Unconventional paring in inversion-symmetry lacking crystal

Electrons pair to carry supercurrent in a crystal. The paring symmetry can be s-wave, p-wave, d-wave and so on, depending on the underlying crystal structure. Those paring possibilities have definite spatial parities: s- and d-wave don't change under inversion while p changes sign and they don't mix in a centrosymmetric crystal. Moreover, they pair constrained by Pauli principle, which requires them to be a singlet for even-parity but triplet for odd-parity. However, for a non-centrosymmetric crystal, mixing is possible:

Unconventional pairs

Unconventional superconducting phase in the weakly correlated noncentrosymmetric Mo₃Al₂C compound: E. Bauer, G. Rogl, Xing-Qiu Chen, R. T. Khan, H. Michor, G. Hilscher, E. Royanian, K. Kumagai, D. Z. Li, Y. Y. Li, R. Podloucky, and P. Rogl ,Phys. Rev. B 82, 064511 (Published August 17, 2010)

Structure and physical properties of the noncentrosymmetric superconductor Mo₃Al₂C: A. B. Karki, Y. M. Xiong, I. Vekhter, D. Browne, P. W. Adams, D. P. Young, K. R. Thomas, Julia Y. Chan, H. Kim, and R. Prozorov: Phys. Rev. B 82, 064512 (Published August 17, 2010)

In superconductors, the appearance of dissipationless current is related to the formation of electron pairs with opposite spin and momentum. The symmetry of these pairs, which is constrained by the symmetries of the underlying crystal structure, defines important aspects of the superconducting state. So what happens to superconductivity when electron pairing occurs in a crystal structure that has no center of inversion?

This interesting question has been investigated in detail theoretically, and it was realized that in such cases the superconducting pairing is unconventional. In conventional (centrosymmetric) superconductors there can only be either spin-singlet or spin-triplet electron pairing, but in the absence of space-inversion symmetry the two can mix by the action of spin-orbit interaction (a relativistic effect), leading to unusual superconducting behavior.

This theoretical prediction has been tested experimentally in two independent articles that appear in Physical Review B. Ernst Bauer and collaborators from the Vienna University of Technology, Austria, with collaborators from China and Japan in one group, and Amar Karki and collaborators from Louisiana State University, US, with collaborators from Iowa State University, US, in the other, successfully grow and characterize Mo₃Al₂C. This material crystallizes in a noncentrosymmetric structure and undergoes a superconducting transition at T_c~9 K. Both groups observe signs of unconventional pairing, hinting at a strong connection between noncentrosymmetry and unconventional superconductivity. – Athanasios Chantis