Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L8: Superconductivity Theory: Strongly Correlated Systems |
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Sponsoring Units: DCMP Chair: Lia Krusin-Elbaum, IBM Research Room: Colorado Convention Center Korbel 1C |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L8.00001: Superconductivity in Cuprates through Loop Current Fluctuations Chandra Varma The quantum-critical fluctuations of the loop-current order parameter discovered [1] in underdoped cuprates has been derived [2] recently to be of the phenomenological form proposed to produce the marginal fermi-liquid [3]properties in the normal state. The coupling function of these fluctuations to fermions is calculated and an effective particle-particle scattering through exchange of such fluctuations is generated. Partial wave decomposition of this scattering shows attractive interaction in the d-wave pairing channel. The coupling constant and the cut-off of the fluctuations is used to estimate the order of magnitude of $T_c$. Variation of $T_c$ with hole density is also discussed. [1] C.M. Varma, Phys. Rev. {\bf B73}, 155113 (2006); B. Fauque et al., Phys. Rev. Lett, {\bf 96}, 197001 (2006); A. Kaminski, et al., Nature {\bf 416}, 610 (2002). [2] Vivek Aji and C. M. Varma, cond-mat/0610646. [3] C.M. Varma, et al. Phys. Rev. Lett., {\bf 63}, 1996 (1989). [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L8.00002: Superfluid Stiffness, Nodal Quasiparticles and Quantum Phase Fluctuations in Underdoped Cuprates Nandini Trivedi, Rajdeep Sensarma, Mohit Randeria We study the low temperature superfluid stiffness $\rho_s(T;x)$ as a function of hole doping $x$ and temperature $T$ for strongly correlated d-wave superconductors. Using Gutzwiller projected wavefunctions and renormalized mean-field theory (RMFT), we calculate $\rho_s(0;x)$ and show that it scales with the quasiparticle spectral weight $Z$. These analytical results are in excellent agreement with earlier variational Monte Carlo studies [1]. We next show that self-consistent inclusion of the zero point motion of phase fluctuations leads to further suppression of $\rho_s(0;x) $, which now vanishes below a doping level of approximately $5\%$. To determine the $T$-dependence of $\rho_s$ we calculate the current carried by nodal quasiparticles (QP) within RMFT and show that the effective charge of the nodal QP is given by $Z m^\ast/m$. Our analytic formula for the effective charge is in excellent agreement with numerical Monte Carlo results of Nave {\it et al.}~[2]. We will conclude by comparing our results with experiments on underdoped cuprates. \\ \noindent [1] A. Paremakanti, M. Randeria and N. Trivedi, Phys. Rev. Lett. {\bf 87}, 217002 (2001) \\ \noindent [2] C. P. Nave, D. A. Ivanov and P. A. Lee, Phys. Rev. B. {\bf 73}, 104502 (2006) [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L8.00003: Phenomenological theory of the underdoped phase of a high-T$_c$ superconductor Alexei Tsvelik, Andrey Chubukov We model the Fermi surface of the cuprates by one-dimensional nested parts near $(0,\pi)$ and $(\pi,0)$ and unnested parts near the zone diagonals. Fermions in the nested regions form 1D spin liquids, and develop spectral gaps below some $\sim T^*$, but superconducting order is prevented by 1D phase fluctuations. We show that the Josephson coupling between order parameters at $(0,\pi)$ and $(\pi,0)$ locks their relative phase at a crossover scale $T^{**}< T^*$. Below $T^{**}$, the system response becomes two-dimensional, and the system displays Nernst effect. The remaining total phase gets locked at $T_c < T^{**}$, at which the system develops a (quasi-) long-range superconducting order. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L8.00004: Competing ferromagnetism in high temperature copper oxide superconductors Sudip Chakravarty, Angela Kopp, Amit Ghosal While much attention has been paid to the underdoped regime of the hole-doped cuprates because of its proximity to a complex Mott insulating phase, little attention has been paid to the overdoped regime. Experiments are beginning to reveal that the phenomenology of the overdoped regime is just as puzzling. For example, the electrons appear to form a Fermi liquid, but this interpretation is problematic; any trace of Mott phenomena, as signified by incommensurate antiferromagnetic fluctuations, is absent, and the uniform spin susceptibility shows a ferromagnetic upturn.Here we show and justify that many of these puzzles can be resolved if we assume that competing ferromagnetic fluctuations are simultaneously present with superconductivity, and the termination of the superconducting dome in the overdoped regime marks a quantum critical point beyond which there should be a genuine ferromagnetic phase at zero temperature. We propose new experiments, and make new predictions, to test our theory and suggest that effort must be mounted to elucidate the nature of the overdoped regime, if the problem of high temperature superconductivity is to be solved. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L8.00005: Theory of Infrared Hall Conductivity of Electron-doped Cuprates Jie Lin, Andrew Millis It has been proposed by several experiments that the electron-doped cuprate Pr$_{2-x}$Ce$_x$CuO$_{4+\delta}$ undergoes a quantum phase transition to an antiferromegnetic state for the doping x smaller than 0.16. Here, we investigate the infrared Hall conductance of the electron-doped cuprates in the commensurate spin density wave state, using the linear response theory. The qualitative agreement between our results and the available experimental data provides strong evidence in favor of the spin density wave scenario and suggests that the magnitude of the gap is large, while quantitative discrepancies point towards additional physics which may be related to scattering of carriers off spin fluctuations. We also discuss the Hall conductivity sum rule and its connection to the experiments. [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L8.00006: Strong correlations lead to protected low energy excitations in disordered d-wave superconductors Arti Garg, Mohit Randeria, Nandini Trivedi We show that strong correlations play a vital role in protecting low energy excitations in disordered high temperature superconductors. The impurity-induced low-energy density of states (DOS) is greatly reduced in the strongly correlated superconductor compared to d-wave Bogoliubov-deGennes theory which ignores strong correlations. The gapless nodal quasiparticles, and the resulting `V' in the low-energy DOS, are much more robust against disorder compared to the large-gap antinodal excitations. We discuss the relevance of our results to angle-resolved photoemission and scanning tunneling spectroscopy experiments. Reference: A. Garg, M. Randeria, and N. Trivedi, cond-mat/0609666 [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L8.00007: Electron-phonon renormalization in Cuprates. Peihong Zhang, Steven G. Louie, Marvin L. Cohen Electron-phonon (e-ph) renormalization effects in a model cuprate system, CaCuO2, are studied by employing density functional theory based methods. Whereas calculations based on the local spin density approximation (LSDA) predicts negligible e-ph coupling effects of the half-breathing Cu-O bond stretching mode, the inclusion of a screened on-site Coulomb interaction (U) in the LSDA+U calculations greatly enhances the e-ph coupling strength of this mode. The full breathing mode, on the other hand, shows a much weaker e-ph renormalization effect. Enhanced oxygen-p character of the top valence states, together with the (local) antiferromagnetic spin ordering, seems to be responsible for a strong e-ph coupling of the half-breathing mode in the LSDA+U calculations. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L8.00008: ``Underlying Fermi surface'' and violation of Luttinger count in strongly correlated superconductors Mohit Randeria, Rajdeep Sensarma, Nandini Trivedi The question of determining the ``underlying Fermi surface'' (FS) that is gapped out by superconductivity (SC) is of great importance in strongly correlated systems, particularly in view of angle-resolved photoemission (ARPES) experiments. We explore various definitions for the FS in the T=0 SC state using the zero-energy Green's function, the excitation spectrum and the momentum distribution. We examine (i) the d-wave SC in high Tc cuprates, and (ii) the s-wave superfluid in the BCS-BEC crossover. In each case we show [1] that the various definitions agree, to a large extent, but all of them violate the Luttinger sum rule and do not enclose the total electron density. We discuss the important role of chemical potential renormalization and incoherent spectral weight in this violation. We show that the magnitude of the violation scales like $(\Delta/E_f)^2$, and its sign correlates with the electron-like or hole-like topology of the FS. These results are in good agreement with ARPES data on LSSCO [2]. \\ \noindent [1] R. Sensarma, M. Randeria, N. Trivedi, cond-mat/0607006. \\ \noindent [2] T. Yoshida {\it et al.}, cond-mat/0510608. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L8.00009: High Energy features in the photoenission spectra of cuprates Arkady Shekhter, Chandra Varma We calculate the real part of the self-energy of fermions scattering off the quantum critical fluctuations derived for cuprates. At the upper cut-off of the quantum critical fluctuation spectra a logarithmic divergence in the real part of the self-energy occurs. The position of the peak of the one-particle spectral function is pinned near this divergence. This explains the recent high energy features observed in angle-resolved photoemission in cuprates superconductors. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L8.00010: The nature of the two energy scales in underdoped superconducting cuprates Elena Bascones, Belen Valenzuela Raman and ARPES experiments have demonstrated that in superconducting underdoped cuprates nodal and antinodal regions are characterized by two energy scales instead of the one expected in BCS. Using the Yang, Rice and Zhang (YRZ) model, in which pseudogap and superconductivity compete below a critical doping, we find that the antinodal Raman pair-breaking peak shifts to higher frequency with underdoping, follows the antinodal ARPES gap and is closely connected with the pseudogap. Its intensity decreases due to the competition between pseudogap and superconductivity. The nodal scale follows the doping dependence of the superconducting order parameter (cond-mat/0611154). [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L8.00011: ``String excitations'' of a hole in a quantum antiferromagnet and ARPES data Efstratios Manousakis Recently, high resolution angle-resolved photoelectron spectra (ARPES) from cuprates have been reported where an anomalous high-energy dispersion was identified. We suggest that these ARPES results reveal the internal structure of the hole quasiparticle in quantum antiferromagnets and more importantly it is evidence for the existence of ``string-excitations'' which validate early predictions based on the $t-J$ and related models. The following features of the ARPES results are all in agreement with predictions without adjusting any parameters: (a) the energy-momentum dispersion of the string-excitations, (b) the manner in which the spectral weight is transfered to higher energy string excitations, and (c) the vanishing of the quasiparticle spectral weight near the $\Gamma$ point. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L8.00012: Doping dependence of quasi-particle gaps at low hole doping in the Hubbard model Markus Aichhorn, Enrico Arrigoni, Michael Potthoff, Werner Hanke Using the variational cluster approach we investigate the doping dependence of the pseudogap and the superconducting gap in the $t$-$t^\prime$-$U$ Hubbard model at low hole doping and zero temperature. The self energy of the system is calculated on a well suitable reference system for the investigated doping range and provides well defined quasi particles in the nodal region. We show that the pseudogap in the paramagnetic regime decreases with increasing hole doping, whereas the superconducting gap in the superconducting solution shows the opposite doping dependence for low hole doping. Furthermore our calculations suggest that the superconduting pseudogap in the antinodal region can be seen as sort of superposition of the paramagnetic pseudogap and the superconducting gap as measured near the nodal region. Thus, we claim that the occurence of two distinct energy gaps recently found in experiments can naturally be explained by the single-band Hubbard model. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L8.00013: Dynamic mean field theory of the Gutzwiller-projected BCS Hamiltonian: phase fluctuation and pseudogap Kwon Park One of the most prominent problems in high temperature superconductivity is the nature of the pseudogap phase in the underdoped regime and its relationship to phase fluctuations. In this context, the Gutzwiller-projected BCS Hamiltonian is a useful model especially suited for the study of high temperature superconductivity in the underdoped regime due to the fact that there is an exact mapping to the Heisenberg model at half filling and a close connection to the t-J model at low doping in general. To be concrete, we have developed a dynamic mean field theory of the d-wave BCS Hamiltonian with on-site repulsion U. The large U limit corresponds to the Gutzwiller- projected BCS Hamiltonian. Effects of the phase fluctuations are studied as a function of on-site repulsion U and doping x. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L8.00014: Dynamical Cluster Approximation results for the effect of long range hoppings on $T_c$ in Cuprates Ehsan Khatami, Alexandru Macridin, Mark Jarrell The Dynamical Cluster Approximation along with the Quantum Monte Carlo (QMC) algorithm are employed to study the effect of long-range hoppings on the superconducting critical temperature of Cuprates. A two-dimensional $t-t'-t''-$U Hamiltonian describes the physics of copper oxide planes in this model. We perform calculations on $4$-site and $16$-site clusters. The results show a weak dependence of the maximum $T_c$ on the long-range hoppings. We see a suppression of $T_c$ due to $t'$ in the hole-doped systems. $t'$ increases the critical doping (the doping beyond which the superconducting phase disappears) in the hole-doped regime, but this doping value is decreased by including $t''$. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L8.00015: Impurity Induced Kondo-like Screening in Cuprates Wei Chen, Marc Gabay, Peter J. Hirschfeld We study the magnetic response of \(t-t'-J\) model to a single nonmagnetic impurity using slave boson mean field theory, with restricted Bogoliubov-de-Gennes(BDG) method which allows us to deal with the strong correlations and reduction of order parameters around the impurity self-consistently. The temperature dependence of the paramagnetic susceptibility \(\chi\) follows a Kondo-like form \(1/(T+\Theta)\), where the screening temperature \(\Theta\) increases with increasing doping. Both this form and the magnitude of \(\chi\) are consistent with NMR experiments in the normal state of Zn doped YBCO. [Preview Abstract] |
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