Anisotropic two-gap superconductivity and the absence of a Pauli paramagnetic limit in single-crystalline LaO0.5F0.5BiS2

Abstract

Ambient-pressure-grown LaO0.5F0.5BiS2 with a superconducting transition temperature Tc ∼ 3 K possesses a highly anisotropic normal state. By a series of electrical resistivity measurements with a magnetic field direction varying between the crystalline c-axis and the ab-plane, we present the first datasets displaying the temperature dependence of the out-of-plane upper critical field H ⊥ c2 (T ), the in-plane upper critical field H / / c2 (T ), as well as the angular dependence of Hc2 at fixed temperatures for ambient-pressure-grown LaO0.5F0.5BiS2 single crystals. The anisotropy of the superconductivity, H / / c2 /H ⊥ c2 , reaches ∼16 on approaching 0 K, but it decreases significantly near Tc. A pronounced upward curvature of H / / c2 (T ) is observed near Tc, which we analyze using a two-gap model. Moreover, H / / c2 (0) is found to exceed the Pauli paramagnetic limit, which can be understood by considering the strong spin-orbit coupling associated with Bi as well as the breaking of the local inversion symmetry at the electronically active BiS2 bilayers. Hence, LaO0.5F0.5BiS2 with a centrosymmetric lattice structure is a unique platform to explore the physics associated with local parity violation in the bulk crystal.

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

“…The T c defined as the midpoint of the transition is 3.2 K and 4.1 K for x = 0.22 and 0.69, respectively; the higher Se concentration causes the higher T c . A weak upward behaviour was observed for x = 0.22 and 0.69, which is similar to previous reports 34 . The amplitude of the upturn behaviour at low temperatures was suppressed by the Se substitution.…”

“…Moreover, the out-of-plane orbital limit also deviates from the WHH curves (see the inset of Fig. 4 ), which is consistent with a previous report 27 , 34 . For another pair-breaking effect, the Pauli limit is estimated from the relation .…”

“…This led to and nm for x = 0.22 and 0.69, respectively. These values are comparable to those reported previously for x = 0 27 , 34 . To estimate the out-of-plane coherence length , we used the defined as the intersection of the extrapolation of the ρ ab ( B ) curves at the minimum temperature of ~ 0.47 K and ρ n (see the insets of Fig.…”

“… was estimated from the midpoint of the resistive transition of the ρ ab ( B ) data (see Supplementary Fig. 2 ); a similar criterion has been used in several studies 6 , 7 , 13 , 14 , 34 . Moreover, we plotted the defined as the beginning of the resistive increase from zero resistivity of the ρ ab ( B ) data and from the ρ ab ( T ) data, respectively, in Supplementary Figs.…”

“…The spin polarisation attributed to local Rashba spin–orbit coupling was observed by spin-ARPES (SARPES) for LaO 0.55 F 0.45 BiS 2 33 . Furthermore, a high was observed, which implies that the local inversion asymmetry can play a significant role in superconductivity 34 . However, research focused on local inversion asymmetry has not been extensively developed in BiCh 2 -based superconductivity.…”