all temperatures, a robust insulating phase resulting from strain-enhanced orthorhombic distortion of the underlying octahedral framework. Photoexcitation initiates a nonthermal IMT to a persistent “hidden” metallic phase. Our measurements reveal that the transition is cooperative and ultrafast, requiring a critical absorbed photon density to activate local charge excitations that mediate magnetic-lattice coupling stabilizing the metallic phase.

Strongly correlated materials possess unique properties which make them strong candidates for a wide range of technological applications. One such material is vanadium dioxide (VO₂) which shows an insulator-to-metal transition (IMT) around 70^oC under ambient conditions making it a possible candidate for low-power steep slope transitions. In concert with the IMT, VO₂ undergoes a structural transition from an insulating monoclinic to a conductive rutile phase. Despite a large amount of research the role of the structural transition in the IMT is currently debated. Most of this research on the IMT has focused on thermally driven IMT in VO₂. Consequently, not much is known about the associated structural dynamics of VO₂ that accompany the electronically driven IMT and their impact on electrical devices incorporating VO₂. Using time-resolved x-ray nanodiffraction microscopy we study the time evolution of the accompanying structural transition to the electronically driven IMT.
In this work we investigate the associated structural evolution of the electronically driven IMT in 10 nm VO₂ ultrathin films grown on TiO₂ (001) substrates by molecular beam epitaxy that have been fabricated into 2-terminal electrical devices. The x-ray diffraction studies were conducted on the 2-ID-D beamline at the Advanced Photon Source at Argonne National Laboratory. In-operando time-resolved x-ray diffraction measurements show aftershocks to the VO₂ atomic structure that persist on a time scale four to six times longer than the simultaneously observed electrical IMT. These aftershocks are inconsistent with the structural changes observed in VO₂ for a thermally driven IMT and are spatially inhomogeneous across the VO₂ 2-terminal device. The largest change in the VO₂ atomic structure is observed where the edge of the electrode meets the patterned VO₂ channel. Additional XRD measurements show that the TiO₂ substrate also undergoes a large structural distortion away from the bulk rutile phase that closely mimic the aftershocks observed in the VO₂ thin film. We find that the changes to the TiO₂ atomic structure are highly asymmetric within the device, can penetrate up to 15 µm under the electrode, and persist on longer time scales up to ten times longer than electrical IMT. Changing the current flow within the device has no impact on the observed asymmetry indicating that the fabricated device geometry plays an important role in shaping the structural distortions. These results highlight the complex interactions that can occur between the substrate and strongly correlated thin films which can play a critical role in device switching times and properties. Lastly, these observed states and may provide another route to stabilize transient states.

Ultrafast light excitation is proving to be a powerful tool to control electronic, spin, and structural behavior in solids. Our studies have revealed that surprising non-equilibrium phases can emerge from targeted, collective excitation at THz frequencies. In certain organic and high-Tc cuprate materials, light can generate transient superconducting-like phases, even at or near room temperature. I will discuss our recent work and possible mechanisms driving the formation of these states.

Ultrafast manipulation of materials by light has recently attracted much attention due to its technological application to optical and electronic devises. Typically, many studies on the ultrafast photo-induced dynamics involve spatially-averaged information, overlooking local variation in the dynamics such as domain-dependent ultrafast response in ferroic materials. Here we report spatially-resolved photo-induced structural dynamics of a multi-domain ferroelectric BaTiO₃ single crystal at room temperature. Although BaTiO₃ is a classic ferroelectrics, it has been recently found that metastable monoclinic phases are coexisting with conventional tetragonal phases at room temperature, at which this material is in the vicinity of “thermotropic” phase boundary [1]. It is important to understand the spatially-resolved photo-induced dynamics towards the achievement of optical control of such metastable phases.
We have performed laser-pump hard x-ray microdiffraction-probe measurements at the beam line 7-ID-C of the Advanced Photon Source in Argonne national laboratory. Upon optical excitation by a 400 nm, 60 fs laser pulses, the x-ray diffraction intensity were monitored using 11 keV x-ray pulses in the specular geometry at various delay times scanning across 90° (a-c) domain walls. The profiles of (200) and (002) reflection intensity, which monitors a and c domains with electric polarization parallel and perpendicular to the sample surface, respectively, are consistent with linear and nonlinear optical microscopic images. The analysis of temporal variation in lattice constants reveals that the tetragonal BaTiO₃ phase becomes more “cubic” by irradiation of 400 nm light, and that the a domains show stronger response compared to the c domain. A damped oscillation was observed due to a strain wave bouncing back and forth at domain boundaries. The mechanisms behind the observed behavior will be discussed in terms of carrier mediated piezoelectric effects, heating effect, and their effects on the domain boundaries.
VS, HW and VG were primarily supported by the Department of Energy grant DE-SC0012375 for the ultrafast synchrotron x-ray work. HF and GS were supported by the National Science foundation DMR-1420620; their participation in this work was partially supported by the DOE DE-SC0012375. Work at Advanced Photon Source was supported by the U.S Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
References
[1] T. T.A. Lummen et al., Nature Comm.5, 3172 (2014).

The dynamics of optically-excited electrons and holes at femtosecond time scales and nanometer length scales is critical in many applications including photovoltaics, photocatalysis and photodetectors. Ultrafast experiments provide insights into the relaxation of non-equilibrium carriers at the tens and hundreds of femtoseconds time scales, but do not yet directly probe shorter times with nanometer spatial resolution. Theoretical calculations can access these scales and complement such experiments, but have so far been primarily restricted to free electron models applicable to simple metals.
We combine first principles calculations of electron-electron and electron-phonon scattering rates with Boltzmann transport simulations to predict the ultrafast dynamics and transport of carriers in real materials. In particular, we calculate the distributions of hot carriers generated by plasmon decay and their transport in aluminum and noble metal nanostructures. We use a fully relativistic DFT+U approach to accurately describe the metal band structures in comparison to photoemission spectra, and calculate carrier generation due to direct and phonon-assisted transitions. At plasmon energies above the interband threshold, copper and gold generate significantly hotter holes, whereas aluminum and silver generate equi-energetic electrons and holes. Below threshold, phonon-assisted transitions generate uniform carrier energy distributions in all metals. We solve a spatially-resolved steady-state linearized Boltzmann equation with ab initio collision integrals from our DFT+U method for the subsequent carrier transport. We find small transport distances < 10 nm for high-energy holes and larger transport distances > 100 nm for the lower energy electrons. These findings help select materials and geometries for plasmonic hot carrier applications depending on the required carrier types and energies.
We also predict the evolution of electron distributions in ultrafast experiments on noble metal nanoparticles from the femtosecond to picosecond time scales, using a modified two-temperature model as well as a full time-dependent Boltzmann equation with ab initio collision integrals. We find that the positions of the d bands and the energy dependence of the electron-phonon matrix elements substantially alter the effective electronic heat capacity and electron-lattice coupling compared to previous simplified theoretical approaches and are important for interpreting high-power ultrafast optical measurements on plasmonic nanoparticles.
This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

Two decades ago it was discovered that excitation of a metallic ferromagnet with an intensive femtosecond laser-pulse causes an ultrafast demagnetization within 300fs [1]. Until today, the mechanism(s) underlying fs laser-induced demagnetization could not be definitely uncovered and remain a controversial issue. Several theories have been proposed - mostly based on the assumption that there must exist some ultrafast dissipation channel of spin angular momentum. One of the proposed fast spin-flip channels is the Elliott-Yafet electron-phonon spin-flip scattering [2], another one is that of relativistic spin-photon interaction [3].
We review the current understanding of the ultrafast demagnetization and outline how the proposed mechanisms could be probed using ab initio calculations. To examine electron-phonon spin-flip scattering we compute ab initio the spin-flip Eliashberg function, from which we calculate the demagnetization for various situations, viz. equilibrium distributions, hot electron distributions in the thermalized regime, and laser-induced nonequilibrium conditions [4]. Thermalized hot electron distributions lead only to a very small demagnetization rate, whereas a larger net demagnetization is computed for nonequilibrium conditions. However, the nonthermal contribution is not sufficient to explain the measured fs demagnetization. We furthermore perform ab initio calculations of the relativistic spin flips due to spin-photon interaction which is found to give only a small magnetization change. Following a different rationale we have developed a model for fs laser-induced magnetization dynamics, based on the high mobility of laser-excited spin-polarized electrons [4]. We establish the influence of fast electron dynamics of excited nonequilibrium electrons and show that this provides spin-transport in the super-diffusive regime, causing effectively demagnetization. We find that super-diffusive flow of hot electrons can account for the experimentally observed demagnetization within 200fs in Ni and explains well observations of ultrafast laser-induced spin currents and THz emission due to femtosecond spin currents.
[1] E. Beaurepaire, J.-C. Merle, A. Daunois, J.-Y. Bigot, Phys. Rev. Lett. 76 (1996) 4250.
[2] B. Koopmans et al, Nature Mater. 9 (2010) 259.
[3] J.-Y. Bigot, M. Vomir, E. Beaurepaire, Nature Phys. 5 (2009) 515.
[3] K. Carva, M. Battiatio, D. Legut, P.M. Oppeneer, Phys. Rev. B 87 (2013) 184425.
[4] R. Mondal, M. Berritta,K. Carva, P.M. Oppeneer, Phys. Rev. B 91 (2015) 174415.
[5] M. Battiato, K. Carva, P.M. Oppeneer, Phys. Rev. Lett. 105 (2010) 027203.

Advances in extreme nonlinear optics make it possible to efficiently upshift tabletop femtosecond lasers into the ultraviolet (EUV) and soft X-ray regions of the spectrum, to wavelengths as short as 8 Å.[1-3] This unique high harmonic (HHG) light source is ideally suited for host of applications in understanding how advanced materials function.[4-8] A host of applications in nanoscience and nanotechnology have now been demonstrated using EUV HHG, including probing the elastic properties of thin films of thicknesses < 5nm, implementing full-field microscopes with record 22nm spatial resolution, quantifying how nanoscale energy flow differs from bulk, measuring how fast a material can change its electronic or magnetic state, or how spin currents can control and enhance magnetization in ultra thin films, and visualizing the dynamic band structure of material.
J. Miao, T. Ishikawa, I. Robinson & M. Murnane, “Beyond Crystallography: Diffractive Imaging with Coherent X-ray Sources”, Science348, 530 (2015).
Popmintchev et al., “Bright Coherent Ultrahigh Harmonics in the keV X-Ray Regime from Mid-Infrared Femtosecond Lasers”, Science336, 1287 (2012).
Kfir et al., “Generation of bright circularly-polarized extreme ultraviolet high harmonics for magnetic circular dichroism spectroscopy”, Nature Photonics9, 99-105 (2015).
Hoogeboom-Pot et al., “A new regime of nanoscale thermal transport: collective diffusion increases dissipation efficiency”, PNAS 112, 4846-4851 (2015).
Turgut et al., “Controlling the Competition between Spin Transport and Optically Induced Demagnetization in Magnetic Multilayers,” Physical Review Letters110, 197201 (2013).
Mathias et al., “Probing the timescale of the exchange interaction in a ferromagnetic alloy”, PNAS109, 4792 (2012).
Hellmann et al., "Collapsing gaps and dominant interactions in layered charge-density-wave compounds", Nature Communications3, 1069 (2012).
Seaberg et al., "Ultrahigh 22 nm Resolution Coherent Diffractive Imaging using a Desktop 13 nm High Harmonic Source," Optics Express19, 22470 (2011).

Charge ordering (CO) in complex oxides plays a vital role in collective phenomena including insulator-to-metal transition, high T_c superconductivity and colossal magneto-resistance. Despite the importance of electronic ordering, the understanding of microscopic mechanisms and the interplay between multiple degrees of freedom remains a challenge. To shed light on the interaction between electronic and lattice order on mesoscopic time scales, we directly characterize the recovery dynamics of charge ordering, lattice and optical conductivity upon optical excitation in a La_1/3Sr_2/3FeO₃ thin film by time-resolved x-ray diffraction and optical pump-probe measurements. The recovery of the commensurate CO dramatically slows down by more than two orders of magnitude when the sample temperature is elevated close to the CO transition temperature. In contrast, the lattice recovery remains at a constant rate independent of sample temperature, governed by the thin film cooling process. The recovery of the optical reflectivity also shows similar slowing down, directly linking the charge ordering to the optical conductivity of the film in the time domain. The deviation of the electronic ordering recovery dynamics from the lattice highlights the nontrivial recovery processes including charge ordered domain growth and interaction.

Strongly correlated materials often undergo ordering transitions where the interactions between electrons, orbitals, and the crystal lattice give rise to the formation of material domains with different physical properties. The manganites in particular are known to undergo orbital ordering, where the physical orientation of the electronic orbitals spatially order into one of two equally probable orientations. Electronic transport in the manganites is highly dependent on the orbital domain orientation. However, it is unclear which degree of freedom drives the formation of the orbitally-ordered state.
To investigate this, we use intense THz light to non-resonantly control the orientation of orbital domains in La_1.5Sr_0.5MnO₃. Our results demonstrate orbital switching in excess of 10% without any direct THz coupling mechanism to the material. To understand the THz coupling mechanism, we construct a Hubbard model, which we use to show that domain control can be achieved either through field assisted hopping of charges or a field-induced modification of bond angles. Both routes enhance the local Coulomb repulsions which ultimately drive orbital ordering. Our results highlight the role of Coulomb interactions in driving orbital order in manganites and demonstrate how THz can be utilized in new ways to understand and manipulate anisotropic phases in a broad range of correlated materials

The interest in manipulating magnetic order by ultrashort laser pulses has thrived since it was observed that such pulses can be used to alter magnetization within picoseconds. In many cases demagnetization by laser heating dominates the dynamics. In ferromagnetic Gd:EuO, however, we demonstrate a non-thermal mechanism that allows the magnetic order of a material to be enhanced or attenuated at will [1]. This is possible in systems simultaneously possessing a low, tuneable density of conduction band carriers and a high density of magnetic moments. In such systems adiabatic processes instead of thermalization according to the three-temperature model dominate the photoinduced change of the magnetic order. On the other hand, a laser field may be used to entirely reverse a magnetic state all-optically by exerting a rectified magnetic poling field which, in complement with laser heating, promotes the switching process. All-optical reversal of an antiferromagnetic state, however, is a challenge because of the lack of a macroscopic magnetic moment to couple to. Here, we show reversible all-optical switching of the antiferromagnetic order in multiferroic TbMnO₃ [2]. We manipulate the antiferromagnetic state via the rigidly coupled ferroelectric state by using a unique relation between the direction of the spontaneous polarization state and the wavelength of the light pulse. We show that such all-optical magnetoelectric switching is a fully reversible process that works unobstructed by the anisotropy of the antiferromagnetic and ferroelectric system. Access to the ultrafast dynamics of the magnetically ordered state in Gd:EuO and TbMnO₃ is provided by second harmonic generation.
[1] M. Matsubara et al., Nature Comm. 6, 6724 (2015)
[2] M. Matsubara, S. Manz et al., Science 348, 1112 (2015

Magnetic skyrmions have been identified in several magnetic materials [1]. Their spatial extent depends on the magnetic interactions involved; some chiral magnets produce skyrmions with the size of 3-200 nm, which are suitable for magnetic information devices. These skyrmions are structurally stable, can exist as isolated particles or form a lattice, and show emergent interactions with electrons and external electromagnetic fields, leading to, e.g., topological and Skyrmion Hall effects [1]. It has been demonstrated that the skyrmion can be driven under an extremely small charge/spin current [2] in metals. Whereas in insulators, electric-fields can induce their motion without dissipation. The optical response/control of this nano-magnetic structure, in an ultrafast manner, is of practical importance.
In this paper, we report real-time dynamics of magnetic skyrmions in Cu₂OSeO₃, an insulating multiferroic ferrimagnet [3], studied by all-optical spin-wave spectroscopy. The spin sector was excited by the inverse-Faraday effect in the optical gap, without heating the sample. We revealed collective spin dynamics in the skyrmion phase distinct from those in the surrounding conical phases, consistent with the photoexcitation of gyration and breathing of the skyrmion [4].
In addition to identifying the topological spin phases, we can infer the interactions between the skyrmions and spin waves. As an example, we demonstrate an optical-drive of magnetic bubbles, which can be topologically equivalent to skyrmions. In this case, magnetoelastic waves, coupled propagation of magnon and phonon, are photoexcited in iron garnet films. These magnetic excitations interact with bubbles and domain walls, more efficiently when the domain wall has steeper curvature.
[1] N. Nagaosa and Y. Tokura, Nature Nanotech.8, 899 (2013). [2] Z. X. Yu et al., Nature Commun.3, 988 (2012). [3] S. Seki et al.,Science336, 198 (2012). [4] N. Ogawa et al., Sci. Rep. 5, 9552 (2015).

The strongly correlated electrons in transition metal perovskites contribute to a wide range of novel electronic phases. LaVO₃ with a partially full d-shell is expected to be metallic, but due to electron-electron interactions a gap emerges and the ground state is a Mott insulator. Such effects depend on the electron overlap, and, therefore, are a strong function of the bonding geometry, and particularly the V-O-V bond angle. Controlling these structural effects on the ultrafast time scale can lead to control over the underlying electronic ground state. Here we report the ultrafast structural dynamics of 25 and 50 nm thick LaVO₃ thin films grown by the hybrid molecular beam epitaxy technique [1] on SrTiO₃ when excited across the bandgap by 400 nm light. Using time-resolved x-ray diffraction on the 100 ps time scale at Sector 7 of the Advanced Photon Source, we directly measured the structural changes with atomic accuracy by monitoring (0 0 3) and (0 1 2) integer Bragg diffraction peaks and find a large out-of-plane strain of 0.18% upon optical excitation; the recovery time is ~1 ns for the 25 nm film and ~2 ns for the 50 nm film, consistent with the thermal transport from the film to the substrate. Further, we will discuss the response of the oxygen octahedral rotation patterns indicated by changes of the (1/2 0 5/2), (0 3/2 5/2) and (3/2 1/2 1/2) half-order diffraction peaks. Understanding such ultrafast structural deformation is important for optimizing optical excitations to create new metastable phases starting from a Mott insulator.
[1] H.-T. Zhang, L. R. Dedon, L. W. Martin, and R. Engel-Herbert, Appl. Phys. Lett. 106, 233102 (2015).
Thin film growth and travel expenses were supported by the Department of Energy under Grant No.DE-SC0012375. The use of Advanced Photon Source was supported by the U.S Department of Energy, Office of Science, Office of Basic Energy Sciences, DE-AC02-06CH11357.

Transparent conducting oxides (TCO) have become critical in a range of applications including photovoltaics, displays and windows. In the last five years there has been a dramatic upswing in the development of new multifunctional transparent contacts with significant improvement in conductivity and processibility. None the less, there is a compelling need for higher transmissivity (wider bandgap) and lower electron affinity alternatives for applications in power electronics, LEDs, and photovoltaics. To this end Ga₂O₃ is a promising TCO material in part to its low electron affinity (3.7 eV for β-Ga₂O₃) and large band gap of 4.8 eV. Conductive Ga₂O₃ is obtained through the extrinsic doping with elements such as Si or Sn but this is not as effective as doping in other oxides such as ZnO and In₂O₃. The reason for this is not entirely clear. To address this, we are working to answer fundamental questions about the conductivity mechanisms. In this work, we will demonstrate a clear understanding of how different fabrication conditions influence the doping environment. We use synchrotron based techniques to characterize Ga₂O₃:Sn to understand the activity of Sn in Ga₂O₃. We find that Sn dopants in the 4+ oxidation state are able to donate electrons and therefore increase conductivity. We show results of x-ray diffraction (XRD) to determine the crystal structure of Ga₂O₃ films, and extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) to determine the local structure of Ga₂O₃ and the oxidation state of the Sn in the samples. These results are compared to density functional theory (DFT) calculations of Sn doping into Ga₂O₃. Films were prepared by pulsed laser deposition (PLD). We employed combinatorial PLD samples which were synthesized with both thermal and compositional gradients across two different axes of the films. These “combi” PLD samples range from amorphous to nanocrystalline along the low (280°C) to high temperature (520°C) gradient and have an approximate 50/50 ratio of 4+ to 2+ Sn oxidation states across this range. Even though the oxidation state of these films does not appear to change as a function of deposition temperature, the conductivity of these films drops as they become amorphous. Additionally, single composition epitaxial Ga₂O₃ samples are prepared on sapphire substrates at a range of deposition temperatures, with the Sn oxidation state being mostly 2+ at low temperatures and transitioning to 4+ at higher temperatures. These results are developing a correlation between conductivity and the local structure and oxidation state of Sn doped Ga₂O₃, which can provide valuable information for targeting synthetic conditions to make more highly doped materials.

The concept of optically breaking crystal symmetries to manipulate electronic properties has garnered significant attention in condensed matter physics. While much experimental emphasis in non-equilibrium physics has so far been placed on transient melting, enhancement or induction of electronic order, a particularly tantalizing prospect concerns the possibility to instead access dynamical steady states of matter with distinct nonequilibrium phase transitions, to affect electronic transport. One paradigmatic theoretical model for phase transitions is graphene, where time-reversal symmetry can be broken via circularly-polarized light to dynamically lift point degeneracies at the Dirac points and the bands acquire a non-zero Chern number.
In this talk, I take a step beyond graphene and instead propose to employ light to dynamically drive topological phase transitions in monolayer transition-metal dichalcogenides (TMDCs) including MoS2 , MoSe2 , WS2 and WSe2. I demonstrate that a circularly-polarized pump pulse can selectively induce topological band inversions at either the band valley by instead bringing into resonance three bands originating in a minimal model of planar transition-metal d-orbitals that is required to capture the non-equilibrium physics. Despite the large band gap, the photo-induced topological band gap scales linearly with the applied pump field, leading to chiral edge modes in a TMDC ribbon already at weak pulse intensities that are expected to be well within reach of present experimental efforts.

The recent advancements in laser technology have dramatically expanded the applications of
lasers to table top experiments in condensed matter physics. Femtosecond time-resolved
spectroscopy techniques are emerging tools in the study of quantum materials, offering new
paths to disentangle coexisting phases with similar energy scale, selectively tune a specific
phase across a quantum critical point and create hidden states that do not exist in equilibrium,
to name a few.
In this talk I will present some of our recent work where ultrafast light is used to manipulate
electron charges, spin and lattice to reveal underlying properties in quantum materials, to
drive metal insulator transition, to depin a charge density wave, to destroy superconductivity
and to control spin texture in topological insulators. Future direction in the field will be
discussed.

Topological insulators are novel materials that do not conduct electricity in their bulk but possess exceptional conducting electronic states on their surface. These surface electrons have a number of highly unusual characteristics: (i) they behave like massless relativistic particles similar to photons (ii) their spin is locked perpendicular to their momentum and (iii) this state is robust against moderate disorder. Understanding and characterizing unique properties of these materials can lead to novel applications such as current induced magnetization or extremely robust quantum memory bits. In this talk, I will first give a brief introduction to these materials and then describe our recent experiments in which we used ultrashort laser pulses to probe and control properties of the topological surface states. In particular, I will describe experiments in which we achieved the first observation of Floquet-Bloch states in these materials.
Floquet-Bloch bands emerge in solid-state systems due to a coherent interaction between light and matter. Using time and angle resolved photoemission spectroscopy; we demonstrate that intense ultrashort midinfrared pulses hybridize with the surface Dirac fermions of a topological insulator to form states periodic in both momentum and energy. These states exhibit band gaps at avoided crossings which are dependent on the incident light polarization and the electron momentum. Circularly polarized light induces an additional gap at the Dirac point by naturally breaking time reversal symmetry. We further characterize these bands as a function of the incident light intensity and pump-probe delay time to understand the excitation and decay mechanisms of electrons in the Floquet-Bloch states.

The theoretical prediction of which of the “missing compounds” in a chemical family are stable and synthesizable and which are unstable is a crucial step in the search for previously unknown functional materials. This active area of contemporary research often involves high throughput examination of the total energy of a given compound in a series of candidate “formal structure types” (FSTs), searching for those with the lowest energy. We find, however, that certain classes of FSTs correspond each to a series of chemically distinct “daughter structure types” (DSTs). We show that the high-throughput ab-initio method widely used for structure prediction of hypothetical materials might overlook chemically distinct realizations (daughter structures) of formal structure prototypes of even moderate complexity.Not including in the list of examined candidate structure types such daughter structures can lead to misidentification of the stable structural form of a target compound, with the possibility of clouding ongoing efforts to predict new materials. We illustrate this by showing that the standard high-throughput approach misses the correct structure of the yet-unreported compounds RbCuS and RbCuSe while the symmetry-unconstrained evolutionary global space-group optimization method retrieves their correct stable structures, which we experimentally observed. As a result of this study we propose that unbiased structure optimization become a routine step in the prediction of new functional materials alongside high-throughput screening and data mining.

Topological insulators, Weyl metals and Skyrmions are a hot topic in condensed matter physics and materials science. The excitement in the physics community is comparable with the excitement when a new superconductor is discovered. Heusler compounds are a remarkable class of materials with more than 1,000 members and a wide range of extraordinary multifunctionalities including tunable topological insulators (TI) half-metallic high-temperature ferri- and ferromagnets, compensated ferrimagnets multiferroic shape memory alloys, and with a high potential for spintronics, energy technologies and magnetocaloric applications. The tunabilty of this class of materials is exceptional and nearly every functionality can be designed. Therefore it is not surprising that we were able to design Heusler compounds with a band inversion and a non-trivial topology for multifunctional TI and Heusler compounds with large anisotropic exchanges and strong Dzyaloshinskii-Moriya interaction. Spontaneous Skyrmion can be found acentric Heusler magnetic compound Mn_1.4PtSn.
The topological state in these zero-gap semiconductors can be created by applying strain or by designing an appropriate quantum well structure, similar to the case of HgTe. Many of these ternary zero-gap semiconductors (LnAuPb, LnPdBi, LnPtSb and LnPtBi) contain the rare-earth element Ln, which can realize additional properties ranging from superconductivity (for example LaPtBi) to magnetism (for example GdPtBi) and heavy fermion behavior (for example YbPtBi). These properties can open new research directions in realizing the quantized anomalous Hall Effect and topological superconductors. C1b Heusler compounds have been grown as single crystals and as thin films. The control of the defects, the charge carriers and mobilities can be optimized. The band inversion is proven by ARPES. Heusler compounds are similar to a stuffed diamond, correspondingly, it should be possible to find the “high Z”' equivalent of graphene in a graphite-like structure or in other related structure types with 18 valence electrons and with inverted bands. Dirac cones and Weyl points can occur at the critical points in the phase diagrams of TI. Weyl points, a new class of topological phases was predicted in NbP, NbAs and TaP. We founds ultrahigh magneto resistance, mobilities and Fermi arcs in this class, proving their topological electronic state.

The quantum anomalous Hall (QAH) effect is a quantum Hall effect induced by spontaneous magnetization instead of an external magnetic field. It is the foundation for applications of dissipationless quantum Hall edge states in low-energy-consuming devices and for realization of other novel quantum phenomena such as chiral topological superconductivity and axion electrodynamics. The QAH phase can be realized a ferromagnetic topological insulator (TI) film as a result of magnetically induced gap at the Dirac surface states. With molecular beam epitaxy techniques, we have prepared thin films of Cr-doped (Bi,Sb)₂Te₃ TI with well-controlled composition, thickness and chemical potential and obtained ferromagnetic insulator phase in them. In such magnetic TI films, we have experimentally observed the quantization of the Hall resistance at h/e² at zero field, accompanied by a considerable drop in the longitudinal resistance, unambiguously demonstrating the occurrence of the QAH effect. The temperature, thickness and magnetic-doping-level dependence of the QAH effect have been systematically studied, which clarifies the roles of the band structure, electron localization and magnetic order in the effect and provides clues for obtaining high temperature QAH materials. These results pave the ways for further studies and practical applications of the QAH-related quantum phenomena.

The electric field induced metallization of insulating oxides is a powerful means of exploring and creating novel electronic states. Recently large internal electric fields from polar surfaces have been used to create emergent metallic, superconducting and magnetic states at interfaces between two insulating oxides. However, the origin of the metallicity is a subject of considerable debate, especially as to whether charged carriers are induced electrostatically. We show that by placing various oxide surfaces and thin films in contact with charged fluids these nominally insulating materials can be transformed into metallic conductors and that the mechanism is rather due to the flow of ionic currents of oxygen to and fro between the oxide surface and the liquid^1-4. We discuss, in particular, the electrolyte gating of epitaxial films of vanadium dioxide (VO₂). VO₂ exhibits a transition from an insulating to a metallic state above a metal-insulator transition temperature, T_MIT, that depends on strain induced in the film by epitaxy with underlayers and/or the substrate material and crystal orientation⁵. Using in-situ gating we use x-ray diffraction to show that the out-of-plane lattice constant can be reversibly changed by more than 3.5% using ionic liquid gating⁴. Similar results are found for WO₃ although an important difference is a strong crystal facet dependence of gating for VO₂ whereas WO₃ is gated to the metallic state independent of crystal facet. The possibility of novel, highly energy efficient “liquid” electronics is also discussed.
1 Jeong, J. et al. Suppression of Metal-Insulator Transition in VO₂ by Electric Field-Induced Oxygen Vacancy Formation. Science339, 1402-1405, (2013).
2 Li, M. et al. Suppression of Ionic Liquid Gate-Induced Metallization of SrTiO₃(001) by Oxygen. Nano Lett.13, 4675-4678, (2013).
3 Schladt, T. D. et al. Crystal-facet dependent metallization in electrolyte-gated rutile TiO₂ single crystals. ACS Nano7, 8074-8081, (2013).
4 Jeong, J. et al. Giant reversible, facet-dependent, structural changes in a correlated-electron insulator induced by ionic liquid gating. Proc. Natl. Acad. Sci.112, 1013-1018, (2015).
5 Aetukuri, N. B. et al. Control of the metal-insulator transition in vanadium dioxide by modifying orbital occupancy. Nat. Phys.9, 661-666, (2013).