Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L23: Focus Session: Search for New Superconductors II: Towards Theoretical Design |
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Sponsoring Units: DMP Chair: Philip Phillips, University of Illinois Room: D165 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L23.00001: Spectral Weight Transfer in a Multi-Orbital Mott System Wei-Cheng Lee, Philip Phillips One of the unique properties in a single band Hubbard model is the spectral weight transfer upon doping. Unlike in a Fermi liquid the redistribution of the spectral weights occurs predominantly near the chemical potential, a significant amount of spectral weights can be transferred from the high energy scales (upper Hubbard band) down to the chemical potential as a Mott insulator is doped. In this talk, we analyze the spectral weight transfer in a multi-orbital Mott system. We find that the spectral weights transferred from the high energy scales are greatly increased due to the multi-orbital structure, leading to a reduction of the critical doping level exhibiting zero thermopower. We argue that this indicates a suppression of the pseudogap phase and also predict the existence of new branches of charge 2e bosons carrying spin 1 at low energy in a multi-orbital Mott system. Relevant experimental consequences will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L23.00002: Electron-electron interaction in superconducting Lithium under pressure Amandeep Kaur, Erik Ylvisaker, Deyu Lu, Yan Li, Giulia Galli, Warren Pickett Lithium is known to exist in different phases and to uperconduct under pressure (P). We investigate ~the screened electron-electron interaction in Li as a function of P by analyzing the dielectric band structures of several phases. These band structures are obtained by iterative diagonalization\footnote{H.Wilson, F.Gygi and G.Galli, Phys. Rev. B, 78,113303 (2008); Hugh F. Wilson, Deyu Lu, Francois Gygi Phys. Rev. B. 79, 245106 (2009).} of the dielectric matrix as a function of wave vector and frequency. Even though the superconductivity in lithium is electron-phonon mediated, lithium is a good test system to study the screened e-e interaction more generally, which might be a primary mechanism for the superconductivity in high T$_{c}$ nitrides of the form MNCl (M=Ti,Zr,Hf). [Preview Abstract] |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L23.00003: An investigation of non-superconducting PuPt2In7 Hahnbidt Rhee, Warren Pickett, Filip Ronning, Jian-Xin Zhu, Eric Bauer PuPt$_2$In$_7$, like the heavy-fermion 115s, is member of a family of systems that are made up of RM$_3$ (R=Ce, Pu, ...; M=In, Ga, ...) building blocks. Superconductivity is observed in many of these materials, and it is understood to arise from an unconventional pairing mechanism due to antiferromagnetic spin fluctuations. Experiments discover that PuPt$_2$In$_7$, however, is an enhanced Pauli paramagnet with a Sommerfeld coefficient of $\sim$250 mJ/mol K$^2$. Here we present a DFT (Density Functional Theory) study of its electronic structure, with direct comparisons made to superconducting PuCoGa$_5$ and PuCoIn$_5$. Fermi surfaces, orbital decomposition of density of states, and band structures reveal many similarities between the two compounds. Our goal is to understand why we observe superconductivity in one but not the other. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:42PM |
L23.00004: Why positive hole carriers and negatively charged planes are conducive to high temperature superconductivity Invited Speaker: The vast majority of superconducting materials have positive Hall coefficient in the normal state, indicating that hole carriers dominate the normal state transport. This was noticed even before BCS theory, and has been amply confirmed by materials found since then: the sign of the Hall coefficient is the strongest normal state predictor of superconductivity. In the superconducting state instead, superfluid carriers are always electron-like, i.e. negative, as indicated by the fact that the magnetic field generated by rotating superconductors is always parallel, never antiparallel, to the body's angular momentum (``London moment''). BCS theory ignores these facts. In contrast, the theory of hole superconductivity, developed over the past 20 years (papers listed in http://physics.ucsd.edu/$\sim $jorge/hole.html) makes charge asymmetry the centerpiece of the action. The Coulomb repulsion between holes is shown to be smaller than that between electrons, thus favoring pairing of holes, and this fundamental electron-hole asymmetry is largest in materials where the conducting structures have \textit{excess negative charge}, as is the case in the cuprates, arsenides and MgB$_{2}$. Charge asymmetry implies that superconductivity is driven by lowering of kinetic energy, associated with expansion of the carrier wavefunction and with \textit{expulsion of negative charge} from the interior to the surface of the material, where it carries the Meissner current. This results in a macroscopic electric field (pointing outward) in the interior of superconductors, and a macroscopic spin current flowing near the surface in the absence of external fields, a kind of macroscopic zero point motion of the superfluid (spin Meissner effect). London's electrodynamic equations are modified in a natural way to describe this physics. It is pointed out that a dynamical explanation of the Meissner effect \textit{requires} radial outflow of charge in the transition to superconductivity, as predicted by this theory and not predicted by BCS. The theory provides clear guidelines regarding where new higher T$_{c}$ superconductors will and will not be found. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L23.00005: The Possibility of Phonon-Mediated Superconductivity in an Iron-Based Material Sheena Shah, Elena Roxana Margine, Aleksey Kolmogorov We have identified a synthesizable candidate FeB4 material with a potential for conventional superconductivity at 15-20 K [1,2]. The strong electron-phonon coupling in the proposed material is unexpected as the recently discovered iron-based superconductors are considered to display an unconventional pairing mechanism. The new nonmagnetic ground state crystal structure has been predicted with an ab initio evolutionary search [3] and shown to be marginally stable at ambient pressures. \\[4pt] [1] A. N. Kolmogorov, S. Shah, E. R. Margine, A. F. Bialon, T. Hammerschmidt, R. Drautz, Phys. Rev. Lett. 105, 217003 (2010). \\[0pt] [2] A. F. Bialon, T. Hammerschmidt, R. Drautz, S. Shah, E. R. Margine, A. N. Kolmogorov (submitted) \\[0pt] [3] A. N. Kolmogorov, MAISE (http://maise-guide.org) [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L23.00006: A DFT (LDA+U) study of the electronic properties of layered, square-planar coordinated, copper monoxide structures Paul M. Grant It is now 25 years and two months since Georg Bednorz observed the onset of high temperature superconductivity in copper oxide perovskites, and yet today its origin remains still largely unresolved. However, it quickly became evident the phenomenon was restricted to those structures possessing a common feature -- square planar coordinated ``sheets,'' or ``layers'' of copper monoxide, and thus now thought to be essential to effect superconductivity in this family of materials. We examine the structural stability and electronic properties of these 2D approximations to the layered CuO compounds as a function of Hubbard U within the DFT (LDA+U) framework, especially for those particular values yielding metallic band formation, and their subsequent fermiology and electron/hole-phonon coupling properties. Although such particular 2D embodiments do not, as yet, exist, we consider their study via DFT as valuable \underline {proxies}\footnote{P. M. Grant, Journal of Physics: Conference Series \textbf{129} (2008) 012042.} to aid eventual understanding of that flavor of superconductivity revealed by the Bednorz-Mueller breakthrough. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L23.00007: Design Algorithms for Novel High Temperature Superconductors O. Paul Isikaku-Ironkwe, Emeka Oguzie, Uko Ofe A grand challenge in superconductivity is to develop a ``materials specific'' theory that enables us to design superconductors from the Periodic Table. Using the Periodic Table properties of electronegativity, valence electrons, formula weight and atomic number, we have been able to quantitatively describe all superconductors in terms of those parameters. We have observed specific correlations with various families of superconductors that enable us to reverse engineer those superconductors. We have developed simple equations, maps and algorithms that facilitate the design of superconductors and predict their approximate transition temperatures. Our design method does not employ density functional theory, even though DFT can be used to verify it. In this paper, we provide many examples of predicted ``materials specific'' novel high temperature superconductors that should test the authenticity of our design algorithms. We also propose a design for possible room temperature superconductivity. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L23.00008: Ionic Plasma Screening and Long-Range Electron Correlations in Quasi-One-Dimensional Conductors Yuri Gartstein, Anvar Zakhidov In quasi-one-dimensional systems with the intercalation-type doping, the dynamical response of dopant ions can substantially affect the interplay of density-wave and superconducting instabilities. We study a generic model of the system of Coulombically coupled Luttinger-liquid chains modified by the Coulomb interaction with displacements of dopant ions. Our interest is in the macroscopic, long wave-length, effects of the ionic response. This three-dimensional electron-ion model system is exactly solvable in the forward-scattering channel allowing us to find the resulting system excitations and electron correlations. For a jellium-like ion response, the effect of the bare electron-electron repulsion on the long- range correlations is essentially canceled by the ions with the effective electron-electron interactions now exhibiting regions of shorter-range repulsion and longer-range attraction. This picture is clarified and reproduced within the macroscopic dielectric function framework. If the system also features a non-polarizational interaction with another optical phonon mode, superconducting correlations are developed already due to the forward-scattering only. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L23.00009: Phase transitions in a three dimensional $U(1) \times U(1)$ lattice London superconductor: Metallic superfluid and charge-4e superconducting states Egil Herland, Egor Babaev, Asle Sudbo We consider a three-dimensional lattice $U(1) \times U(1)$ and $[U(1)]^N$ superconductors in the London limit, with individually conserved condensates. The $U(1) \times U(1)$ problem, generically, has two types of intercomponent interactions of different characters. First, the condensates are interacting via a minimal coupling to the same fluctuating gauge field. A second type of coupling is the direct dissipationless drag represented by a local intercomponent current-current coupling. We study phase transitions and two types of competing paired phases which occur: (i) a metallic superfluid phase, (ii) a composite superconducting phase where there is order in the phase sum of the order parameters which has many properties of a single-component superconductor but with a doubled value of electric charge. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L23.00010: Exact calculations of phase separation instabilities and pairing in two-dimensional Betts nanoclusters Armen Kocharian, Gayanath Fernando, Kun Fang The energy eigenvalues and eigenstates of the Hubbard model with nearest and next nearest neighbor hoppings are calculated by exact diagonalization and Lanczos (algorithm) techniques in isotropic Betts nanoclusters with the square symmetry and periodic boundary conditions. The electron pairing instabilities and quantum critical points for one hole off half filling are evaluated by monitoring the charge and spin pairing gaps and level crossings instabilities in the ground state and at finite temperatures. The calculated spin and charge energy gaps and quantum critical points in optimized 8 and 10 site Betts clusters of square symmetry pertain universal critical behavior and are fully consistent with the exact results obtained for an ``elementary'' bipartite square geometry [Kocharian et al., Phys. Rev. B 78, 075431 (2008)]. We found the strong particle-hole assymetry effect in the electron pairing instability due to the presence of the next nearest neighbor hopping term. Correlated lectrons in various contrasting bipartite and non-bipartite two- and three- dimensional cluster topologies display a number of inhomogeneous, coherent and non-coherent nanoscale phases seen by scanning tunneling microscopy in high Tc cuprates, iron pnictides, manganites, etc. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L23.00011: Electromagnetic Interactions between Electrons moving in the Layered Conductors with a Dielectric Interlayer Kenji Tanahashi Electromagnetic interactions between two electrons moving in the two layered conductors separated with a dielectric interlayer have been estimated. We assume a simple situation in which the two electrons in the layered conductors move with the constant velocity in the same direction. The electric and magnetic fields of a moving electron are derived from the scalar and the vector potentials in the non-relativistic frame. The total electromagnetic force exerted between two electrons is obtained by the Lorentz expression, and the force depends on the velocity of the moving electrons. With increasing the velocity of the electrons, the magnetic force increases and the magnetic attractive force exceeds the electric repulsive force, when $v/c\;\ge 1 \mathord{\left/ {\vphantom {1 {\sqrt {\varepsilon _r \,\mu _r } }}} \right. \kern-\nulldelimiterspace} {\sqrt {\varepsilon _r \,\mu _r } }$, where v is the velocity of the two electrons moving parallel in the same direction, c is the speed of light, and$\varepsilon _r \;$is the relative dielectric constant in the direction of the perpendicular to the plane of the layers, and$\mu _r $ is the in-plane permittivity of the conduction layers. In vacuum the magnetic interaction between moving electrons never surpasses the electric interaction. However, in the highly anisotropic structures in conductivity, the magnetic interaction between moving electrons should be taken into consideration to investigate the behavior of the electrons. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L23.00012: Electrodynamics of Nearly Ferroelectric Superconductors in the non-local Pippard limit Upali Aparajita, Joseph Birman We report the structure of the magnetic field and secular current in a Nearly Ferroelectric Superconducting (NFE-SC) thin film. It was shown that unlike in conventional superconducting films, the external radiation causes alternating pattern of current strips. The strength of the innermost current torrents is governed by the laser field intensity as well as resonance with the ferroelectric component. The latter is modeled by secular reflection and random scattering in the Pippard non-local limit. Our calculations suggest that corresponding magnetic field pattern affects vortex formation in such material. [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L23.00013: Planar-coordinated nickelates, isoelectronic to overdoped cuprates: an LDA+DMFT comparison Chuck-Hou Yee, Gabriel Kotliar, Kristjan Haule We show the Ni-O planes in the bilayer and trilayer T'-type nickelates, recently synthesized by Poltavets, {\it et al.} [1], are electronically analogous to the Cu-O planes in overdoped superconducting cuprates. The density of states, Fermi surface, nickel valence and mass renormalization, computed using LDA+DMFT, are in good agreement with available experiment, and indicate that the compounds are well-described by multilayer Hubbard physics. Significant interlayer coupling generates bonding-antibonding Fermi surfaces, similar to those seen in the cuprates. We investigate the possibility that interlayer coupling can explain the presence of a phase transition with $R \log 2$ entropy in the trilayer, and the absence of such a transition in the bilayer. \\[4pt] [1] Poltavets, {\it et al.}, Phys. Rev. Lett. {\bf 104}, 206403 (2010). [Preview Abstract] |
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