Critical current density (Jc), thermal activation energy (U0), and upper critical field (Hc2) of La1-xSmxO0.5F0.5BiS2 (x = 0.2, 0.8) superconductors are investigated from magnetic field dependent r{ho}(T) studies. The estimated upper critical field (
Hc2) has low values of 1.04 T for x = 0.2 and 1.41 T for x = 0.8. These values are lower than Sm free LaO0.5F0.5BiS2 superconductor (1.9 T). The critical current density (Jc) is estimated to be 1.35*105 A/cm2 and 5.07 *105 A/cm2 (2 K) for x = 0.2 and 0.8 respectively, using the Beans model. The thermal activation energy (U0/kB) is 61 K for x = 0.2 and 140 K for x =0.8 as calculated from Arrhenius plots at low magnetic field (1 T) and indicates a strong flux pinning potential which might be co-existing with applied magnetic field.
We have investigated the pressure effect on the newly discovered samarium doped La1-xSmxO0.5F0.5BiS2 superconductors. More than threefold increase in Tc (10.3 K) is observed with external pressure (at ~1.74 GPa at a rate of 4.08 K/GPa)) for x = 0.2 c
omposition. There is a concomitant large improvement in the quality of the superconducting transition. Beyond this pressure Tc decreases monotonously at the rate of -2.09 K/GPa. In the x = 0.8 sample, we do not observe any enhancement in Tc with application of pressure (up to 1.76 GPa). The semiconducting behavior observed in the normal state resistivity of both of the samples is significantly subdued with the application of pressure which, if interpreted invoking thermal activation process, implies that the activation energy gap of the carriers is significantly reduced with pressure. We believe these observations should generate further interest in the La1-xSmxO0.5F0.5BiS2 superconductors.
Polycrystalline Eu0.5La0.5BiS2F was synthesized by solid state reaction which crystallizes in the tetragonal CeOBiS2 structure (P4/nmm). We report here enhancement of Tc to 2.2 K in Eu0.5La0.5BiS2F (by electron doping in EuBiS2F with Tc ~ 0.3 K). Eu0
.5La0.5BiS2F is semiconducting down to 3 K and an onset of superconductivity is seen at 2.2 K at ambient pressure. Upon application of pressure the Tc could be enhanced upto 10 K. Step like features are seen in the resistivity curves at intermediate pressures (0.5 - 1 GPa) which hints towards the possible existence of two phases with different Tc. At a pressure above 1.38GPa, the Tconset remains invariant at 10 K but the Tc(r{ho}=0) is increased to above 8.2 K. There is a possible transformation from a low Tc phase to a high Tc phase by application of pressure.
Crystal structure and properties of a new member of oxy-bismuth-sulfide SmO1-xFxBiS2 are reported here. The compounds SmO1-xFxBiS2 (x = 0.0 and 0.5) are found to be isostructural with LaOBiS2 and crystallize in the CeOBiS2 type structure (P4/nmm). Sm
substitution in LaO0.5F0.5BiS2, (La1-ySmyO0.5F0.5BiS2), leads to a gradual decrease in a-lattice constant however the c-lattice constant does not show such a gradual trend. Enhancement in Tc is achieved upon partially substituting La by smaller Sm ion. Maximum Tc ~ 4.6 K was observed for composition with y = 0.8. Disobeying this trend Tc disappears unexpectedly in composition SmO0.5F0.5BiS2 (y = 1.0). Both the undoped and F-doped (x = 0.0 and 0.5) compounds are paramagnetic exhibiting semiconducting behavior down to 2 K.
