Volume 32, Issue 4, April 2006
Index of content:
32(2006); http://dx.doi.org/10.1063/1.2199442View Description Hide Description
Pseudogap physics in strongly correlated systems is essentially scale dependent. We generalize the dynamical mean-fieldtheory (DMFT) by introducing into the DMFT equations dependence on the correlation length of pseudogap fluctuations via an additional (momentum-dependent) self-energy . This self-energy describes nonlocal dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT approach is used for the numerical solution of the weakly doped one-band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest- and next-nearest-neighbor hopping. The effective single impurity problem is solved by the numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) the doped Mott insulator and (ii) the case of bandwidth ( is the value of local Coulomb interaction) are considered. Densities of states, spectral functions and ARPES spectra calculated within DMFT show a pseudogap formation near the Fermi level of the quasiparticle band. We also briefly discuss effects of random impurity scattering. Finally we demonstrate a qualitative picture of Fermi surface “destruction” due to pseudogap fluctuations and formation of “Fermi arcs” which agrees well with ARPES observations.
Applying BCS–BEC crossover theory to high-temperature superconductors and ultracold atomic Fermi gases (Review Article)32(2006); http://dx.doi.org/10.1063/1.2199443View Description Hide Description
This review is written at the time of the twentieth anniversary of the discovery of high-temperature superconductors, which nearly coincides with the important discovery of the superfluid phases of ultracold trapped fermionic atoms. We show how these two subjects have much in common. Both have been addressed from the perspective of the BCS—Bose–Einstein condensation (BEC) crossover scenario, which is designed to treat short coherence length superfluids with transition temperatures which are “high” with respect to the Fermi energy. A generalized mean field treatment of BCS–BEC crossover at general temperatures, based on the BCS–Leggett ground state, has met with remarkable success in the fermionic atomic systems. Here we summarize this success in the context of four different cold atom experiments, all of which provide indications, direct or indirect, for the existence of a pseudogap. This scenario also provides a physical picture of the pseudogap phase in the underdoped cuprates which is a central focus of high research. We summarize successful applications of BCS–BEC crossover to key experiments in high systems, including the phase diagram, specific heat, and vortex core STM data, along with the Nernst effect, and exciting recent data on the superfluid density in very underdoped samples.
Pseudogap and high-temperature superconductivity from weak to strong coupling. Towards a quantitative theory (Review Article)32(2006); http://dx.doi.org/10.1063/1.2199446View Description Hide Description
This is a short review of the theoretical work on the two-dimensional Hubbard model performed in Sherbrooke in the last few years. It is written on the occasion of the twentieth anniversary of the discovery of high-temperature superconductivity. We discuss several approaches, how they were benchmarked and how they agree sufficiently with each other that we can trust that the results are accurate solutions of the Hubbard model. Then comparisons are made with experiment. We show that the Hubbard model does exhibit -wave superconductivity and antiferromagnetism essentially where they are observed for both hole- and electron-doped cuprates. We also show that the pseudogap phenomenon comes out of these calculations. In the case of electron-doped high temperature superconductors, comparisons with angle-resolved photoemission experiments are nearly quantitative. The value of the pseudogap temperature observed for these compounds in recent photoemission experiments had been predicted by theory before it was observed experimentally. Additional experimental confirmation would be useful. The theoretical methods that are surveyed include mostly the two-particle self-consistent approach, variational cluster perturbation theory (or variational cluster approximation), and cellular dynamical mean-fieldtheory.
The role of the Coulomb interaction in the formation of superconducting and pseudogap states in cuprate metal-oxides32(2006); http://dx.doi.org/10.1063/1.2199447View Description Hide Description
It is shown that the key role in the mechanism of high-superconductivity in the layered cuprate metal-oxides with anisotropic quasi-two-dimensional electronic spectrum and -wave symmetry of the superconducting order parameter is played by the retarded screened Coulomb interaction and many-body correlations. We argue that the pseudogap observed in these materials is the anisotropicdielectric gap, which appears due to the electron-hole pairing on the flat portions of the Fermi surface in the vicinity of the extended saddle points in the quasiparticle spectrum. This gap coexists with the superconducting gap and is partially suppressed by scattering of current carriers on the charged point defects. The suppression of dielectric gap is analogous to the suppression of superconducting gap by magnetic impurities in “gapless” superconductors. The complete destruction of the pseudogap by charged impurities is assumed to increase significantly.