Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session S11: Theory of Superconductivity in Cuprates I |
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Sponsoring Units: DCMP Chair: Alexander Balatsky, Los Alamos National Laboratory Room: Morial Convention Center RO9 |
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S11.00001: Cuprate higher harmonic gap structure: theory vs. experiment David Parker, Alexander Balatsky We present a detailed comparison to experiment of the generalized gap symmetry predictions of spin-fluctuation mediated superconductivity theory in the hole-doped and electron-doped cuprates, within a weak-coupling BCS framework. We comment on the implications of these results for the ongoing ``one gap vs two gap'' controversy in the cuprates, and discuss the impact of these results on the quasiparticle lifetime model of the cuprate Fermi arcs. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S11.00002: Sublattice model of atomic scale pairing inhomogeneity in a superconductor Vivek Mishra, P.J. Hirschfeld, Yuri S. Barash We study a toy model for a superconductor on a bipartite lattice, where intrinsic microscopic inhomogeneity is produced by two different pairing coupling constants on each sublattice. We consider effects of the inhomogeneity on the transition temperature, the density of states, the specific heat and superfluid density in the framework of the Bogoliubov-de Gennes equations, which may be solved analytically in several interesting cases. The phase diagram in the plane of two pairing coupling constants is found to include a state of gapless superconductivity. [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S11.00003: Suppression of $d$-wave superconductivity in the weakly inhomogeneous checkerboard Hubbard Model D.G.S.P. Doluweera, M. Jarrell, Th. Maier, A. Macridin, Th. Pruschke Using a dynamical cluster quantum Monte Carlo approximation we investigate the $d$-wave superconducting transition temperature T$_{c}$ of the doped 2D Hubbard model with a weak inhomogeneity in the form of checkerboard pattern in the hoppings. The hopping within a 2 $\times $ 2 cluster (plaquette) is $t$ and the hopping between the plaquettes is $t'$ ( 0.8$t \le $ \textit{t' $\le $ t }). We find T$_{c}$ decreases monotonically with decreasing $t' $ for both fixed $U/t$ or $U/W$ ($U$ the on site Hubbard interaction and $W$ the bandwidth). The characteristic spin excitation energy scale and the strength of $d$-wave pairing interaction decrease with decreasing T$_{c}$ suggesting a strong correlation between these two quantities. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S11.00004: Electronic properties investigation of YBaCuO using the PAW formalism Simon Pesant, Michel C\^ot\'e Using density-functional theory (DFT) and the projector-augmented wave (PAW), we characterize the electronic properties of the YBa$_{2}$Cu$_{3}$O$_{7-x}$. These systems are metallic or antiferromagnet at room temperature depending on the hole doping induced by oxygen atoms in the basal plane. Also, the impact of an onsite coulomb repulsion term on the DFT know as LDA+U, is investigated in the different structures to take into account the highly correlated character of the electrons of those systems. The addition of an onsite coulomb repulsive term does not strongly alter the electronic properties of the YBa$_{2}$Cu$_{3}$O$_{7}$, and YBa$_{2}$Cu$_{3}$O$_{6.5}$ but gives more accurate details about the electronic structure. In the case of YBa$_{2}$Cu$_{3}$O$_{6}$, the impact of the U term is primordial, the anti-ferromagnetism phase being recovered when LDA+U is used, compared to the standard LDA where it is metallic. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S11.00005: Electronic properties and chain conductivity of underdoped YBa$_2$Cu$_3$O$_{6+x}$ by First-Principles calculations Vincenzo Fiorentini, Alessio Filippetti, Giorgia Lopez, Mauro Mantega Metal-insulating transitions in cuprates represent an historical challenge for first-principles calculations. Here we present results obtained through the pseudo-self-interaction free density functional scheme (PSIC) that is capable to correct the gross failures of LSDA at just a moderate increase of computing effort, and works well in both strong-correlated and metallic limit. Here we describe the properties of the end-point systems YBa$_2$Cu$_3$O$_6$ and YBa$_2$Cu$_3$O$_7$ as well as the chemistry of insulating-metal transition occurring in the CuO chains of underdoped YBa$_2$Cu$_3$O$_{6+x}$ in the region x=[0,0.5]. Coherently with the one-dimensional metallic percolative regime observed at low-doping, we find that the metal-insulating transition occurring at low doping in the non-magnetic Cu(1)Ox chains is induced by chain-like alignment of the doping oxygens within the chains, whereas disorder (i.e. non chain-aligned) distributions are always insulating. In the Cu(2)O2 planes the insulating antiferromagnetic state remains stable up to x=0.25, while at x=0.5 a normal-metal state, similar to that seen for YBa$_2$Cu$_3$O$_7$, take place. The in-plane antiferromagnetic- paramagnetic competition depends on x but is almost unaffected by the intra-chain order-disorder competition. [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S11.00006: Holes localization and Fermi Surface morphology of Y$_{1-x}$Ca$_x$BaCu$_3$O$_6$ by First-Principles Giorgia Lopez, Alessio Filippetti, Vincenzo Fiorentini The basic chemistry of underdoped Y$_{1-x}$Ca$_x$BaCu$_3$O$_6$ (and of high-T$_c$ superconductors) is permeated by unexplained features. One, in particular, concerns the nature of the non- superconducting phase and its Fermi Surface (FS) whose character is not univocally described by angle-resolved photoemission and Hall measurements, which detect disconnected arcs and pockets, respectively. But what is really missing for a sound interpretation of these data is a robust link between the observed FS and the corresponding electronic structure. As the description of the underdoped regime is hardly accessible through standard First-Principles calculations (FPC), here we use the innovative pseudo-self-interaction corrected local spin density (PSIC) approach to trace an accurate overview of underdoped Y$_{1-x}$Ca$_x$BaCu$_3$O$_6$, with special emphasis on the FS morphology. In particular, the PSIC can predict the formation of Zhang-Rice singlets (ZRS), and we show that electronic states characterized by a mixture of ZRS and antiferromagnetic CuO$_2$ units present, in fact, a variegated series of differently-shaped, disconnected FS that may reconcile the experimental data with a sound interpretation of the underdoped Y$_{1-x}$Ca$_x$BaCu$_3$O$_6$ properties. [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S11.00007: Nature of high-temperature superconductivity John D. Dow Using muon spectroscopy, the high-temperature superconductivity of YBa(2)Cu(3)O(7) is shown to reside in its BaO layers, not in its cuprate planes. The symmetry of the hole-pairing is s-like, not d-like. The family of superconductors Pb(2)Sr(2)Y(1-x)R(x)Cu(3)O(8) can be doped p-type (with R=Ca) or n-type (with R=Ce or Am). The n-type versions do not superconduct, but the p-type compounds do superconduct. The doped ruthenate Ba(2)YRuO(6) begins superconducting in its BaO layers at 92 K. A successful theory of high-temperature superconductivity must explain the ruthenates, the superconducting organic compounds, and the superconducting cuprates. Presently none do. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S11.00008: Percolation Theory of the Pseudogap Phase Alexei Abrikosov A concept of the pseudogap state in high-$T_c $ layered cuprates on the basis of percolation theory is proposed. Contrary to the self-consistent BCS critical temperature, which defines $T^{\ast}$ - the upper boundary of the psesudogap state, the real critical temperature, $T_c$, is defined, as the percolation threshold, where the infinite cluster appears. This permits to obtain the exact formula for $T_c$, as function of doping and understand its ``dome-like'' shape. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S11.00009: Pairing in Non-Fermi Liquids in Terms of Bethe-Salpeter Equation Yuriy Malozovsky, J.D. Fan The pairing between two fermionic excitations in a non-Fermi liquid is considered in terms of the Bethe-Salpeter equation. We consider the pairing in the Fermi systems with vanishing spectral weight. It is well known that the quasiparticle pole in the single-particle Green's function in non-Fermi liquids is absent or weak. The examples of such systems can be viewed as the ``Marginal'' Fermi liquid and Luttinger liquid. Another example that has also been considered is the Fermi system with pseudogap behavior in the spectral weight. Although the pairing between two excitations in non-Fermi liquids is, in general, absent, yet we show that the Cooper's type pairing can occur and the conditions for such pairing are discussed. The results have also application to the systems with smeared or non-monotonic Fermi distribution. The system that transits from the Fermi type to Bose type behavior can be a showcase of such systems as discussed. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S11.00010: Multiple Pairing in the BCS Model J.D. Fan, Yuriy Malozovsky We study Cooper's paring for more than two particles in terms of the BCS model. We consider the multiple pairing in terms of the BCS Hamiltonian in a quiescent Fermi sea model and in the BCS ground state. Although there is no interaction between Cooper pairs in terms of the BCS Hamiltonian, yet we show that four particles are paired and form a bound state in the singlet state with just twice the bound state energy of a single Cooper's pair. The four-particle bound state only exists as the result of the Pauli principle and the sharp Fermi edge. We have also shown that the smearing of the Fermi edge due to $\Delta \left( k \right)$ as it is in the BCS ground state weakens the pairing of either two or four particles. We show that in the particle-hole channel there exists the multiple particle-hole resonance for four particles and four holes in a quiescent Fermi sea model similar to the case of two particles and two holes resonance. There is no particle-hole resonance in the BCS ground state as shown, which means that the particle-hole resonance is removed by $\Delta \left( k \right)$ due to smearing of the Fermi distribution. The wave function for the multiple-pairing model is discussed as well. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S11.00011: Extraction of the pairing glue spectra of high Tc superconductors Han-Yong Choi, Jaehyun Yoon, Takeshi Kondo, Adam Kaminski, Chandra Varma We report the current progress of extracting the ``pairing glue'' spectra of high Tc superconductors. This is done by inverting the d-wave Eliashberg equation, which is an extension of the McMillan-Rowell analysis of the tunneling conductance for conventional s-wave superconductors. A major difference is that there are two distinct $\alpha ^2F$ functions for the d-wave superconductors. Consequently, we need twice more experimental inputs to perform this analysis; the pairing function $\Delta (\omega )$ and self-energy $\Sigma (\omega )$. This experimental information is currently not available. We therefore first generate $\Delta (\omega )$ and $\Sigma (\omega )$ theoretically using the marginal Fermi liquid like glue spectra. Then, using the generated functions as ``experimental inputs'' we invert the Eliashberg equation to extract the glue spectra. We will compare the input and extracted glue spectra to demonstrate the applicability of the approach. We will next describe how to obtain experimental $\Delta (\omega )$ and $\Sigma (\omega )$ from ARPES data. Then, using the obtained experimental inputs we invert the Eliashberg equation to extract the glue spectra. The results will be reported for several temperatures above and below Tc. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S11.00012: A competing order scenario of two-gap behavior in hole doped cuprates Tanmoy Das, R.S. Markiewicz, A. Bansil Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature $T_c$, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations[1] which show that most of the characteristic features observed in the recent angle-resolved photoemission (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a non-superconducting origin in a competing phase. Our analysis indicates that, near optimal doping, superconductivity can quench the competing order at low temperatures, and that some of the key differences observed between the STM and ARPES results can give insight into the superlattice symmetry of the competing order. Work is supported in part by the USDOE. \newline [1] Tanmoy Das, R. S. Markiewicz, and A. Bansil, arXiv/0711.0480. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S11.00013: Local quasiparticle lifetimes in a d-wave superconductor S. Graser, P.J. Hirschfeld, D.J. Scalapino Recently, scanning tunnelling spectroscopy (STS) measurements have exhibited good fits to conductance spectra at the surface of Bi-2212 using a BCS-type model for a d-wave superconductor and assuming a local quasiparticle scattering rate varying linearly with energy. Employing a model of quasiparticle scattering by impurities and spin fluctuations we argue that the broadening of the local density of states is in general given by the self-energy of the system averaged over a small region. The size of this region at low energies is shown to be significantly larger than a gap ``patch'', a region over which the gap is roughly constant in this system; states measured by STS are therefore very homogeneous in this energy range. At energies above a scale determined by disorder, STS averages over states localized within a gap ``patch'', and lifetimes are correspondingly inhomogeneous. We show that the local self-energy in the impurity-plus-spin fluctuation model can explain the data as well as the phenomenological linear scattering rate extracted from experiment. [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S11.00014: Disordered superconductors: role of interaction strength Raimundo dos Santos, Felipe Mondaini, Thereza Paiva, Richard Scalettar We have considered the half-filled disordered attractive Hubbard model, in which the on-site attraction is switched off on a fraction $f$ of sites, while keeping a finite $U$ on the remaining ones. The configurationally-averaged equal-time pair structure factor has been calculated as a function of temperature, through Quantum Monte Carlo simulations for several $f$ and $U$, and a finite-size scaling {\it ansatz} has been used for the zero-temperature gap. We have found that the system sustains superconductivity in the ground state up to a critical impurity concentration, $f_c$, which increases with $U$, at least up to the largest values of $U$ we have considered. Also, the normalized zero-temperature gap as a function of $f$, for fixed $U$, shows a maximum near $f_m$, within a range of $U$ values, thus indicating that a small amount of disorder can initially enhance superconductivity. We argue that, overall, the observed behavior results from both the breakdown of CDW-superconductivity degeneracy and the fact that free sites tend to ``push'' electrons towards attractive sites. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S11.00015: A singlet-pairing superconductor is always also a super-spin-current-conductor. Chia-Ren Hu A heuristic argument and a simple theory are used to show that, as a fundamental difference between BEC and BCS condensation of fermion pairs, \textit{the later, even for singlet pairing, can carry a sizable dissipation-less spin-current below practically the same T}$_{C}$. The heuristic argument is based on the similarity between a spin-current carried by a singlet-pairing condensate and (coherent) partner changing in a dancing hall. Simple theory: We consider singlet pairing in a normal metal carrying a moderate spin-current, which causes the spin-up- and -down Fermi surfaces (FSs) to be shifted in the momentum space by $\pm $\textbf{q}/2. [(k,$\uparrow )$,(-k,$\downarrow )$]-pairing is clearly still possible over the entire FSs. To favor a spin current in the system, we introduce a vector Lagrange multiplier \textbf{v}$_{sp}$, and add -\textbf{v}$_{sp}$\textbf{$\cdot \Sigma $ }$_{k, \sigma }\sigma $ h\textbf{k c}$_{ k,\sigma }^{\dag }$\textbf{c}$_{ k, \sigma }$to the Hamiltonian. Since time-reversal invariance is not broken, negligible changes to all properties of the singlet-pairing state follow, and the system remains fully gapped. No depairing can be induced even for a sizable spin current. Two experimental tests of this prediction will be discussed. [Preview Abstract] |
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