Upper critical magnetic field of LnO0.5F0.5 BiS2 (Ln = La, Nd) superconductors at ambient and high pressure

The upper critical fields Hc(2) of polycrystalline samples of LnO(0.5)F(0.5)BiS(2) (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 Hc(2)(T) curves for all the samples show an uncharacteristic concave upward curvature at temperatures below Tc, which cannot be described by the conventional one-band WerthamerHelfand-Hohenberg theory. For the LaO0.5F0.5BiS2 sample under HP, as temperature is decreased, the upper critical field Honset, 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 LaO0.5F0.5BiS2 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 NdO0.5F0.5BiS2 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 NdO0.5F0.5BiS2 is not as large as that for LaO0.5F0.5BiS2.