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

The discovery of superconductivity in quasi-one-dimensional Cr-based pnictides A₂Cr₃As₃ (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 H_c2(φ) profile. Finally we conclude with remarks on the possible unconventional superconducting pairing symmetry.