Superconductivity with peculiar upper critical fields in quasi-one-dimensional Cr-based pnictides

The discovery of superconductivity in quasi-one-dimensional Cr-based pnictides A(2)Cr(3)As(3) (A = alkali metals) has generated considerable research interest, primarily owing to their reduced dimensionality, significant electron correlations, and possible unconventional superconductivity. The upper critical field (H-c2) provides important information on the superconducting pairing. In this paper, we first briefly overview the latest research progress on the Cr-based superconductors. Then, we introduce typical H-c2 (T) behaviors of type-II superconductors in relation with the pair-breaking mechanisms. After a description of the measurement method for H-c2, we focus on the analysis of H-c2 data, especially for the temperature and angle dependence, in K2Cr3As3 crystals. The result indicates (i) an absence of Pauli-paramagnetic pair breaking for field perpendicular to the Cr3As3 chains, and (ii) a unique threefold modulation for the in-plane Hc(2) (f) profile. Finally we conclude with remarks on the possible unconventional superconducting pairing symmetry.