New Paradigm of Triplet Superconductivity

Abstract

Since 1980, more than 10 or so triplet superconductors have been discovered. Now we can put them into two separate classes. Type A consists of e.g. (TMTSF)2PF6, (TMTSF)2ClO4, UPt3, Sr2RuO4, PrOs4Sb12. These triplet superconductors are characterized with the extreme smallness of the spin{orbit coupling energy Eso (« δ, where δ is the superconducting gap). Also like superfluid 3He-A, the order parameter of these superconductors are characterized by l̂ (the chiral vector) and d̂ (the spin vector). In these superconductors, an Abrikosov vortex splits into a pair of half quantum vortices at low temperatures. Type A1 comprises most of non-centrosymmetric triplet superconductors discovered recently, e.g. CePt3Si, CeIrSi3, CeRhSi 3, and Li2Pt3B. They are characterized by l̂ and d̂1 + id̂2 like superfluid 3He-A1. The spin{orbit coupling energy Eso is extremely large Eso ≈ 103 K. Therefore, as noted by Frigeri et al., the Fermi surface splits for the up-spin one and the down-spin one. However, contrary to Frigeri et al., the superconductivity should occupy only the larger Fermi surface (say for spin-up). The other Fermi surface remains in the normal state. Also in type A1 superconductors, an Abrikosov vortex does not split into a pair of half quantum vortices. Further all thase triplet superconductors (both type A and type A1) harbor the zero mode or the Majorana fermion attached to each vortex, of which implication should be further explored.