Based on the self-consistent T -matrix approximation, the quantum interference (QI) effect is studied with the diagrammatic technique in weaklydisordered two-dimensional crystals with nearly half-filled bands. In addition to the usual 0-mode cooperon and diffuson, there exist π-mode cooperon and diffuson in the unitary limit due to the particle-hole symmetry. The diffusive π-modes are gapped by the deviation from the exactly-nested Fermi surface. The conductivity diagrams with the gapped π-mode cooperon or diffuson are found to give rise to unconventional features of the QI effect. Besides the inelastic scattering, the thermal fluctuation is shown to be also an important dephasing mechanism in the QI processes related with the diffusive π-modes. In the proximity of the nesting case, a power-law antilocalization effect appears due to the π-mode diffuson. For large deviation from the nested Fermi surface, this anti-localization effect is suppressed, and the conductivity remains to have the usual logarithmic weak-localization correction contributed by the 0-mode cooperon. As a result, the dc conductivity in the unitary limit becomes a non-monotonic function of the temperature or the sample size, which is quite different from the prediction of the usual weak-localization theory.PACS number(s): 73.20.Fz, 71.30.+h, 72.15.Rn, 73.20.Jc

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The pressure dependence of electronic structure, phonon frequencies, electron -phonon coupling constant, and the superconducting transition temperature T c in MgB 2 are calculated via zone-center frozen phonon modes calculations using the FP-LAPW method. The calculations are performed from pressure dependent lattice parameters measured experimentally up to 8 GPa. Under quasi-hydrostatic pressure, T c is found to decrease with an approximately quadratic dependence, deviating significantly from the linearity. It is shown that the pressure-induced anisotropic change of lattice parameters, the increasing E 2g phonon frequency, and nonlinear electron -phonon coupling are three key factors of producing nonlinear T c (P) observed in MgB 2 .

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The van Hove model of a two-dimensional band structure with nearest-neighbor and next-nearest-neighbor hopping integrals is used to account for unusual temperature and doping dependence of the thermoelectric power (TEP) in cuprate superconductors. It is shown that the present theory can reproduce global trends of TEP data, including a positive peak of TEP in the underdoped regime, a decrease in TEP with increasing doping, a change in sign of TEP nearby the optimal doping, and an approximately linear decrease in TEP with temperature.