Abstract. Measurements of electrical resistivity were performed between 3 and 300 K at various pressures up to 2.8 GPa on the BiS 2 -based superconductors LnO 0.5 F 0.5 BiS 2 (Ln = Pr, Nd).At lower pressures, PrO 0.5 F 0.5 BiS 2 and NdO 0.5 F 0.5 BiS 2 exhibit superconductivity with critical temperatures T c of 3.5 and 3.9 K, respectively. As pressure is increased, both compounds undergo a transition at a pressure P t from a low T c superconducting phase to a high T c superconducting phase in which T c reaches maximum values of 7.6 and 6.4 K for PrO 0.5 F 0.5 BiS 2 and NdO 0.5 F 0.5 BiS 2 , respectively. The pressure-induced transition is characterized by a rapid increase in T c within a small range in pressure of ∼0.3 GPa for both compounds. In the normal state of PrO 0.5 F 0.5 BiS 2 , the transition pressure P t correlates with the pressure where the suppression of semiconducting behaviour saturates. In the normal state of NdO 0.5 F 0.5 BiS 2 , P t is coincident with a semiconductor-metal transition. This behaviour is similar to the results recently reported for the LnO 0.5 F 0.5 BiS 2 (Ln = La, Ce) compounds. We observe that P t and the size of the jump in T c between the two superconducting phases both scale with the lanthanide element in LnO 0.5 F 0.5 BiS 2 (Ln = La, Ce, Pr, Nd).

A novel family of superconductors based on BiS 2 -based superconducting layers were discovered in 2012. In short order, other BiS 2 -based superconductors with the same or related crystal structures were discovered with superconducting critical temperatures T c of up to 10 K. Many experimental and theoretical studies have been carried out with the goal of establishing the basic properties of these new materials and understanding the underlying mechanism for superconductivity. In this selective review of the literature, we distill the central discoveries from this extensive body of work, and discuss the results from different types of experiments on these materials within the context of theoretical concepts and models.