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October 2018
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  1. Condensed-Matter Physics & Materials Science Seminar

    11 am, ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Ian Robinson

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


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  1. NOV



    Condensed-Matter Physics & Materials Science Seminar

    "Phase transitions, emergent symmetries and quantum critical behavior of Dirac systems"

    Presented by Michael Scherer, University of Cologne, Germany

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

    Thursday, November 8, 2018, 1:30 pm

    Hosted by: Laura Classen


  1. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Bo Chen, Tongji University, China

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

    Hosted by: Ian Robinson

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

  2. Condensed-Matter Physics & Materials Science Seminar

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

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

    Hosted by: Igor Zaliznyak

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

  3. Condensed-Matter Physics & Materials Science Seminar

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

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

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

    Hosted by: Andreas Weichselbaum

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

  4. Condensed-Matter Physics & Materials Science Seminar

    "Pair-breaking quantum phase transition in superconducting nanowires"

    Presented by Andrey Rogachev, University of Utah

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

    Hosted by: Ivan Bozovic

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

  5. Condensed-Matter Physics & Materials Science Seminar

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

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

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

    Hosted by: Alexei Tsvelik

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

  6. Condensed-Matter Physics & Materials Science Seminar

    "Advances in high energy electron holography"

    Presented by Dr. Toshiaki Tanigaki, Hitachi, Japan

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

    Hosted by: MG Han

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

  7. Condensed-Matter Physics & Materials Science Seminar

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

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

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

    Hosted by: Ian Robinson

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

  8. Condensed-Matter Physics & Materials Science Seminar

    "Atomic level structural characterization of materials by electron microscopy"

    Presented by Shize Yang, Center for Functional Nanomaterials

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

    Hosted by: Yimei Zhu

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

  9. Condensed-Matter Physics & Materials Science Seminar

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

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

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

    Hosted by: Alexei Tsvelik

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

  10. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Linze Li, University of California @ Irvine

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

    Hosted by: Yimei Zhu

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

  11. Condensed-Matter Physics & Materials Science Seminar

    "Room-temperature magnetic spiral order induced by disorder"

    Presented by Christopher Mudry, Paul Scherrer Institut, Switzerland

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

    Hosted by: Alexei Tsvelik

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

  12. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Dmitry Karpov, New Mexico State University

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

    Hosted by: Ian Robinson

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

  13. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Ryuichi Shindo, Peking University, China

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

    Hosted by: Alexei Tsvelik

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

  14. Condensed-Matter Physics & Materials Science Seminar

    "Fermi-Surface Reconstruction in Nd-doped CeCoIn5"

    Presented by Elizabeth Green, Dresden High Magnetic Field Laboratory

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

    Hosted by: Cedomir Petrovic

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

  15. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Robert Koch, Alfred University

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

    Hosted by: Ian Robinson

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

  16. Condensed-Matter Physics & Materials Science Seminar

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

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

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

    Hosted by: Andreas Weichselbaum

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

  17. Condensed-Matter Physics & Materials Science Seminar

    "Defects and their functional properties in multiferroic hexagonal systems"

    Presented by Shaobo Cheng, McMaster University Canada

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

    Hosted by: Yimei Zhu

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

  18. Condensed-Matter Physics & Materials Science Seminar

    "Developing theoretical understanding of non-equilibrium phenomena"

    Presented by Alexander Kemper, North Carolina State University

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

    Hosted by: Peter D. Johnson

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

  19. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Rhonda Stroud, US Naval Research Laboratory

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

    Hosted by: Yimei Zhu

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

  20. Condensed-Matter Physics & Materials Science Seminar

    "Chemistry beyond the crystal- advanced Fourier techniques"

    Presented by Simon Kimber, Oak Ridge National Laboratory

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

    Hosted by: Ian Robinson

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

  21. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Jedediah Pixley, Rutgers

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

    Hosted by: Laura Classen

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

  22. Condensed-Matter Physics & Materials Science Seminar

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

    Presented by Adrian Feiguin, Northeastern University

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

    Hosted by: Alexei Tsvelik

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

  23. Condensed-Matter Physics & Materials Science Seminar

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

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

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

    Hosted by: Cedomir Petrovic

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

  24. Condensed-Matter Physics & Materials Science Seminar

    "Non-abelian symmetries and applications in tensor networks"

    Presented by Andreas Weichselbaum, Brookhaven National Lab

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

    Hosted by: Igor Zaliznyak

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

  25. Condensed-Matter Physics & Materials Science Seminar

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

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

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

    Hosted by: Robert Konik

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

  26. Condensed-Matter Physics & Materials Science Seminar

    "Spatial Resolution of Low-Loss EELS"

    Presented by R.F. Egerton, University of Alberta, Canada

    Tuesday, March 20, 2018, 2 pm
    Building 480 Conference Room

    Hosted by: Yimei Zhu

    Recent-generation TEM/STEM instruments fitted with an electron monochromator provide an energy resolution down to 0.01 eV for electron energy-loss spectroscopy (EELS) and are themselves capable of achieving a spatial resolution approaching 0.1 nm. Besides offering the possibility of vibrational-mode EELS for examining chemical bonds, these instruments could be useful for mapping the electronic properties (e.g. band gap) of insulators and semiconductors. However, basic physics imposes a spatial resolution of few nm (or tens of nm) for energy loss below 10 eV, due to delocalization of the inelastic scattering. We will discuss what might be done to improve the spatial resolution, to make low-loss EELS competitive with other techniques.

  27. Condensed-Matter Physics & Materials Science Seminar

    "Quantum dimer models for high temperature superconductors"

    Presented by Garry Goldstein, Cambridge University, United Kingdom

    Friday, March 16, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Robert Konik

    In this talk we review the slave boson meanfield formulation of the fermion+boson quantum dimer model for the pseudogap phase of the high temperature superconductors. We show that in the presence of weak slowly varying external magnetic and electric fields the fermionic dimers undergo semiclassical motion in the external field. As a result in the presence of magnetic fields strong enough to destroy superconductivity the dimers undergo quantum oscillations. Indeed they satisfy Onsager quantization for their orbits and Lifshtiz-Kosevich formula for the amplitude of oscillations. We also compute the effective charges of the dimers in the presence of external magnetic fields as a function of temperature. We show that the effective magnetic charge changes sign from negative −e at low temperature to positive +e at high temperature. This leads to a change of the sign of the Hall coeÿcient as a function of temperature. We also compute the magnetoresistance as a function of the external field and temperature within a linearized Boltzmann equation approximation for the fermionic dimers. Furthermore we further show that the dimers undergo a Lifshitz transition as a function of doping with a van Hove singularity appearing at the Fermi surface near optimal doping ∼ 20%. Indeed the van Hove singularity leads to a divergence of the density of states and as such an optimum Tc. We study the interplay of nematic fluctuations and the van Hove singularity both of which occur near optimal doping. We show that the van Hove singularity modifies the critical properties of the QCP (quantum critical point) for nematic fluctuations and that the QCP may be described by Hertz Millis like theory with z = 4. This allows us to calculate the critical exponents of the nematic fluctuations and to show that the fermionic dimers have non-Fermi liquid behavior near the QCP with the self energy diverging ∼ |ω3/4| near the QCP.

  28. Condensed-Matter Physics & Materials Science Seminar

    "Splitting of electrons and violation of the Luttinger sum rule"

    Presented by Eoin Quinn, University of Amsterdam, Netherlands

    Thursday, March 15, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Robert Konik

    We obtain a controlled description of a strongly correlated regime of electronic behaviour. We argue that there are two ways to characterise the electronic degree of freedom, either by the canonical fermion algebra or by the graded Lie algebra su(2|2). The first underlies the Fermi liquid description of correlated matter, and we identify a novel regime governed by the latter. We obtain the electronic spectral function within a controlled approximation, and find a splitting in two of the electronic band. The Luttinger sum rule is violated and a Mott metal-insulator transition is exhibited.

  29. Condensed-Matter Physics & Materials Science Seminar

    "Enabling emergent spin-orbit magnetism in iridate-based heterostructures"

    Presented by Jian Liu, The University of Tennessee, Knoxville

    Thursday, March 15, 2018, 11 am
    ISB Bldg. 734 Seminar Room 201 (upstairs)

    Hosted by: Mark Dean

    5d transition metal oxides have emerged as a novel playground for some of the most outstanding and challenging problems in condensed matter physics, such as metal-insulator transition and quantum magnetism. In particular, layered iridates hosting square lattices of IrO6 octahedra have drawn significant interests due to the electronic and magnetic analogy with high-Tc cuprates. However, materials of this kind are limited to a few Ruddlesden-Popper (RP) compounds. In this talk, I will discuss our recent work on overcoming this bottleneck by constructing such two-dimensional (2D) structures confined in superlattices grown by heteroepitaxy. By leveraging the layering control of epitaxial growth, we are not only able to develop new structural variants of layered iridates, but also unravel and exploit the intriguing spin-orbit-driven 2D magnetism beyond the cuprate physics yet invisible in the RP iridates. The results demonstrate the power of this approach in tailing the exchange interactions, enabling new magnetic controls, and providing unique insights into the emergent phenomena of 5d electrons.

  30. Condensed-Matter Physics & Materials Science Seminar

    "3D non-Fermi liquid behavior from 1D critical local moments"

    Presented by Laura Classen, BNL

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

    Hosted by: Igor Zaliznyak

    We study the temperature dependence of the electrical resistivity in a system composed of critical spin chains interacting with three dimensional conduction electrons and driven to criticality via an external magnetic field. The relevant experimental system is Yb2Pt2Pb, a metal where itinerant electrons coexist with localized moments of Yb-ions which can be described in terms of effective S = 1/2 spins with dominantly one-dimensional exchange interaction. The spin subsystem becomes critical in a relatively weak magnetic field, where it behaves like a Luttinger liquid. We theoretically examine a Kondo lattice with different effective space dimensionalities of the two interacting sub-systems. We characterize the corresponding non-Fermi liquid behavior due to the "local criticality" from the spins by calculating the electronic relaxation rate and the dc resistivity and establish its quasi linear temperature dependence.

  31. Condensed-Matter Physics & Materials Science Seminar

    "Topological Spin Excitations in a Highly Interconnected 3D Spin Lattice"

    Presented by Yuan Li, International Center for Quantum Materials, Peking University, China

    Thursday, February 22, 2018, 11 am
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Mark Dean

    The recent discovery of topological semimetals, which possess distinct electron-band crossing with non-trivial topological characteristics, has stimulated intense research interest. By extending the notion of symmetry-protected band crossing into one of the simplest magnetic groups, namely by including the symmetry of time-reversal followed by space-inversion, we predict the existence of topological magnon-band crossing in three-dimensional (3D) collinear antiferromagnets. The crossing takes on the forms of Dirac points and nodal lines, in the presence and absence, respectively, of the conservation of the total spin along the ordered moments. In a concrete example of a Heisenberg spin model for a "spin-web" compound, we theoretically demonstrate the presence of Dirac magnons over a wide parameter range using linear spin-wave approximation, and obtain the corresponding topological surface states [1]. Inelastic neutron scattering experiments have been carried out to detect the bulk magnon-band crossing in a single-crystal sample. The highly interconnected nature of the spin lattice suppresses quantum fluctuations and facilitates our experimental observation, leading to remarkably clean experimental data and very good agreement with the linear spin-wave calculations. The predicted topological band crossing is confirmed [2]. [1] K. Li et al., PRL 119, 247202 (2017). [2] W. Yao et al., arXiv:1711.00632.

  32. Condensed-Matter Physics & Materials Science Seminar

    "Nematic superconductivity in topological materials"

    Presented by Matt Smylie, Argonne National Laboratory

    Tuesday, February 13, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Genda Gu

    In a topological superconductor, a bulk superconducting gap induces a symmetry-protected gapless superconducting surface state. This surface state can host exotic Majorana zero modes, which are expected to revolutionize computation technology through energy-efficient fault-tolerant quantum computing. In this talk, we will discuss the search for bulk topological superconductors and the discovery of nematic superconductivity in MxBi2Se3 (M=Cu,Sr,Nb), where the superconducting system spontaneously breaks rotational symmetry at Tc. The nematic superconducting state and possible origins of the rotational symmetry breaking will be explored, with many conventional causes being eliminated.

  33. Condensed-Matter Physics & Materials Science Seminar

    "Establishing Jeff =3/2 Ground State in a Lacunar Spinel GaTa4Se8"

    Presented by Myung Joon Han, Korea Advanced Institute of Science and Technology (KAIST)

    Wednesday, January 31, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Sangkook Choi

    In this talk, after briefly introducing the research activities in my group, I will present our recent progress on GaTa4Se8 which is known as a 'paramagnetic Mott' insulator and exhibits superconducting transition under pressure. Its low temperature behaviors found in susceptibility and specific heat measurement have not yet been clearly understood. The important first step to study these intriguing phenomena and the relationship between them is to clarify the nature of its electronic and magnetic property. By using first-principles band structure calculation and resonant inelastic x-ray scattering technique, we show that GaTa4Se8 is a novel 'Jeff=3/2 Mott' insulator in which spin-orbit interaction plays a key role to form a gap together with electronic correlation. The excitations involving the Jeff = 1/2 molecular orbital are absent only at the Ta L2 edge, manifesting the realization of the molecular Jeff = 3/2 ground state in GaTa4Se8. Based on this finding, the possible consequences of the Jeff = 3/2 state will be discussed

  34. Condensed-Matter Physics & Materials Science Seminar

    "Spin-orbit coupling and electronic correlations in Hund's metals: Sr2RuO4"

    Presented by Minjae Kim, École Polytechnique, France

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

    Hosted by: Gabi Kotliar

    We investigate the interplay of spin-orbit coupling (SOC) and Hund's rule coupling driven electronic correlations in Sr2RuO4 using dynamical mean-field theory. We find that the orbital diagonal components of the dynamical electronic correlations are unaffected by the SOC, which validates the concept of a Hund's metal in the presence of SOC. In contrast, SOC itself is enhanced by approximately a factor of two by electronic correlations. We introduce the concept of an energy dependent quasiparticle SOC, which is found to be essential in accounting simultaneously for: (i) the Fermi surface (ii) the low-energy dispersion of quasiparticles and (iii) the splitting between bands at higher binding energy. Our calculations are in good agreement with available experimental data. References: [1-4] [1] C. Veenstra et al., Physical Review Letters 112, 127002 (2014) [2] M. Haverkort et al., Physical Review Letters 101, 026406 (2008) [3] J. Mravlje et al., Physical Review Letters 106, 096401 (2011) [4] M. Kim et al., arXiv preprint arXiv:1707.02462 (2017)

  35. Condensed-Matter Physics & Materials Science Seminar

    ""In situ characterization of the phase behavior of metal oxides at extreme conditions""

    Presented by Leighanne Gallington, Argonne National Laboratory

    Wednesday, January 17, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Room 201 (upstairs)

    Hosted by: Ian Robinson

    In situ characterization of the phase behavior of materials in the lab is complicated by the difficulty of designing compatible sample environments as well as the long time scales required to acquire diffraction data with sufficient counting statistics for crystallographic analyses. The high energy x-rays available at synchrotron sources allow for penetration of most sample environments, while high flux allows for rapid acquisition of diffraction patterns, thereby allowing construction of detailed phase diagrams. Low and negative thermal expansion (NTE) materials have been studied extensively, as they can potentially be used to create composites with finely controlled thermal expansion characteristics, improved resistance to thermal shock, and a broader range of operating temperatures.1-4 While the thermal expansion behavior of the NTE materials ZrW2O8 and HfW2O8 was well-described at ambient pressures,4-6 knowledge of the effects of stress on their thermal expansion was limited.7 In situ synchrotron powder diffraction was utilized to explore the role of orientational disorder in determining both the phase behavior and the thermoelastic properties of these materials. An especially designed pressure cell allowed for simultaneous sampling of temperatures up to 513 K and pressures up to 414 MPa.8 Reversible compression-induced orientational disordering of MO4 tetrahedra occurred concomitantly with elastic softening on heating and enhanced negative thermal expansion upon compression in ZrW2O8 and HfW2O8, but only in the ordered phase.9, 10 In light of the comparatively recent nuclear disaster in Fukushima, understanding interactions and phase behavior in nuclear fuels under severe accident conditions is of paramount interest. While diffraction measurements have been performed on materials recovered from melts of corium (UO2-ZrO2), there is a lack of in situ characterization of this material at elevated temperatures. Achieving the extreme temperatures required

  36. Condensed-Matter Physics & Materials Science Seminar

    "Singular density fluctuations in the strange metal phase of Bi2Sr2CaCu2O8+x observed with momentum-resolved EELS (M-EELS)"

    Presented by Peter Abbamonte, University of Illinois at Urbana Champaign

    Friday, January 12, 2018, 11 am
    ISB Bldg. 734, Conf. Room 201 (upstairs)

    Hosted by: Peter D. Johnson

    High-temperature superconductivity arises out of an anomalous normal state commonly referred to as a "bad" or "strange" metal, since it lacks the usual signatures of electron quasiparticles. In ordinary metals, such quasiparticles manifest as propagating collective modes encoded in the dynamic charge susceptibility ??(q,?), which describes the response of the system to applied fields. However, the analogous collective modes of a strange metal are currently unknown. Here, we present the first measurement of ??(q,?) for a prototypical strange metal, Bi2.1Sr1.9CaCu2O8+x (BSCCO), using momentum-resolved inelastic electron scattering (M-EELS). We discover a surprising energy- and momentum-independent continuum of fluctuations extending up to 1 eV, at odds with the dispersive plasmons expected in normal metals. This spectrum is found to be temperature-independent across the superconducting phase transition at optimal doping. Tuning the composition to overdoping, where a crossover to Fermi liquid behavior is expected, this momentum-independent continuum is found to persist, though a 0.5 eV gap-like feature now emerges at low temperature. Our results indicate that the phenomenon underlying the strange metal is a singular form a charge dynamics of a new kind, that does not fit into any known picture of quantum critical scaling.

  37. Condensed-Matter Physics & Materials Science Seminar

    "Bose condensation of excitons in TiSe2"

    Presented by Peter Abbamonte, University of Illinois at Urbana–Champaign

    Thursday, January 11, 2018, 1:30 pm
    ISB Bldg. 734, Conf. Room 201 (upstairs)

    Hosted by: Peter D. Johnson

    Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. But definitive evidence for a thermodynamically stable exciton condensate has never been achieved. In this talk I will describe our use of momentum-resolved electron energy-loss spectroscopy (M-EELS) to study the valence plasmon in the transition metal dichalcogenide semimetal, 1T-TiSe2. Near the phase transition temperature, TC = 190 K, the plasmon energy falls to zero at nonzero momentum, indicating dynamical slowing down of plasma fluctuations and crystallization of the valence electrons into an exciton condensate. At low temperature, the plasmon evolves into an amplitude mode of this electronic crystal. Our study represents the first observation of a soft plasmon in any material, the first definitive evidence for exciton condensation in a three-dimensional solid, and the discovery of a new form of matter, "excitonium."

  38. Condensed-Matter Physics & Materials Science Seminar

    ""Photoemission studies of the electronic properties of rare-earth intermetallics and oxide interface""

    Presented by Alla Chikina, Paul Scherrer Institute, Switzerland

    Monday, January 8, 2018, 3 pm
    ISB Bldg. 734 Seminar Room 201 (upstairs)

    Hosted by: Cedomir Petrovic

    Longer than 70 years solid state research has been focused on the study of materials with strong electron correlations due to their remarkable electronic and magnetic properties. In such systems, the average energy of the Coulomb interaction is greater or comparable to its kinetic energy and electrons tend to be localized. This localization is strong enough that electrons can be considered in the framework of the atomic approach. Interaction with itinerant electrons makes the interpretation of their physical properties more complicated. A typical example of a strongly-correlated system contains transition and rare earth (RE) elements. Here, I present both theoretical and experimental insight into the itinerant-localized electron interaction in rare-earth 122 silicides (RERh2Si2). The properties of RERh2Si2 change from the heavy-fermion behavior in YbRh2Si2 up to well-pronounced magnetic properties in EuRh2Si2 and GdRh2Si2. The competition between the Kondo effect and the magnetic RKKY interactions determines the properties of a large class of materials which have localized 4f magnetic moments coupled to itinerant valence electrons. The strong electron correlations, also well known in the transition metal oxides, rise up their remarkable functional and magnetic properties. It gives a route in a manipulation of electron, spin, orbital and lattice degrees of freedom for novel electronic and spintronic devices based on oxide interfaces. An important role in the electronic and magnetic properties of this interface is played by oxygen vacancies which form a dichotomic electron system where strongly correlated localized electrons in the in-gap states (IGSs) coexist with less correlated ones constituting the mobile two-dimensional electron system (2DES). On the example of the interface between LaAlO3 and SrTiO3 we consider a complex band ordering in the dichotomic LAO/STO electron system that goes beyond the conventional eg vs t2g picture.

  39. Condensed-Matter Physics & Materials Science Seminar

    "Illuminating rationally engineered complex oxides"

    Presented by Derek Meyers, BNL

    Friday, January 5, 2018, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Mark Dean

    Advances in unit cell scale synthesis have unlocked the ability to create artificial materials at the interface of complex oxides. This opens the door to the rational design of materials properties. To explore the spin, charge, and orbital character of these synthetic materials, resonant x-ray scattering techniques are utilized which unveil their long-range ordering and low energy excitations. In this talk, we will explore several recent examples of this new methodology and provide an outlook on the future of this emergent field.

  40. Condensed-Matter Physics & Materials Science Seminar

    "Spin fluctuations in 122 transition metal arsenides measured using inelastic neutron scattering technique"

    Presented by Aashish Sapkota, Ames Laboratory

    Thursday, December 21, 2017, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: John Tranquada

    122 transition metal compounds with ThCr2Si2-type structure have been extensively studied because of their wide range of interesting physical properties like superconductivity, valence fluctuations, various magnetic ground states, etc. A subset (ATM2Pn2) of this class consisting of alkaline earth metals (A), 3d transition metals (TM) and pnictogen (Pn) attracted significant interest after discovery of an unconventional superconductivity in 122 iron arsenide compounds. In 122 iron arsenide superconductors, magnetism is in close proximity to the superconductivity and the spin fluctuations are considered as a key component for the pairing mechanism for superconductivity. These properties as well as the wide range of magnetic ground states, found in ATM2As2, motivated a detail studies of the magnetism in these compounds and neutron scattering technique has been extensively used for the study. In this seminar, I will discuss our results of inelastic neutron scattering measurements of the spin fluctuations in two compounds [CaCo1.86As2 and Ca(Fe1-xCox)2As2] of ATM2Pn2 class. First, I will discuss extremely extended spin fluctuations along two directions of reciprocal space in CaCo1.86As2, which shows A-type antiferromagnetic ground states. The result suggests that CaCo1.86As2 is highly-frustrated and is a unique example of highly-frustrated square-lattice system. Next, I will discuss the evolution of the spin fluctuations in Co-doped CaFe2As2 and compare it to that of Co-doped BaFe2As2. In this part, I will also discuss a peculiar suppression of the spin fluctuations with temperature observed in Ca(Fe1-xCox)2As2, x = 0.030 compound, which shows superconducting ground state.

  41. Condensed-Matter Physics & Materials Science Seminar

    "Examples of translational research using thermoelectric oxides"

    Presented by Ryoji Funahashi, National Institute of Advanced Industrial Science & Technology, Japan

    Wednesday, December 6, 2017, 3 pm
    Conference Room, Building 480

    Hosted by: Qiang Li

    We have been relishing a lot of affluence thanks to energy. Fossil energy provides us fun to drive, warmth to escape from cold, brightness of illumination, etc. However consumption of the fossil fuel produces CO2. The amount of CO2 emission will increase with increasing consumption of fossil energy, gas, oil, and coal year by year. The average of total utilizing efficiency of the primary energy is as low as 30 %, with 70 % exhausted to the air as waste heat. It is clear that improved efficiencies of energy conversion systems could have a significant impact on energy consumption and carbon dioxide emission rate. Where a large sum of heat is localized, mechanical conversion systems can be used to generate electricity. However, most sources of waste heat are widely dispersed. Although technologies of storage and transport of such the dilute heat energy have been developed, most waste heat can't be used effectively. Electricity is a convenient form of energy that is easily transported, redirected, and stored, thus there are a number of advantages to the conversion of waste heat emitted from our living and industrial activities to electricity. Thermoelectric conversion is paid attention as the strongest candidate to generate electricity from dilute waste heat. Oxide materials are considered to be promising ones because of their durability against high temperature, low cost for producing etc. The misfit CoO2 compounds show high thermoelectric efficiency at high temperature in air. Thermoelectric modules using p-type Ca3Co4O9, one of the CoO2 compounds and n-type CaMnO3 have been produced [1, 2]. The maximum power density against area of the substrate of the module reaches 4.3 kW/m2 at 973 K of the heat source temperature [3]. Portable power generation units composed of an oxide thermoelectric module. Water circulation and batteries for air cooling are unnecessary for thermoelectric conversion. The units can generate 2-5 W using heat energy with temperature of 300-8

  42. Condensed-Matter Physics & Materials Science Seminar

    "Complementary response of static spin-stripe order and superconductivity to non-magnetic impurities and pressure in cuprates"

    Presented by Zurab Guguchia, Columbia University

    Thursday, November 16, 2017, 1:30 pm
    ISB Bldg. 734, Conference Room 201 (upstairs)

    Hosted by: Emil Bozin

    Cuprate high-temperature superconductors (HTSs) have complex phase diagrams with multiple competing ordered phases. Understanding to which degree charge, spin, and superconducting orders compete or coexist is paramount for elucidating the microscopic pairing mechanism in the cuprate HTSs. In this talk, i will report some novel results of muonspin rotation (μSR), neutron Scattering and magnetization experiments on non-magnetic Zn impurity and hydrostatic pressure effects on the static spin-stripe order and superconductivity in the La214 cuprates [1,2]. Namely, in La2−xBaxCu1−yZnyO4 (0.11 ≤ x ≤ 0.17) and La1.48Nd0.4Sr0.12Cu1−yZnyO4. Remarkably, it was found that in these systems the spin-stripe ordering temperature Tso decreases linearly with Zn doping y and disappears at y ≈ 4 % , demonstrating the extreme sensitivity of static spin-stripe order to impurities within a CuO2 plane. Moreover, Tso is suppressed in the same manner as the superconducting transition temperature Tc by Zn impurities. We also observed the same pressure evolution of both Tc and Tso in La2−xBaxCuO4, while there is an antagonistic pressure evolution of magnetic volume fraction and superfluid density [1,2,3]. These results indicate that static spin-stripe order and SC pairing correlations develop in a cooperative fashion in La214 cuprates. In other words, the existence of the stripe order requires intertwining with the SC pairing correlations, such as occurs in the proposed pair-density wave (PDW) state [4]. [1] Z. Guguchia et. al., Phys. Rev. B 94, 214511 (2016). [2] Z. Guguchia et. al., Phys. Rev. Lett. 119, 087002 (2017). [3] Z. Guguchia et. al., Phys. Rev. Lett. 113, 057002 (2014). [4] E. Fradkin, S.A. Kivelson, and J.M. Tranquada, Rev. Mod. Phys. 87, 457 (2015).

  43. Condensed-Matter Physics & Materials Science Seminar

    "Quasiparticle spectra from stochastic many-body methods"

    Presented by Vojtech Vlcek, University of California, Los Angeles

    Thursday, November 9, 2017, 1:30 pm
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Gabi Kotliar

    I will present new developments and applications of stochastic approaches to electronic structure and many-body perturbation theory, which overcome the steep scaling of conventional deterministic schemes. The general principles of linear-scaling stochastic methods for TDDFT, GW and BSE will be discussed and exemplified on realistic nanoscale systems with more than 5000 valence electrons. The stochastic approaches enable mapping the evolution of optical absorption, spectral functions and quasiparticle energies and lifetimes, as well as the emergence of collective excitations, over the full range from molecules to large bulk-like 3D nanoclusters and 2D layers.

  44. Condensed-Matter Physics & Materials Science Seminar

    "Proximity effects in cuprate/manganite multilayers"

    Presented by Christian Bernhard, University of Fribourg, Switzerland

    Monday, November 6, 2017, 1:30 pm
    ISB Bldg. 734 Seminar Room 201 (upstairs)

    Hosted by: Chris Homes

    Recently we observed an intriguing, magnetic-filed-induced insulator-to-metal transition in YBa2Cu3O7/Pr1-xCaxMnO3 (YBCO/PCMO) multilayers [1]. In the low field regime, the response of these multilayers is highly resistive and resembles the one of granular superconductors or frustrated Josephson-networks. Notably, a coherent superconducting response can be restored with a large magnetic field. The latter also suppresses the charge/orbital order of the PCMO layers towards a ferromagnetic state. This coincidence suggests an intimate relationship between the insulator-to-superconductor transition in the YBCO layer and the suppression of the charge/orbital order in the PCMO. I will discuss the evidence, based on resonant x-ray scattering experiments, that the latter induces (or strongly enhances) a static Cu-CDW order in YBCO that is intertwined with superconductivity. [1] B.P.P. Mallett et al., Phys. Rev. B 94, 180503(R) (2016).

  45. Condensed-Matter Physics & Materials Science Seminar

    "Wandering amongst the Feynamn diagrams"

    Presented by Nikolay Prokofiev, University of Massachusetts-Amherst

    Friday, November 3, 2017, 11 am
    ISB Bldg. 734 Conf. Rm. 201 (upstairs)

    Hosted by: Igor Zaliznyak

    Feynman diagrams are the most celebrated and powerful tool of theoretical physics usually associated with the analytic approach. I will argue that diagrammatic expansions are also an ideal numerical tool with enormous and yet to be explored potential for solving interacting many-body systems by direct simulation of Feynman diagrams (bare or skeleton) for the proper self-energies and polarization operators up to high order. Though the original series based on are propagators are sign-alternating and often divergent one can determine the answer behind them by using proper series re-summation techniques and working with skeleton diagrams, i.e. by making the entire scheme self-consistent. The bottom line is that the diagrammatic Monte Carlo approach generically solves the computational complexity for interacting fermionic systems. In terms of physical applications, I will disucss results for the Hubbard model, resonant fermi gas at unitarity, and stability of Dirac liquid against strong Coulomb interaction in graphene.

  46. Condensed-Matter Physics & Materials Science Seminar

    "Theory and Computation Guided Discovery of New Thermoelectric Materials"

    Presented by Vladan Stevanovic, Colorado School of Mines & National Renewable Energy Laboratory

    Wednesday, October 25, 2017, 1:30 pm
    ISB Bldg. 734, Conf. Rm. 201 (upstairs)

    Hosted by: Cedomir Petrovic

    Progress in the widespread adoption of all solid heat-to-electricity technologies has largely been hindered by the absence of suitable thermoelectric materials. In pursuit for new thermoelectrics recent advances in large-scale deployment of first principles calculations could be useful in identifying new promising material systems. However, the need to predict electron and phonon transport properties with sufficient accuracy renders direct assessment of the thermoelectric figure of merit (zT) for large numbers of systems unfeasible. This is true even in the case of relatively simple semiconductor materials, which could be described by the computationally inexpensive single particle theories such as density functional theory (DFT). While the state-of-the-art DFT based approaches to charge carrier and heat transport of semiconductors can deliver desired accuracy, they are currently limited to relatively simple chemistries and/or case-by-case studies. In this talk I will discuss integrated theory-computation-experiment efforts in developing a robust set of material descriptors that: (1) are rooted in the Boltzmann transport theory, but do not rely on classic and largely inapplicable constant relaxation time or constant mean free path approximations, (2) are computationally tractable allowing material searches across large chemical spaces, and (3) are sufficiently accurate to provide reliable predictions. Our approach is demonstrated to correctly identify known thermoelectric materials1 and reliably suggest new and promising candidate semiconductors.2 At the end, I will review successes and failures in our quest for new thermoelectrics, and discuss dopability of semiconductors as the critical outstanding challenge in achieving high zT materials. 1. Yan, P. Gorai, B. Ortiz, S. Miller, S. A. Barnett, T. Mason, V. Stevanovic, and E. S. Toberer, "Material descriptors for thermoelectric performance", Energy Environ. Sci. 2. P. Gorai, V. Stevanovic, and E. Tobe

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