The study of noncentrosymmetric superconductors is an interesting subject.
Noncentrosymmetric superconductors have been extensively investigated since the discovery of superconductivity in
CePt3Si. Materials without spatial inversion symmetry has a unique property that the degeneracy of bands with
spin up and spin down is lifted in the presence of spin-orbit coupling. In this case, spin singlet and spin triplet Cooper pairs
can coexist to form a mixed pairing state.
Examples of noncentrosymemtric superconductors are
CePt3Si (Tc = 0.75K)
LaPt3Si
UIr (under pressure)
CeRhSi3
Li2Pd3B
Li2Pt3B
CeIrSi3
LaNiC2 (Tc = 2.7K)
The existence of line nodes in the gap function is suggested for CePt3Si, Li2Pd3B and
Li2Pt3B.
We are interested in LaNiC2 which exhibits relatively high temperature superconductivity with
Tc = 2.7K among these compounds.
The isostructural compound YNiC2 also shows superconductivity when Th
is substituted for Y. The maximum of Tc is Tc &sim 8.3K in
Y0.5Th0.5NiC2 [2].
We have carried out the first principles calculation for LaNiC2 [3].
Recently, the time reversal symmetry (TRS) breaking has been observed through zero-field muon spin resonance [4].
This result is very attractive since the TRS breaking strongly suggests unconventional superconductivity.
References
[1] W. H. Lee et al.: Solid State Commun. 101 (1997) 323.
[2] K. J. Syu et al.: Solid State Commun. 141 (2007) 519.
[3] I. Hase and T. Yanagisawa: J. Phys. Soc. Jpn. 78 (2009) 084724.
"Electronic Structure of RNiC2 (R = La, Y, and Th)"
[4] A. D. Hiller et al.: Phys. Rev. Lett. 102 (2009) 117007.
[5] J. Quintanilla et al.: Phys. Rev. B82 (2010) 174511.
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