Extremely high upper critical field in BiCh2-based (Ch: S and Se) layered superconductor LaO0.5F0.5BiS2−xSex (x = 0.22 and 0.69)

Abstract

Centrosymmetric compounds with local inversion symmetry breaking have tremendously interesting and intriguing physical properties. In this study, we focus on a BiCh2-based (Ch: S, Se) layered superconductor, as a system with local inversion asymmetry, because spin polarisation based on the Rashba–Dresselhaus-type spin–orbit coupling has been observed in centrosymmetric BiCh2-based LaOBiS2 systems, while the BiCh2 layer lacks local inversion symmetry. Herein, we report the existence of extremely high in-plane upper critical fields in the BiCh2-based system LaO0.5F0.5BiS2−xSex (x = 0.22 and 0.69). The superconducting states are not completely suppressed by the applied magnetic fields with strengths up to 55 T. Thus, we consider that the in-plane upper critical field is enhanced by the local inversion symmetry breaking and its layered structure. Our study will open a new pathway for the discovery of superconductors that exhibit a high upper critical field by focusing on the local inversion symmetry breaking.

The upper critical field in the BiCh2-based superconductors CeOBiS1.7Se0.3 and PrO0.5F0.5BiS2-x Se x (x = 0, 0.3)

“…However, the experimental data derived from the WHH curves in the low-temperature region are shown in figures 8(b) and (d). A similar behavior was observed for LaO 0.5 F 0.5 BiS 2-x Se x [23,50]. Although the origin of this behavior is currently unclear, the two-gap model fits well with the experimental data for LaO 0.5 F 0.5 BiS 2 [24].…”

“…However, the specific heat measurement of LaO 0.5 F 0.5 BiSSe single crystals indicates a single-gap nature [50]. We suggested that local inversion asymmetry enhances the Pauli limit in LaO 0.5 F 0.5 BiS 2-x Se x (x=0.22, 0.69) single crystals [23]. In tetragonal systems with the Rashba-type ASOC, some components of magnetic susceptibility do not change after the superconducting transition, resulting in protection of the Cooper pairs from de-pairing against the applied fields [18, 24, 53].…”

“…Very recently, extremely high B c2 was observed in LaO 0.5 F 0.5 BiS 2-x Se x (x=0.22, 0.69) single crystals [23]. In these compounds, the superconducting states were not destroyed even at a magnetic field strength of 55 T. This result implies that local inversion asymmetry plays a significant role in superconductivity, and the paramagnetic pair-breaking effect is suppressed by the Rashba-type ASOC.…”

Superconductivity and Local Inversion-Symmetry Breaking

“…With increasing magnetic field, the layers become increasingly decoupled, such that δ → 0 and the spatial modulation decreases, similar to the J = 0 case. This picture is consistent with similar results seen within a BdG approach (91,92) and could be realized in LaO0.5F0.5BiS2−xSex (x = 0.22, 0.69) (34). Finally, note that incorporation of orbital effects in a multi-layer system may stabilize an odd-parity superconducting state similar to that discussed in Sec.…”

“…A symmorphic example, illustrated in Figure 2a, is the quasi-tetragonal cuprate Bi2Sr2CaCu2O 8+δ (Bi2212) (29,30,31,32). This class further includes the tetragonal CeRh2As2 (33) and the family of BiS2−nSen superconductors, such as LaO0.5Fe0.5BiSe2 (34), all showing intriguing magnetic properties. They crystalize in the nonsymmorphic P4/nmm space group (#129) with generating point group D 4h , see Figure 2b for examples.…”