Date of Award

12-2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

James R. Thompson

Committee Members

Victor Barzykin, Adriana Moreo, Veerle Keppens

Abstract

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 ΔCel / Cel ≈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 Hc2) 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 Hc1(T), penetration field Hp(T), and thermodynamic critical field Hc(T). The developed technique is fast, reliable and requires virtually no processing time.

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