Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S12: Focus Session: Superconductivity: Theory and Computation II |
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Sponsoring Units: DCOMP DCMP Chair: Oleg Starykh, University of Utah Room: LACC 402A |
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S12.00001: Eliashberg Theory of Multiband Superconductors Invited Speaker: The application of the Eliashberg strong-coupling theory to multiband superconductors is discussed. The results of calculations are presented for the electronic densities of states in different bands, the tunneling conductance, the free energy and the specific heat. Effects of impurity scattering are discussed with particular emphasis on the two-band case relevant for new superconductor MgB2. The results are compared with the experimental data for doped MgB2 samples. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S12.00002: Nanostructures in Unconventional Superconductors Dirk Morr, Nikolaos Stavropoulos Nanoscale structures by design provide the unique opportunity to reveal the wave-like nature of electrons in condensed matter systems. This has led to the discovery of many new quantum phenomena, such as quantum imaging using electronic waves. In this talk I argue that a new class of quantum effects can be expected to emerge when nanoscale impurity structures are coupled to host systems with complex electronic correlations, such as superconductors. Using some recently developed theoretical approaches, I show that this coupling leads to new types of quantum imaging which possess geometry based selection rules. As a result, one can create nanoscale copying machines, suppress pair-breaking effects of magnetic impurities, and project quantum images ``around the corner". Moreover, nanostructure consisting of more complex building blocks, such as molecules, can be used to manipulate the electronic structure of unconventional $d_{x^2-y^2}$-superconductors in a controlled way. This in turn allows us to gain insight into the nature of electronic correlations and to study the role played by collective modes in the appearance of unconventional superconductivity. Finally, I discuss the relevance of these results for recent scanning tunneling experiments on the high-temperature superconductors. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S12.00003: Numerical study of transport through a mesoscopic superconducting device Lucian Covaci, Frank Marsiglio Starting from the tight-binding description of a superconductor, with the use of the extended Hubbard Hamiltonian, we rely on real-space methods to describe the properties of a superconducting device. The Bogoliubov de Gennes equation are solved for the superconducting device and the Keldysh Green's functions are calculated. We use a perturbation method, first introduced by Caroli et al., which considers the connection of two semi-infinite leads to the superconducting device as a perturbation. The leads can be either in the normal state or in the superconducting state, leading to interesting effects on the Andreev processes inside the superconductor. Using this approach we calculate microscopic currents through a 2D superconducting device. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S12.00004: Mesoscopic fluctuations in disordered superconductors with broken time-reversal symmetry Shinsei Ryu, Akira Furusaki, Andreas Ludwig, Christopher Mudry The mesoscopic regime in the problem of Anderson localization is a scaling regime in which disorder effects remain weak. It can be realized in quasi-one and higher dimensional systems. Fluctuations in the global density of states, the local density of states, as well as in the conductance were first studied for conventional metals in the pioneering works of Stone and Lee on the one hand, and Altshuler, Kravtsov, and Lerner, on the other hand. Here we extend the analysis of the conductance fluctuations by Altshuler, Kravtsov, and Lerner to dirty superconductors with broken time-reversal symmetry in near two spatial dimensions. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S12.00005: Effect of superconducting fluctuations on ultrasound in unconventional superconductor M. Mar'enko, C. Bourbonnais, A.-M.S. Tremblay We study the renormalization of sound attenuation and sound velocity by fluctuation Cooper pairs in layered superconductors. We consider the influence of $s$- and $d$-wave symmetry of the fluctuating order parameter, on both longitudinal and transverse phonon modes. We show that both unconventional order parameter symmetry and transverse sound polarization suppress the AL and MT terms, while the DOS contribution is the least affected. The combination of these effects can change the sign of the overall fluctuation corrections above $T_c$. We also compare the results obtained using the Ginzburg-Landau formalism with a microscopic derivation of the fluctuation corrections in $s$- wave superconductors with a momentum-independent scalar electron-phonon vertex. These calculations are motivated by ongoing ultrasound measurements in organic superconductors. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S12.00006: LDA+DCA calculations of cuprate superconductors Paul Kent, Alexandru Macridin, Mark Jarrell, Thomas Schulthess, Ole Krogh Andersen, Tanusri Dasgupta, Ove Jepsen We present calculations of the properties of realistic models of single-layer cuprate superconductors. A multi-band Hubbard model is obtained from downfolded material specific local density approximation (LDA) density functional theory (DFT) calculations. The on-site U is obtained from constrained DFT calculations. The resulting model is solved using the dynamic cluster approximation (DCA) and quantum Monte Carlo, for small clusters. Some of us have previously shown that DCA calculations of the single band Hubbard model, with empirical parameters, reproduce key features of the experimental phase diagram, including the d-wave superconducting region and pseudogap. In the multi-band model, we find a superconducting region, and discuss how the computed transition temperature depends on the downfolded band structure. In model calculations, we test the sensitivity of the transition temperature to changes in the individual hopping terms, including the copper-oxygen and oxygen-oxygen hybridization. Work supported by the Division of Materials Science and Engineering, U.S. Department of Energy, under Contract DE-AC05-00OR22725 with UT-Battelle LLC. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S12.00007: Short-Range Correlation Induced Pseudogap in Strongly Correlated Systems Bumsoo Kyung, Sarma Kancharla, David Senechal, Andre-Marie Tremblay, Marcello Civelli, Gabriel Kotliar We investigate the correlation-driven Mott transition as well as the evolution of the Mott-Hubbard insulator into a correlated metal upon doping in the 2D Hubbard model by means of the Cellular Dynamical Mean Field Theory (CDMFT). By comparing the solutions with and without antiferromagnetic long-range order, it is found that at intermediate to strong coupling the dominant physics is well captured by short range spin correlations in the normal state. These short range spin correlations create two 'additional' bands apart from the familiar lower and upper Hubbard bands in the single particle spectrum. Even a tiny doping into the Mott-Hubbard insulator causes dramatic effects - a jump of the Fermi energy to one of these additional bands and an immediate suppression of the spectral weight in the region that is now at the Fermi energy. This suggests a mechanism for the pseudogap phenomenon observed in the normal state of several cuprates. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S12.00008: Quantum phase fluctuations in a correlated lattice $d$-wave superconductor and Cooper pair density-wave in the underdoped cuprates Ashot Melikyan, Zlatko Tesanovic We introduce and study an XY-type model of thermal and quantum phase fluctuations in a two-dimensional correlated lattice $d$-wave superconductor. We investigate the origin of the charge density-wave of Cooper pairs (CPCDW), which we argue is the state behind the periodic modulation of LDOS discovered in recent STM experiments. We illustrate how Mott-Hubbard correlations near half-filling suppress superfluid density and favor an incompressible state which breaks translational symmetry of the underlying atomic lattice. The formation of CPCDW in such a strongly quantum fluctuating superconductor can naturally be understood as an Abrikosov-Hofstadter problem in a type-II dual superconductor, with the role of the dual magnetic field played by the electron density. The resulting Abrikosov lattice of dual vortices translates into the periodic modulation of the BdG gap function and the electronic density. A 4 $\times$ 4 checkerboard modulation pattern naturally arises as an energetically favored ground state at and near the $x=1/8$ doping and produces LDOS in good agreement with experimental observations. Z. Te\v sanovi\' c, Phys. Rev. Lett. {\bf 93}, 217004 (2004), A. Melikyan and Z. Te\v sanovi\' c, cond-mat/0408344. Supported in part by the NSF grant DMR00-94981. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S12.00009: Gaussian fluctuation corrections to the BCS mean field gap amplitude at zero temperature Simon Kos, Andrew Millis, Anatoly Larkin The leading (Gaussian) fluctuation correction to the weak-coupling zero temperature BCS superconducting gap equation is computed. We find that the dominant contribution comes from the high energies and momenta (compared to the gap) and gives a correction smaller by the weak-coupling factor $gN_0$ than the mean-field terms. This correction is small due to cancellation of singular contributions from the amplitude and phase mode at high energies and momenta. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S12.00010: Quantum Fluctuations and Pairing in Strongly Interacting Fermi Systems: A Tractable Crossing-symmetric Approach Khandker Quader, T.L. Ainsworth We carry out a general study of quantum fluctuations and pairing in 3D Fermi systems using a tractable crossing-symmetric approach. The underlying fermionic interactions are taken to be finite-range, non-local, and of arbitrary strength. Our method allows us to obtain quantum fluctuations, such as density, current, spin, and spin-current fluctuations. Pairing interactions in various pairing channels are obtained self-consistently from the competing quantum fluctuations. Here we explore s, p, and d -wave pairing. The sign of the non-local interaction is found to be important for the types of quantum fluctuations exhibited by a system, and consequently the channel of pairing. Consequences for static spin susceptibility and the effective mass are also discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S12.00011: Metal-superconductor quantum phase transition in ultrathin superconducting cylinders Oskar Vafek, Malcom Beasley, Steven Kivelson We consider a metal-superconductor quantum phase transition induced by a magnetic field in a doubly connected mesoscopic sample i.e. in the geometry of a long cylinder. Since the dynamical critical exponent z=2, the eventual 1D quantum criticality can be accessed controllably by $\epsilon$-expansion starting from the upper critical dimension d=2. We calculate the critical exponents as well as the crossover function for the conductivity and compare it with the existing data of Liu et. al. Science, 294, 2332 (2001). Finally, we discuss the nature of the quantum multi-critical point. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S12.00012: Metal-Insulator Transition from Isotope Effects in the Strongly Correlated Local Jahn-Teller Phonon Systems Jong E. Han Electron-phonon coupling in the strong correlated regime has received a great deal of attention in the past few years, particularly motivated by the suggestion that the phonons in cuprate HTSC systems could play significant role in the $d$-wave pairing. Here we investigate a related model of local Jahn-Teller (JT) phonons interacting with electrons in the strong Coulomb limit, using the dynamical mean-field theory with quantum Monte Carlo technique. It is shown that the isotope effects become important with local JT phonons at low phonon frequency as the local polaron regime is approached. The system eventually goes through a metal-insulator transition driven by the isotope effects in such regime. The JT system is qualitatively different from the the non-JT phonon systems in that the metal-insulator transition in JT coupling is not driven by the strong charge fluctuation, but by internal multiplet fluctuation. [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S12.00013: Specific Heat and Resistivity of materials with complex Fermi surface topology David Quesada New families of superconducting materials share a common property; they all have a complex Fermi surface topology. The later is a result of the complex dynamics of carriers in these systems and the layered crystalline structure of most of them. In this paper, the electronic specific heat and resistivity have been computed in the normal and superconducting state for three models of Fermi surface: 1. a tetragonal tight binding dispersion law including second order hopping element, 2. a tetragonal tight binding dispersion law including second order hopping element and interlayer hopping that depends only on plane momentum, and 3. a hexagonal tight binding dispersion law. The effect of the proximity of the Fermi level to the Van Hove singularity is analyzed, as well as its relevance to the phenomenon of superconductivity. The calculations in the superconducting state are done for d-wave symmetry and d+s order parameters. Results are compared with angle resolved specific heat measurements and angle resolved photoemission spectroscopy. [Preview Abstract] |
Wednesday, March 23, 2005 5:30PM - 5:42PM |
S12.00014: Chiral symmetry breaking in ${QED}_{3}$ in presence of irrelevant interactions: a renormalization group study Kamran Kaveh, Igor Herbut Motivated by recent theoretical approaches to high temperature superconductivity, we study dynamical mass generation in three dimensional quantum electrodynamics (${QED}_{3}$) in presence of irrelevant four-fermion quartic terms. The problem is reformulated in terms of the renormalization group flows of certain four-fermion couplings and charge, and then studied in the limit of large number of fermion flavors $N$. We find that the critical number of fermions $N_c$ below which the mass becomes dynamically generated depends continuously on a weak chiral-symmetry-breaking interaction. One-loop calculation in our gauge-invariant approach yields $N_{c0} = 6$ in pure ${QED} _3$. We also find that chiral-symmetry-preserving mass cannot become dynamically generated in pure ${QED}_{3}$. [Preview Abstract] |
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