Physical Properties of Unconventional Superconductors

In the following development, we attempt to obtain basic physical properties of majority of unconventional superconductors through a Bardeen-Cooper-Schrieffer-type weak-coupling approach. The method we adopted in our study is model-independent. The weak-coupling approach depends only on the symmetry of the coupling, not on its origin. It is known that a general analysis of physical properties of unconventional superconductors is possible, but has not been done yet. The goal of our work is to develop and test such analysis, using available experimental data.

Since paramagnetic limiting for spin-singlet superconductors can be suppressed in superconductors with broken inversion symmetry of crystal lattice, a possibility for triplet-pairing opens up. However, it contradicts a P.W. Anderson theorem that spin-triplet is unlikely to occur in a material without an inversion center. Nevertheless, experimental evidence (in CePt₃Si, for example) strongly supports the p-wave pairing mechanism in A12 (non-centrosymmetric) compounds. In our research, we found Re₃W to be the most attractive material from the A12 family for experimental investigation and decided to concentrate on it. Although theoretically favorable in many aspects for some degree of p-wave superconductivity, annealed bulk Re₃W exhibited no evidence for triplet pairing upon experimental study. Instead, we discovered behaviors usually attributed to BCS-like s-wave pairing in SC state, such as ΔC_el / C_el ≈BCS value, exponential T-dependence of the specific heat and dirty-limit BCS-like normalized superfluid density,λ² (0) / λ² (T), at low temperatures. We report remarkably large (up to 60% of the upper critical magnetic field H_c2) reversible and linear in field magnetization, M(H), in Re₃W. Subsequent magnetic measurements with application of modified flux line lattice relaxation method allowed us to construct/determine a reliable SC state phase diagram in this mildly hysteretic material, that includes data for the lower critical field H_c1(T), penetration field H_p(T), and thermodynamic critical field H_c(T). The developed technique is fast, reliable and requires virtually no processing time.