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November 2020
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  1. Condensed-Matter Physics & Materials Science Seminar

    11 am, Virtual Seminar

    Hosted by: Mark Dean

    The spatially anisotropic character of d-orbitals in transition metal compounds can lead to many and diverse 'quantum' phases of matter when spin orbit coupling is on equal footing with with electronic correlations. These phases include multi-polar orders, spin liquids, and/or spin orbitals liquids. The most celebrated example is the Kitaev model for j=1/2 Kramers doublets on the honeycomb lattice. When orbital degeneracy is introduced, the resulting orbital fluctuations open up many unexplored possibilities. In this seminar, I will introduce the cluster Mott insulating lacunar spinels as class of model materials where correlations, spin orbit coupling, and orbital degeneracy act in concert to produce several interesting magnetic phases. Following this, I will present a set of neutron scattering results on one member of this family: GaTa4Se8, a material that realizes j=3/2 model on the FCC lattice. We observe that the ground state of GaTa4Se8 is a spin-orbital dimer singlet phase and capture the collective magnetic excitations from that state. Intriguingly, the dimerization transition is preceded by a regime of dynamic vibronic fluctuations extending to temperatures four times higher than the global symmetry breaking transition. I will discuss these results in relation to the quadrupolar ordering transition recently proposed for the sister compound GaNb4Se8. Zoom Meeting https://bnl.zoomgov.com/j/1608389385?pwd=QWNXbnFTOXVyQkFBU2o3ZGFjZ2pIdz09&from=msft

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  1. Condensed-Matter Physics & Materials Science Seminar

    "Spin Orbital Dimer Singlets and Vibronic Fluctuations in a Cluster Mott Insulator"

    Presented by Kemp Plumb, Brown University

    Thursday, November 5, 2020, 11 am
    Virtual Seminar

    Hosted by: Mark Dean

    The spatially anisotropic character of d-orbitals in transition metal compounds can lead to many and diverse 'quantum' phases of matter when spin orbit coupling is on equal footing with with electronic correlations. These phases include multi-polar orders, spin liquids, and/or spin orbitals liquids. The most celebrated example is the Kitaev model for j=1/2 Kramers doublets on the honeycomb lattice. When orbital degeneracy is introduced, the resulting orbital fluctuations open up many unexplored possibilities. In this seminar, I will introduce the cluster Mott insulating lacunar spinels as class of model materials where correlations, spin orbit coupling, and orbital degeneracy act in concert to produce several interesting magnetic phases. Following this, I will present a set of neutron scattering results on one member of this family: GaTa4Se8, a material that realizes j=3/2 model on the FCC lattice. We observe that the ground state of GaTa4Se8 is a spin-orbital dimer singlet phase and capture the collective magnetic excitations from that state. Intriguingly, the dimerization transition is preceded by a regime of dynamic vibronic fluctuations extending to temperatures four times higher than the global symmetry breaking transition. I will discuss these results in relation to the quadrupolar ordering transition recently proposed for the sister compound GaNb4Se8. Zoom Meeting https://bnl.zoomgov.com/j/1608389385?pwd=QWNXbnFTOXVyQkFBU2o3ZGFjZ2pIdz09&from=msft

  2. Condensed-Matter Physics & Materials Science Seminar

    "Nonequilibrium Dynamics of Collective Excitations in Quantum Materials"

    Presented by Edoardo Baldini, Massachusetts Institute of Technology

    Thursday, June 18, 2020, 11 am
    https://bluejeans.com/956965585

    Hosted by: Mark Dean

    Revealing the dynamics of collective excitations in strongly correlated electron systems is a subject of pivotal importance, as collectivity lies at the origin of several cooperative phenomena that cause profound transformations, instabilities, and phase transitions. In this talk, I will discuss the dynamics of collective excitations (e.g., excitons, magnons, phonons) from the perspective of ultrafast science [1-3]. In particular, I will focus on the role that specific collective excitations play in the formation of hidden phases of matter, i.e. phases that do not have counterparts in the equilibrium phase diagrams of quantum materials. As an example, I will describe our recent discovery of a transient antiferromagnetic metallic phase in a prototypical layered Mott insulator [4]. We observed this phase upon photoexciting a sub-Mott gap exciton-magnon mode, an exotic state of bound electron-hole pairs dressed with the spin degree of freedom [5]. Driving this peculiar exciton also allowed us to realize the coherent control of the underlying antiferromagnetic order for tens of picoseconds, a feature that can lead to the development of novel all-optical magnonic devices. [1] E. Baldini, Nonequilibrium Dynamics of Collective Excitations in Quantum Materials, Springer (2018) [2] E. Baldini*, C. A. Belvin* et al., Nat. Phys. 16, 541-545 (2020) [3] X. Li et al., Science 364, 1079-1082 (2019) [4] C. A. Belvin*, E. Baldini* et al., in review (2020) [5] S. Kang et al., Nature, in press (2020) https://bluejeans.com/956965585

  3. Condensed-Matter Physics & Materials Science Seminar

    "Towards Novel Quantum Materials: Design, Synthesis and Characterizations"

    Presented by Ruidan Zhong, Princeton University

    Thursday, May 7, 2020, 1:30 pm
    Zoom CMPMSD Seminar

    Hosted by: Qiang Li

    Quantum materials are materials where the extraordinary effects of quantum mechanics give rise to exotic and often incredible properties. To understand their basic behavior if we are to enable optimization for a specific purpose, discovering novel quantum materials is a primary task for scientists in the field of condensed matter physics and materials science. In this seminar, research strategies toward novel quantum materials in experiment will be introduced from the perspective of chemistry, materials science and physics. Such research process leads to fruitful results, and two recent discovered candidates of quantum spin liquid (QSL) will be presented as an example. The first candidate is a geometric frustrated magnet, Na2BaCo(PO4)2, which is structurally perfect without intrinsic disorder. Experimental results, including magnetization, specific heat, and neutron scattering, have indicated that this compound is an ideal QSL candidate. The second compound, BaCo2(AsO4)2, are believed to be a Kitaev QSL candidate. This is the first time that Kitaev physics are proposed to be realized in 3d-transition-metal honeycomb in experiment. Non-Kitaev interactions can be fully suppressed by low field, yielding nonmagnetic ground state and many other similarities with the well-studied Kitaev QSL α-RuCl3. https://us02web.zoom.us/j/89625989696

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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

  12. 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.

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