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 LnOFBiS2 (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 LaOFBiS2 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 LaOFBiS2 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 NdOFBiS2 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 NdOFBiS2 is not as large as that for LaOFBiS2.