BNL Home
March 2019
Sunday Monday Tuesday Wednesday Thursday Friday Saturday

1

  1. No events scheduled

2

  1. No events scheduled

3

  1. No events scheduled

4

  1. No events scheduled

5

  1. No events scheduled

6

  1. No events scheduled

7

  1. No events scheduled

8

  1. No events scheduled

9

  1. No events scheduled

10

  1. No events scheduled

11

  1. Condensed-Matter Physics & Materials Science Seminar

    1:30 pm, ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Mark Dean

    In strongly correlated systems, interactions between various microscopic degrees of freedom with similar energy scales can induce strong competition and frustration, leading to exotic phenomena. Here we use neutron scattering technique to study several strongly correlated systems to show how the competition and interplay between these degrees of freedom can induce different phases and properties. 1) In the pressure-induced superconductor CrAs, the competition between various magnetic interactions lead to a noncollinear helimagnetic order. In addition, CrAs exhibits a spin reorientation at a critical pressure (Pc ~ 0.6 GPa), which is accompanied by a lattice anomaly and coincides with the emergence of bulk superconductivity, indicating the strong interplay between magnetic, structural and electronic degrees of freedom. 2) FeSe, the structurally simplest iron-based superconductor, shows nematic order at 90 K, but no magnetic order in the parent phase. Our neutron scattering experiments reveal both stripe and Neel spin fluctuations that are coupled to the nematicity. The competition between these two phases suppress the magnetic order and drive the system into a nematic quantum disordered paramagnet. Similar phenomenon is observed in YFe2Ge2, in which the magnetic order is suppressed by the competition between stripe type AFM phase and in-plane FM phase. 3) In the heavily electron-doped FeSe based superconductor Li0.8Fe0.2ODFeSe (Tc=41 K), a twisted dispersion of spin excitations is observed which may be caused by the competition between itinerant and local electrons, analogous to the hole-doped cuprates which host remarkably high Tc as well. 4) In the two-dimensional triangular lattice antiferromagnet YbMgGaO4, due to the strong spin-orbit coupling and crystalline electric field (CEF), the low-lying crystal field ground state is a Kramers doublet. The geometric frustration is enhanced by the anisotropic interactions and a quantum sp

12

  1. No events scheduled

13

  1. No events scheduled

14

  1. No events scheduled

15

  1. No events scheduled

16

  1. No events scheduled

17

  1. No events scheduled

18

  1. No events scheduled

19

  1. No events scheduled

20

  1. No events scheduled

21

  1. No events scheduled

22

  1. No events scheduled

23

  1. No events scheduled

24

  1. No events scheduled

25

  1. No events scheduled

26

  1. No events scheduled

27

  1. No events scheduled

28

  1. No events scheduled

29

  1. No events scheduled

30

  1. No events scheduled

31

  1. No events scheduled

  1. APR

    4

    Thursday

    Condensed-Matter Physics & Materials Science Seminar

    "TBA"

    Presented by Markus Holzmann, Laboratoire de Physique et Modélisation des Milieux Condensés

    1:30 pm, ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Thursday, April 4, 2019, 1:30 pm

    Hosted by: Robert Konik

    TBD

  2. APR

    11

    Thursday

    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

    1:30 pm, Bldg. 734, ISB Conference Room 201 (upstairs)

    Thursday, April 11, 2019, 1:30 pm

    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

  1. Condensed-Matter Physics & Materials Science Seminar

    "Neutron scattering study of strongly correlated systems"

    Presented by Yao Shen, Fudan University, China

    Monday, March 11, 2019, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Mark Dean

    In strongly correlated systems, interactions between various microscopic degrees of freedom with similar energy scales can induce strong competition and frustration, leading to exotic phenomena. Here we use neutron scattering technique to study several strongly correlated systems to show how the competition and interplay between these degrees of freedom can induce different phases and properties. 1) In the pressure-induced superconductor CrAs, the competition between various magnetic interactions lead to a noncollinear helimagnetic order. In addition, CrAs exhibits a spin reorientation at a critical pressure (Pc ~ 0.6 GPa), which is accompanied by a lattice anomaly and coincides with the emergence of bulk superconductivity, indicating the strong interplay between magnetic, structural and electronic degrees of freedom. 2) FeSe, the structurally simplest iron-based superconductor, shows nematic order at 90 K, but no magnetic order in the parent phase. Our neutron scattering experiments reveal both stripe and Neel spin fluctuations that are coupled to the nematicity. The competition between these two phases suppress the magnetic order and drive the system into a nematic quantum disordered paramagnet. Similar phenomenon is observed in YFe2Ge2, in which the magnetic order is suppressed by the competition between stripe type AFM phase and in-plane FM phase. 3) In the heavily electron-doped FeSe based superconductor Li0.8Fe0.2ODFeSe (Tc=41 K), a twisted dispersion of spin excitations is observed which may be caused by the competition between itinerant and local electrons, analogous to the hole-doped cuprates which host remarkably high Tc as well. 4) In the two-dimensional triangular lattice antiferromagnet YbMgGaO4, due to the strong spin-orbit coupling and crystalline electric field (CEF), the low-lying crystal field ground state is a Kramers doublet. The geometric frustration is enhanced by the anisotropic interactions and a quantum sp

  2. Condensed-Matter Physics & Materials Science Seminar

    "Unconventional superconductivity and complex tensor order in half-Heusler superconductors"

    Presented by Igor Boettcher, University of Maryland

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

    Hosted by: Laura Classen

    A revolutionary new direction in the field of superconductivity emerged recently with the synthesis of superconductors with strong inherent spin-orbit coupling such as the half-Heusler alloys. Due to band inversion, the low-energy degrees of freedom are electrons at a three-dimensional quadratic band touching point with an effective spin 3/2, which allows for higher-spin Cooper pairing and potentially topological superconductivity. I will illuminate some possibilities for unconventional superconductivity in this system, in particular a novel superconducting quantum critical point and the transition into a phase with complex tensor order, which is a superconducting state captured by a complex second-rank tensor valued order parameter describing Cooper pairs having spin 2. Here the interplay of both tensorial and complex nature results in a rich and intriguing phenomenology. I will highlight how optical response measurements can shed light on the phase structure of individual compounds.

  3. Condensed-Matter Physics & Materials Science Seminar

    ": Electronic structure of d-metal systems as revealed by ab initio modeling of resonant inelastic X-ray scattering"

    Presented by Lei Xu, Leibniz Institute for Solid State and Materials Research Dresden, Germany

    Friday, February 15, 2019, 11 am
    ISB Bldg. 734 Conference Room 201 (upstairs)

    Hosted by: Weiguo Yin

    I will present our work on the theoretical investigation of the electronic structure, magnetic interactions and resonant inelastic X-ray scattering (RIXS) in 3d or 4d-5d transition metal (TM) compounds by using wave-function-based many-body quantum chemistry (QC) methods. My presentation contains two parts. In the first part, I will discuss the magnetic properties of 4d and 5d TM ions with a formally degenerate t12g electron configuration in the double-perovskite (DP) materials Ba2YMoO6, Ba2LiOsO6 and Ba2NaOsO6. Our analysis indicates that the sizable magnetic moments and g-factors found experimentally are due to both strong TM d – ligand p hybridization and dynamic Jahn-Teller effects. Our results also point out that cation charge imbalance in the DP structure allows a fine tuning of the gap between the t2g and eg levels. In another example of t12g electron configuration, spin-Peierls (SP) TiPO4 compound, we assign excitation peaks of experimental RIXS spectra and find that the d1 ground state is composed of an admixture of dz2 and dxz orbital character. In the second part, I will discuss a computational scheme for computing intensities as measured in X-ray absorption and RIXS experiments. We take into account the readjustment of the charge distribution in the 'vicinity' of an excited electron for the modeling of RIXS. The computed L3-edge RIXS spectra for Cu2+ 3d9 ions in KCuF3 and for Ni2+ 3d8 ions in La2NiO4 reproduce trends found experimentally for the incoming-photon incident-angle and polarization dependence.

  4. Condensed-Matter Physics & Materials Science Seminar

    "Resonant inelastic X-ray scattering (RIXS) as a probe of exciton-phonon coupling"

    Presented by Andrey Geondzhian, European Synchrotron Radiation Facility (ESRF), France (UTC+1)

    Thursday, February 14, 2019, 9:30 am
    ISB Bldg. 734 Conference Room 201 (upstairs)

    Hosted by: Weiguo Yin

    Phonons contribute to resonant inelastic X-ray scattering (RIXS) as a consequence of the coupling between electronic and lattice degrees of freedom. Unlike other techniques that are sensitive to electron-phonon interactions, RIXS can give access to momentum dependent coupling constants. This Information is highly desirable in the context of understanding anisotropic conventional and unconventional superconductivity. In my talk, I will consider the phonon contribution to RIXS from the theoretical point of view. In contrast to previous studies, we emphasize the role of the core-hole lattice coupling. Our model, with parameters obtained from first principles, shows that even in the case of a deep core-hole, RIXS probes exciton-phonon coupling rather than a direct electron-phonon coupling. Further, to address the needs of predictive approach and overcome limitations of the model studies we developed a Green's function formalism to capture electron-phonon contributions to RIXS and other core-level spectroscopies (X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS)). Our approach is based on the cumulant expansion of the Green's function combined with many-body theory calculated vibrational coupling constants. In the case of the XAS and RIXS, we use a two-particle exciton Green's function, which accounts implicitly for particle-hole interference effects that have previously proved difficult. Finally, to demonstrate the methodology, we successfully applied our formalism to small molecules, for which unambiguous experimental data exist.

  5. Condensed-Matter Physics & Materials Science Seminar

    "Phase transition in functional materials and structural dynamics as studied by UTEM"

    Presented by Ming Zhang, Institute of Physics, Chinese Academy of Sciences

    Tuesday, February 12, 2019, 10 am
    ISB Bldg. 734 Conference Room 201 (upstairs)

    Hosted by: Jing Tao

    My presentation contains 3 parts. First, I will briefly introduce the pump-probe technique and Ultrafast Transmission Electron Microscopy (UTEM). Then I will demonstrate our development project of the UTEM, including the modifications of the configuration, the establishment of optical system, the generation of photoelectrons, and specific cases are discussed to show the capability of our UTEM. At last, I will highlight the application of UTEM via two examples: (1) The photoinduced martensitic (MT) transition and reverse transition in a shape memory alloy Mn50Ni40Sn10 have been examined by UTEM, and imaging and diffraction observations clearly show a variety of structural dynamic features at picosecond time scales; (2) The Lorentz UTEM for direct imaging photoinduced ultrafast magnetization dynamics, revealing remarkable features of magnetic transient states after a femtosecond pulsed laser excitation, and three successive dynamical processes involving four distinct magnetic states are evidently observed in MnNiGa crystals.

  6. Condensed-Matter Physics & Materials Science Seminar

    "Stimulation of quantum phases by time-dependent perturbations"

    Presented by Victor Galitski, University of Maryland

    Thursday, February 7, 2019, 11 am
    ISB Bldg. 734, Conf. Room 201 (upstairs)

    Hosted by: Mark Dean

    I will review our theory work on dynamic stimulation of various quantum phases. A key idea here is that the equilibrium distribution is rarely optimal for occurrence of a given quantum state and dynamic perturbations can be used to "deform" an electron population in a favorable way in order to enhance quantum coherence. To illustrate this idea, I will show how both Cooper pairing and phase coherence can be dynamically enhanced in both conventional superconductors and bosonic superlfuids. Then, I will discuss dynamic enhancement of high-temperature superconductivity in the cuprates, as it reported in experiments by the Andrea Cavalleri group in Hamburg. It will be shown that an optical pump can suppress charge order and simultaneously enhance superconductivity, due to the inherent competition between the two. In the second part of my talk, I will generalize these ideas to quantum cavities, where the light-matter coupling can be strongly enhanced. In particular, I will discuss the hybridization of cavity photons with collective modes in interacting two-dimensional materials, including the formation of Higgs polaritons and the closest analogue to excitons in a superconductor - Bardasis-Schrieffer modes - hybridized with light.

  7. Condensed-Matter Physics & Materials Science Seminar

    "Novel Electrochemistry for Fuel Cell Reactions: Efficient Synthesis and New Characterization Methods"

    Presented by Zhixiu Liang

    Wednesday, February 6, 2019, 4:30 pm
    Bldg. 480 Conference Room

    Hosted by: Jing Tao

    The ever increasing consumption of fossil fuels for transportation causes climate change causing a growing concern about their future availability and further adverse environmental effects. To address this issue, the concept of CO2 neutral fuels-based energy cycle was brought out. The key reactions in that concept are electrochemical methanol oxidation (MOR), ethanol oxidation reaction (EOR), and CO2 reduction reaction (CO2RR). These all are elctrocatalysis research challenges being slow even at the best catalyst that hamper application of fuel cells, and bring environmental benefits. My research made these improvements of catalysts for the key reactions. In-situ electrochemical infrared reflective absorbance spectrum (EC-IRRAS) reveals that at lower temperature, such reaction is not complete and generates more formate; at elevated temperature, such reaction is complete to carbonate. Ethanol is one of the ideal fuels for fuel cells, but requires highly improved catalysts. Au@PtIr/C catalyst was synthesized with a surfactant-free wet-chemistry approach. Transmission electron microscope (TEM) characterization confirms the monolayer/sub-monolayer Pt-Ir shell, gold core structure. The catalyst has a very high mass activity of 58 A/mg at peak current. In situ EC-IRRAS reveals that C-C bond is cleaved upon contact with the catalyst surface leading to ethanol complete oxidation to CO2. Related researches on methodologies, included in situ TEM to help obtaining catalysts improvements, give morphologic, structural and spectroscopic information at wide range from hundreds of microns to sub-nanometer coupled with various detectors. Microelectromechanical System (MEMS) based chips technology enables TEM observation in operando, with liquid-flow-cell chips and electrochemistry chips designed and fabricated. Ag@Au hollow cubes synthesis via galvanic replacement of Au on Ag cubes was investigated with in situ TEM. The results demonstrate abnormal react

  8. Condensed-Matter Physics & Materials Science Seminar

    "Strongly-correlated systems: Controllable field-theoretical approach"

    Presented by Igor Tupitsyn, University of Massachusetts Amherst

    Tuesday, January 29, 2019, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    Accurate account for interactions in theoretical models for strongly correlated many-body systems is the key for understanding real materials and one of the major technical challenges of modern physics. To accept this challenge, new and more effective methods, capable of dealing with interacting systems/models in an approximation-free manner, are required. One of such methods is the field-theoretical Diagrammatic Monte Carlo technique (DiagMC). While a conventional Quantum Monte Carlo samples the configuration space of a given model Hamiltonian, the DiagMC samples the configuration space of the model-specific Feynman diagrams and obtains final results with controlled accuracy by accounting for all the relevant diagrammatic orders. In contrast to conventional QMC, it does not suffer from the fermionic sign problem and can be applied to any system with arbitrary dispersion relation and shape of the interaction potential (both doped and undoped). In the first part of my talk I will introduce the technique, based on its bold-line (skeleton) implementation, and benchmark it against known results for the problem of semimetal-insulator transition in suspended graphene. In the second part I will briefly demonstrate its applications to various strongly-correlated systems/problems (stability of the 2d Dirac liquid state against strong long-range Coulomb interaction; interacting Chern insulators; phonons in metals; 1d chain of hydrogen atoms; uniform electron gas (jellium model), optical conductivity, etc).

  9. Condensed-Matter Physics & Materials Science Seminar

    "Recent Progress in Non-perturbative methods for QFTs"

    Presented by Lorenzo Vitale, Boston University

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

    Hosted by: Alexei Tsvelik

    Quantum field theories (QFT) are notoriously hard to solve in the strongly coupled regime, and few tools are available in space dimension larger than one. In this talk I discuss recent progress and ideas in characterizing certain QFTs in dimension d >= 1, based on the Hamiltonian Truncation and S-matrix bootstrap techniques. Some of the applications I will mention are Landau-Ginzburg theories and the Chern-Simons-matter theories.

  10. Condensed-Matter Physics & Materials Science Seminar

    "Effect of ion irradiation on the mechanical behavior and microstructural evolution of nanoscale metallic alloys"

    Presented by Gowtham Sriram Jawaharram, University of Illinois at Urbana - Champaign

    Wednesday, January 16, 2019, 11 am
    ISB Bldg. 734 Conference Room 201 (upstairs)

    Hosted by: Jing Tao

    Nanostructured alloys are considered as potential candidates for next generation (Generation IV) nuclear reactors because the high densities of interfacial defect sinks present in these materials. The effect of irradiation on the mechanical behavior of such alloys has received limited attention, likely resulting from the experimental challenges associated with performing such experiments. The first part of the talk will report on our recent efforts to perform high temperature irradiation induced creep (IIC) measurements in focused ion beam fabricated FCC alloys (single crystalline Ag nanopillars and nanocrystalline high entropy alloys (HEA) microbeams) by combining in-situ TEM based small-scale mechanical testing with ion irradiation and in-situ laser heating using the in-situ ion irradiation transmission electron microscope (I3TEM) at Sandia National Laboratories. The effect of pillar size, grain size, and temperature on the observed creep mechanism will be discussed. The second part of the talk will focus on the microstructural evolution of model highly immiscible CuW alloys during thermal annealing and high temperature irradiation characterized using high angle annular dark field (HAADF) imaging. The results will be discussed from the context of evolution and spatial distribution of W precipitates and its effect on hardness as a function of irradiation dose and temperature.

  11. Condensed-Matter Physics & Materials Science Seminar

    "Exact Solution and Semiclassical Analysis of BCS-BEC Crossover in One Dimension"

    Presented by Tianhao Ren, Columbia University

    Monday, January 7, 2019, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    In this talk, I will introduce a new type of model for two-component systems in one dimension subject to exact solutions by Bethe ansatz. It describes the BCS-BEC crossover in one dimension and its integrability is obtained by fine-tuning the model parameters. The new model has rich many-body physics, where the Fermi momentum for the ground state distribution is constrained to be smaller than a certain value and the zero temperature phase diagram with an external field has a critical field strength for polarization. Also the low energy excitation spectra of the new model present robust features that can be related to solitons at BCS-BEC crossover in one dimension, as shown by the semiclassical analysis.

  12. Condensed-Matter Physics & Materials Science Seminar

    "Uncovering the interactions behind quantum phenomena"

    Presented by Keith Taddei, Oak Ridge National Laboratory

    Tuesday, December 18, 2018, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson/Mark Dean

    Quantum computing, spintronics and plasmonics are nascent fields with potential to radically change our technological landscape. Fundamental to advancing these technologies is a mastery of quantum materials such as superconductors, quantum-spin-liquids and multiferroics. Ideally, we would know exactly what interactions give rise to these phenomena and design materials suitable for applications however, such an understanding as of yet eludes us. Instead we are stuck digging around in the phase space of known quantum materials slowly uncovering pertinent details to their design, filling in pieces of our incomplete picture. In this presentation, I will discuss recent bits I have found in my use of neutron scattering to study quantum materials. Starting with a novel new family of quasi-one-dimensional (Q1D) superconductors (A1,2TM3As3 with A = alkali metal and TM = Cr, Mo) I will present findings of short-range structural order and a proximate magnetic instability which, due the radically different structure, allow for new insights to the pertinence to such orders to superconductivity. Importantly, in these materials the two orders break different symmetries and so their interactions with the superconducting order can be studied independently. Next, I will discuss an interesting yet neglected family of frustrated magnetic materials – the rare-earth pyrogermanates (REPG). We find the Er2Ge2O7 REPG to exhibit 'local-Ising' type magnetism in direct analogy to the spin-ice pyrochlores suggesting effects of local anisotropies and dipole interactions. Finally, I will present ongoing work investigating spin-driven polarization effects in the magnetically and structurally straightforward multiferroic BiCoO3. These results demonstrate the essential role of neutron and x-ray scattering techniques in studying these complex materials and the fruitful opportunities these systems present to advance our understanding of quantum materials.

  13. Condensed-Matter Physics & Materials Science Seminar

    "Discussion of opportunities related to Quantum Information initiative"

    Presented by Alexei Tsvelik, BNL

    Friday, December 7, 2018, 3 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Alexei Tsvelik will be sharing his thoughts on how we can answer to the DOE initiative on Quantum Information

  14. Condensed-Matter Physics & Materials Science Seminar

    "Laser induces dynamics in complex oxides with visible/NIR and X-ray probe (Note: This will be a skype presentation)"

    Presented by Sergii Parchenko, Swiss Light Source, Paul Scherrer Institute, Switzerland

    Tuesday, December 4, 2018, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson/Mark Dean

    **********Note: This will be a Skype Presentation************ recent achievements in generation of ultrashort and intense light pulses allow observation of the physical process on the ultrafast regime. exploring fundamental physical processes on the time scales of interactions, responsible for them, is the key for future understanding of the physical principles and implementation then to the technological application. with this talk, i'm going to present the study of laser induced dynamics in complex oxides with focus on several physical objects: magnetic exchange interaction, insulator to metal transition and magneto-electric coupling. it will be discussed how the study of laser induced changes with different probing methods could help to understand the microscopic mechanisms of physical processes on the ultrafast time scale.

  15. Condensed-Matter Physics & Materials Science Seminar

    "First-principles description of correlated materials with strong spin-orbit coupling: the analytic continuation and branching ratio calculation"

    Presented by Jae-Hoon Sim, Department of Physics, KAIST, Korea, Republic of (South)

    Monday, December 3, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Sangkook Choi

    The DFT+DMFT combined with the continuous-time quantum Monte Carlo (CT-QMC) impurity solver is one of the successful approaches to describe correlated electron materials. However, analytic continuation of the QMC data written in the imaginary frequency to the real axis is a difficult numeric problem mainly due to the ill-conditioned kernel matrix. While the maximum entropy method is one of the most suitable choices to gain information from the noisy input data, its applications to the materials with strong spin-orbit coupling are limited by the non-negative condition of the output spectral function. In the first part of this talk, I will discuss the newly developed methods for analytic continuation problem, the so-called maximum quantum entropy method (MQEM) [1]. It is the extension of the conventional method, introducing quantum relative entropy as a regularization function. The application of the MQEM for a prototype j_eff=1/2 Mott insulator, Sr2IrO4, shows that it provides a reasonable band structure without introducing a material specific base set. I will also introduce the application of machine learning technique to the same problem [2]. In the second part, a simple technique to branching ratio from the first-principles calculation will be discussed [3]. The calculated ?L·S? and branching ratio of the different 5d iridates, namely Sr2IrO4, Sr2MgIrO6, Sr2ScIrO6, and Sr2TiIrO6 are in good agreement with recent experimental data. Its reliability and applicability also be carefully examined in the recent study. [1] J.-H. Sim and M. J. Han, Phys. Rev. B 98, 205102 (2018). [2] H. Yoon, J.-H. Sim, and M. J. Han, Phys. Rev. B (in press). [3] J.-H. Sim, H. Yoon, S. H. Park, and M. J. Han, Phys. Rev. B 94, 115149 (2016).

  16. Condensed-Matter Physics & Materials Science Seminar

    "Localized-to-itinerant crossovers in Kondo materials"

    Presented by Daniel Mazzone, Brookhaven National Laboratory, NSLS-II

    Monday, December 3, 2018, 11 am
    ISB Bldg. Conf. Room 201 (upstairs)

    Hosted by: Ian Robinson/Mark Dean

    While charge carriers in crystalline structures can be located close to the nuclei or establish a delocalized character, they often epitomize strong fluctuations at intermediate regimes where emergent quantum phases show an intricate coupling among various degrees of freedom. Kondo materials are particularly interesting model systems to investigate strongly correlated phenomena, because they often possess small energy scales that are highly susceptible to macroscopic constraints. I will present recent neutron and X-ray scattering results on the series Nd1-xCexCoIn5 and Sm1-xYxS, where the ground state properties were tuned either via chemical substitution or magnetic field. We find that Nd substitution in CeCoIn5 affects the magnetic coupling parameters, triggering a change in the magnetic symmetry that is offset from the emergence of coherent heavy bands and unconventional superconductivity. Intriguingly, another magneto-superconducting phase with altered coupling is observed in Nd0.05Ce0.95CoIn5 at large magnetic fields. Sm1-xYxS features a transition towards an intermediate valence state under yttrium doping. Our results unravel a Kondo-triggered Lifshitz-transition in the mixed-valence state, which dives an unusually strong charge localization at low temperatures.

  17. Condensed-Matter Physics & Materials Science Seminar

    "Dirac fermions and critical phenomena: exponents and emergent symmetries"

    Presented by Michael Scherer, University of Cologne, Germany

    Thursday, November 8, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Laura Classen

    Dirac fermions appear as quasi-particle excitations in various condensed-matter systems for example in graphene or as surface states of topological insulators. Close to a quantum phase transition they exhibit a series of exotic properties, e.g., emergent symmetries, fluctuation-induced critical points, the appearance of two length scales and a hierarchy of mass gaps. I discuss mechanisms that are behind these phenomena from a quantum field-theoretical point of view. Further, I present a four-loop renormalization group study for the determination of the Dirac fermions' critical behavior and compare to the predictions of complementary approaches such as quantum Monte Carlo and the conformal bootstrap. Finally, I will also comment on the possibility to test duality conjectures with these calculations.

  18. Condensed-Matter Physics & Materials Science Seminar

    "In-situ Investigation of Crystallization of a Metallic Glass by Bragg Coherent X-ray Diffraction"

    Presented by Bo Chen, Tongji University, China

    Monday, October 15, 2018, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson

    The crystallization behaviour of metallic glass (MG) has long been investigated ever since the discovery of these important functional materials [1]. Compared with crystalline and amorphous extremes, mate-rials containing crystalline precipitates within an otherwise amorphous MG or partially crystallized ma-terials have distinct properties that could be a way of tuning the materials' characteristics. Several methods including powder X-ray diffraction (XRD), transmission electron microscope (TEM) and se-lected area electron diffraction (SAED) are usually combined to characterize the degree of crystalline structure in amorphous materials. Until now, these methods, however, have failed to show the crystal-lization of individual crystal grains in three dimensions. In this work, the in-situ Bragg coherent X-ray diffraction imaging (BCDI) [2, 3] reveals the grain growth and the strain variation of individual crystals up to the sizes of a few hundred nanometers from the pure Fe-based MG powder during heating. We have found that there is preferential growth along one direction during the crystal formation; there is fractal structure around the developing crystal surface; there is also strain relaxation within the growing crystals while cooling. The work supports a two-step crystallization model for the Fe-based MG during heating. This could help to pave the way for designing partially crystalline materials with their at-tendant soft magnetic, anti-corrosive and mechanical properties. References [1] D. H. Kim, W. T. Kim, E. S. Park, N. Mattern, and J. Eckert, Prog. Mater. Sci. 2013, 58, 1103. [2] M. A. Pfeifer, G. J. Williams, I. A. Vartanyants, R. Harder and I. K. Robinson, Nature 2006, 442, 63. [3] I. K. Robinson and R. Harder, Nat. Mater. 2009, 8, 291.

  19. Condensed-Matter Physics & Materials Science Seminar

    "Universality and quantum criticality of the one-dimensional spinor Bose gas"

    Thursday, September 27, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201

    Hosted by: Igor Zaliznyak

    We investigate the universal thermodynamics of the two-component one-dimensional Bose gas with contact interactions in the vicinity of the quantum critical point separating the vacuum and the ferromagnetic liquid regime. We find that the quantum critical region belongs to the universality class of the spin-degenerate impenetrable particle gas which, surprisingly, is very different from the single-component case and identify its boundaries with the peaks of the specific heat. In addition, we show that the compressibility Wilson ratio, which quantifies the relative strength of thermal and quantum fluctuations, serves as a good discriminator of the quantum regimes near the quantum crit- ical point. Remarkably, in the Tonks-Girardeau regime the universal contact develops a pronounced minimum, reflected in a counterintuitive narrowing of the momentum distribution as we increase the temperature. This momentum reconstruction, also present at low and intermediate momenta, signals the transition from the ferromagnetic to the spin-incoherent Luttinger liquid phase and can be detected in current experiments with ultracold atomic gases in optical lattices.

  20. Condensed-Matter Physics & Materials Science Seminar

    "Multiloop functional renormalization group: Exact flow equations from the self-consistent parquet relations"

    Presented by Fabian Kugler, Ludwig-Maximilians-Universitat Munchen, Germany

    Thursday, September 20, 2018, 1:30 pm
    ISB 734 Conference Room 201

    Hosted by: Andreas Weichselbaum

    The functional renormalization group (fRG) is a versatile, quantum-field-theoretical formulation of the powerful RG idea and has seen a large number of successful applications. The main limitation of this framework is the truncation of the hierarchy of flow equations, where typically effective three-particle interactions are neglected altogether. From another perspective, the parquet formalism consists of self-consistent many-body relations on the one- and two-particle level and allows for the most elaborate diagrammatic resummations. Here, we unify these approaches by deriving multiloop fRG flow equations from the self-consistent parquet relations [1]. On the one hand, this circumvents the reliance on higher-point vertices within fRG and equips the method with quantitative predictive power [2]. On the other hand, it enables solutions of the parquet equations in previously unaccessible regimes. Using the X-ray-edge singularity as an example, we introduce the formalism and illustrate our findings with numerical results [3]. Finally, we discuss applications to the 2D Hubbard model [4] and the combination of multiloop fRG with the dynamical mean-field theory. [1] F. B. Kugler and J. von Delft, arXiv:1807.02898 (2018) [2] F. B. Kugler and J. von Delft, PRB 97, 035162 (2018) [3] F. B. Kugler and J. von Delft, PRL 120, 057403 (2018) [4] A. Tagliavini, C. Hille, F. B. Kugler, S. Andergassen, A. Toschi, and C. Honerkamp, arXiv:1807:02697 (2018)

  21. Condensed-Matter Physics & Materials Science Seminar

    "Pair-breaking quantum phase transition in superconducting nanowires"

    Presented by Andrey Rogachev, University of Utah

    Friday, September 7, 2018, 11 am
    ISB Bldg. 734 Conference Room 201 (upstairs)

    Hosted by: Ivan Bozovic

    Quantum phase transitions (QPT) between distinct ground states of matter are widespread phenomena, yet there are only a few experimentally accessible systems where the microscopic mechanism of the transition can be tested and understood. In this talk we will report on discovery that a magnetic-field driven quantum phase transition in MoGe superconducting nanowires can be fully explained by the critical theory of pair-breaking transitions characterized by a correlation length exponent v≈1 and dynamic critical exponent z≈ 2. We find that in the quantum critical regime, the electrical conductivity is in agreement with a theoretically predicted scaling function and, moreover, that the theory quantitatively describes the dependence of conductivity on the critical temperature, field magnitude and orientation, nanowire cross-sectional area, and microscopic parameters of the nanowire material. At the critical field, the conductivity follows a T^(d–2)/z dependence predicted by phenomenological scaling theories and more recently obtained within a holographic framework. Our work uncovers the microscopic processes governing the transition: the pair-breaking effect of the magnetic field on interacting Cooper pairs overdamped by their coupling to electronic degrees of freedom. It also reveals the universal character of continuous quantum phase transitions. In the talk we will also briefly comment on reliability of the finite-size scaling analysis, origin of zero-bias anomaly in wires and implication of our finding for QPT in superconducting films.

  22. Condensed-Matter Physics & Materials Science Seminar

    "Spinon Confinement and a Longitudinal Mode in One Dimensional Yb2Pt2Pb"

    Presented by Bill Gannon, Department of Physics and Astronomy, Texas A&M University

    Thursday, August 23, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    Abstract: The Yb3+ magnetic moments in Yb2Pt2Pb are seemingly classical, since the large spin-orbit coupling of the 4f-electrons and the crystal electric field dictate a J = +/-7/2 Yb ground state doublet. Surprisingly, the fundamental low energy magnetic excitations in Yb2Pt2Pb are spinons on one dimensional chains, shown to be in good agreement with the behavior expected with the XXZ Hamiltonian for nearly isotropic, S = +/-1/2 magnetic moments. We have performed new high resolution neutron scattering measurements to examine the properties of these excitations in a magnetic field. In fields larger than 0.5 T, the chemical potential closes the gap to the spinon dispersion, modifying the quantum continuum through the formation of a spinon Fermi surface. This leads to the formation of spinon bound states along the chains, coupled to a longitudinally polarized interchain mode at energies below the quantum continuum. The ground state doublet nature of the Yb ions ensures that at all fields, transverse excitations are virtually nonexistent, allowing direct measurement of the mode dispersion.

  23. Condensed-Matter Physics & Materials Science Seminar

    "Advances in high energy electron holography"

    Presented by Dr. Toshiaki Tanigaki, Hitachi, Japan

    Friday, August 10, 2018, 1:30 pm
    Conference room in building 480

    Hosted by: MG Han

    Advances in High-Voltage Electron Holography T. Tanigaki Research & Development Group, Hitachi, Ltd. Email: toshiaki.tanigaki.mv@hitachi.com Electron holography can observe electromagnetic field inside materials and devices at high-resolution around atomic scale. The high penetration power of a high energy electron wave is crucial to observing magnetic structures, which exist only in thick samples. It is particularly crucial in three-dimensional (3D) observations, which require a series of sample observations with the sample increasingly tilted so that the projected sample thickness increases with the tilt angle. As an example of this, magnetic vortex cores confined in stacked ferromagnetic (Fe) discs were observed three-dimensionally by using vector-field electron tomography with a 1.0 MV holography electron microscope [1]. To invent new functional materials and devices for establishing a sustainable society, methods for controlling atomic arrangements in small areas such as interfaces have become important [2,3]. Electron holography is a powerful tool for analyzing the origins of functions by observing electromagnetic fields and strains at high resolutions. The advantages of high-voltage electron holography are high resolution and penetration power due to high energy electron waves. The quest for finding the ultimate resolution through continuous improvements on holography electron microscopes led to the development of an aberration corrected 1.2 MV holography electron microscope [4,5] (Figure 1). We describe recent results obtained by using the high-voltage electron holography. Spatial resolution of 1.2 MV holography electron microscope reached 0.043 nm at high-resolutions, when the sample was placed in a high magnetic field of the objective lens [4]. Under the observation conditions, in which the sample was placed in a field-free position for observing a magnetic field, the spatial res

  24. Condensed-Matter Physics & Materials Science Seminar

    "Imaging Non-equilibrium Dynamics in Two-Dimensional Materials"

    Presented by Kenneth Beyerlein, Max Planck Institute for the Structure and Dynamics of Matter, Germany

    Wednesday, August 1, 2018, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson

    The interfaces in thin film heterostructures dictate the performance of an electronic device. Understanding their behavior upon exposure to light is important for advancing photovoltaics and spintronics. However, producing an atomic image of these dynamics is an under-determined problem without a unique solution. In this talk, I will show how a set of ultrafast soft X-ray diffraction rocking curves can be spliced together to add constraints to the phase retrieval problem. In doing so, the anti-ferromagnetic order through a NdNiO3 film after illumination of the substrate with a mid-Infrared laser pulse will be imaged. Notably, a disordered phase front initiated at the substrate interface is shown to evolve at twice the speed of sound. This time-spliced imaging technique opens a new window into the correlated dynamics of two-dimensional materials.

  25. Condensed-Matter Physics & Materials Science Seminar

    "Atomic level structural characterization of materials by electron microscopy"

    Presented by Shize Yang, Center for Functional Nanomaterials

    Thursday, July 26, 2018, 4 pm
    Bldg. 480, Conference Room

    Hosted by: Yimei Zhu

    In recent years, with the development of technologies, electron microscopy techniques have been widely developed. Important advancement has been achieved on in-situ electron microscopy, cryogenic electron microscopy, electron tomography, advanced electron energy loss spectroscopy etc. In this talk I will briefly introduce and show how those techniques provide a vital role in the structural characterization of 2D materials, catalysts and battery materials.

  26. Condensed-Matter Physics & Materials Science Seminar

    "Mechanism of strange metal and strange metal state near a heavy fermion quantum critical point"

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

    Wednesday, July 18, 2018, 1:30 pm
    ISB Bldg. 734 Conference Room 201

    Hosted by: Alexei Tsvelik

    Strange metal (SM) behaviors with non-Fermi liquid (NFL) properties, generic features of heavy fermion systems near quantum phase transitions, are yet to be understood microscopically. A paradigmatic example is the magnetic field-tuned quantum critical heavy fermion metal YbRh2Si2 (YRS), revealing a possible SM state over a finite range of fields at low temperatures when substituted with Ge. Above a critical field, the SM state gives way to a heavy Fermi liquid with Kondo correlation. The NFL behavior shows most notably a linear-in-temperature electrical resistivity and a logarithmic-in-temperature followed by a power-law-in-temperature in the specific heat coefficient at low temperatures [1]. We propose a mechanism to explain it: a quasi-2d fluctuating anti-ferromagnetic short-range resonating-valence-bond (RVB) spin-liquid competing with the Kondo correlation (Fig. 1) [2]. Applying renormalization group analysis on an effective field theory beyond a large-N approach to an antiferromagnetic Kondo-Heisenberg model, we identify the critical point, and explain remarkably well the SM behavior. Our theory goes beyond the well-established framework of quantum phase transitions and serves as a new basis to address open issues of the non-Fermi liquid behavior in quantum critical heavy-fermion compounds, such as: the strange superconductivity observed in the "115" family CeMIn5 (M=Co, Rh)[3]. References: [1] J. Custers et al., Nature 424, 524 (2003); J. Custers et al., Phys. Rev. Lett. 104, 186402 (2010). [2] Yung-Yeh Chang, Silke Paschen, and Chung-Hou Chung, Phys. Rev. B 97, 035156 (2018). [3] Y. Y. Chang,, F. Hsu, S. Kirchner, C. Y. Mou, T. K. Lee and C. H. Chung (un-published).

  27. Condensed-Matter Physics & Materials Science Seminar

    "Electron-microscopy-guided designing of ferroelectric materials for nonvolatile memories and multifunctional nanodevices"

    Presented by Linze Li, University of California @ Irvine

    Thursday, July 12, 2018, 11 am
    Bldg. 480, Conference Room

    Hosted by: Yimei Zhu

    As a prototypical example of functional oxides, ferroelectric materials have been utilized in a broad range of electronic, optical, and electromechanical applications and hold the promise for the design of future high-density nonvolatile memories and multifunctional nanodevices. The utilities of ferroelectrics are derived from the structures and switching of ferroelectric domains, or from their coupling to other material functionalities. In recent years, advanced imaging techniques based on aberration-corrected scanning transmission electron microscopy (STEM) and in situ transmission electron microscopy (TEM) have become powerful methods to characterize ferroelectric oxides, allowing nanoscale polarization states to be unambiguously determined with sub-Angstrom resolution, and allowing domain switching processes to be directly resolved in real time. In this presentation I will show several examples of applying advanced STEM or TEM-based techniques to the study of the static and dynamic properties of domains and domain walls in ferroelectric and multiferroic BiFeO3 thin films. Atomic structures and electrical switching behaviors of charged domain walls have been observed. A strong interaction between the ferroelectric polarization and nanoscale impurity defects has been discovered, and a new route to the production of exotic polarization states by utilizing such interaction has been proposed and established. These findings open up the possibility for the designing of novel ferroelectric materials and multifunctional devices with nanoscale structural defects or charged domain walls as essential components.

  28. Condensed-Matter Physics & Materials Science Seminar

    "Room-temperature magnetic spiral order induced by disorder"

    Presented by Christopher Mudry, Paul Scherrer Institut, Switzerland

    Monday, July 2, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    Upon cooling, the compound YBaCuFeO5 undergoes a phase transition to an antiferromagnetic long-range ordered phase. Upon further cooling, a second phase transition to a magnetic spiral phase takes place. The latter transition temperature depends on the sample preparation and can reach room temperature. We propose a mechanism to explain the transition to a magnetic spiral ordered phase due to frustrating magnetic interactions that are introduced randomly along a single crystallographic direction as caused by a particular type of chemical disorder. This mechanism could open the way to high-temperature multiferroism.

  29. Condensed-Matter Physics & Materials Science Seminar

    "Imaging of Local Structure and Dynamics in Hard and Soft Condensed Matter Systems"

    Presented by Dmitry Karpov, New Mexico State University

    Friday, June 22, 2018, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson

    With advancement of coherent probes there is a shift from integral studies to highly localized studies in either spatial or temporal domains. Nanostructures and low dimensional phenomena, correlated fluctuations and associated transitions directly benefit from new instrumental capabilities. Studies of ferroelectric and magnetic materials and of their local behavior allow both to test fundamental physics concepts and provide access to technologies with direct practical applications. Topological phase transitions and topological defects are among the topics that are actively pursued in modern materials science. In recent study [1] conducted by our group we were able to visualize three-dimensional topological vortex structure in a volume of individual ferroelectric nanoparticle of barium titanate under external electric field using Bragg coherent diffractive imaging technique. Among other things we observed: (i) electric field induced structural transition from mixture of tetragonal and monoclinic phases to dominant monoclinic phase; (ii) controllable switching of vortex chirality; (iii) vortex mediated behavior of the nano-domains in the particle; (iv) and that the core of the vortex in the volume behaves as a nanorod of zero ferroelectric polarization which can be rotated by external electric field and can serve as a conducting channel for charge carriers. These findings can be used in the design of novel nanoelectronics devices and for creating artificial states of matter. Better understanding of the materials behavior at the nanoscale requires ways of probing anisotropies of the refractive index. Using polarized laser light, we've developed a method [2] termed birefringent coherent diffractive imaging that allows to extract projections of dielectric permittivity tensor in nematic liquid crystal. Further expanding this tool into full-vectorial mode shows that the method can be applied for imaging of magnetic domains, cellular structures, and ot

  30. Condensed-Matter Physics & Materials Science Seminar

    "Theories of transport scaling in disordered semimetals and topological spin-nematic excitonic insulators in graphite under high magnetic field"

    Presented by Ryuichi Shindo, Peking University, China

    Thursday, June 21, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    In the first part of my talk, I will talk about transport scaling theories in disordered Weyl semimetal [1,2]. In electronic band structure of solid state material, two band touching points with linear dispersion (called as `Weyl node') appear in pair in the momentum space. When they annihilate with each other, the system undergoes a quantum phase transition from Weyl semimetal (WSM) phase to a band insulator (BI) phase. The continuous phase transition is recently discovered in solid state materials [3]. The phase transition is described by a critical theory with a `magnetic dipole' like object in the momentum space. The critical theory hosts a disorder-driven quantum multicritical point, which is encompassed by three quantum phases, WSM phase, BI phase, and diffusive metal (DM) phase. Based on the renormalization group argument, we clarify transport scaling properties around the Weyl node at the quantum multicritical point as well as all phase boundaries among these three phases [1,2]. In the second part of my talk, I will argue that three-dimensional topological excitonic insulator is realized in graphite under high magnetic field [4,5]. Graphite under high magnetic field exhibits consecutive metal-insulator (MI) transitions as well as re-entrant insulator-metal (IM) transition at low temperature. We explain these enigmatic insulator phases as manifestation of excitonic insulator phases with spin nematic orderings ("SNEI" phases). Especially, we explain unusual field-dependences of in-plane resistivity in the graphite experiment by surface transports via 2+1 massless surface Dirac fermion in one of the SNEI phases [4,5]. [1] https://arxiv.org/abs/1803.09051, under review [2] https://arxiv.org/abs/1710.00572, selected as PRB editors' suggestion [3] Tian Liang, et.al., Science Advances, 3, e1602510 (2017) [4] https://arxiv.org/abs/1802.10253, under review [5] in preparation &

  31. Condensed-Matter Physics & Materials Science Seminar

    "Fermi-Surface Reconstruction in Nd-doped CeCoIn5"

    Presented by Elizabeth Green, Dresden High Magnetic Field Laboratory

    Thursday, June 21, 2018, 11 am
    ISB Bldg. 734 Seminar Room 201 (upstairs)

    Hosted by: Cedomir Petrovic

    Heavy fermion compounds are well known to exhibit novel properties when exposed to high magnetic fields. Most notably CeCoIn5 exhibits a field-induced superconducting state at high magnetic fields known as the Qphase. Recent neutron scattering measurements show a similar Q-vector for the 5% Nd-doped CeCoIn5 at zero applied magnetic field [1] which has initiated intense theoretical and experimental work on this doping series. In this talk I will present de Haas-van Alphen effect measurements which indicate a drastic Fermi-surface reconstruction occurs between 2 and 5% Nd-doping levels. The cylindrical Fermi surface, believed to play a crucial role in superconductivity in these materials, develops a quasi-three-dimensional topology with increased doping levels thus reducing the likelihood of an enhanced nesting scenario, previously given as a possible explanation for the Q-phase. However, effective masses remain relatively unchanged up to 10% Nd indicating the crossing of a spin density wave type of quantum critical point. In addition, I will present evidence that by substituting Ce with Nd the electronic pairing potential may be altered. These results help elucidate the reasoning for the emergence of the Q-phase seen in the 5% Nd sample and may be relevant to other heavy fermion compounds. [1] S. Raymond et al., JPSJ 83, 013707 (2014).

  32. Condensed-Matter Physics & Materials Science Seminar

    "X-ray Scattering as a Tool for Understanding Nanostructured Materials"

    Presented by Robert Koch, Alfred University

    Tuesday, June 19, 2018, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson

    Materials with significant local distortions from the bulk average structure often show novel and useful properties. Identifying and quantifying the nature and extent of this correlated disorder1 is however quite challenging, as traditional crystallography and the associated tools often do not adequately describe such nanostructured materials. This is a manifestation of the "nanostructure problem"2 and the solution requires complex modelling incorporating multiple techniques. This talk focuses on the application of X-ray scattering and complex modelling as tools for understanding various nanostructured materials, including nanocrystalline nickel with large clusters of planar defects, interlayered non-silicon photovoltaics, geometrically frustrated ternary alkaline earth hexaborides, manganese dioxide nanosheet assemblies, and nanostructured noble metal alloys. Complex modelling leveraging both standard techniques as well as genetic algorithms, Markov chain Monte Carlo, and machine learning together provide synergistic understanding spanning length scales from a few Ångstrom to hundreds of nanometers. Additionally, an example of how complex modelling can be used to shed understanding on the nature of crystallographic disorder in superconducting alloys of 2H-TaSe2−xSx is discussed. A potential model is proposed whereby alloys of 2H-TaSe2−xSx are composed of interlayered sheets with two unique c-axes. This model is consistent with the observed 00l Bragg profile broadening trend and may help explain the suppression of charge density waves and maximization of superconductivity in these systems. 1. Keen, D. A. & Goodwin, A. L. The crystallography of correlated disorder. Nature 521, 303–309 (2015). 2. Billinge, S. J. L. & Levin, I. The problem with determining atomic structure at the nanoscale. Science 316, 561–565 (2007).

  33. Condensed-Matter Physics & Materials Science Seminar

    "Doublon-holon origin of the subpeaks at the Hubbard band edges"

    Presented by Seung-Sup Lee, Ludwig-Maximilians-University, Germany

    Thursday, June 14, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Andreas Weichselbaum

    Dynamical mean-field theory (DMFT) studies frequently observe a fine structure in the local spectral function of the SU(2) Fermi-Hubbard model (i.e., one-band Hubbard model) at half filling: In the metallic phase close to the Mott transition, subpeaks emerge at the inner edges of the Hubbard bands. Here we demonstrate that these subpeaks originate from the low-energy effective interaction of doublon-holon pairs, by investigating how the correlation functions of doublon and holon operators contribute to the subpeaks [1, 2]. We use the numerical renormalization group (NRG) as a DMFT impurity solver to obtain the correlation functions on the real-frequency axis with improved spectral resolution [3]. A mean- field analysis of the low-energy effective Hamiltonian [2] provides results consistent with the numerical result. The subpeaks are associated with a distinctive dispersion that is different from those for quasiparticles and the Hubbard bands. Also, the subpeaks become more pronounced in the SU(N) Hubbard models for larger number N of particle flavors, due to the increased degeneracy of doublon-holon pair excitations. Hence we expect that the sub-peaks can be observed in the photoemission spectroscopy experiments of multi-band materials or in the ultracold atom simulation of the SU(N) Hubbard models. [1] S.-S. B. Lee, J. von Delft, and A. Weichselbaum, Phys. Rev. Lett. 119, 236402 (2017). [2] S.-S. B. Lee, J. von Delft, and A. Weichselbaum, Phys. Rev. B 96, 245106 (2017). [3] S.-S. B. Lee and A. Weichselbaum, Phys. Rev. B 94, 235127 (2016).

  34. Condensed-Matter Physics & Materials Science Seminar

    "Defects and their functional properties in multiferroic hexagonal systems"

    Presented by Shaobo Cheng, McMaster University Canada

    Monday, June 11, 2018, 2:30 pm
    Bldg. 480, Conference Room

    Hosted by: Yimei Zhu

    As a main component of quantum materials, multiferroic materials, which simultaneously have multiple orderings, hold promise for use in the next generation of memory devices. Taking advantage of the state-of-the-art transmission electron microscopy techniques, we have systematically studied the defects induced emergent phenomena in multiferroic hexagonal systems. Two single phase multiferroic hexagonal systems will be covered in this talk: YMnO3 and LuFe2O4. YMnO3 is a classic single phase multiferroic material with geometric ferroelectricity. The effects oxygen vacancies, partial edge dislocations, and interfacial atomic reconstructions will be presented. LuFe2O4 is a well-known multiferroic system with charge ordering origin. The effects of twins and interstitial oxygen in LuFe2O4 single crystalline sample will be discussed. The structural-property relationship for both systems has been tried to be established in our studies. Our findings demonstrate the structural flexibility of both manganites and ferrites, and open the door to new tunable multifunctional applications.

  35. Condensed-Matter Physics & Materials Science Seminar

    "Developing theoretical understanding of non-equilibrium phenomena"

    Presented by Alexander Kemper, North Carolina State University

    Thursday, May 24, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Peter D. Johnson

    In this talk, I will present an overview of some of our recent results in the area of non-equilibrium many-body theory. Experimental developments are enabling the study of electrons and atoms in the time domain with ever increasing resolution. The theoretical development has been somewhat lacking, and remains mostly rooted in extensions of equilibrium models. Our work has been to put the theoretical modeling on a firmer footing. Through numerical solution of the equations of motion, we can directly evaluate experimentally relevant spectra. These may be analyzed with the benefit of knowing the precise model and correlation functions that underlie the spectra. Most of the talk will focus on the interaction between a system of electrons interacting with several degrees of freedom, including the lattice, impurity scattering, and each other. Typically, non-equilibrium results are analyzed through a framework that relies on equilibrium intuition. Our results show that the validity of this type of analysis falls on a spectrum that varies from correct to wholly incorrect, which I will illustrate with specific examples. This line of thinking will be further developed by considering the flow of energy between various subsystems.

  36. Condensed-Matter Physics & Materials Science Seminar

    "Picoastronomy: an electron microscopist's view of the history of the Solar System"

    Presented by Rhonda Stroud, US Naval Research Laboratory

    Friday, May 4, 2018, 2 pm
    Bldg. 480, Conference Room

    Hosted by: Yimei Zhu

    A wide range of astrophysical processes, from condensation of dust particles in circumstellar envelopes to space weathering on airless bodies, are inherently pico-to-nanoscale phenomena. Thus an electron microscope, used for direct observation of planetary materials in the laboratory, can be as much of an astronomical tool as a telescope pointed at the sky. The energy resolution of state-of-the-art monochromated scanning transmission electron microscopes (STEMs), as low as 10 meV, makes it possible to directly observe the infra-red optical properties of individual cosmic dust grains in the 5 to 25 um range. Thus, distinguishing the 10-um and 18-um features of individual bonafide astrosilicates is now possible. The identity of volatiles, trapped in individual nanoscale vesicles, can be determined with STEM-EELS to better constrain space weathering processes in lunar soils. Finally, STEM-EDS offers to possibility of constraining noble gas contents of primitive carbonaceous materials, including nanodiamond, and "phase Q", thus thus constrain their formation histories.

  37. Condensed-Matter Physics & Materials Science Seminar

    "Chemistry beyond the crystal- advanced Fourier techniques"

    Presented by Simon Kimber, Oak Ridge National Laboratory

    Monday, April 30, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson

    Chemical crystallography nowadays makes structure determination and refinement trivial. However, advances in x-ray and neutron sources mean that we should revisit some of the basic assumptions that shape our experiments. For example, most chemical reactivity in e.g. catalysis, self-assembly etc, occurs in the solution phase. Why are we as crystallographers then wedded to the solid state? In this presentation, I will show how total scattering can be used to determine changes in cluster structure during photochemical reactions and to probe the role of the solvent in 'magic size' cluster formation. I will then describe how neutron scattering techniques can be used to challenge another basic assumption- the static approximation in total scattering. We have successfully applied so-called 'dynamic-PDF' techniques to simple chalcogenide materials. This allows to determine the time scale on which local distortions appear, providing insight into the role of highly anharmonic phonons in e.g. phase change and thermoelectric materials. Time allowing, I will also provide a short update on progress at ORNL, including the upcoming restart of the SNS, and new instrumentation for diffraction, total and diffuse scattering.

  38. Condensed-Matter Physics & Materials Science Seminar

    "Topological properties of Weyl semimetals in the presence of randomness"

    Presented by Jedediah Pixley, Rutgers

    Wednesday, April 25, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Laura Classen

    We will discuss the effects of short-range disorder on three-dimensional Weyl semimetals with a focus on the topological Fermi arc surface states and the existence of the axial anomaly in the presence of parallel electric and magnetic fields. We will briefly review the bulk properties of disordered Weyl semimetals concentrating on the proposed quantum critical point separating a semimetal and diffusive metal phase driven by disorder. We show that quasi-localized, rare eigenstates contribute an exponentially small but non-zero density of states at the Weyl node energy. This destabilizes the semimetal phase and converts the semimetal-to-diffusive metal transition into a cross over (dubbed an avoided quantum critical point). In turn, it is no longer obvious how robust the topological properties are in these materials. We will therefore discuss the effects disorder has on the robustness of Weyl Fermi arc surface states and the axial anomaly. We find that the Fermi arcs, in addition to having a finite lifetime from disorder broadening, hybridize with the non-perturbative bulk rare states, which unbinds them from the surface (i.e. they lose their purely surface spectral character). Nonetheless, the surface chiral velocity is robust and survives in the presence of strong disorder. Lastly, we will discuss the robustness of the axial anomaly for a single Weyl cone in the presence of disorder. We will show that deep in the diffusive limit, when a band structure picture of dispersing (chiral) Landau levels no longer applies, the axial anomaly survives.

  39. Condensed-Matter Physics & Materials Science Seminar

    "Building and understanding magnetic nano-structures, one atom at a time"

    Presented by Adrian Feiguin, Northeastern University

    Tuesday, April 24, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Alexei Tsvelik

    In the past decade we have witnessed enormous progress in experiments that consist of placing magnetic atoms at predetermined positions on substrates and building magnetic nanostructures one atom at a time. The effective interaction between spins is mediated by the conduction electrons in the substrate. In order to understand these interactions, we rely on a theory developed decades ago by Ruderman, Kittel, Kasuya, and Yosida, dubbed "RKKY theory", which applies when the spins are classical. The quantum nature of the electronic spin introduces another degree of complexity and competition with another quantum phenomenon: the Kondo effect. This competition is quite subtle and non-trivial, and can only be studied by numerical means. We investigate this mechanism on different lattice geometries in 2 and 3 dimensions by introducing an exact mapping onto an effective one-dimensional problem that we can solve with the density matrix renormalization group method (DMRG). We show a clear and departure from the conventional RKKY theory, and important differences that can be attributed to the dimensionality and geometry. We have found that there is a critical distance at which the Kondo effect dominates, translating into a finite range for the RKKY interaction. In particular, for dimension d>1, Kondo physics dominates even at short distances, while the ferromagnetic RKKY state is energetically unfavorable. Remarkably, in the case of impurities with higher spin S=1, both effects can co-exist: while the impurities are partially screened by the conduction electrons, an effective dangling spin S=1/2 is responsible for the entanglement between impurities.

  40. Condensed-Matter Physics & Materials Science Seminar

    "Plastic Deformation at the Nanoscale and Superconductivity Enhancement in Decompression"

    Presented by Bin Chen, Shanghai Laboratory of Center for High Pressure Science & Technology Advanced Research (HPSTAR), China

    Thursday, April 5, 2018, 1:30 pm
    ISB Bldg. 734 Seminar Room 201 (upstairs)

    Hosted by: Cedomir Petrovic

    Plastic Deformation at the Nanoscale: Understanding the plastic deformation of nanocrystalline materials is a longstanding challenge [1,2]. Various controversial observations, mainly on the existence of dislocations and the mechanisms for a reversed Hall–Petch effect, have been reported. However, in situ observation of plastic deformation in ultrafine (sub-10 nm) nanocrystals has long been difficult, precluding the direct exploration of mechanics at the nanometer scale. By using a radial diamond-anvil cell (rDAC) x-ray diffraction technique we plastically deformed nickel to pressures above 35 GPa and observed that 1) dislocation-mediated deformation was still operative in as small as 3 nm nickel particles [3]; 2) 70 nm nickel particles were found to rotate more than any other grain size, signaling the reversal in the size dependence of grain rotation [4,5]; 3). Hall-Petch effect in nickel can be extended to 3 nm [6]. These observations demand considering the role of defects in the physical behaviors of nanomaterials. Superconductivity Enhancement in Decompression: An unexpected superconductivity enhancement was recently observed in decompressed In2Se3 [7]. The onset of superconductivity in In2Se3 occurred at 41.3 GPa with a critical temperature (Tc) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting during decompression down to 10.7 GPa. More surprisingly, the highest Tc in decompression was 8.2 K, a twofold increase in the same crystal structure as in compression. The novel decompression-induced superconductivity enhancement implies that it is possible to maintain pressure-induced superconductivity at lower or even ambient pressures with better superconducting performance. References: [1] B. Chen, et al., MRS Bulletin 41, 473 (2016). [2] H. K. Mao, et al., Matter and Radiation at Extremes, 1, 59 (2016). [3] B. Chen, et al., Science 338, 1448 (2012). [4] B. Chen, et al., Proc. Natl. Ac

  41. Condensed-Matter Physics & Materials Science Seminar

    "Non-abelian symmetries and applications in tensor networks"

    Presented by Andreas Weichselbaum, Brookhaven National Lab

    Thursday, March 29, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Igor Zaliznyak

    I will give a brief introduction to tensor network states with focus on exploiting all symmetries, abelian and non-abelian alike. I will briefly motivate a generic framework for finite-dimensional Lie algebras, which has been fully implemented in the tensor library QSpace [1]. The latter was already put under extensive scrutiny over the past couple of years. Along it already also gave rise to a range of excellent applications. Here, in particular, I will briefly highlight 1D density matrix renormalization group (DMRG) calculations on SU(N) Heisenberg ladders, 2D projected entangled pair state (PEPS) simulations on Spin-1 Kagome [2], and infinite-dimensional dynamical mean-field theory (DMFT) simulations on Hund's metals [3]. [1] A. Weichselbaum, Annals of Physics 327, 2972 (2012) [2] Liu et al., PRB 91 (R), 060403(R) (2015) [3] Stadler et al. PRL 115, 136401 (2015)

  42. Condensed-Matter Physics & Materials Science Seminar

    "Accurate spectral calculations for testing electronic structures, low energy excitations, and vibronic interactions"

    Presented by Keith Gilmore, The European Synchrotron Radiation Facility, France

    Thursday, March 29, 2018, 11 am
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Robert Konik

    Resonant inelastic x-ray scattering (RIXS) is a relatively new technique for probing low energy excitations in materials. In addition to traditional techniques, such as angle resolved photoemission, it has become an important, high precision characterization tool of strongly correlated electron materials. To calculate RIXS, and related core and valence level spectra, we solve the Bethe-Salpeter equation (BSE) based on a self-energy corrected density functional theory electronic structure. I outline our implementation of the BSE and use SrVO3 for demonstration. The sensitivity of spectral features to the self-energy approximation – whether G0W0, qpscGW, or DMFT – is highlighted. To include interactions beyond the usual BSE I introduce the cumulant expansion. Spectral functions derived from a GW self-energy are typically inadequate when the dressed Green's function is built via the Dyson equation. With the same GW self-energy, a superior Green's function and spectral function, implicitly including vertex corrections, is obtained through the cumulant expansion. I consider application of the GW-cumulant expansion to photoemission, photoabsorption, and X-ray scattering. Lastly, vibronic coupling has important impacts on these spectra. I show how to calculation the phonon contribution to photoemission, absorption and scattering with a vibronic cumulant.

Currently showing events from the past year. See all past events »