Narrow Results

The temperature and field dependence of the penetration depth was determined from muon spin rotation (μ⁺SR) measurements on a single crystal of YBa₂Cu₃O₇ having a superconducting transition at T_c ≈ 91.3 K. Data were acquired at applied magnetic fields of 0.05, 1.0, 3.0, and 6.0 Tesla, yielding results inconsistent with any pairing state requiring nodes, including d-wave pairing. These data are, however, completely consistent with s-wave (or extended s-wave) superconductivity, with clear evidence of field-dependent, temperature-activated vortex pinning. Our results confirm the s-wave character originally observed in 1989, and show that the features of μ⁺SR (and microwave) data used by other authors as evidence for d-wave superconductivity are instead due to temperature- and field-dependent vortex pinning/reordering, resulting in significant distortion of the flux lattice.

Experiments reveal the existence of metallic bands at surfaces of metals and insulators. The bands can be doped externally. We review properties of surface superconductivity that may set up in such bands at low temperatures and various means of superconductivity defection. The fundamental difference as compared to the ordinary superconductivity in metals, besides its two-dimensionality lies in the absence of the center of space inversion. This results in mixing between the singlet and triplet channels of the Cooper pairing.

The idea of spontaneous symmetry breaking in many-body physics from personal perspective (Bose-gas, nuclear structure and a new approach of Generalized Density Matrix).

Treating competing fluctuations, e.g., density, spin, current, need a tractable, self-consistent approach. One method that treats particle-particle and particle-hole correlations self-consistently is the diagrammatic "crossing-symmetric equations" method. In a general calculation for pairing, non-local interaction plays an important role in enhancing certain quantum fluctuations and thereby determining the pairing symmetry.