Upper critical magnetic field of LnO${}_{0.5}$F${}_{0.5}$BiS₂ (Ln = La, Nd) superconductors at ambient and high pressure

The upper critical fields H _c2 of polycrystalline samples of LnO${}_{0.5}$F${}_{0.5}$BiS₂ (Ln = La, Nd) at ambient pressure (tetragonal structure) and high pressure (HP) (monoclinic structure) have been investigated via electrical resistivity measurements at various magnetic fields up to 8.5 T. The H _c2(T) curves for all the samples show an uncharacteristic concave upward curvature at temperatures below T _c, which cannot be described by the conventional one-band Werthamer–Helfand–Hohenberg theory. For the LaO${}_{0.5}$F${}_{0.5}$BiS₂ sample under HP, as temperature is decreased, the upper critical field H _onset, estimated from the onset of the superconducting transitions, increases slowly between 4.9 and 5.8 T compared with the slope of H _onset(T) below 4.9 T and above 5.8 T. This anomalous behavior reveals a remarkable similarity in superconductivity between LaO${}_{0.5}$F${}_{0.5}$BiS₂ samples measured under HP and synthesized under HP, although the crystal structures of the two samples were reported to be different. A reasonable explanation is that local atomic environment, which can be tuned by applying external pressure, is essential to the enhancement of T _c for BiS₂-based superconductors. On the other hand, such anomalous behavior is very subtle in the case of NdO${}_{0.5}$F${}_{0.5}$BiS₂ under HP, suggesting that the anisotropy of the upper critical field in the ab-plane and the possible lattice deformation induced by external pressure is weak. This explains why the pressure-induced enhancement of T _c for NdO${}_{0.5}$F${}_{0.5}$BiS₂ is not as large as that for LaO${}_{0.5}$F${}_{0.5}$BiS₂. (paper)