Pressure-induced enhancement of superconductivity and suppression of semiconducting behavior in LnO sub(0.5) F sub(0.5) BiS sub(2) (Ln = La,Ce) compounds

Electrical resistivity measurements as a function of temperature between 1 and 300 K were performed at various pressures up to 3 GPa on the superconducting layered compounds LnO sub(0.5) F sub(0.5) BiS sub(2) (Ln = La, Ce). At atmospheric pressure, LaO sub(0.5) F sub(0.5) BiS sub(2) and CeO sub(0.5) F sub(0.5) BiS sub(2) have superconducting critical temperatures T sub(c) of 3.3 and 2.3 K, respectively. For both compounds, the superconducting critical temperature T sub(c) initially increases, reaches a maximum value of 10.1 Kfor LaO sub(0.5) F sub(0.5) BiS sub(2) and 6.7 K for CeO sub(0.5) F sub(0.5) BiS sub(2), and then gradually decreases with increasing pressure. Both samples also exhibit transient behavior in the region between the lower T sub(c) phase near atmospheric pressure and the higher T sub(c) phase at higher pressures. This region is characterized by a broadening of the superconducting transition, in which T sub(c) and the transition width Delta T sub(c) are reversible with increasing and decreasing pressure. There is also an appreciable pressure-induced and hysteretic suppression of semiconducting behavior up to the pressure at which the maximum value of T sub(c) is found. At pressures above the value at which the maximum in T sub(c) occurs, there is a gradual decrease of T sub(c) and further suppression of the semiconducting behavior with pressure, both of which are reversible.