Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V39: Focus Session: Superconductivity-Theory and Computation III |
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Sponsoring Units: DCOMP DCMP Chair: Alex Gurevich, University of Wisconsin-Madison Room: Baltimore Convention Center 342 |
Thursday, March 16, 2006 11:15AM - 11:27AM |
V39.00001: Non-Fermi liquid and pairing in electron-doped cuprates Andrey Chubukov, Pavel Krotkov In electron-doped cuprates near optimal doping we show that in the normal state the fermionic self-energy has a non-Fermi liquid form leading to peculiar frequency dependencies of the conductivity and the Raman response. We solve the pairing problem and demonstrate that $T_{c}$ is determined by the curvature of the Fermi surface, and the pairing gap $\Delta(\mathbf{k},\omega)$ is strongly non-monotonic along the Fermi surface. The normal state frequency dependencies, the value of $T_{c}\sim10K$ and the $\mathbf{k}$-dependence of the gap agree with the experiment. [Preview Abstract] |
Thursday, March 16, 2006 11:27AM - 11:39AM |
V39.00002: Antiferromagnetic superconducting state in the electron-doped cuprates? Tanmoy Das, Robert S. Markiewicz, Arun Bansil Recent angle-resolved photoemission (ARPES) studies of the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO)[1] have been interpreted in terms of a uniform antiferromagnetic (AF) metal, with doping into the upper magnetic band and gap collapse close to optimal doping[2]. An open question is whether the system remains uniform in the simultaneous presence of AF and (d- wave) superconducting (SC) order. Here, we explore the properties of a {\it uniform} AF-SC model for NCCO, to ascertain to what extent we can explain anomalous features, such as the nonmonotonic angle dependence of the superconducting gap[3]. Work supported by the USDOE. \newline \newline [1] N.P. Armitage, {\it et al.,} PRL {\bf 87}, 147003 (2002). \newline [2] C. Kusko, {\it et al.,} PRB{\bf 66}, 140513 (2002); A.-M.S. Tremblay, {\it et al.,} cond-mat/0511334. \newline [3] H. Matsui, {\it et al.,} PRL {\bf 95}, 017003 (2005). [Preview Abstract] |
Thursday, March 16, 2006 11:39AM - 11:51AM |
V39.00003: Spin response in high-$T_{c}$ superconductors at higher energies Babak Seradjeh, Igor Herbut We discuss our recent results on the spin sector of the theory of underdoped cuprates as fluctating d-wave superconductors (I. Herbut, PRL 94, 237001 (2005)). We improve on an earlier computation of spin response (I. Herbut and D. Lee, PRB 68, 104518 (2003)) by including the effects of non-relativistic terms in the dispersion of nodal quasipartciles and thus by going beyond the low-energy approximation employed in the earlier work. We find that non-relativistic effects tend to reduce the spin gap at $(\pi, \pi)$. This could be relevant for recent neutron scattering measurements. [Preview Abstract] |
Thursday, March 16, 2006 11:51AM - 12:03PM |
V39.00004: Screening in anisotropic superfluids and the superfluid density in underdoped cuprates Matthew Case, Igor Herbut The effects of quantum and classical phase fluctuations on the superfluid density of underdoped cuprates can be modeled by an anisotropic system of Coulomb interacting bosons. In such a system, the collective excitations screen the Coulomb interaction whose effective strength, together with the strength of the anisotropy, determines the temperature dependence of the superfluid density. We will argue that the underdoped cuprates are in the short-ranged, weakly interacting regime characterized by an approximately linear superfluid density. [Preview Abstract] |
Thursday, March 16, 2006 12:03PM - 12:15PM |
V39.00005: Impurity effects in metals and superconductors Kurt Scharnberg, Simon Scheffler, Carsten Rieck In the theoretical analyses of impurity effects in superconductors the assumption is usually made that all quantities are slowly varying functions of energy except for the Green functions, whose energy dependence is integrated out. The energy-integrated normals state Green function for the NFE model is $-i$ and the divergent real part is omitted. The argument of any other momentum dependent quantity is replaced by the Fermi momentum. When defects modelled by scattering potentials of finite range, which lead to scattering in infinitely many angular momentum channels, are treated within this so-called Fermi Surface Restricted Approximation, unacceptable results for the selfenergies, the pair breaking parameters and others follow in the strong scattering limit. It has been found necessary to consider the principal value integral involving the real part of the Green function in the $T$-matrix equation. In this way, the range of the potential is re-introduced as an essential parameter and resonant scattering in different $\ell$-channels can occur for moderately large potentials, while $\delta$-function potentials, even when there strength is infinite, do not scatter at all. This generalized theory of impurity effects will be applied to calculations of the local density of states near an impurity and, using the selfconsistent $T$-matrix approximation for an ensemble of impurities, to calculations of spectral functions. [Preview Abstract] |
Thursday, March 16, 2006 12:15PM - 12:27PM |
V39.00006: Type-IV Superconductivity Phenomenon: Cooper Pairs with Broken Parity and Spin-Rotational Symmetries in D- and S-wave Singlet Superconductors Andrei Lebed Paramagnetic effects are shown to result in the appearance of a triplet component of order parameter in vortex phases of d- and s-wave singlet superconductors in the absence of impurities. This component, which breaks both parity and spin-rotational symmetries of Cooper pairs, is expected to be of the order of unity in a number of modern superconductors such as high-Tc, organic, MgB$_{2}$, and some others. A generic phase diagram of such type-IV superconductors [1], which are singlet ones at H=0 and in the Meissner phase and characterized by singlet-triplet mixed Copper pairs, $\Delta _s +i\Delta _t $, in a vortex phase, is suggested. \newline \newline [1] A.G. Lebed, Physical Review Letters, accepted (2006). [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 12:39PM |
V39.00007: Superfluid density and competing orders in $d$-wave superconductors Sergei Sharapov, Jules Carbotte We derive expressions for the superfluid density $\rho_s$ in the low-temperature limit $T \to 0$ in $d$-wave superconductors, taking into account the presence of competing orders such as spin-density waves, $is$-pairing, etc. Recent experimental data for the thermal conductivity and for elastic neutron scattering in La$_{2-x}$Sr$_x$CuO$_4$ suggest there are magnetic field induced anomalies that can be interpreted in terms of competing orders. We consider the implications of these results for the superfluid density and show in the case of competing spin-density wave order the usual Volovik-like $\sqrt{H}$ depletion of $\rho_s(H)$ is replaced by a slower dependence of $\rho_s$ on the applied magnetic field $H$. We show that it is crucial to include the competing order parameter in the self-consistent equation for the impurity scattering rate. [Preview Abstract] |
Thursday, March 16, 2006 12:39PM - 12:51PM |
V39.00008: Index theorem and quantum order of d-wave superconductors in the vortex state Oskar Vafek, Ashot Melikyan It is argued that the ground state of the lattice d-wave superconductor in the lattice of singly quantized $hc/2e$ vortices can posses quantum order. This is due to simultaneous crystallization of both the superflow and the fluxes of the $Z_2$ gauge field, the second of which has no classical analog. In the presence of particle hole symmetry we prove an index theorem which puts a lower bound on the number of zero energy modes. This bound is equal to 4 times the number of lattice sites coinciding with the centers of inversion within the magnetic unit cell. Generic cases are constructed in which this bound exceeds the number of zero modes of an equivalent lattice of doubly quantized vortices (as well as singly quantized vortices without Z2), despite identical classical symmetries. The spectrum around the zero modes is doubly degenerate and Dirac-like, with velocities that become universal functions of $\Delta/t$ in the limit of low magnetic field. For small, but finite strength of particle-hole symmetry breaking, the gapped state can be characterized by a topological quantum number, related to spin Hall conductivity, which differs in the case of $hc/2e$ and $hc/e$ vortex lattice. [Preview Abstract] |
Thursday, March 16, 2006 12:51PM - 1:03PM |
V39.00009: Electronic Density of States of a Superconductor with a Spatially Varying Gap and Phase Fluctuations: A Monte Carlo Simulation Daniel Valdez-Balderas, David Stroud Recent experiments have shown that the superconducting energy gap in some high-T$_c$ superconductors is spatially inhomogeneous. Motivated by these experiments, and using exact diagonalization of a model d-wave Hamiltonian (T. Eckl et al. PRB 66 140510), combined with Monte Carlo simulations of a Ginzburg-Landau free energy functional, we have calculated the electronic density of states $n(E)$ of a model high-T$_c$ superconductor with an inhomogeneous gap. The free energy functional incorporates both phase and amplitude fluctuations together with quenched disorder. It leads to a superconducting transition temperature $T_c$ well below the pseudogap temperature $T_{c0}$, and has a spatially varying gap at very low $T$, both consistent with experiments in underdoped Bi2212. Our calculated $n(E)$ shows coherence peaks for $T < T_c$, which disappear for $T > T_c$. We will also present calculated results for both the global and local $n(E)$ as a function of temperature and disorder. [Preview Abstract] |
Thursday, March 16, 2006 1:03PM - 1:15PM |
V39.00010: Raising Bi-O bands above the Fermi energy level of hole-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and other cuprate superconductors Hsin Lin, S. Sahrakorpi, R.S. Markiewicz, A. Bansil The Fermi surface (FS) of Bi$_2$Sr$_2$CaCu$_2$O$_8$ (Bi2212) predicted by band theory displays Bi-related pockets around the $(\pi,0)$ point, which have never been observed experimentally. We show that when the effects of hole doping by substituting Pb for Bi or by adding excess O in Bi2212 are included, the Bi-O bands are lifted above the Fermi energy ($E_F$) and the resulting first-principles FS is in remarkable accord with measurements. With decreasing hole-doping the Bi-O bands drop below the $E_F$ and the system self-dopes below a critical hole concentration. Computations on other Bi- as well as Tl- and Hg-based compounds indicate that lifting of the cation-derived band with hole doping is a general property of the electronic structures of the cuprates. Work supported by the USDOE. [Preview Abstract] |
Thursday, March 16, 2006 1:15PM - 1:27PM |
V39.00011: Evolution of Mid-gap States and Residual 3-Dimensionality in Cuprates S. Sahrakorpi, M. Lindroos, R. Markiewicz, A. Bansil We have carried out extensive first principles doping-dependent computations of angle-resolved photoemission (ARPES) intensities in Bi$_2$Sr$_2$CaCu$_2$O$_8$ (Bi2212) and La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over a wide range of binding energies.$^{1,2,3}$ Intercell hopping and the associated 3-dimensionality, which is usually neglected in discussing cuprate physics, is shown to play a key role in shaping the ARPES spectra. Despite the obvious importance of strong coupling effects (e.g. the presence of a lower Hubbard band coexisting with mid-gap states in the doped insulator), a number of salient features of the experimental ARPES spectra of LSCO are captured to a surprising extent when $k_z$-dispersion is properly included in the analysis. \\ \\ 1. S. Sahrakorpi, et al., Phys. Rev. Lett. 95, 157601 (2005).\\ 2. A. Bansil, et al., Phys. Rev. B 71, 012503 (2005).\\ 3. A. Bansil, et al., New Journal of Physics 7, 140 (2005); http://www.iop.org/EJ/abstract/1367-2630/7/1/140. [Preview Abstract] |
Thursday, March 16, 2006 1:27PM - 1:39PM |
V39.00012: Dirac-Bogoliubov-de Gennes quasiparticles in the mixed state of lattice d-wave superconductors Ashot Melikyan, Zlatko Tesanovic In the mixed state of cuprate superconductors the low energy quasiparticles can be effectively described as Dirac fermions moving in the field of singular scalar and vector potentials. Although the effective linearized Hamiltonian operator formally does not depend on the structure of vortex cores, singular nature of the perturbation requires imposing additional boundary conditions at vortex locations. Each vortex is described by a parameter $\theta$ that effectively represents all effects beyond linearization. We identify the parameters $\theta$ of the solutions found earlier, analyze the relation between fully self-consistent formulation of the problem and the linearized model, and construct the low-field scaling form of the quasiparticle spectra incorporating the parameters $\theta$ explicitly and generalizing the conventional Simon-Lee scaling. We present a detailed numerical study of the lattice $d$-wave superconductor model and examine its low energy, low magnetic field behavior. The low energy limit is faithfully represented by Dirac-like fermions which are either truly massless, massless at the linearized level or massive, depending on the commensuration of magnetic length and lattice spacing. [Preview Abstract] |
Thursday, March 16, 2006 1:39PM - 1:51PM |
V39.00013: Phase Fluctuations in Strongly Coupled d-Wave Superconductors Matthias Mayr, Gonzalo Alvarez, Cengiz Sen, Elbio Dagotto We present a numerically exact solution for the BCS Hamiltonian at any temperature, including the degrees of freedom associated with classical phase, as well as amplitude fluctuations via a Monte Carlo integration. This allows for an investigation over the whole range of couplings: from weak attraction, as in the well-known BCS limit, to the mainly unexplored strong-coupling regime of pronounced phase fluctuations. In the latter, two characteristic temperatures T$^*$ and T$_c$, associated with short- and long-range ordering, respectively, can be identified in a mean-field-motivated Hamiltonian. T$^*$ at the same time corresponds to the opening of a gap in the excitation spectrum. In addition to introducing a novel procedure to study strongly coupled $d$-wave superconductors, our results indicate that classical phase fluctuations are not sufficient to explain the pseudogap features of high-temperature superconductors. [Preview Abstract] |
Thursday, March 16, 2006 1:51PM - 2:03PM |
V39.00014: Excitonic pairing between nodal fermions William Shively, Dmitri Khveshchenko We discuss excitonic pairing in nodal fermion systems, characterized by a vanishing quasiparticle density of states at the point-like Fermi surface and a concomitant lack of screening for long-range interactions. We solve the gap equation and obtain the free energy and critical values of the interaction strength for a variety of algebraically interactions and power-law densities of states. On the basis of this insight, we analyze possible phase transitions in this interesting class of non-fermi-liquid systems. [Preview Abstract] |
Thursday, March 16, 2006 2:03PM - 2:15PM |
V39.00015: Nodal Quasiparticles at a Nematic Quantum Critical Point Paul Oreto, Eun-Ah Kim, Steven Kivelson, Eduardo Fradkin The nodal quasiparticles in the high-Tc superconductors are robust consequences of the d-wave symmetry. Under most circumstances, the properties of these Dirac-like particles are generally simple, since most interactions involving them are ``irrelevant.'' However, they are strongly scattered by the symmetry breaking fluctuations near an electronic nematic quantum critical point. In the quantum critical regime, an anomalous damping and enhancement of the dispersion anisotropy of the quasi-particles results. Possible relevance of this to the so-called ``Fermi arc'' seen in the underdoped cuprates is discussed. [Preview Abstract] |
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