BNL Home
June 2017
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  1. Condensed-Matter Physics & Materials Science Seminar

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

    Hosted by: 'Alexei Tsvelik'

    We construct a quantum field theory in (2+1)-dimensional spacetime for strongly interacting Majorana fields that is amenable to a mean-field approximation. The mean-field phase diagram predicts the existence of two competing phases, one of which supports chiral non-Abelian topological order, while the other supports chiral Abelian topological order. The two mean-field phases are separated by a continuous phase transition. This quantum field theory captures the low-energy physics of quantum spin-1/2 localized on the sites of a lattice whose interactions are $SU(2)$ symmetric but break time-reversal symmetry. The lattice geometry can be interpreted as a one-dimensional stacking of two-leg ladders or as a bilayer of two square lattices. Both incompressible ground states can thus be thought of as chiral spin liquids in two-dimensional space supporting non-Abelian and Abelian topological order, respectively.

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19

  1. Condensed-Matter Physics & Materials Science Seminar

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

    Hosted by: '''Emil Bozin'''

    The unique advantages of neutrons for characterization of nuclear fuel materials [1] are applied at the pulsed spallation neutron source at LANSCE to accelerate the development and ultimately licensing of new nuclear fuel forms. Neutrons allow to characterize the crystallography of phases consisting of heavy elements (e.g. uranium) and light elements (e.g. oxygen, nitrogen, or silicon) [2]. The penetration ability in combination with comparably large (e.g. cm sized) beam spots provide microstructural characterization of typical fuel geometries for phase composition, strains, and textures from neutron diffraction. In parallel, we are developing energy-resolved neutron imaging and tomography with which we can complement diffraction characterization. This unique approach not only allows to visualize cracks, arrangement of fuel pellets in rodlets etc., but also characterization of isotope or element densities by means of neutron absorption resonance analysis [3]. Laser-driven pulsed neutron sources [4] have the potential to provide these capabilities "pool-side", e.g. at the Advanced Test Reactor at Idaho National Laboratory. Compared to proton accelerator driven spallation sources, requiring investments exceeding $1B, the investment cost for a laser-driven neutron source would be of the order of several $10M with the potential of similar flux to that of a smaller, earlier generation spallation neutron source. Compared to electron accelerator-driven neutron sources, the flux of a laser-driven source would be at least one order of magnitude higher. Compared to reactor neutron sources, the pulse structure of the laser-driven neutron source would enable unique characterization not possible with steady-state reactor neutrons. In this presentation, we provide an overview of our recent accomplishments in fuel characterization for accident-tolerant fuel consisting of uranium nitride/uranium silicide composite fuels as well as metallic fuels.

20

  1. Condensed-Matter Physics & Materials Science Seminar

    11 am, Bldg. 480, Conference Room

    Hosted by: 'Yimei Zhu'

    Capitalizing on the energy competition of charge itineracy with the strong electron-electron and electron-phonon couplings, nanoscale manipulation of the charge and lattice degrees of freedom in strongly correlated oxides can often lead to new functionalities that are inaccessible in the bulk form. In this talk, I will present our studies of the emerging phenomena at epitaxial correlated oxide nanostructures and hetero-interfaces that result from the nanoscale lattice and charge control. By creating nanoscale periodic depth modulation, we have achieved a 50-fold enhancement of the magnetic crystalline anisotropy in ultrathin colossal magnetoresistive (La,Sr)MnO3, which is attributed to a non-equilibrium strain distribution established in the nanostructures [1]. I will also discuss the intricate interplay between epitaxial strain and electric field effect in determining the correlated transport of the charge transfer type Mott insulator (Sm,Nd)NiO3 [2,3], and how the interfacial charge transfer between two correlated oxides can be exploited to effectively engineer the performance of ferroelectric-gated Mott transistors [4]. [1] A. Rajapitamahuni et al., PRL 116, 187201 (2016). [2] L. Zhang et al., JPCM 27, 132201 (2015). [3] L. Zhang et al., APL 107, 152906 (2015). [4] X. Chen et al., Adv. Mater, in press (2017).

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22

  1. Condensed-Matter Physics & Materials Science Seminar

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

    Hosted by: ''''Mark Dean''''

    The resonant inelastic X-ray scattering (RIXS) technique is best known for significant breakthroughs in the investigation of strongly correlated materials such as cuprates. However, the rapid advancement of RIXS spectrographs has made it increasingly attractive to apply the technique to a broad range of quantum materials outside of this comfort zone. This talk will review lessons learned from our recent measurements on material systems that feature a balance of correlated and itinerant physics, including VO2, the hidden order compound URu2Si2, and Prussian blue analogue battery electrodes. RIXS spectra enable the first observation of important collective modes for these systems, and provide a look into how correlated electron symmetries are melted - or persist! - in relatively itinerant and covalent environments. The data also highlight the need for improved theoretical modeling and higher photon throughput to achieve deeper insights.

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  1. AUG

    23

    Wednesday

    Condensed-Matter Physics & Materials Science Seminar

    "TBA"

    Presented by Andrew Mckenzie, Max-Planck-Institute, Germany

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

    Wednesday, August 23, 2017, 1:30 pm

    Hosted by: 'Cedomir Petrovic'

    TBA

  1. Condensed-Matter Physics & Materials Science Seminar

    "Resonant inelastic X-ray scattering on "moderately correlated" quantum materials"

    Presented by L. Andrew Wray, New York University

    Thursday, June 22, 2017, 1 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: ''''Mark Dean''''

    The resonant inelastic X-ray scattering (RIXS) technique is best known for significant breakthroughs in the investigation of strongly correlated materials such as cuprates. However, the rapid advancement of RIXS spectrographs has made it increasingly attractive to apply the technique to a broad range of quantum materials outside of this comfort zone. This talk will review lessons learned from our recent measurements on material systems that feature a balance of correlated and itinerant physics, including VO2, the hidden order compound URu2Si2, and Prussian blue analogue battery electrodes. RIXS spectra enable the first observation of important collective modes for these systems, and provide a look into how correlated electron symmetries are melted - or persist! - in relatively itinerant and covalent environments. The data also highlight the need for improved theoretical modeling and higher photon throughput to achieve deeper insights.

  2. Condensed-Matter Physics & Materials Science Seminar

    "Tailoring Lattice and Charge at Complex Oxide Nanostructures and Interfaces"

    Presented by Xia Hong, University of Nebraska-Lincoln

    Tuesday, June 20, 2017, 11 am
    Bldg. 480, Conference Room

    Hosted by: 'Yimei Zhu'

    Capitalizing on the energy competition of charge itineracy with the strong electron-electron and electron-phonon couplings, nanoscale manipulation of the charge and lattice degrees of freedom in strongly correlated oxides can often lead to new functionalities that are inaccessible in the bulk form. In this talk, I will present our studies of the emerging phenomena at epitaxial correlated oxide nanostructures and hetero-interfaces that result from the nanoscale lattice and charge control. By creating nanoscale periodic depth modulation, we have achieved a 50-fold enhancement of the magnetic crystalline anisotropy in ultrathin colossal magnetoresistive (La,Sr)MnO3, which is attributed to a non-equilibrium strain distribution established in the nanostructures [1]. I will also discuss the intricate interplay between epitaxial strain and electric field effect in determining the correlated transport of the charge transfer type Mott insulator (Sm,Nd)NiO3 [2,3], and how the interfacial charge transfer between two correlated oxides can be exploited to effectively engineer the performance of ferroelectric-gated Mott transistors [4]. [1] A. Rajapitamahuni et al., PRL 116, 187201 (2016). [2] L. Zhang et al., JPCM 27, 132201 (2015). [3] L. Zhang et al., APL 107, 152906 (2015). [4] X. Chen et al., Adv. Mater, in press (2017).

  3. Condensed-Matter Physics & Materials Science Seminar

    "Laser-driven Pulsed Neutron Sources as a Potential Pool-side Characterization Tool for Nuclear Fuels"

    Presented by Sven Vogel, Los Alamos National Laboratory

    Monday, June 19, 2017, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: '''Emil Bozin'''

    The unique advantages of neutrons for characterization of nuclear fuel materials [1] are applied at the pulsed spallation neutron source at LANSCE to accelerate the development and ultimately licensing of new nuclear fuel forms. Neutrons allow to characterize the crystallography of phases consisting of heavy elements (e.g. uranium) and light elements (e.g. oxygen, nitrogen, or silicon) [2]. The penetration ability in combination with comparably large (e.g. cm sized) beam spots provide microstructural characterization of typical fuel geometries for phase composition, strains, and textures from neutron diffraction. In parallel, we are developing energy-resolved neutron imaging and tomography with which we can complement diffraction characterization. This unique approach not only allows to visualize cracks, arrangement of fuel pellets in rodlets etc., but also characterization of isotope or element densities by means of neutron absorption resonance analysis [3]. Laser-driven pulsed neutron sources [4] have the potential to provide these capabilities "pool-side", e.g. at the Advanced Test Reactor at Idaho National Laboratory. Compared to proton accelerator driven spallation sources, requiring investments exceeding $1B, the investment cost for a laser-driven neutron source would be of the order of several $10M with the potential of similar flux to that of a smaller, earlier generation spallation neutron source. Compared to electron accelerator-driven neutron sources, the flux of a laser-driven source would be at least one order of magnitude higher. Compared to reactor neutron sources, the pulse structure of the laser-driven neutron source would enable unique characterization not possible with steady-state reactor neutrons. In this presentation, we provide an overview of our recent accomplishments in fuel characterization for accident-tolerant fuel consisting of uranium nitride/uranium silicide composite fuels as well as metallic fuels.

  4. Condensed-Matter Physics & Materials Science Seminar

    "A model of chiral spin liquids with tunable edge states"

    Presented by Christopher Mudry, Paul Scherrer Institute, Switzerland

    Thursday, June 15, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: 'Alexei Tsvelik'

    We construct a quantum field theory in (2+1)-dimensional spacetime for strongly interacting Majorana fields that is amenable to a mean-field approximation. The mean-field phase diagram predicts the existence of two competing phases, one of which supports chiral non-Abelian topological order, while the other supports chiral Abelian topological order. The two mean-field phases are separated by a continuous phase transition. This quantum field theory captures the low-energy physics of quantum spin-1/2 localized on the sites of a lattice whose interactions are $SU(2)$ symmetric but break time-reversal symmetry. The lattice geometry can be interpreted as a one-dimensional stacking of two-leg ladders or as a bilayer of two square lattices. Both incompressible ground states can thus be thought of as chiral spin liquids in two-dimensional space supporting non-Abelian and Abelian topological order, respectively.

  5. Condensed-Matter Physics & Materials Science Seminar

    "Thin-Film Alchemy: Using Epitaxial Engineering to Unleash the Hidden Properties of Oxides"

    Presented by Darrell G. Schlom, Cornell University

    Monday, May 15, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: 'Ivan Bozovic'

    Guided by theory, unparalleled properties—those of hidden ground states—are being unleashed by exploiting large strains in concert with the ability to precisely control dimensionality and stabilize metastable phases in epitaxial oxide heterostructures. For example, materials that are not ferroelectric or ferromagnetic in their unstrained state can be transmuted into materials that are both at the same time. Similarly, new tunable dielectrics with unparalleled performance have been created as well as a new single-phase multiferroic material where ferroelectricity and strong magnetic ordering are coupled near room-temperature. These are just three examples of the unparalleled properties—those of hidden ground states—being unleashed in epitaxial oxide heterostructures utilizing thin film alchemy

  6. 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

    Thursday, May 4, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    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.

  7. Condensed-Matter Physics & Materials Science Seminar

    "Spin-liquids in novel triangular and kagome rare-earth magnets"

    Presented by Martin Mourigal, Georgia Tech

    Friday, April 28, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: '''Igor Zaliznyak'''

    Insulating magnets combining the effects of geometrical frustration with strong spin-orbit coupling offer a prime route to realize correlated quantum states with exotic ground-states and excitations. Spin-space anisotropy and bond-directional magnetic exchange interactions are naturally present in rare-earth oxides. One of the most celebrated consequence is the existence of classical and quantum "spin-ice" physics in rare-earth pyrochlores, materials in which magnetic ions occupy a three-dimensional network of corner-sharing tetrahedra. In this talk, I will present the discovery of distinct flavors of exotic magnetic matter in families of rare-earth oxides with two-dimensional kagome [1] and triangular [2] geometries. This experimental work relies on recent advances in materials synthesis and combines thermodynamic characterization with state-of-the-art neutron scattering experiments to unravel the classical or quantum nature of these newly discovered quasi-two-dimensional spin-liquids. [1] Emergent order in the kagome Ising magnet Dy3Mg2Sb3O14, J. A. M. Paddison, H. S. Ong, J. O. Hamp, P. Mukherjee, X. Bai, M. G. Tucker, N. P. Butch, C. Castelnovo, M. Mourigal, and S. E. Dutton, Nature Communications 7, 13842 (2016). [2] Continuous excitations of the triangular-lattice quantum spin liquid YbMgGaO4, J. A. M. Paddison, M. Daum, Z. L. Dun, G. Ehlers, Y. Liu, M. B. Stone, H. D. Zhou, and M. Mourigal, Nature Physics AOP (2016).

  8. Condensed-Matter Physics & Materials Science Seminar

    "Magnetometry Study of Underdoped Cuprate YBa2Cu3O6.55"

    Presented by Fan Yu, University of Michigan

    Friday, April 28, 2017, 11 am
    Bldg. 734, ISB. Conf. Rm. 168

    Hosted by: '''''''Qiang Li'''''''

    This talk would be focused on my study of the phase diagram of underdoped cuprate YBa2Cu3O6.55 using torque magnetometry as well as my exploration of extending magnetometry method into even higher magnetic fields (>45T) using pulsed magnet. The complex phase diagrams of cuprates are sometimes referred to as "competing orders", where a large variety of ordering tendencies are known to (co-)exist. Our experiment managed to reveal an anomaly on the magnetic susceptibility, which we believe was related to charge density wave transition. Particularly interesting is that this anomaly is observed in the strong diamagnetic regime where vortex liquid exists. We believe this should be considered as a direct experimental evidence for the picture of "competing orders". To further our understanding of the quantum vortex liquid, experiments at mK temperatures and at magnetic field exceeding 40 Tesla are necessary. During my PhD study, considerable amount of time was devoted to developing a reliable magnetometry method utilizing the pulsed magnet at NHMFL, Los Alamos. I would like to present my trail-and-error as well as the proposition of "time-delayed probe design", which should be able to bypass the inherent noise of a pulsed environment.

  9. 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

    Thursday, April 20, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    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

  10. Condensed-Matter Physics & Materials Science Seminar

    "Listening to the hydrodynamic noise of Dirac fluid in graphene"

    Presented by Kin Chung Fong, Raytheon BBN Technologies and Harvard University

    Tuesday, April 18, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: 'Qiang Li'

    Interactions between the Dirac fermions in graphene can lead to new collective behavior described by hydrodynamics. By listening to the Johnson noise of the electrons, we are able to probe simultaneously the thermal and electrical transport of the Dirac fluid and observe how it departs from Fermi liquid physics. At high temperature near the neutrality point, we find a strong enhancement of the thermal conductivity and breakdown of Wiedemann-Franz law in graphene. This is attributed to the non-degenerate electrons and holes forming a strongly coupled Dirac fluid. At lower temperatures beyond the hydrodynamic behavior, the Dirac fermions are in extreme thermal isolation with minute specific heat that can be exploited for ultra-sensitive photon detection. We will present our latest experimental result towards observing single microwave photons and explore its role in scaling up the superconducting qubit systems. Our model suggests the graphene-based Josephson junction single photon detector can have a high-speed, negligible dark count, and high intrinsic quantum efficiency for applications in quantum information science and technologies. Ref: Science 351, 1058 (2016)

  11. Condensed-Matter Physics & Materials Science Seminar

    "Electronic Squeezing of Pumped Phonons: Negative $U$ and Transient Superconductivity"

    Presented by Dante Kennes, Columbia University

    Thursday, April 13, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: '''Neil Robinson'''

    Advances in light sources and time resolved spectroscopy have made it possible to excite specific atomic vibrations in solids and to observe the resulting changes in electronic properties but the mechanism by which phonon excitation causes qualitative changes in electronic properties has remained unclear. Here we show that the dominant symmetry-allowed coupling between electron density and dipole active modes implies an electron density-dependent squeezing of the phonon state which provides an attractive contribution to the electron-electron interaction, independent of the sign of the bare electron-phonon coupling and with a magnitude proportional to the degree of laser-induced phonon excitation. Reasonable excitation amplitudes lead to non-negligible attractive interactions that may cause significant transient changes in electronic properties including superconductivity. The mechanism is generically applicable to a wide range of systems, offering a promising route to manipulating and controlling electronic phase behavior in novel materials.

  12. Condensed-Matter Physics & Materials Science Seminar

    "Explore Mesoscopic Physics in Strongly Correlated Electron Materials with IR near-field microscopy and spectroscopy"

    Presented by Mengkun Liu, Stony Brook University

    Thursday, March 30, 2017, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: 'Cedomir Petrovic'

    In strongly correlated electron materials, the delicate interplay between spin, charge, and lattice degrees of freedom often leads to extremely rich phase diagrams exhibiting intrinsic phase inhomogeneities. The key to understanding such complexities usually lies in the characterization and control of these materials at fundamental energy, time and length scales. I will use this opportunity to report the recent advances in the IR and THz near-field microscopy and spectroscopy, and explain how they can be used to probe electronic/structural phase transitions with unprecedented spatial and temporal resolutions. Specifically, with scanning near-field infrared microscopy we resolved the insulator to metal phase transitions in 3d (VO2), 4d (Ca2RuO4) and 4f (SmS) materials with ~10 nm resolution over a broad spectral range. The results set the stage for future spectroscopic investigations to access the fundamental properties of complex materials.

  13. 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

    Tuesday, March 28, 2017, 11 am
    Bldg. 734, ISB Conference Room 201 (upstairs)

    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.

  14. Condensed-Matter Physics & Materials Science Seminar

    "Resonant Inelastic X-ray Scattering and X-ray Emission Spectroscopy of Iron Pnictide Superconductors"

    Presented by Jonathan Pelliciari, Paul Scherrer Institute, Switzerland

    Monday, March 27, 2017, 10 am
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: 'Mark Dean'

    I will describe Resonant Inelastic X-Ray Scattering (RIXS) experiments performed at the Swiss Light Source focusing on the detection of high-energy spin fluctuations on iron pnictides. I will show that RIXS has been successfully used to extract the spin excitation spectrum on NaFeAs, BaFe2As2, EuFe2As2 and SmFeAsO, parent compounds [1-3]. We investigated electron-doped NaFe1-xCoxAs observing the persistence of broad dispersive magnetic excitations in optimal and overdoped samples [1]. The energy of such modes is unaffected by doping and the magnetic weight per iron atom of magnons / paramagnons remains constant, demonstrating the impurity role of Co doping. The persistence of magnetic spectral weight is also caught by theoretical calculations. In the second part of the talk, I will present a combined Fe-L3 RIXS and Fe-Kβ X-rays emission spectroscopy (XES) study of isovalently doped BaFe2(As1-xPx)2 spanning a large portion of the phase diagram. RIXS measurements find the persistence of broad dispersive magnetic excitations for all doping levels. Remarkably, the energy of such modes is strongly hardened by doping differently from the cases of electron- and hole-doped BaFe2As2 [5]. On the other hand, XES experiments show a gradual quenching of the local magnetic moment, which is intriguing if compared to the behavior of spin correlations. We link the unconventional evolution of magnetism to the shift from 2- to 3-dimensional electronic structure of the system, hand in hand with the warping of the Fermi surface. Combined together these findings help to shed light on the real degree of electronic correlations in Fe pnictides. References [1] J. Pelliciari et al., Phys. Rev. B, 93, 134515 (2016); [2] J. Pelliciari et al., Appl. Phys. Lett. 109, 122601 (2016); [3] J. Pelliciari et al., "Local and collective magnetism of EuFe2As2" accepted in Phys. Rev. B (2017); [4] K. J. Zhou et al, Nat. Comm., 4, 1470 (2013)

  15. Condensed-Matter Physics & Materials Science Seminar

    "Shining a light on high-Tc superconductivity"

    Presented by Peter Johnson, BNL

    Friday, March 24, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: '''Igor Zaliznyak'''

    TBD

  16. Condensed-Matter Physics & Materials Science Seminar

    "Nematic quantum paramagnet and possible application to FeSe"

    Presented by Fa Wang, International Center for Quantum Materials Peking University, China

    Thursday, March 23, 2017, 11 am
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: ''Weiguo Yin''

    The nematic phases in iron pnictides are in close proximity to the stripe antiferromagnetic order, suggesting that magnetism is the driving force for the spontaneous 4-fold crystal rotation symmetry breaking. In contrast, bulk FeSe shows a nematic phase below 90K at ambient pressure, but has no magnetic long range order down to very low temperature. This prompts suggestions that the nematicity in FeSe is driven by some other mechanism. We argue that magnetic correlation can still drive nematic order in the absence of magnetic long-range order. By field theoretical considerations and exact diagonalization results on finite size lattices, we conclude that the paramagnetic phase in frustrated spin-1 J_1-J_2 model on square lattice is likely a "nematic quantum paramagnet", which breaks only the crystal 4-fold rotation symmetry. The prototype wavefunctions of such quantum ground states are horizontal(vertical) aligned spin-1 AKLT chains. We suggest that the local spins in FeSe may form this phase due to strong frustration. One unique consequence of this proposal is that the nematic paramagnetic phase will be close to both stripe and Neel antiferromagnetic order, and will thus host low but finite energy spin fluctuations at both ordering wavevectors. Reference: Fa Wang, S. A. Kivelson, and Dung-Hai Lee, Nat. Phys. 11, 959 (2015)

  17. Condensed-Matter Physics & Materials Science Seminar

    "Transport and signatures of Mottness versus Hundness in strongly correlated metals"

    Presented by Xiaoyu Deng, Rutgers

    Thursday, March 9, 2017, 11 am
    Bldg. 734, ISB Conference Room 201 (upstairs)

    Hosted by: ''Gabi Kotliar''

    In this seminar I will focus two fundamental aspects of strongly correlated metals: the transport properties and the origin of correlation. Recent advances enables us to study quantitatively various properties of two archetypal correlated oxides, vanadium oxides and ruthenates, using the LDA+DMFT method. Both are strongly correlation, these two materials are quite different in their origins of correlation: V2O3 is proximate to a Mott state while Sr2RuO4 is not. Thus V2O3 is regarded as a prototype Mott system, while recent studies emphasize that Sr2RuO4 belongs to new category termed "Hund's metal" in which Hund's coupling is responsible for the correlations. We carried out a systematical theoretical study on the transport properties of V2O3 and ruthenates family. Our computed resistivity and optical conductivity are in very good agreement with experimental measurements, which clearly demonstrates that the strong correlation dominates the transport of this material , despite their origin of correlation. We demonstrated that "resilient quasiparticles" dominates the transport. Furthermore by expressing the resistivity in terms of an effective plasma frequency and an effective scattering rate, we uncover the so-called "hidden Fermi liquid" behavior. We identified signatures of Mottness and Hundness by a comparative study of V2O3 and Sr2RuO4. In V2O3 the low temperature coherent resonance emerges from the pseudogap regime appearing at high temperature between incoherent peaks, while in Sr2RuO4, it emerges from a single incoherent peak with large finite value at the Fermi level.. We show that these two contrasting scenarios features interesting behaviors in the local properties of correlated atoms including charge fluctuations, spin and orbit susceptibility and entropy. The findings shed new lights on the understanding of strongly correlated metals.

  18. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Andrei Kutepov, Rutgers University

    Thursday, March 2, 2017, 1:30 pm
    Bldg. 734, ISB Conference Room 201 (upstairs)

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

    TBA

  19. 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.

  20. 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

  21. 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''

  22. 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).

  23. 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.

  24. 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.

  25. 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.

  26. 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

  27. 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

  28. 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

  29. 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.

  30. 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.

  31. 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.

  32. 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).

  33. 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.

  34. 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.

  35. 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.

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