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February 2017
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  1. Condensed-Matter Physics & Materials Science Seminar

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

    Hosted by: '''Robert Konik'''

    Thermalization, a common phenomenon in various physical settings, can naturally fail in certain isolated disordered quantum systems, challenging basic tenets of quantum statistical mechanics. Many body localization (MBL) is a canonical example of such an intriguing scenario and, therefore, attracted tremendous attention from condensed matter, statistical physics, and atomic physics communities. Considerable effort has recently gone into establishing the existence of the MBL phase, and the nature of dynamical phase transition from MBL to the thermal phase. However, understanding instabilities to the MBL phase that may lead to the complete or partial restoration of thermalization is still an open question. In this talk, I would focus on two such instabilities to the MBL phase coming from single particle mobility edge and the presence of extensive degeneracy in the many body spectrum. The goal is to identify the most robust form of MBL (in the presence of instabilities) to gain insight into the mechanisms of quantum thermalization.

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  1. MAR

    2

    Thursday

    Condensed-Matter Physics & Materials Science Seminar

    "Ab Initio electronic structure of solids: correlation effects beyond the GW method"

    Presented by Andrei Kutepov, Rutgers University

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

    Thursday, March 2, 2017, 1:30 pm

    Hosted by: ''''Gabi Kotliar''''

    TBA

  2. MAR

    9

    Thursday

    Condensed-Matter Physics & Materials Science Seminar

    "TBD"

    Presented by Xiaoyu Deng, Rutgers

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

    Thursday, March 9, 2017, 11:00 am

    Hosted by: 'Gabi Kotliar'

    TBD

  3. MAR

    28

    Tuesday

    Condensed-Matter Physics & Materials Science Seminar

    "Thermalization and light cones in a model with weak integrability breaking"

    Presented by Stefan Groha, University of Oxford, United Kingdom

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

    Tuesday, March 28, 2017, 1:30 pm

    Hosted by: ''Neil Robinson''

    We employ equation of motion techniques to study the non-equilibrium dynamics in a lattice model of weakly interacting spinless fermions. Our model provides a simple setting for analyzing the effects of weak integrability breaking perturbations on the time evolution after a quantum quench. We establish the accuracy of the method by comparing results at short and intermediate times to time-dependent density matrix renormalization group computations. For sufficiently weak integrability-breaking interactions we always observe prethermalization plateaux, where local observables relax to non-thermal values at intermediate time scales. At later times a crossover towards thermal behaviour sets in. We determine the associated time scale, which depends on the initial state, the band structure of the non-interacting theory, and the strength of the integrability breaking perturbation. Our method allows us to analyze in some detail the spreading of correlations and in particular the structure of the associated light cones in our model. We find that the interior and exterior of the light cone are separated by an intermediate region, the temporal width of which appears to scale with a universal power-law t 1/3.

  4. APR

    20

    Thursday

    Condensed-Matter Physics & Materials Science Seminar

    "Unpaired Spins in Superconductors: From Assassin to Enabler"

    Presented by Jeffrey Lynn, NIST Center for Neutron Research, National Institute of Standards and Technology

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

    Thursday, April 20, 2017, 11:00 am

    Hosted by: '''Igor Zaliznyak'''

    The magnetic properties of superconductors have a rich and interesting history, and we will briefly review some highlights. Early work showed that even tiny concentrations of magnetic impurities destroyed the superconducting pairing through the exchange-driven spin depairing mechanism, prohibiting any possibility of magnetic order coexisting with superconductivity. The first exceptions to this rule were provided by the cubic rare-earth substituted CeRu2 alloys, followed by the ternary Chevrel-phase superconductors (e.g. HoMo6S8) and related compounds, where long range magnetic order coexists or competes with superconductivity. The very low magnetic ordering temperatures (~1 K) suggested that dipolar rather than exchange interactions dominate, thus (it was thought) allowing the coexistence. These materials also provided the first examples of the competition between ferromagnetism and superconductivity. In the newer borocarbide class of magnetic superconductors (e.g. ErNi2B2C), however, it became clear that the magnetic order is in fact exchange driven. The borocarbides also provided the first example of the spontaneous formation of flux quanta (vortices). For the cuprate and iron-based superconductors (formerly known as "high Tc") we now have come full circle, as the spins are not only tolerated but are intimately tied to the superconductivity. The "parent" cuprate systems are Mott-Hubbard antiferromagnetic insulators with very strong magnetic interactions that are two-dimensional in nature. These strong exchange interactions survive into the superconducting state, yielding highly correlated electrons that participate directly in the superconducting pairing. The "parent" materials of the new iron-based high TC superconductors are also antiferromagnets with very energetic spin excitations, and in the superconducting regime they form a "magnetic resonance" that is directly tied to the superconducting order parameter, ju

  5. APR

    28

    Friday

    Condensed-Matter Physics & Materials Science Seminar

    "TBD"

    Presented by Martin Mourigal, Georgia Tech

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

    Friday, April 28, 2017, 1:30 pm

    Hosted by: ''Igor Zaliznyak''

    TBD

  1. Condensed-Matter Physics & Materials Science Seminar

    "Thermalization and chaos in quantum systems"

    Presented by Sriram Ganeshan, Stony Brook University

    Tuesday, February 14, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: '''Robert Konik'''

    Thermalization, a common phenomenon in various physical settings, can naturally fail in certain isolated disordered quantum systems, challenging basic tenets of quantum statistical mechanics. Many body localization (MBL) is a canonical example of such an intriguing scenario and, therefore, attracted tremendous attention from condensed matter, statistical physics, and atomic physics communities. Considerable effort has recently gone into establishing the existence of the MBL phase, and the nature of dynamical phase transition from MBL to the thermal phase. However, understanding instabilities to the MBL phase that may lead to the complete or partial restoration of thermalization is still an open question. In this talk, I would focus on two such instabilities to the MBL phase coming from single particle mobility edge and the presence of extensive degeneracy in the many body spectrum. The goal is to identify the most robust form of MBL (in the presence of instabilities) to gain insight into the mechanisms of quantum thermalization.

  2. Condensed-Matter Physics & Materials Science Seminar

    "Anion-based approaches to engineering functionality in perovskite oxide heterostructures"

    Presented by Steve May, Drexel University

    Thursday, January 26, 2017, 1:30 pm
    ISB Bldg. 734, Conf. Room 201 (upstairs)

    Hosted by: ''Mark Dean''

    Scientific interest in ABO3 perovskite oxides remains intense due to the wide range of physical behavior present in these materials. The ability to control the position, occupation, and composition of the anion site has recently emerged as a new route to tune properties in epitaxial perovskites. This talk will focus on recent and ongoing efforts aimed at developing anion-based approaches to tailor electronic, optical and magnetic properties in oxide heterostructures. First, I will discuss how the position of the oxygen anions can be controlled to stabilize non-bulk-like bond angles and lengths, thereby modifying electronic and magnetic behavior in manganite films and superlattices. In the second half of the talk, I will describe efforts focused on controlling the occupation and composition of the anion site, including reversible oxidation/reduction in thin La1/3Sr2/3FeO3-? films and topotactic fluorination reactions to realize oxyfluoride films

  3. Condensed-Matter Physics & Materials Science Seminar

    "Ultrafast Dynamical Phenomena in Nanostructural Materials by 4D Electron Microscopy"

    Presented by Xuewen Fu, California Institute of Technology

    Tuesday, January 24, 2017, 2 pm
    Building 480, Conference Room

    Hosted by: ''Yimei Zhu''

  4. Condensed-Matter Physics & Materials Science Seminar

    "Creation and Control of Low Dimensional Electron System in Transition Metal Oxides"

    Presented by Milan Radovic, Paul Scherer Institut, Switzerland

    Monday, January 23, 2017, 11 am
    Building 734, conference room 201

    Hosted by: '''Cedomir Petrovic'''

    Transition Metal Oxides (TMOs) exhibit unique and multifunctional electronic properties (such as high-temperature superconductivity, colossal magnetoresistance, metal-insulator transitions, etc.) directly related to the spin and orbital degrees of freedom of the transition metal d-states. Furthermore, their iso-structural nature permits realization of heterostructures where novel unexpected electronic properties take place. Engineering transition metal oxide surfaces and interfaces carries the potential for achieving new physical properties that radically differ from those of the constituent bulk materials. This is the case of oxide-lowDEGs, which recently showed extraordinary occurrences, including interfacial superconductivity, magnetism, large tuneable spin-orbit coupling and indications of topological states. In my talk, I will present recent spin resolved Angle Resolved Photoemission Spectroscopy (ARPES) measurements of the low dimensional electron gas at SrTiO3 [1, 2, 3], TiO2-anatase and Sr1-xBaxTiO3 showing that these materials have capability for the realization of TMO based electronic device. References: [1] N. C. Plumb, M. Salluzzo, E. Razzoli, M. Månsson, M. Falub, J. Krempasky, C. E. Matt, J. Chang, J. Minár, J. Braun, H. Ebert, B. Delley, K.-J. Zhou, C. Monney, T. Schmitt, M. Shi, J. Mesot1, C. Quitmann, L. Patthey, M. Radovic, Phys. Rev. Lett. 113, 086801 (2014). [2] A. F. Santander-Syro, F. Fortuna, C. Bareille, T. C. Rodel, G. Landolt, N. C. Plumb, J. H. Dil, and M. Radovic, Nature Materials, 13, 1085–1090 doi:10.1038/nmat4107 (2014). [3] Z. Wang, S. McKeown Walker, A. Tamai, Z. Ristic, F.Y. Bruno, A. de la Torre, S. Ricco, N.C. Plumb, M. Shi, P. Hlawenka, J. Sanchez-Barriga, A. Varykhalov, T.K. Kim, M. Hoesch, P.D.C. King, W. Meevasana, U. Diebold, J. Mesot, M. Radovic, and F. Baumberger, Nature Materials 15, 835–839 (2016) doi:10.1038/nmat4623 (2016).

  5. Condensed-Matter Physics & Materials Science Seminar

    "Transient Dynamics of Strongly Correlated Electrons After Sudden Excitations"

    Presented by Marco Schiro, Institut de Physique Theorique (IPhT), CEA, Saclay, France

    Friday, January 13, 2017, 1:30 pm
    Seminar Room 2nd Floor Bldg 734

    Hosted by: 'Robert Konik'

    The development of pump-probe spectroscopies with femtosecond time resolution, which allows to track the dynamics of electronic degrees of freedom in solids under optical excitations, opens up a new window to understand strongly correlated materials and offers the intriguing possibility of controlling their properties with light, on ultra-fast time scales. Triggered by these advances, the interest around time dependent phenomena in quantum many body systems has recently substantially grown. In this talk will review recent progress in understanding transient dynamics of electrons in correlated metals, Mott Insulators and superconductors. I will show that quite generically these systems display very sharp dynamical transitions as a function of the external perturbation, in correspondence of which the lattice response and the sensitivity to density inhomogeneities can be greatly enhanced.

  6. Condensed-Matter Physics & Materials Science Seminar

    "Complexity in Spin-Frustrated Rock-Salt Manganites"

    Presented by Alexandros Lappas, Institute of Electronic Structure and Laser, Foundation for Research & Technology, Greece

    Thursday, December 1, 2016, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: 'Emil Bozin'

    Complexity in transition metal oxides is the outcome of simultaneously active electron degrees of freedom (spin-charge-orbital) and their evolution under the restrictions imposed by the geometry of the underlined crystal lattice. Consequently, the materials' response to competing states requires that we assess structural correlations across a wide range of length and time scales. Taking advantage of cutting-edge structural facilities accessed at neutron [1, 2], synchrotron X-ray [3] and electron microscopy [4] labs we address current limitations in understanding the crystallographic structure of layered rock-salt type triangular-lattice manganites of the AMnO2 type (A= Na, Cu). The unexpected coexistence of long- and short-range magnetic correlations [3, 5] due to two major opposing effects (elastic vs. magnetic exchange) of similar magnitude, lead to nearly equivalent, competing structural phases enabling infinitesimal quenched disorder to locally lift the differing degree of inherent frustration in the parent AMnO2 phase. These manganites provide a paradigm of a rarely observed nanoscale inhomogeneity in an insulating spin system, an intriguing complexity of competition due to geometrical frustration. The dramatic impact of topology and site-disorder on frustrated magnetism is further demonstrated by the hydrated variant of the NaMnO2 antiferromagnet, which gives way to a strongly interacting spin-glass state, indicative of the subtle balance of competing processes in multivalent two-dimensional systems [6]. [1] M. Giot et al., Phys. Rev. Lett. 2007, 99, 247211. [2] C. Vecchini et al., Phys. Rev. B 2010, 82, 094404. [3] A. Zorko et al., Nat. Commun. 2014, 5, 3222. [4] A.M. Abakumov et al., Chem. Mater. 2014, 26, 3306. [5] A. Zorko et al., Sci. Rep. 2015, 5, 9272. [6] I. Bakaimi et al., Phys. Rev. B 2016, 93, 184422.

  7. Condensed-Matter Physics & Materials Science Seminar

    "X-ray Photon Correlation Spectroscopy at Large Angles"

    Presented by Mark Sutton, McGill University

    Tuesday, November 22, 2016, 1:30 pm
    ISB Bldg. 734, Conf. Room 201 (upstairs)

    Hosted by: 'Mark Dean'

    Xray photon correlation spectroscopy (XPCS) has proven to be a powerful way to study time correlations in equilibrium systems. The straight forward extension to two-time correlations has also proven very useful. To date, most XPCS work has been done using small-angle x-ray scattering (SAXS). As with conventional x-ray diffraction, the information in disordered Bragg peaks (large angle scattering) often contains more information but it can be harder to interpret. In this talk, I will discuss several results using large angle XPCS which explore some of the complications and the resulting extra information obtained.

  8. Condensed-Matter Physics & Materials Science Seminar

    "Probing the magnetic structure of EuPtIn4 via x-ray resonant magnetic scattering"

    Presented by Jose Renato Mardegan, Deutsche Elektronen-Synchrotron (DESY), Germany

    Tuesday, November 22, 2016, 11 am
    ISB Bldg. 734, Seminar Rm. 201 (upstairs)

    Hosted by: ''Ian Robinson''

    The search for fascinating materials with interesting electronic and magnetic properties has led to an enormous development in diverse areas of condensed matters physics. In particular, the Indium-rich materials containing rare-earth elements can host exotic physical phenomena emerging from the competition and/or cooperation of several physical mechanisms such as the Ruderman-Kittel-Kasuya-Yosida (RKKY) magnetic interaction, heavy fermion (HF) behavior, crystalline electric field (CEF) and Kondo effects[1,2].Since the magnetic ordering and the screening of f-electrons have an important role in the ground state properties of these materials, the magnetic structure determination can be a powerful tool to understand how the moments of the magnetic ions are interacting among each other. In this sense, x-ray resonant magnetic scattering (XRMS) technique was employed to solve the magnetic structure at low temperature of the new intermetallic EuPtIn4 compound. At the resonant energy of the Eu ion (7617 eV – L2 edge), magnetic incommensurate (ICM) reflections with propagation vector type (1/2, 1/2, τ) with τ ~ 0.427 were observed. Temperature and magnetic field dependence performed at the magnetic reflections reveal an AFM coupling with a Néel temperature TN = 13.1 K and a spin flop transition above 3 T, respectively. In addition, we do not observe any magnetic anomalies related to a second phase transition as suggested in the previously reported macroscopic measurements [3,4]. The ICM phase observed at low temperature is due to geometric frustration of the Eu ions in which the RKKY exchange interaction cannot be simultaneously satisfied. Although the EuPtIn4 compound displays similar properties to a heavy fermion compound such as exotic magnetic structure and enhancement of Sommerfeld coefficient, further investigation must be performed in this new series of materials.[1] Z. Fisk, et al., Proc. Natl. Acad. Sci. USA 92, 6663 (1995).[2] P. Coleman, Handb

  9. Condensed-Matter Physics & Materials Science Seminar

    "Tracking chemical reactions with time-resolved x-ray spectroscopic techniques"

    Presented by Tadese Abebaw Assefa, European XFEL Laboratory, Germany

    Monday, November 21, 2016, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: ''Ian Robinson''

    Transition metal compounds play a significant role in many chemical and biologically relevant processes. Hereby charge transfer, ligand detachment and attachment processes are fundamental ingredients, which often determine the outcome of a given chemical reaction. We investigated aqueous ferrocyanide ([FeII(CN)6]4-) ions, which undergoes charge transfer and ultrafast ligand dissociation upon irradiation of 266 and 355 nm laser light. Time-resolved (TR) x-ray absorption and emission spectroscopies (XAS and XES) deliver information about structural and electronic changes in real-time implemented to follow the chemical reaction. Synchrotron-based studies are limited with 100ps time resolution enables us to disentangle simultaneous photoproducts formed after 266 nm laser excitation. Furthermore, we investigated the ultrafast ligand dissociation of aqueous ferrocyanide ions upon irradiation of 355 nm laser light at the x-ray Free Electron Laser facility (SACLA, Japan). Based on a comparison of the simulated pre-edge peaks of 1s→3d transition with the experimental data, we concluded that the reaction pathway commences via ligand detachment resulting pentacoordinated intermediate complex ([FeII(CN)5]3-), followed by the formation of the long-lived photoaquated complex ([FeII(CN)5(H2O)]3-). The ligand detachment and attachment process takes 12.43 ± 5.77 ps. TR XES results also reveal spin state change in the intermediate state. Combining these findings we interpret the consecutive steps of ligand exchange mechanism for ferrocyanide ions. Also, we characterise the molecular structure of photoexcited [FeII(terpy)2]2+ molecule via TR Extended X-ray absorption fine structure (EXAFS). The data analysis in energy space used two structural model expansions which are the representations of DFT predicted 5E and 5B2 quintet high spin states. After statistical evaluation of the two models, the 5E high spin state model is in better agreement with experimental data. The ener

  10. Condensed-Matter Physics & Materials Science Seminar

    "Surface X-ray Diffraction for Operando Characterization of Chemical Reactions on Surfaces""

    Presented by Roberto Felici, Istituto SPIN - CNR, Italy

    Friday, November 4, 2016, 11 am
    ISB Bldg. 734, Sem. Rm. 201 (upstairs)

    Hosted by: ''''Ian Robinson''''

    X-rays are an ideal probe for studying structural properties of matter and, thanks to the brilliance of synchrotron sources, they are also employed to determine the atomic structure and morphology of surfaces and interfaces. Surface x-ray diffraction has been originally developed to determine the static structure of surfaces. However with the development of x-ray sources, detectors and analysis tools it is now possible to characterise in detail processes which occur at surfaces. Aim of this talk is to present recent results obtained at the id03 surface diffraction beamline of the ESRF dealing with the in-situ characterization of the structure and morphology of a catalyst during a surface reaction. Examples will deal with heterogenous catalytic oxidation of CO on single crystal surfaces /1,2/ and supported nanoparticles /3/ References 1 R. van Rijn et al., Phys. Chem. Chem. Phys. 13 (2011) 13167 2 B.L. Hendriksen et al., Nat. Chem. 2 (2010) 730 3 O. Balmes, et al., Phys. Chem.Chem. Phys. 14 (2012) 4796

  11. Condensed-Matter Physics & Materials Science Seminar

    "Driven Dirac Materials"

    Presented by Alexander Balatsky, Los Alamos National Laboratory

    Thursday, October 27, 2016, 1:30 pm
    Bldg. 734, ISB Seminar Rm. 201 (upstairs)

    Hosted by: 'Robert Konik'

    Dirac Materials exhibit nodes in the spectra that result in the strong energy dependence of the Density of States (DOS). Collective many body instabilities in Dirac Materials are controlled by the dimensionless DOS. Hence the driven and nonequilibrium Dirac Materials offer a platform for investigation of collective instabilities of Dirac nodes via controlled tuning of the coupling constants with drive. I will present the results of investigation of the many body instabilities, like excitonic instabilities, in driven Dirac Materials. Recent optical pump experiments are consistent with the creation of long lived states away from equilibrium in Dirac Materials.

  12. Condensed-Matter Physics & Materials Science Seminar

    "Creating Spatially Ordered States in Monolayer Graphene"

    Presented by Abhay Pasupathy, Columbia University

    Friday, October 21, 2016, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: 'Cedomir Petrovic'

    Electrons in graphene at the Fermi level have chirality or handedness that arises from the honeycomb structure in real space. This chirality is responsible for many of the fascinating electronic properties of graphene such as Klein tunneling. In this talk, I will describe two related scanning tunneling microscopy experiments that probe the chiral nature of the electronic states in graphene. First, I will describe an experiment where we observe the chiral symmetry of graphene to be broken, resulting in a bond-ordered phase called Kekule order. I will show that this new phase in monolayer graphene can be induced by adatoms on the surface of graphene which interact electronically with each other. In a related experiment, I will describe the electronic structure of graphene in the presence of a circular potential well that separates the sheet into p (hole) and n (electron) doped regions. Electrons in these wells spend a finite amount of time before transitioning out of the well, resulting in quasibound states that can be measured in scanning tunneling spectroscopy. Due to the chirality of the electrons in graphene, the transition probabilities at the p-n junction are governed by the physics of Klein tunneling, which can be understood from the details of the energies and wavefunctions of the quasibound states observed in experiment.

  13. Condensed-Matter Physics & Materials Science Seminar

    "X-ray Imaging via Bragg CDI: From Ultrafast Physics to Defect Dynamics"

    Presented by Andrew Ulvestad, Argonne National Laboratory

    Friday, October 7, 2016, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: ''Ian Robinson''

    Bragg coherent diffractive imaging is an emerging x-ray imaging technique capable of resolving both defect and ultrafast dynamics in nanocrystals with three-dimensional detail and nanometer resolution. This ability to study single nanocrystals in their reactive environments opens new insight into a broad range of materials science questions, including how to improve materials that convert heat into electricity, understanding degradation in advanced battery cathodes, and probing the structure-stability relationship in fuel cell catalysts. Here I will discuss Bragg CDI studies of phonon dynamics in Zinc Oxide and defect dynamics in thin film grains driven by temperature. Finally, I will touch on future directions for BCDI with the anticipated increase in coherent flux at upgraded synchrotrons.

  14. Condensed-Matter Physics & Materials Science Seminar

    "The numerical renormalization group as a viable multi-band impurity solver for dynamical mean-field theory"

    Presented by Katharina Stadler, Ludwig-Maximilians-Universitaet Muenchen, ASC, Germany

    Wednesday, October 5, 2016, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: 'Gabi Kotliar'

    In my talk I will present the numerical renormalization group (NRG) as a viable multi-band impurity solver for dynamical mean-field theory (DMFT). NRG offers unprecedented real-frequency spectral resolution at arbitrarily low energies and temperatures. It is thus perfectly suited to study "Hund metals" [1], which show - in experiments and theoretical DMFT calculations - puzzling behavior at unusually low energy scales, like Fermi-liquid behavior at low temperatures, a coherence-incoherence crossover with increasing temperature [2, 3] and fractional power laws for the imaginary part of the Matsubara self-energy in the incoherent regime, discovered already early on with continuous time quantum Monte Carlo (CTQMC) as DMFT solver [3]. I will explicitly demonstrate the advantages of NRG+DMFT in the context of a channel-symmetric three-band Anderson-Hund model on a Bethe lattice at 1/3 filling (with NRG exploiting the non-abelian SU(3) channel symmetry to reduce numerical costs) [4]. In contrast to CTQMC, our NRG+DMFT calculations finally settled the existence of a Fermi-liquid ground state. We further revealed new important insights: our real-frequency one-particle spectral function shows a coherence-incoherence crossover (driven by Hund J rather than Hubbard U) and strong particle-hole asymmetry, which leads to the above-mentioned apparent fractional power laws; two-stage screening, where spin screening occurs at much lower energies than orbital screening ("spin-orbital separation"); and zero-temperature spectral properties that are similar with or without DMFT self-consistency, in contrast to Mott-Hubbard systems, where the DMFT self-consistency opens a gap. A recent reformulation of NRG, called "interleaved NRG" (iNRG) [5, 6] allows to tackle more realistic models of Hund metals where channel symmetries are generally broken (for example, due to crystal field splitting).

  15. Condensed-Matter Physics & Materials Science Seminar

    "Interplay of structure, magnetism and superconductivity in the 112 Fe based superconducting family"

    Presented by Ni Ni, UCLA

    Thursday, September 15, 2016, 1:30 pm
    Seminar Room, 2nd Fl, ISB Bldg. 734

    Hosted by: ''Robert Konik''

    Both cuprates and Fe-based superconductors, the two known high Tc superconducting families, show rich emergent phenomena near the superconductivity (SC). To understand the mechanism of unconventional SC, it is crucial to unravel the nature of these emergent orders. The 112 Fe pnictide superconductor (FPS), Ca1−xRExFeAs2 (CaRE112), shows SC up to 42 K, the highest bulk Tc among all nonoxide FPS. Being an exceptional FPS where the global C4 rotational symmetry is broken even at room temperature, it is important to extract the similarities and di?erences between 112 and other FPS so that critical ingredients in inducing SC in FPS can be ?ltered. In this talk, I will review current progress in the study of 112. The comparison between Co doped CaLa112 and Co doped 10-3-8 will be made and the importance of interlayer coupling will be discussed.

  16. Condensed-Matter Physics & Materials Science Seminar

    "The first-principles study of structural, electronic, and magnetic properties of strongly correlated materials: DFT+DMFT approach."

    Presented by Hyowon Park, University of Illinois

    Thursday, August 25, 2016, 3 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: ''Neil Robinson''

    Strongly correlated materials including transitional metal oxides and heavy fermion materials exhibit novel structural, electronic, and magnetic properties. The first-principles study of these unusual properties requires a theoretical description that goes beyond density functional theory to treat strong correlation effects properly. In this talk, I will show that the density functional theory plus dynamical mean field theory (DFT+DMFT) method enables realistic and quantitative calculations of those properties in good agreement with experimental spectroscopic measurements. First, I will clarify the nature of the insulating phase in bulk rare-earth nickelates using DFT+DMFT and determine the structural and metal-insulator phase diagram. I will also present DFT+DMFT results of structural and electronic properties in artificially structured LaNiO3/LaAlO3 superlattices under strains. Calculation results of layer-resolved orbital polarization will be compared to recent X-ray absorption spectroscopy data and analyzed in terms of structural and quantum confinement effects. Finally, I will show the momentum and frequency dependent magnetic excitation spectra in CePd3 computed using DFT+DMFT and explain that the calculated spectra based on realistic band excitations are in good agreement with the inelastic neutron scattering data measured in this material.

  17. Condensed-Matter Physics & Materials Science Seminar

    "Controlling the metal-insulator transition in LaNiO3"

    Presented by Frederick Walker, Yale University

    Thursday, August 18, 2016, 1:30 pm
    Bldg. 734, ISB Bldg., Conf Room 201 (upstairs)

    Hosted by: 'Mark Dean'

    New materials are needed to advance electronic, optical and energy materials beyond current technology trends. Perovskite oxides can potentially meet these needs due to their flexibility and unique functional properties. In bulk materials, these properties are accessed through modifications of physical and electronic structure through cation substitution in the perovskite lattice. An even larger phase space of properties and functionalities is possible when these materials are combined in thin film heterostructure form using molecular beam epitaxy. The sensitivity of the resulting properties on interface structure often dominates device function. Uncovering a microscopic understanding of emergent properties at such interfaces is challenging due to the small volume of material present. In this talk, we show how a combination of first principles theory and experiment can be used to develop a non-volatile, three terminal switch. The device is implemented by using the perovskite LaNiO3 as a conducting channel and a ferroelectric gate. The approach to developing this switch involves synchrotron x-ray characterization of picoscale structural distortions for LaNiO3 heterostructures, including LaNiO3-vacuum, LaNiO3-band insulator, and LaNiO3-ferroelectric. The consequences of the picoscale distortions are strong modulations of the measured electronic transport as a function of interface and ferroelectric polarization direction. Quantitative comparisons of the structure with first principles theory show excellent agreement. Theory provides an understanding of how the picoscale distortions at the interface result in changes in orbital occupation and band properties of both the nickelate and ferroelectric. These insights inspire new principles for designing ferroelectric heterostructures that show record non-volatile resistance modulations.

  18. Condensed-Matter Physics & Materials Science Seminar

    "Quantiative Determination of the the Fluctuations Leading to Superconductivity in Cuprates"

    Presented by Chandra Varma, University of California, Riverside

    Tuesday, June 14, 2016, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: 'Peter D. Johnson'

    I will report on Laser based ARPES of unprecedented accuracy and stability (taken by the group of Xingjiang Zhou, IOP, Beijing), together with a method of analysis suggested by me (and carried out with the group of Han-Yong Choi, Asia Pacific Center for Theoretical Physics, Korea), to quantitatively extract the e↵ective frequency and momentum dependent interactions of fermions in both the full symmetry (normal) and the d-wave (pairing) symmetry in a family of cuprates. The results are remarkably simple. The principal interactions are of the form: I(k, k',w)~g0 [(1−cos(20k)cos(20'k)]F(w) They are separable functions of momentum and frequency, the first part is the repulsive part and the second part is the attractive d-wave part. F(w) is nearly constant with an upper-energy cutoff of about 0.4eV. The dimensionless coupling constant g0 ~ 0.15. These results were predicted in a theory of superconductivity and of the strange metal phase by quantum-critical fluctuation of loop-currents. They also rule out several alternatives proposed. I will also comment on the normal state and superconductivity in the Fe-based compounds.

  19. Condensed-Matter Physics & Materials Science Seminar

    "Equilibrium States and Dynamics of Spin Assemblies in Magnetic Thin Films, Heterostructures and Nanostructured Entities"

    Presented by Ramesh B. Budhani, Indian Institute of Technology Kanpur

    Tuesday, May 17, 2016, 11 am
    Building 480, Conference Room

    Hosted by: 'Lijun Wu'

    The orientation of spin assemblies in ferromagnetic thin films and nanostructures can take a variety of shapes depending on the relative strength of factors contributing to their magnetic free energy. These factors are derived from the direct quantum mechanical exchange between the electronic spins or those mediated by impurities, and those associated with the size, shape, crystallographic structure, strain, dipolar interactions and external fields. Here we present three cases where the orientational dynamics has been studied as functions of temperature, magnetic field strength and the elapsed time after acquiring a particular configuration. These studies are based on magnetic force microscopy and bulk magnetometry measurements on strain epitaxial films of La0.67Ca0.33MnO3, and lithographically patterned submicron size ring assemblies of permalloy and Co/Pd multilayers, which also form artificial spin ices. Towards the end of this lecture we will discuss interface driven magnetic and electronic phenomena in magnetic thin films.

  20. Condensed-Matter Physics & Materials Science Seminar

    "Dirac Materials"

    Presented by Alexander Balatsky, Los Alamos National Laboratory

    Monday, May 16, 2016, 11 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: 'Peter D. Johnson'

    Discoveries of superfluid phases in 3He, high Tc superconductors, graphene and topological insulators have brought into focus materials where quasiparticles are described by same Dirac equation that governs behavior of relativistic particles. I will discuss how this class of materials, called Dirac materials, exhibits unusual universal features seen in numerous realizations: Klein tunneling, chiral symmetries and impurity resonances. Goal of this talk is to explore these similarities and discuss the unique role of symmetries that protect Dirac spectrum and possible routes to generate gaps due to many body instabilities. We will also discuss ongoing investigation of the symmetries of Dirac materials, quantum imaging, and means to control their properties. At the end we will propose to use modern tools to design artificial Dirac Materials. One example would be the design Bosonic Dirac materials that host bosonic Dirac excitations, something that would not be possible in particle physics.

  21. Condensed-Matter Physics & Materials Science Seminar

    "Topological Insulators and Dirac Semimetals - Recent Progress in New Materials"

    Presented by Robert J. Cava, Princeton University

    Thursday, March 31, 2016, 10 am
    Large Seminar Room, Bldg. 510

    Hosted by: ''Peter D. Johnson''

    "New materials give new properties" describes the goal of our solid state chemistry research program. This goal would be much easier to attain if there was a reliable way to predict the stability of an unknown non-molecular solid, then predict what its properties would be, then make it as a real material and then finally test it, but unfortunately that is not the case; establishing such a process is the grand challenge in contemporary solid state chemistry, and so we have to operate differently. Our discussions with experimental and theoretical physicists teach us about current issues in the electronic and magnetic properties of matter, and our chemistry background teaches us how to think about crystal structures and bonding; our work is about trying to put these two cultures together to find new materials. In this talk I will describe some of our recent results in Topological Insulators and Dirac and Weyl Semimetals.

  22. Condensed-Matter Physics & Materials Science Seminar

    "Phase transitions in strongly correlated systems from diagrammatic multi-scale methods"

    Presented by Andrey Antipov, University of Michigan

    Thursday, March 24, 2016, 1:30 pm
    Bldg. 734, ISB Conf. Rm. 201 (upstairs)

    Hosted by: ''Alexei Tsvelik''

    The dynamical mean field theory (DMFT) has become the standard tool in describing strongly correlated electron materials. While it captures the quantum dynamics of local fields, it neglects spatial correlations. To describe e.g. anti-ferromagnetism, unconventional superconductivity or frustration a proper treatment of non-local correlations is necessary. Diagrammatic multi-scale approaches offer an elegant option to accomplish this: the difficult correlated part of the system is solved using a non-perturbative many-body method, whereas 'easier', 'weakly correlated' parts of the problem are tackled using a secondary perturbative scheme. Here we employ such a method, the dual fermion approach, to problems of charge and spin ordering in Falicov-Kimball and Hubbard models by constructing a systematic diagrammatic extension on top of DMFT. Near the critical point model we study the interplay between charge and spin excitations and long-range fluctuations. We show that such multi-scale approach is indeed capable of capturing the non mean-field nature of the critical point of the lattice model and correctly describes the transition to mean-field like behavior as the number of spatial dimensions increases. Our numerical method is available as a freely distributed open-source code.

  23. Condensed-Matter Physics & Materials Science Seminar

    "SU(N) symmetric tensor network simulations of strongly correlated quantum many-body systems"

    Presented by Andreas Weichselbaum, Ludwig Maximilians University

    Tuesday, March 22, 2016, 1:30 pm
    Small Seminar Room, Bldg. 510

    Hosted by: ''Alexei Tsvelik''

    Tensor network simulations have emerged as a powerful algebraic framework for the simulation of strongly correlated quantum many-body systems. Their great appeal lies in the fact that they are exact in that they do no rely on small parameters. They significantly extend exact diagonalization to much larger system sizes in (effective) 1D or 2D all the way to the thermodynamic limit. I will give a brief introduction based on the hugely successful methods such as the numerical renormalization group (NRG) or the density matrix renormalization group (DMRG) with focus on multi-orbital systems, both symmetric and non-symmetric. A versatile numerical tool in that respect is my recently developed tensor library QSpace that can efficiently deal with generic symmetry settings including SU(N). After a brief motivation via the prototypical symmetric multi-orbital system of iron impurities in gold or silver, I will present recent results on a dynamical mean-field theory (DMFT) study concerning the coherent-incoherent crossover in iron-pnictides, followed by recent work on the spin-1 Heisenberg kagome lattice and preliminary results on SU(N) spin ladders.

  24. Condensed-Matter Physics & Materials Science Seminar

    "From Molecular Beam Epitaxy to high magnetic field Quantum Oscillations"

    Presented by Yoshiharu Krockenberger, NTT Basic Research Laboratories

    Monday, March 21, 2016, 1:30 pm
    Bldg.480 Conf. Rm

    Hosted by: 'Ivan Bozovic'

    Cuprate superconductors present a major challenge in condensed matter physics not only due to their electron correlations but also due to their complex crystal structure. Complex crystal structures, i.e. various cations at various lattice positions, demand for the utmost caretaking when synthesizing them. In particular, Molecular Beam Epitaxy is the foremost versatile tool and technique that allows for the synthesis of such materials without the necessity to compromise on impurity phases. First, I introduce our custom designed Molecular Beam Epitaxy equipment which is empowered by e-guns, not effusion cells, and controlled by electron impact emission spectroscopy. After presenting several material systems I present high magnetic field quantum oscillation data on films synthesized by our Molecular Beam Epitaxy systems.

  25. Condensed-Matter Physics & Materials Science Seminar

    "Real-space Visualization of the Superconducting Proximity effect and Josephson tunneling on Nano-sized Pb thin film"

    Presented by Howon Kim, Institute for Solid State Physics, University of Tokyo, Japan

    Wednesday, March 9, 2016, 11 am
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Kazuhiro Fujita

    The proximity and Josephson effects are well-known phenomenon and widely used terms in superconductivity. Due to the recent advances in fabrication techniques those phenomenon has been extensively studied to give a new insight to the superconductor-based device applications. Nevertheless, many of these works addressed macroscopic properties of the samples, and thus local information is still missing. In this talk, I will discuss how we can realize the proximity effect and Josephson effect using scanning tunneling microscope at a nanometer scale. In the first part of the talk, we address how the local surface structure can influence on the proximity effect at the interface between superconducting two-dimensional Pb islands and a single-atomic-layer metal by performing local tunneling spectroscopy. From the spectroscopic mapping taken around the Pb-based S/N interface, we observed the gap at the Fermi energy, reminiscent of the superconducting gap, propagating into the metal region (proximity effect) and its depth decaying with the distance from the interface. Additionally, we observed that the propagation of the gap is terminated by the steps of the substrate and enhancement of the gap-depth in the area between the interface and the step edge. The experimental results are compared with the results of quasi-classical theory based on the Usadel equation. [1] The second part of the talk addresses atomic-scale S-S junctions by using scanning tunneling microscopy and spectroscopy. In our local conductance measurements between superconducting Pb islands on Si(111) or Ge(111) and Pb layers on the end of PtIr tip apex, we observed evolution of not only the normal-state conductance [2] but also a zero-bias peak (ZBP) from tunnel to atomic contact, which corresponds to the Josephson current, with a decrease in the tip-substrate distance on the different atomic sites on the surface crystalline lattice of the substrate. With a help of multiple Andreev reflectio

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