Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G39: Focus Session: Superconductivity: Theory and Computation I |
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Sponsoring Units: DCMP DCOMP Chair: Alexander Balatsky, Los Alamos National Laboratory Room: Baltimore Convention Center 342 |
Tuesday, March 14, 2006 8:00AM - 8:36AM |
G39.00001: d-wave Pairing in the Two-Dimensional Hubbard Model Invited Speaker: In this talk we will review numerical evidence showing that the 2D Hubbard model can exhibit antiferromagnetism, stripes, d(xx-yy) pairing and pseudogap behavior. These numerical studies show that the different phases are delicately balanced with respect to small changes in parameters, reminding one of the cuprate materials. We will then discuss the structure of the pairing mechanism in this model. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G39.00002: Andreev states in 2D finite size systems Lucian Covaci, Frank Marsiglio Andreev reflection occurs at the interface between a normal metal and a superconductor. An electron with energy lower than the superconducting gap is reflected back as a hole while a pair enters the superconductor. In 1D systems, for energies smaller than the superconducting gap, the allowed states in the normal metal layer are quantized. For 2D systems these energies become continuous due to the extra degree of freedom in the direction parallel to the interface. With the use of a mean field extended Hubbard Hamiltonian to describe s-wave and d-wave superconductors, we employ a Lanczos recursion method to calculate the local density of states in these systems. Different shapes are considered, such as squares, triangles and circles. We observe the finite size properties of the Andreev bound states and present the different modes of the localized states. We also consider the effect of rough surfaces on the formation of the bound states. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G39.00003: $d$-wave Pairing Amplitude and Particle-hole Asymmetry of the $t-J$ type models: Implication from the Spectral Weight Analysis Chung-Pin Chou, Ting-kuo Lee, Chang-Ming Ho By using the projected variational wave functions (VWF), we study the quasiparticle spectral weight (QPSW) of hole-doped cuprates in the $t-J$ type models. The QPSW $Z^{+}$ for adding an electron shows a pocket structure outside the Fermi surface that is related to the momentum distribution function $n_{\mathbf{k}}$. Based on our VWF, $Z^{-}$ for removing an electron can be proven to have a simple relation among the pairing amplitude and $Z^{+}$. In addition, we also find that strong correlations mainly causes the particle- hole asymmetry of the low-energy tunneling spectra. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G39.00004: The Structure of the Pairing Interaction in the 2D Hubbard Model Thomas Maier, Mark Jarrell, Douglas Scalapino We present a detailed analysis of the effective pairing interaction in the doped two-dimensional Hubbard model. Using dynamical cluster Monte Carlo calculations we have studied the irreducible particle-particle vertex responsible for pairing in this model. The leading low temperature eigenvalue of the Bethe-Salpeter equation for the particle-particle channel is shown to have $d_{x^2-y^2}$-wave symmetry. The irreducible particle-particle vertex increases with increasing momentum transfer and decreases when the energy transfer exceeds a scale associated with the $Q=(\pi,\pi)$ spin susceptibility. Using an exact decomposition of this vertex into a fully irreducible two-fermion vertex and charge and magnetic exchange channels, the dominant contribution to the effective pairing interaction is found to come from the magnetic, spin $S=1$ exchange channel. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G39.00005: Fluctuation exchange theory for superconductivity of $f$-electron systems with multipole degrees of freedom Katsunori Kubo, Takashi Hotta Recently, an interesting possibility of superconductivity induced by multipole fluctuations has been discussed in some $f$-electron materials. In order to clarify such exotic superconductivity from a microscopic viewpoint, we apply fluctuation exchange (FLEX) approximation to an $f$-electron model with active orbital degree of freedom on the basis of a $j$-$j$ coupling scheme. In this study, we consider a square lattice, for simplicity. First, we evaluate orbital dependent fluctuations and effective paring interactions within the FLEX approximation. Then, we determine the symmetry of the gap function among possible superconducting states by solving the Eliashberg equation. In particular, we pay our attention to the effect of crystalline electric field on the appearance of superconductivity. By further decomposing complex orbital dependent fluctuations into multipole components, we also discuss possible relevance of multipole fluctuations to exotic superconductivity. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G39.00006: New Phases, Superfluid Weights, Free Carrier Densities: RG Theory of Hubbard, tJ Models A. Nihat Berker, Michael Hinczewski We have studied the Hubbard and tJ models in d=3 with renormalization-group theory, obtaining phase diagrams and thermodynamic properties for all temperatures, densities, and coupling strengths. In the Hubbard model, at low temperatures and around half filling, the antiferromagnetic phase is obtained. (1) At strong coupling and 30-35\% doping from half filling, (2) at weak and intermediate coupling and 10-18\% doping, two novel phases ($\tau$ phases) were found. In these phases, the hopping expectation value is non-zero at all length scales. The weak-intermediate coupling $\tau$ phase exhibits, as in BCS superconductivity, an excitation spectrum gap and, in the specific heat, a low-temperature exponential decay and a cusp phase transition singularity. The strong coupling $\tau$ phase exhibits, as in BEC superconductivity, in the specific heat, an $\alpha \sim -1 $ phase transition singularity and a pair-formation peak above the phase transition temperature. In the tJ model, we find that the superfluid weight increases with hole doping, passes through a maximum within the $\tau$ phase at 32-37\% doping, and decreases, and that the free carrier density also increases to a maximum value at 32-37\% doping but remains at this value under further doping. Calculations with spatially anisotropic d=3 systems yield a chemical potential shift as a function of hole doping in good agreement with experiments. Recent results with magnetic and non-magnetic impurities will also be presented. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G39.00007: Pseudogap and antiferromagnetic correlations in the Hubbard model Alexandru Macridin, Mark Jarrell, Thomas Maier, Paul Kent, Carsten Honerkamp Using the dynamical cluster approximation we calculate the single-particle spectra of the Hubbard model with next-nearest neighbor hopping t' at small doping. We find that the pseudogap along the zone diagonal in the electron doped systems is due to long range antiferromagnetic correlations. The physics in the proximity of $(0,\pi)$ is dramatically influenced by t'and determined only by the short range correlations. The effect of t' on the low energy ARPES spectra is weak except close to the zone edge. The short range correlations are sufficient to yield a pseudogap in the magnetic susceptibility and produce occupied states in the second antiferromagnetic Brillouin zone which develop a gap with decreasing temperature. We compare our self-energy with the one obtained form renormalization group (RG) calculations. In order to analyze the importance of scattering in different channels we analyze the self energy using a simple ansatz motivated by RG calculations. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G39.00008: Nonmonotonic gap in the coexisting antiferromagnetic and superconducting state for electron-doped cuprates Qingshan Yuan, Feng Yuan, Chin-Sen Ting Recent measurements on the superconducting (SC) electron-doped cuprates Nd$_{1.85}$Ce$_{0.15}$CuO$_4$ and Pr$_{0.89}$LaCe$_{0.11}$CuO$_4$ (both at optimal doping) have revealed that the excitation gap does not fit the simplest commonly assumed $d$-wave function $\cos k_x - \cos k_y$, but exhibits a nonmonotonic behavior with the gap maxima locating midway between the Brillouin zone boundaries and the zone diagonals. This observed gap was naturally regarded as the SC gap and the previous theoretical explanations were limited to extending the SC gap out of the simplest $d$ wave. Here we propose a new idea that the observed gap at optimal doping is the lowest quasiparticle excitation energy in the {\it coexisting} antiferromagnetic (AF) and SC state, which is not purely the SC gap. The idea is implemented by simply studying the coexistence of AF and SC orders within the slave-boson mean-field approach based on the $t$-$t'$-$t''$-$J$ model. Although the pairing gap itself is assumed to be the simplest $d$ wave which is monotonic, we have found that the quasiparticle excitation gap in the coexisting state is nonmonotonic, with the maxima around the hot spots where the Fermi surface is missing due to the AF gap. Within the same coexisting state the spectral function is also calculated at optimal doping. The obtained results are all consistent with experiments. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G39.00009: Hole correlation in the $t-J$ model with four holes on a 32-site lattice P. W. Leung We study the $t-J$ model with four holes on a 32-site square lattice using exact diagonalization. This system corresponds to doping level $x$=1/8. At the ``realistic'' parameter $J/t$=0.3, holes in the ground state of this system are unbound. They have short range repulsion due to lowering of kinetic energy. There is no antiferromagnetic spin order and the electron momentum distribution function resembles hole pockets. Furthermore, we show evidence that in case antiferromagnetic order exists, holes form $d$-wave bound pairs and there is mutual repulsion among hole pairs. This presumably will occur at low doping level. This scenario is compatible with a checkerboard-type charge density state proposed to explain the ``1/8 anomaly'' in the LSCO family, except that it is the ground state only when the system possesses strong antiferromagnetic order. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G39.00010: LDOS and Angle-Resolved Photoemission Spectral Function of An inhomogeneous Superconductor Ming Cheng, W.P. Su Nanoscale inhomogeneity seems to be a central feature of the d-wave superconductivity in the cuprates. Such a feature can strongly affect the local density of states (LDOS) and the spectral weight functions. Within the Bogoliubov-de Gennes formalism we examine various inhomogeneous configurations of the superconducting order parameter to see which ones better agree with the experimental data. Nanoscale large amplitude oscillations in the order parameter seem to fit the LDOS data for the underdoped cuprates. The one-particle spectral function for a general inhomogeneous configuration exhibits a coherent peak in the nodal direction. In contrast, the spectral function in the antinodal region is easily rendered incoherent by the inhomogeneity. This throws new light on the dichotomy between the nodal and antinodal quasiparticles in the underdoped cuprates. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G39.00011: Distinguishing Patterns of Charge Order in 2 dimensions; Stripes or Checkerboards John Robertson, Steven Kivelson, Aharon Kapitulnik, Eduardo Fradkin Charge ordered states are common in strongly correlated materials, including especially the cuprate high temperature superconductors. Identifying where such phases occur in the phase diagram, and where they occur as significant fluctuating orders is a critical step in understanding what role they play in the physics. However, the both the presence of quenched disorder, and the smallness of the charge modulations make the detection of such order difficult in experiments. We discuss strategies for identifying the nature of the underlying ``clean'' order (such as ``stripes'' or ``checkerboards'') in experiments, particularly in STM. We use a model of an effective Hamiltonian in the presence of quenched disorder to simulate experimental data, and further apply our techniques to real experimental data. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G39.00012: Inhomogeneous states with checkerboard order in the t-J Model Chunhua Li, Sen Zhou, Ziqiang Wang We study inhomogeneous states in the $t$-$J$ model using an unrestricted Gutzwiller approximation. We find that $pa\times pa$ checkerboard order, where $p$ is a doping dependent number, emerges from Fermi surface instabilities of both the staggered flux phase and the Fermi liquid state with realistic band parameters. In both cases, the checkerboard order develops at wave vectors $(\pm 2\pi/pa,0)$, $(0,\pm2\pi/pa)$ that are tied to the peaks of the wave-vector dependent susceptibility, and is of the Lomer-Rice-Scott type. The properties of such periodic, inhomogeneous states are discussed in connection to the checkerboard patterns in the local tunneling density of states discovered in underdoped cuprates. [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G39.00013: Hidden D-Wave Checkerboard Order in Cuprates Jiangping Hu, Kangjun Seo We propose a new hidden order, D-wave checkerboard order, to explain the local charge checkerboard ordering observed in STM studies of the High Tc superconductors. We show that even a weak D-wave checkerboard order can have a strong effect on the STM spectrum in superconducting states and can nicely explain experimental observations. The D-wave checkerboard order also generates a Fermi arc with little dispersion around nodal points, which are consistent with results from angle resolved photoemission spectroscopy measurements. Therefore, the D-wave checkerboard order can naturally connect the pseudo-gap physics with the checkerboard structure featured in STM measurements. [Preview Abstract] |
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