TRR 360 Seminar:

Paramagnetic limit of spin-triplet superconductors

Thomas Bernat

Paramagnetic limit of spin-triplet superconductors

Dr. Thomas Bernat

Department of Physics and Astronomy, Uppsala University, Sweden

An upper critical field above the Pauli limit is often taken as a hint of spin-triplet pairing, since equal-spin triplet pairs are insensitive to the Zeeman field. In real materials, however, strong spin-orbit coupling pins the d-vector to the crystal axes. Generically, the condensate is then a mixture of equal-spin and opposite-spin pairs for any field direction.
I’ll present two main results. First, all triplet states are Pauli limited at finite temperature, but this limit diverges as a power law when T ¡æ 0. Second, for a d-vector with fixed direction and a slightly tilted field, the phase diagram becomes surprisingly rich. Two tricritical points appear along the upper critical line, with a first-order transition between them, and a first-order line between two distinct superconducting phases can also emerge. I’ll show how a Landau expansion to eighth order in the gap explains this structure.
Finally, we’ll discuss UPt3, where the model predicts a strongly temperature-dependent H_c2 anisotropy at low T. This calls for measurements at lower temperatures than those done so far.