Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V32: Focus Session: Superconductivity: Theory and Computation III |
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Sponsoring Units: DCOMP DCMP Chair: A. Bansil, Northeastern University Room: LACC 507 |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V32.00001: Explanation of the Tao effect J.E. Hirsch R. Tao and coworkers discovered that in an applied electric field superconducting microparticles aggregate to form balls of macroscopic dimensions$^{(1)}$. The phenomenon appears to be as general as the Meissner effect. Within the conventional theory of superconductivity electrostatic fields do not penetrate into superconductors and the observed effect would not be expected. We propose an explanation of the effect based on an alternative description of the electrodynamics of superconductors recently proposed$^{(2)}$, that results from the unconventional theory of `hole superconductivity'. In our theory a spontaneous electrostatic field exists inside superconductors and if the sample is not spherical also outside. Experiments to test the theory will be discussed. (1) R. Tao, X. Xu and E. Amr, Physica C 398, 78 (2003) and references therein. (2) J.E. Hirsch, Phys.Rev. B 69, 214515 (2004) and references therein. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V32.00002: Topological Theory of Electron-Phonon Interactions in High Temperature Superconductors J.C. Phillips The large isotope effects in the phonon kinks observed in photoemission spectra (ARPES) of optimally doped cuprate high temperature superconductors (HTSC) seem to suggest that other particles (such as magnons) must be contributing to HTSC. Here we use topological (non-Hamiltonian) methods to discuss the data, emphasizing nanoscale phase separation and the importance of a narrow band of quantum percolative states near the Fermi energy that is spatially pinned to a nonmagnetic \textit{self-organized }filamentary dopant array. Topological discrete, noncontinuum, nonperturbative methods explain the ``miracle'' of an ideal nearly free electron phonon kink in sharply defined nodal quasiparticle states in LSCO at the metal-insulator transition. Finally the universality of the kink energy and Fermi velocity in different cuprates is the result of the marginally elastic nature of these materials, and specifically the isostatic character of the CuO$_2$ planes. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V32.00003: Low Temperature Quasiparticle Transport in a d-Wave Superconductor with Coexisting Charge Density Wave Order Adam Durst, Subir Sachdev, Steven Girvin In light of recent experiments suggesting the coexistence of d-wave superconductivity (dSC) and charge density wave order (CDW) in the high-$T_{c}$ cuprate superconductors, we consider a phenomenological model in which the relative magnitude of the two order parameters can be varied by hand. For a CDW of wavevector $(\pi,0)$, there exists a critical value of the CDW order parameter beyond which the nodes in the energy spectrum of the dSC quasiparticles become fully gapped. Using a Nambu formalism generalized to include the combined effect of the dSC and CDW order parameters, we calculate the low temperature thermal conductivity across this nodal transition. Surprising results are obtained which may be relevant to low temperature thermal transport measurements in the underdoped cuprates. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V32.00004: Incommensurate Order Proximate to a DDW Phase Ivailo Dimov, Chetan Nayak We investigate the conditions under which incommensurate (IC) order can arise in the proximity of a $d_{x^2-y^2}$-density wave (DDW) phase, within the framework of pseudogap models involving competition of DDW and d-wave superconductivity (DSC). Analogously to the case of an antiferromagnetic (AF) Fermi surface instability, we argue that IC DDW order is energetically favorable away from half filling. Various possibilities of IC DDW order are presented. The resulting phase diagrams derived from extended Hubbard models with correlated hopping terms are constructed. We discuss our results in the context of the recent discovery by FT-STS of bond-oriented IC order in the Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ pseudogap as well as the checkerboard patterns found in Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V32.00005: Universality of transition temperatures in families of copper oxide superconductors Angela Kopp, Sudip Chakravarty The transition temperature ($T_c$) of multi-layer cuprate superconductors has an unusual dependence on the number of layers ($n$) per unit cell: it forms a bell-shaped curve peaked at $n=3$. An explanation of this behavior is due to the combined effects of interlayer tunneling and a competing order, the latter effect being enhanced for $n \geq 3$ by a charge imbalance between the layers. We explore this proposal further by examining the mean-field theory of a superconducting order parameter and a competing $d$-density wave (DDW) order parameter. We focus on three effects: interlayer DDW coupling, increased charge imbalance in the five-layer system, and fluctuations of the superconducting order parameter. We find that (1) the DDW order parameters in neighboring layers prefer to couple ``anti-ferromagnetically''---and, surprisingly, the coupling vanishes identically for two layers with order parameters that are ``ferromagnetically'' aligned; (2) both the interlayer DDW coupling and the increased charge imbalance bring the calculation into better agreement with the experimental results; and (3) fluctuations can have a more pronounced effect when they occur in the presence of a competing order parameter. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V32.00006: Series Study of Superconducting Correlations in the 2D t-J Model W. O. Putikka By calculating 12th order high temperature series for all spin singlet pairing correlation functions (s-wave, d$_{x^2-y^2}$ and d$_{xy}$), I investigate the possiblity of superconductivity in the 2D t-J model. In generating the series I allow the pairs to be any size that contributes to the 12th order series. Summing up all contributions, I find a strong $R=0$ response for d$_{x^2-y^2}$ symmetry, but with $R>0$ the correlation function is very small. The $R=0$ response can be shown to be primarily a sum of two point correlators and is thus not indicative of long range superconducting order. By holding the internal degrees of freedom of the pairs fixed I further investigate the $R>0$ correlations. I will discuss the results of these calculations for all symmetries mentioned above. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V32.00007: A fully fermionic mean field theory of the cuprate superconductors Tiago C. Ribeiro, Xiao-Gang Wen We introduce a new, fully fermionic, mean field approach to the extended $tJ$ model, which incorporates both hole-like quasiparticle and spin-charge separated excitations, and argue that it is relevant to both hole and electron doped cuprates. We show this approach leads to a mean field phase diagram that closely resembles the one of the cuprate materials and compare it to additional experimental evidence. This theory provides a new non-BCS mechanism leading to superconductivity. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V32.00008: t-J model on the Shastry-Sutherland lattice Y. F. Cheng, P. W. Leung The spin-1/2 Heisenberg model on the Shastry Sutherland (SS) lattice is a frustrated system that has a spin gap state. Our previous study has shown that the $t$-$J$ model on the SS lattice does not exhibit hole-pairing when the diagonal hopping term t' is positive.[Phys.Rev.B{\bf 69}, 180403 (2004)]. It demonstrates that the existence of a spin gap does not necessarily lead to hole pairing. In this project, we investigate the case where t' is negative. The method we use is numerical diagonalization on a 32-site SS lattice with periodic boundary conditions. Our result shows that in the present case the distortion on the spin background due to a mobile hole is different from the previous case where t' is positive. The frustration due to hopping motion of the hole does not favor dimerized spin order which exists at half-filling. As a result, other spin orders competing with the dimerized spin order may be enhanced. We will also study the possibility of hole-pairing in the present case. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V32.00009: Study of gossamer superconductivity and antiferromagnetism in the extended $t$-$J$-$U$ model Feng Yuan, Qingshan Yuan, C. S. Ting, T. K. Lee We have analytically studied the d-wave superconductivity (dSC) and antiferromagnetism for a two dimensional extended $t$-$J$-$U$ model, by use of a renormalized mean field theory with the Gutzwiller approximation. The purpose of introducing the $U$ term is to partially impose the no double occupancy constraint. The phase diagrams as functions of doping $\delta$ and $U$ are studied. Using the standard value of $t/J=3.0$ and in the large $U$ limit, we show that the antiferromagnetic order emerges and coexists with the dSC in the underdoped region below the doping $\delta\sim0.1$. The dSC order parameter increases from zero as the doping increases and reaches a maximum near the optimal doping $\delta\sim0.15$. In the small $U$ limit, only the dSC order survives while the AF order disappears. The effect of the long-range hopping terms $t^{'}$ and $t^{''}$ on the dSC and antiferromagnetism is also discussed. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V32.00010: Quasiparticle excitation spectra in the doped 2D Hubbard model Yoshiro Kakehashi, Peter Fulde The 2D Hubbard model has been much investigated as the simplest model of the high-Tc cuprate. Nonetheless, the fundamental properties of single-particle excitations from the ground-state have not yet been discussed because of the limitations of the momentum and energy resolutions in the usual numerical techniques. We present here our results of calculations for the ground-state excitation spectra based on the self-consistent projection operator method which has high resolution in both momentum and energy. Calculated excitation spectra for the intermediate Coulomb interaction strength are consistent with those obtained by the QMC technique at finite temperatures from the underdoped region to the overdoped region. The results also verify the hole-like Fermi surface in the underdoped region and the electron-like Fermi surface in the overdoped region. Our approach is consistent with the marginal Fermi liquid (MFL) picture. We found that the MFL with localized Mott-Hubbard subband can persist up to several percent of the doping concentration, and changes with doping discontinously to the normal Fermi-liquid states with the collapse of the lower Hubbard bands. This justifies the violation of Luttinger's theorem in the underdoped region, which was suggested by many previous calculations. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V32.00011: Superconductivity in the 2D Hubbard model Sarma Kancharla, Massimo Capone, Marcello Civelli, Andre-Marie Tremblay, David Senechal, Gabriel Kotliar The superconducting (SC) ground state in the hole- and electron-doped two dimensional Hubbard model is investigated by means of the Cellular Dynamical Mean Field Theory. It is found to extend over a range of doping broadly consistent with the cuprates. The order parameter assumes a dome shape and scales with the magnetic exchange coupling J for U comparable to the bandwidth. Suppression of the order parameter with frustration suggests the pairing is driven by antiferromagnetic (AF) correlations. Increasing proximity to the Mott insulator resulting in a decrease of available carriers outweighs the effect of increasing AF correlations on the underdoped side to suppress the order parameter in comparison to optimal doping. Whereas, on the overdoped side, adding carriers frustrates the AF correlations to suppress the SC order. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V32.00012: Antiferromagnetism and d-wave superconductivity in the one-band Hubbard model using V-CPT A.-M.S. Tremblay, David S\'en\'echal, Pierre-Luc Lavertu, Marc-Andr\'e Marois Using variational cluster perturbation theory (V-CPT) we study the competition between d-wave superconductivity and antiferromagnetism in the the $t$-$t'$-$t''$-$U$ Hubbard model. Large scale computer calculations reproduce the overall ground state phase diagram of the high-temperature superconductors as well as the one-particle excitation spectra for both hole- and electron-doping. We identify clear signatures of the Mott gap as well as of antiferromagnetism and of d-wave superconductivity that should be observable in photoemission experiments. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V32.00013: Surface charging in d-wave superconductors Tamara Nunner, Irina Bobkova, Yuri Barash, Peter Hirschfeld Near surfaces d-wave superconductivity is suppressed and competing order parameters can emerge. Of particular interest is the formation of antiferromagnetism which can be accompanied by a charging of the surface similar to the vortex case. We analyze this scenario for different surface orientations by solving the Bogoliubov-de Gennes equations for a model Hamiltonian with competing antiferromagnetic and d-wave superconducting interactions. In special limits analytical solutions are possible and help to identify the role of Andreev bound states. Finally, we discuss the implications of an angle-dependent surface charge for the critical current through interfaces. [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V32.00014: Disorder-induced Superconductivity in Two-Dimensional Fermionic Model Ji-Woo Lee, Shailesh Chandrasekharan, Harold U. Baranger We study the possibility of inducing superconductivity by introducing a disordered chemical potential in a strongly interacting model of fermions. We work at half filling in two dimensions with a Hamiltonian that has the same symmetries of the Hubbard model. By increasing the on-site attraction we can convert the fermionic model into a Heisenberg spin model. It is well known that at half filling the enhanced SU(2) symmetry of the Hubbard model prevents superconductivity at finite temperatures - a uniform chemical potential is necessary to induce superconductivity. In our previous work, we have used our model to induce superconductivity using such a uniform chemical potential and have studied how disorder uppresses this superconductivity (cond-mat/0411306). Here we study if a disordered chemical potential with zero mean can also induce superconductivity. Our results show that this is indeed possible. While the winding susceptibility decreases monotonically as a function of system size in the absence of a chemcial potential, it flattens out in the presence of disorder. Interestingly, there is more structure to the functional dependence of the susceptibility on system size indicating the existence of a length scale related to the disorder in the problem. This work was supported in part by NSF (DMR-0103003). [Preview Abstract] |
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