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  1. Condensed-Matter Physics & Materials Science Seminar

    "Tuning of spin-orbital interactions and charge gap by epitaxial strain in Sr2IrO4"

    Presented by Thorsten Schmitt, Photon Science Division, Paul Scherrer Institut, Switzerland

    Monday, February 24, 2020, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Mark Dean

    The nature of the highly spin-orbit coupled Mott state of Sr2IrO4 suggests the ground state as well as the collective entangled spin and orbital excitations to be strongly dependent on the lattice degree of freedom. For this reason, Sr2IrO4 provides an ideal platform for controlling the physical properties of a correlated material by inducing local lattice distortions. We use epitaxial strain to modify the Ir-O bond geometry and perform momentum-dependent Resonant Inelastic X-ray Scattering (RIXS) both at the metal and ligand sites to unveil the response of the low energy elementary excitations. By applying tensile strain, we observe a large softening of the spin(-orbital) wave dispersion along the [h,0] direction and a simultaneous hardening along the [h,h] direction. This evolution entails a strain-driven crossover from anisotropic to isotropic interactions between the magnetic moments. We also show how the charge excitations are coupled to the lattice in Sr2IrO4. To this end, using O K-edge RIXS, we unveil the evolution of a dispersive electron-hole pair excitonic mode which shifts to lower (higher) energies upon compressive (tensile) strain, manifesting a reduction (increase) in the size of the charge gap. We show that this behavior originates in the modified hopping elements between the t2g orbitals induced by strain. Our work highlights the central role played by the lattice in determining both the spin(-orbital) as well as the charge excitations of Sr2IrO4 and confirms epitaxial strain as a promising route towards the control of the ground state of complex oxides in the presence of high spin-orbit coupling.

  2. Condensed-Matter Physics & Materials Science Seminar

    "Field-theoretical approach to strongly-correlated problems: RIXS in metals and Spin fermion model"

    Presented by Igor Tupitsyn, University of Massachusetts Amherst

    Monday, February 24, 2020, 11 am
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    In this talk I am going to touch two interesting strongly-correlated problems: Resonant inelastic x-ray scattering (RIXS) in metals and Spin fermion (SF) model. RIXS is a very promising technique for studying collective excitations in condensed matter systems. However, extraction of information from the RIXS signal is a difficult task and the standard approach to solution of RIXS problem is based on approximations that are inaccurate in metals (short-range/contact potentials and non-interacting Fermi-sea). Simultaneously, the SF model has a wide range of applications in the physics of cuprates and iron-based superconductors. However, all developments and applications of the SF model are also based on various, often uncontrollable, approximations. In my talk I am going to address both problems within the general "field-theoretical approach to strongly-correlated problems" framework. In the first part I will consider the RIXS in metals problem within a diagrammatic approach that fully respects the long-range Coulomb nature of interactions between all charged particles. In particular, I will demonstrate how the single-plasmon dispersion can be extracted from the multi-excitation RIXS spectra. In the remaining time I will briefly discuss how to deal with the SF model in the approximation-free manner by employing the Diagrammatic Monte Carlo technique, combining the advantages of Feynman diagrammatic techniques and Quantum Monte Carlo simulations. I will also show what one can get in the first skeleton order – in the widely used in materials science GW approximation.

  3. Condensed-Matter Physics & Materials Science Seminar

    "Emergent phenomena from disorder on a 3D Topological Insulator surface"

    Presented by Yishuai Xu, New York University

    Friday, February 7, 2020, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Mark Dean

    Three-dimensional topological insulators are bulk insulators with Z2 topological order that gives rise to Dirac surface states. These surface states are well protected against weak perturbations that do not break time-reversal symmetry, such as non-magnetic scalar potential disorder. However recent studies have shown that non-magnetic point defects can introduce new in-gap states. We developed a numerical model to simulate point defects on a TI surface, and performed linear-dichroic angle resolved photoemission (ARPES) to image these states in the surface electronic structure. We find that resonance states associated with the defects can hybridize with the Dirac cone surface state and create a kink-like feature in the band structure near the Dirac point. These resonance states are not Anderson localized even though they cluster around the defects sites, and at higher densities, the kink feature is predicted to evolve into a new distinct band that can support diffusive transport. We also present ARPES spectromicroscopy measurements that more clearly resolve the interplay of Dirac surface states with real-space structure.

  4. Condensed-Matter Physics & Materials Science Seminar

    "RIXS study of charge dynamics in cuprates"

    Presented by Jiaqi Lin, Chinese Academy of Sciences / BNL

    Thursday, February 6, 2020, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Mark Dean

    The charge dynamics in cuprates is not comprehensively studied due to the limitation of experimental methods. Resonant inelastic x-ray scattering (RIXS) is a technique that is capable of coupling to various types of excitations and recently has been established as a new way to probe charge density response. Here we use RIXS technique to study two types of charge excitations, the plasmon excitations in electron-doped cuprates and the charge density wave (CDW) excitations in hole-doped cuprates. 1) We track the doping dependence of charge excitations in electron-doped cuprates La2-xCexCuO4. From the resonant energy dependence and the out-of-plane momentum dependence, the charge excitations are identified as three-dimensional plasmons, which reflect the nature of the electronic structure and Coulomb repulsion on both short and long length scale. With increasing electron doping, the plasmon excitations increase monotonically in energy and life time, which reflects the reduction of short-range electronic correlation. 2) We report a comprehensive RIXS study of La2−xSrxCuO4 finding that CDW effects persist up to a remarkably high doping level of x = 0.21 before disappearing at x = 0.25. The inelastic excitation spectra remain essentially unchanged with doping despite crossing a topological transition in the Fermi surface. This indicates that the spectra contain little or no direct coupling to electronic excitations near the Fermi surface, rather they are dominated by the resonant cross-section for phonons and CDW-induced phonon-softening. We interpret our results in terms of a CDW that is generated by strong correlations and a phonon response that is driven by the CDW-induced modification of the lattice.

  5. Condensed-Matter Physics & Materials Science Seminar

    "Effect of Zeeman coupling on the Majorana vortex modes in iron-based topological superconductors"

    Presented by Pouyan Ghaemi, The City College of New York

    Tuesday, January 28, 2020, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Peter D. Johnson

    In the superconducting regime of FeTe(1−x)Sex, there exist two types of vortices which are distinct by the presence or absence of zero energy states in their core. To understand their origin,we examine the interplay of Zeeman coupling and superconducting pairings in three-dimensional metals with band inversion. Weak Zeeman fields are found to suppress the intra-orbital spin-singlet pairing, known to localize the states at the ends of the vortices on the surface. On the other hand, an orbital-triplet pairing is shown to be stable against Zeeman interactions, but leads to delocalized zero-energy Majorana modes which extend through the vortex. In contrast, the finite-energy vortex modes remain localized at the vortex ends even when the pairing is of orbital-triplet form. Phenomenologically, this manifests as an observed disappearance of zero-bias peaks within the cores of vortices upon increase of the applied magnetic field. The presence of magnetic impurities in FeTe(1−x)Sex, which are attracted to the vortices, would lead to such Zeeman-induced delocalization of Majorana modes in a fraction of vortices that capture a large enough number of magnetic impurities. Our results provide a possible explanation to the dichotomy between topological and non-topological vortices recently observed in FeTe(1−x)Sex.

  6. Condensed-Matter Physics & Materials Science Seminar

    "Interlayer charge dynamics in metallic transition metal dichalcogenides"

    Presented by Edoardo Martino, Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland

    Tuesday, January 21, 2020, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Christopher Homes

    Layered metallic transition metal dichalcogenides (TMDs) are conventionally seen as two-dimensional conductors, despite a scarcity of systematic studies of the interlayer charge transport. Motivated by the prevailing strategy of functionalizing 2D materials by creating van der Waals heterostructures, we initiated an in-depth study of out-of-plane charge dynamics and emergent properties arising from interlayer coupling. Unprecedented results have been obtained thanks to employing Focused-ion-beam-assisted 3D microfabrication of samples, which enables tailoring geometry and current paths with submicron precision [1]. In this talk, I will present the first transport data revealing c-axis-oriented quasi-one- dimensional electronic states in 1T-TaS2, —a compound with the richest charge density wave phase diagram among TMDs. Temperature dependence of resistivity shows a robust coherent out-of-plane transport, while in-plane conduction is hindered by the presence of a unique nanoarray of charge density wave domains. Consequently, we interpret the highly debated metal-insulator transition in 1T-TaS2 as a Peierls-like instability of the c-axis-oriented orbital chains, in opposition to the long-standing Mott localization picture [2]. Among other highlights of our current research are the anomalous transport properties observed in natural heterostructures or arising from stacking faults. [1] Moll, P. J. (2018). Focused ion beam microstructuring of quantum matter. Annual Review of Condensed Matter Physics, 9, 147-162. [2] Martino, E., Pisoni, A., Ciric, L., Arakcheeva, A., Berger, H., Akrap, A., ... & Forró, L. (2019). Preferential out-of-plane conduction and quasi-one-dimensional electronic states in layered van der Waals material 1T-TaS2. arXiv preprint arXiv:1910.03817.

  7. Condensed-Matter Physics & Materials Science Seminar

    "Symmetry Protected Topological Semimetals"

    Presented by Jennifer Cano, SUNY-Stony Brook

    Thursday, January 16, 2020, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Mark Dean

    Topological semimetals can exhibit gapless Fermi arc surface states and unusual transport properties. I will discuss new aspects of the bulk-edge correspondence that elucidate the topological nature of Dirac fermions. I will then present the classification of nodal fermions in both magnetic and non-magnetic space groups. Finally, I will present an outlook for finding material realizations.

  8. Condensed-Matter Physics & Materials Science Seminar

    "Entropic elasticity and negative thermal expansion in crystalline solids"

    Presented by Igor Zaliznyak, Brookhaven National Laboratory

    Thursday, January 9, 2020, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201

  9. Condensed-Matter Physics & Materials Science Seminar

    "In-situ TEM sample-management solutions Wildfire and Lightning Heating and Biasing – capturing real dynamics in TEM"

    Presented by Yevheniy Pivak, DENSsolutions

    Monday, December 9, 2019, 11 am
    Bldg. 480, Conference Room

    Hosted by: Shaobo Cheng

    DENSsolutions offers a complete suite of in-situ sample management solutions for unrivalled high resolution imaging in Transmission Electron Microscopes (TEM) under varying environmental conditions including Heating, Biasing, Gases & Liquids. This presentation is aimed at researchers that want to observe their materials in varying real-time dynamic In-situ TEM environments at high resolution. The format of the presentation will include an explanation to the theory behind DENSsolutions' MEMS-based technology, along with brief products introduction. The main topic of the presentation is the Heating and/or Biasing system and its application in the fields of materials science, chemistry and microelectronics. Application examples such as solar cells, ceramics, ReRam, batteries, 2D materials and more will be covered.

  10. Condensed-Matter Physics & Materials Science Seminar

    "Topological transition on anisotropic hexagonal lattices and effective phonon model for the Quantum Hall transition"

    Presented by Andreas Sinner, University of Augsburg, Germany

    Tuesday, November 12, 2019, 1 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    The topology of the band structure, which is determined by the lattice symmetries, has a strong influence on the transport properties. We consider an anisotropic honeycomb lattice and study the effect of a continuously deformed band structure on the conductivity and optical properties. We find a strong suppression of the conductivity in one direction and increment by several orders in another which leand to a considerable change of optical properties. We further study a gap generation in a two-dimensional Dirac fermion system which are coupled to in-plane phonons. At sufficiently strong electron-phonon interaction a gap appears in the spectrum of fermions. The structure of elementary excitations above the gap in the corresponding phase reveals the presence of scale invariant parity breaking terms which resemble Chern-Simons excitations. The Kubo formula remyields quantized Hall plateaux. References: EPL 119, 27001 (2017); PRB 97, 235411 (2018); PRB 93, 125112 (2016); Ann. Phys. 400, 262 (2018); arxiv:1908.00442.

  11. Condensed-Matter Physics & Materials Science Seminar

    "Engineering magnetism with light with the novel photovoltaic perovskite CH3NH3PbI3"

    Presented by László Forró, Ecole Polytechnique Fédérale de Lausanne, Switzerland

    Thursday, October 24, 2019, 2:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Cedomir Petrovic

    The demand for ever-increasing density of information storage and speed of manipulation boosts an intense search for new magnetic materials and novel ways of controlling the magnetic bit. Here, we report the synthesis of a ferromagnetic photovoltaic CH3NH3(Mn:Pb)I3 material in which the photo-excited electrons rapidly melt the local magnetic order through the Ruderman–Kittel–Kasuya–Yosida interactions without heating up the spin system (1). Similar effect was observed in La1-xSrxMnO3/CH3NH3PbI3 heterostructure in which Tc can be tuned by x (2). The observed optical melting of magnetism could be of practical importance, for example, in a magnetic thin film of a hard drive, where a small magnetic writing field could change the magnetic bit. Our method needs only a low-power visible light source, providing isothermal switching, and a small magnetic guide-field to overcompensate the stray field of neighboring bits. Acknowledgment: The work has been performed in collaboration with B. Náfrádi, E. Horváth, A, Arakcheeva, P. Szirmai, M. Spina, H. Lee, O.V. Yazyev, D. Chernyshov, and many others. The research was partially supported by the ERC Advanced Grant (PICOPROP#670918). Reference : 1. Nafradi et al, Nature Communications, 7, 13406, (2016) 2. Nafradi et al, submitted to PNAS

  12. Condensed-Matter Physics & Materials Science Seminar

    "Heavy-fermion quantum criticality and unconventional superconductivity"

    Presented by Frank Stegllich, Max-Planck-Institute for Chemical Physics & Solids, Germany

    Tuesday, October 15, 2019, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Cedomir Petrovic

    Heavy-fermion (HF) metals, i.e., intermetallic compounds of certain lanthanides and actinides, have been subject of intensive investigations over the last few decades. These research activities have furnished important discoveries, such as of unconventional superconductivity (SC) ("beyond BCS") and unconventional quantum criticality ("beyond Landau"). About fifty HF superconductors are currently known, more than half of which exhibiting a quantum critical point (QCP) where antiferromagnetic (AF) order is smoothly suppressed by tuning a non-thermal control parameter like pressure or magnetic field. Two variants of HF AF-QCPs have yet been established, i.e., a conventional ("3D SDW") and an unconventional, partial Mott ("Kondo destroying") QCP [1, 2]. In clean, stoichiometric HF metals, the huge entropy accumulated at such an AF QCP is commonly removed by forming an unconventional superconducting phase. The apparent validity of this 'quantum critical paradigm' will be illustrated in the first part of the talk by addressing exemplary quantum critical materials, i.e., the isostructural compounds YbRh2Si2 and CeCu2Si2. The former system exhibits a partial-Mott QCP as reflected by, e.g., an abrupt jump of the Fermi-surface volume [3- 5] and a violation of the Wiedemann-Franz law [6, 7]. For this compound, no SC had been detected down to 10 mK, the lowest temperature accessible in a commercial 3He-4He dilution refrigerator [8]. However, recent magnetic and specific-heat measurements performed in a nuclear demagnetization cryostat down to about 1 mK revealed HF, i.e., unconventional, SC below Tc = 2 mK [9]. CeCu2Si2, the first HF superconductor [10], exhibits SC at a 3D SDW-QCP and was considered a (one-band) d-wave superconductor until a few years ago, when its specific heat was found to exhibit two-gap behavior and exponential temperature dependence at very low temperatures [11]. Based on atomic substitution [12],

  13. Condensed-Matter Physics & Materials Science Seminar

    "Nematic superconductivity in twisted bilayer graphene"

    Presented by Laura Classen, University of Minnesota

    Tuesday, September 10, 2019, 1 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    Tunable insulating and superconducting phases have recently been induced in several twisted graphene-based heterostructures. These correlated phases are ascribed to an exceptional band flattening, which comes along with a very large hexagonal moiré pattern in real space. We study this interplay of orders in a phenomenological model for the moiré superlattice with a focus on superconductivity. Motivated by the presence of van-Hove instabilities, we approach the pairing problem as an interaction-induced instability of the Fermi surface in terms of the unbiased functional renormalization group. We find two pairing instabilities with different symmetries being close in energy and show that a similar situation arises in a model specific for twisted bilayer graphene. In view of recent experimental observations that the threefold lattice rotational symmetry is broken in the superconducting state of hole-doped twisted bilayer graphene, we analyze the corresponding Landau-Ginzburg free energy with two superconducting order parameters. The result is, indeed, a mixed ground state that breaks rotation symmetry and leads to nematic superconductivity. Time-reversal symmetry can simultaneously be broken.

  14. Condensed-Matter Physics & Materials Science Seminar

    "Tailoring the twinning of DyBa2Cu3O7-x thin films with atomic-layer-by-layer molecular beam epitaxy"

    Presented by Daniel Putzky, Max Planck Institute for Solid State Research, Germany

    Monday, August 26, 2019, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Tony Valla/Ilya Drozdov

    In this talk I will present the work on high-quality, epitaxial DyBa2Cu3O7-x (DBCO) thin films grown by molecular beam epitaxy (MBE). In contrast to the previous DBCO growth by MBE using co-deposition technique, we have employed an atomic-layer-by-layer shuttering sequence with in-situ RHEED feedback. Films grown on LSAT (100), NGO (110) and STO (100) have a sharp superconducting transition above 80 K. Scanning-transition electron microscopy (STEM) shows atomically sharp substrate-film interface and the absence of stacking faults, unlike films previously grown by PLD. In the second part of the talk I will focus on the structural investigation using x-ray diffraction (XRD). In-plane scans at the KARA synchrotron confirm the epitaxial relationship to the substrate. In addition the formation of twin domains with the bulk-like orthorhombic crystal structure were observed. By reducing the film thickness the tetragonal to orthorhombic phase transition can be suppressed while the films still remain superconducting.

  15. Condensed-Matter Physics & Materials Science Seminar

    "The 2-spinon contribution to the longitudinal structure factor in the XXZ model"

    Presented by Isaac Perez Castillo, Institute of Physics, UNAM and London Mathematical Laboratory

    Thursday, August 22, 2019, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    In this work we derive exactly the two-spinon contribution to the longitudinal dynamical structure factor of the anisotropic Heisenberg spin-1/2 chain in the gapped regime by using quantum group approach. We will briefly discuss some of the mathematical difficulties when confronting form factor formulas given by quantum group approach and how to overcome these obstacles. We end up by contrasting our results with those coming from perturbation theory, while comparison to DMRG and experiments are currently underway.

  16. Condensed-Matter Physics & Materials Science Seminar

    "Fermi arcs, nodal and antinodal gaps in cuprates : the 'pairon' model to the rescue"

    Presented by William Sacks, Sorbonne University, France

    Friday, July 26, 2019, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Ivan Bozovic

    Angle-resolved photoemission, in addition to tunneling, has provided key information on the cuprate pairing on the microscopic scale. In particular, in the underdoped regime, the angular dependence of the gap function Δ(θ) deviates from a pure d-wave form such that the antinodal gap value ΔAN and the nodal gap value ΔN completely diverge. On another front, ARPES has firmly established that the enigmatic Fermi arcs, i.e. normal electron excitations around the nodes, exist even below Tc. In this work, we will interpret these experiments based on the 'pairon' model [1] in which the fundamental object is a hole pair bound by its local antiferromagnetic environment on the scale of the coherence length ξAF. The pairon model agrees quantitatively with both the gap function Δ(θ) and the Fermi arcs seen at finite temperature.

  17. Condensed-Matter Physics & Materials Science Seminar

    "Strange superconductivity near an antiferromagnetic heavy-fermion quantum critical point"

    Presented by Chung-Hou Chung, Department of Electrophysics, National Chiao-Tung University, Taiwan

    Wednesday, July 24, 2019, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201

    Hosted by: Alexei Tsvelik

    The heavy fermion systems CeMIn5 with M = Co, Rh, Ir, the "115" family, provide a prototypical example of an exotic "strange superconductivity" where unconventional d-wave Cooper pairs get condensed out of an incoherent strange metal normal state, displaying non-Fermi liquid behavior such as: T-linear-resistivity, T-logarithmic specific heat coefficient and a T-power-law singularity in magnetic susceptibility, near an antiferromagnetic quantum critical point [1]. The microscopic origin of strange superconductivity and its link to antiferromagnetic quantum criticality of the strange metal state are still long-standing open issues. We propose a microscopic mechanism for strange superconductors, based on the coexistence and competition between the Kondo correlation and the quasi-2d short-ranged antiferromagnetic resonating-valence-bond (RVB)spin-liquid near the antiferromagnetic quantum critical point via a large-N (Sp(N)) Kondo-Heisenberg model and renormalization group analysis beyond the mean-field level [2]. In the absence of superconductivity, this effective field theory [3] can describe various aspects of strange metal state observed in Ge-substituted YbRh2Si2 [4] close to the field-tuned Kondo breakdown quantum critical point. The interplay of these two effects between the Kondo and RVB physics provides a qualitative understanding on how superconductivity emerges from the strange metal state and the observed superconducting phase diagrams for CeMIn5 [1,2]. References: [1] C. Petrovic et al. J. Phys. Condens. Matt. 13, L337 (2001); S. Zaum et al. Phys. Rev. Lett. 106, 087003 (2011). [2] Y. Y. Chang, F. Hsu, S. Kirchner, C. Y. Mou, T. K. Lee, and C. H. Chung, Phys. Rev. B 99, 094513 (2019). [3] Y. Y. Chang, S. Paschen, and C. H. Chung, Phys. Rev. B 97, 035156 (2018). [4] J. Custers et al, Nature (London) 424, 524 (2003); J. Custers et al. Phys. Rev. Lett. 104, 186402 (2010).

  18. Condensed-Matter Physics & Materials Science Seminar

    "Electron beam effects on organic ices"

    Presented by Marco Beleggia, Technical University of Denmark, Denmark

    Monday, July 22, 2019, 11 am
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Yimei Zhu

    While beam damage is often considered detrimental to our quantitative imaging capabilities, the energy and charge injected into the sample as a result of inelastic scattering can be exploited beneficially. This is especially true in radiation-chemistry-type experimental setups in the electron microscope where the beam promotes local wanted chemical reactions. We have observed that by exposing to the electron beam a layer of small volatile organic molecules condensed over a cold substrate results in the formation of a solid product. Evidence suggests that the exposure mechanism driving the formation of a solid product is partial dehydrogenation of the molecules, removal of H2, and progressive increase of the average molecular weight. Contrary to focused electron beam induced deposition, that relies on surface absorption followed by aggregation of mobile species, at cryogenic temperature organic ice molecules are largely immobilized, and act as targets for the incoming electrons throughout the entire thickness of the layer. Therefore, the exposure occurs throughout the volume of the frozen precursor, and the features are essentially determined by the electron distribution, with diffusion/transport parameters bearing little or no relevance. Since larger molecules are less volatile, if the molecular weight increases sufficiently, upon raising the temperature the unexposed areas leave the sample, while the exposed molecules assemble into a solid product in the form of hydrogenated amorphous carbon.

  19. Condensed-Matter Physics & Materials Science Seminar

    "Excitonic condensation of strongly correlated electrons"

    Presented by Professor Jan Kunes, Vienna University of Technology, Austria

    Wednesday, June 26, 2019, 2:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: Keith Gilmore

    Spontaneous symmetry breaking is a prominent demonstration of the collective behavior of strongly correlated systems. Besides ordering of charge or of spin dipoles, more exotic types of long-range order are possible, which do not couple to conventional probes and are therefore sometimes called the hidden order. Excitonic magnets, or excitonic condensates, are examples of such systems. I will introduce the concept of excitonic condensate from the strong coupling perspective and discuss the rich variety of excitonic phases arising from the internal (spin, orbital) degrees of freedom of the excitons. I will present some numerical results obtained with dynamical mean-field theory for models as well as for specific materials, which we suspect to be excitonic magnets. The presentation will include the recently obtained results for dynamical susceptibilities in phases with long-range order and some proposals on how to detect excitonic condensates with today's experimental techniques.

  20. Condensed-Matter Physics & Materials Science Seminar

    "Tracking phase textures in complex oxides using coherent x-rays"

    Presented by Xiaoqian Chen, Lawrence Berkeley Laboratory

    Monday, June 17, 2019, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson/Mark Dean

    In complex oxides, coupled interactions result in unpredictable and novel emerging orders that are yet to be understood. With the recent advancement in x-ray and laser sources, exploration of equilibrium fluctuation and nonlinear dynamics have become an effective approach to understand these intertwined orders. In particular, coherent x-rays are a simultaneous probe of the order parameter, phase texture, and dynamics. In the first part of my talk, I will use underdoped cuprate La2-xBaxCuO4 as an example to show how x-ray speckle correlation can be a test for (lattice degree of freedom and charge) order coupling and dynamics. However, can we image domain dynamics in real time? In the second part of my talk, I will use antiferromagnetically ordered artificial lattice to demonstrate that phase retrieval lensless imaging can be used to image charge and magnetic orders. Using Bragg coherent diffraction imaging, we revealed a single domain wall motion with 100ms time resolution. References [1] X. M. Chen et al. Phys. Rev. Lett. 117, 167001 (2016) [2] V. Thampy et al. Phys. Rev. B 95, 241111 (2017) [3] X. M. Chen et al. Nat Commun. 10 1435 (2019) [4] X. M. Chen et al. under review, arXiv:1809.05656 [cond-mat.mes-hall]

  21. Condensed-Matter Physics & Materials Science Seminar

    "Probing quantum materials with multiple spectroscopic techniques"

    Presented by Eduardo H. da Silva Neto, University of California, Davis

    Thursday, June 6, 2019, 1:30 pm
    ISB - Bldg. 734

    Hosted by: Robert Konik

    Resonant X-ray Scattering (RXS), Scanning Tunneling Spectroscopy (STS) and Angle-Resolved Photo-Emission Spectroscopy (ARPES) measurements have been at the forefront of several advances in the studies of quantum materials. Our group specializes in these techniques, looking to leverage their combination to the study of quantum materials. I will discuss two projects where we have used these state-of-the-art techniques to study high-temperature superconductors and topological materials. Charge order has now been ubiquitously observed in cuprate high-temperature superconductors. However, it remains unclear if the charge order is purely static or whether it also features dynamic correlations. I will discuss a polarization-resolved soft x-ray inelastic RXS experiment with unprecedented resolution that demonstrates the existence of a coupling between dynamic magnetic and charge-order correlations in the electron-doped cuprate Nd2−xCexCuO4 [1-3]. I will also discuss a combined ARPES-STS study of the topological material Hf2Te2P. Similar to the reports by H. Ji, et al. on Zr2Te2P [4], band structure calculations and ARPES by Hosen et al. [5] also suggest multiple topological surface states in Hf2Te2P. However, some topological surface states still lacked direct spectroscopic evidence due the inability of ARPES experiments to probe the unoccupied band structure. Using the combination of STS and ARPES with surface K-doping, we probe the unoccupied band structure of Hf2Te2P and demonstrate the presence of multiple surface states with a linear Dirac-like dispersion, consistent with the predictions from previously reported band structure calculations [6]. [1] E. H. da Silva Neto, et al. Science 347, 282 (2015). [2] E. H. da Silva Neto, et al. Science Advances 2 (8), e1600782 (2016). [3] E. H. da Silva Neto, et al. Physical Review B, Rapid Communication 98, 161114(R) (2018). [4] H. Ji, et al. Physical

  22. Condensed-Matter Physics & Materials Science Seminar

    "Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory"

    Presented by Xiaozhe Shen, SLAC National Accelerator Laboratory

    Monday, June 3, 2019, 2 pm
    Bldg. 480, Conference Room

    Hosted by: Jing Tao

    Ultrafast electron diffraction (UED) is a transformative tool for probing atomic structural dynamics in ultrafast science to understand the correlation between materials' structure and their functionalities, with the ultimate goal of controlling energy and matter. The advent of high-brightness relativistic electron beams from photocathode radio frequency (RF) gun provides a great opportunity to push the resolving power of UED onto atomic length and time scales. With the expertise in electron beam physics and ultrafast laser technology, SLAC has dedicated enormous efforts to develop a world-leading UED using mega-electron-volt (MeV) electron beams since 2014. Over the years, SLAC MeV UED has achieved great instrument performance and delivered numerous scientific outcomes for ultrafast science. In 2019, SLAC MeV UED has officially transformed into a user facility. In this talk, performance of SLAC MeV UED will be reviewed, including characterization of the instrument resolution and machine stability. The unique capabilities of SLAC MeV UED to accommodate various sample environments for a broad range of scientific interests, including condense matter physics and chemical science, will be presented, with highlighted scientific results. Research and development efforts to improve the performance of SLAC MeV UED will be discussed.

  23. Condensed-Matter Physics & Materials Science Seminar

    "In situ imaging of gold nanocrystals during the CO oxidation reaction studied by Bragg Coherent Diffraction Imaging"

    Presented by Ana Flavia Suzana, Brazilian Association of Synchrotron Light Technology-ABTLUS, Brazil

    Thursday, May 23, 2019, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson

    The fundamental aim of heterogeneous catalysis research is to understand mechanisms at the nanoparticle level, and then to design and synthesize catalysts with desired active sites. In this regard, the in situ/operando characterization of defects is crucial as they are preferential catalytic sites for the reaction occurrence. In this seminar I will talk about the main part of the work developed during my PhD: the investigation of the morphology and structure evolution of gold nano-catalysts supported on titanium dioxide. Those catalytic materials were evaluated for the model CO oxidation reaction, chosen for its environmental relevance and "simplicity" to be reproducible within our X-ray imaging study. We used the Bragg Coherent Diffraction Imaging technique to follow in situ the 3D morphology changes under catalytic reaction conditions. We correlated the 3D displacement field and strain distribution of the gold nanoparticles to the catalytic properties of the material. In particular, for a 120 nm gold nanoparticle, we quantified under working conditions the adsorbate-induced surface stress on the gold nanocrystal, which leads to restructuration and defects identified as a nanotwin network.

  24. Condensed-Matter Physics & Materials Science Seminar

    ""Superconductivity and magnetism at ferroelectric critical point""

    Presented by Alexander Balatsky, UConn Nordita

    Thursday, April 18, 2019, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Ilya Drozdov

    It is well established that multiple entangled orders emerge in quantum materials at criticality: eg superconducting states develop in the vicinity of magnetic phases. I will make the case that similar phenomena occur in quantum paraelectrics. Recent observations of strain and O18 isotope substitution in doped STO support the view of the key role critical ferroelectric fluctuations play in producing superconductivity. Looking beyond superconductivity, I will illustrate how quantum ferroelectric fluctuations can induce magnetic fluctuations due to recently proposed phenomenon of dynamic multiferroicity.

  25. Condensed-Matter Physics & Materials Science Seminar

    "Tailoring electronic and thermal properties of bulk Cu26T2(Ge,Sn)6S32 colusite through defects engineering and functionalization of the conductive network"

    Presented by Emmanuel Guilmeau, CRISMAT Laboratory, Caen, France

    Thursday, April 11, 2019, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: Qiang Li

    A complete study of the structure and thermoelectric properties of colusite Cu26T2(Ge,Sn)6S32 (T = V, Cr, Mo, W) is presented. A brief introduction provides a state-of-theart/survey of thermoelectric sulfides, with a special focus on the structural features and transport properties relationship in Cu-based sulfides. In the first part of this presentation, we highlight the key role of the densification process on the formation of short-to-medium range structural defects in Cu26V2Sn6S32 [1]. A simple and powerful way to adjust carrier concentration combined with enhanced phonon scattering through point defects and disordered regions is described. By combining experiments with band structure and phonons calculations, we elucidate, for the first time, the underlying mechanisms at the origin of the intrinsically low thermal conductivity in colusite samples as well as the effect of S vacancies and antisite defects on the carrier concentration. In the second part, we demonstrate the spectacular role of the substitution of V5+ by hexavalent T6+ cations (Cr, Mo and W) on the electronic properties, leading to high power factors [2]. In particular, Cu26Cr2Ge6S32 shows a value of 1.53 mW m-1 K-2 at RT that reaches a maximum value of 1.94 mW m-1 K-2 at 700 K. The rationale is based on the concept of conductive "Cu-S" network, which in colusites corresponds to the more symmetric parent sphalerite structure. The interactions within the mixed octahedral-tetrahedral [TS4]Cu6 complexes are shown to be responsible for the outstanding electronic transport properties. [1] C. Bourgès et al., J. Amer. Chem. Soc. 140 (2018) 2186 [2] V. Pavan Kumar et al., Adv. Energy Mater. 9 (2019) 1803249

  26. Condensed-Matter Physics & Materials Science Seminar

    "Importance of electron interactions in understanding the photo-electron spectroscopy and the Weyl character of MoTe2"

    Presented by Niraj Aryal, Florida State University

    Thursday, April 11, 2019, 11 am
    ISB Bldg. 734 Conference Room 201 (upstairs)

    Hosted by: Weiguo Yin

    Weyl semimetals are crystalline materials that host pairs of chiral Weyl Fermions (WFs) as low energy excitations. Such WFs act as sources and sinks of Berry curvature and can contribute to many exotic transport properties. Recently, inversion symmetry broken transition metal dichalcogenide materials like MoTe2 and WTe2 have been predicted to host type-II WFs by DFT calculations and ARPES experiments. However, quantum oscillation experiments (QOE) disagree with the DFT calculations thus raising doubt about the existence of Weyl physics in these materials [1]. In order to address this discrepancy, we studied the role of electron interactions in Td-MoTe2 by employing DFT where the onsite Coulomb repulsion (Hubbard U) for the Mo 4d states is included within the DFT+U scheme. We found that in addition to explaining the QOE, inclusion of electron interaction is needed to explain the light-polarization dependence measured by ARPES [2]. We also found that while the number of Weyl points (WPs) and their position in the Brillouin Zone change as a function of U, a pair of such WPs very close to the Fermi level survive the inclusion of these important corrections. Our calculations suggest that the Fermi surface of Td-MoTe2 is in the vicinity of a correlations-induced Lifshitz transition which can be probed experimentally. If time allows, I will also present briefly our study of the interface between topological insulator and non-topological materials which are important for band engineering and studying emergent fundamental phenomena. References [1] D. Rhodes, R. Schonemann, N. Aryal, Q.R. Zhou et al., Bulk Fermi surface of the Weyl type-II semimetallic candidate ?-MoTe2, Phys. Rev. B 96, 165134 (2017). [2] N. Aryal and E. Manousakis, Importance of electron correlations in understanding the photo-electron spectroscopy and the Weyl character of MoTe2, Phys. Rev. B 99, 035123 (2019).

  27. Condensed-Matter Physics & Materials Science Seminar

    ""Charge and lattice entanglement in quantum materials observed by TEM: Tb2Cu0.83Pd0.17O4 and Cu2S""

    Presented by Wei Wang, Institute of Physics, Chinese Academy of Sciences, Beijing

    Tuesday, April 9, 2019, 3 pm
    Bldg. 480, Conference Room

    Hosted by: Jing Tao

    Plenty of physical properties in strongly electron correlated system are thought to arise from intricate interplay among charge, spin, orbital and lattice. Understanding the structural origin of these functionalities, such as superconductivity, multiferroics, etc, has attracted tremendous attention for decades. Using electron diffraction technique in TEM, we recently studied the modulated structure in Tb2Cu0.83Pd0.17O4 compound and phase transition in Cu2S. After a brief introduction of TEM techniques that I have employed for the study, I will report observations of electron-beam-induced smectic-nematic phase transitions in Tb2Cu0.83Pd0.17O4. Electron diffraction and HAADF-STEM images indicate a superlattice structure with Cu/Pd displacements perpendicular to the Cu-O plane on Cu sites. In addition, the superlattice modulation undergoes a reversible smectic-nematic phase transition under the electron beam illumination. Our in situ TEM results imply that the modulated structure root in a charge ordering at Cu sites. Then I will switch to an on-going study of the Cu2S at high temperature. Previous reports show that the crystal structural of Cu2S can be manipulated by electron beam illumination, suggesting a strong coupling between charge and lattice. To explore the structural phase transition and to have a better understanding of superionic behavior in this material, we focus on diffuse scattering in the electron diffraction patterns obtained at high temperatures, which results from short range ordering of Cu atoms. Electron diffraction tomographyic data were collected in order to reconstruct the real-space structure for Cu atoms. Preliminary results will be shown followed by a discussion with early-stage interpretations.

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