  Electrical resistivity measurements as a function of temperature between 1 K
and 300 K were performed at various pressures up to 3 GPa on the
superconducting layered compounds Ln(O0.5F0.5)BiS2 (Ln = La, Ce). At
atmospheric pressure, La(O0.5F0.5)BiS2 and Ce(O0.5F0.5)BiS2 have
superconducting critical temperatures, Tc, of 3.3 K and 2.3 K, respectively.
For both compounds, the superconducting critical temperature Tc initially
increases, reaches a maximum value of 10.1 K for La(O0.5F0.5)BiS2 and 6.7 K for
CeO(0.5F0.5)BiS2, and then gradually decreases with increasing pressure. Both
samples also exhibit transient behavior in the region between the lower Tc
phase near atmospheric pressure and the higher Tc phase. This region is
characterized by a broadening of the superconducting transition, in which Tc
and the transition width, delta Tc, 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 Tc is found. At pressures above the value at which the
maximum in Tc occurs, there is a gradual decrease of Tc and further suppression
of the semiconducting behavior with pressure, both of which are reversible.
