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)_{2}PF_{6}, (TMTSF)_{2}ClO_{4}, UPt_3, Sr_{2}RuO_{4}, PrOs_{4}Sb_{12}. These triplet superconductors are characterized with the extreme smallness of the spin-orbit coupling energy E_{so} (≪ Δ, where Δ is the superconducting gap). Also like superfluid ^{3}He-A, the order parameter of these superconductors are characterized by l^ (the chiral vector) and d^ (the spin vector). In these superconductors, an Abrikosov vortex splits into a pair of half quantum vortices at low temperatures. Type A_{1} comprises most of non-centrosymmetric triplet superconductors discovered recently, e.g. CePt_{3}Si, CeIrSi_3, CeRhSi_3, and Li_{2}Pt_{3}B. They are characterized by l^ and d^_{1}+id^_{2} like superfluid ^{3}He-A_{1}. The spin-orbit coupling energy E_{so} is extremely large E_{so}≈ 10^3 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 A_1 superconductors, an Abrikosov vortex does not split into a pair of half quantum vortices. Further all thase triplet superconductors (both type A and type A_1) harbor the zero mode or the Majorana fermion attached to each vortex, of which implication should be further explored.
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