BiCh2-based superconductors (Ch: S, Se) are a new series of layered superconductor. However, mechanisms for the emergence of superconductivity in BiCh2-based superconductors have not been clarified. In this study, we have investigated crystal structu
re of two series of optimally-doped BiCh2-based superconductors, Ce1-xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1-ySey)2, using powder synchrotron x-ray diffraction in order to reveal the relationship between crystal structure and superconducting properties of the BiCh2-based family. We have found that an enhancement of in-plane chemical pressure would commonly induce bulk superconductivity in both systems. Furthermore, we have revealed that superconducting transition temperature for REO0.5F0.5BiCh2 superconductors could universally be determined by degree of in-plane chemical pressure.
We have systematically investigated the crystal structure, magnetic susceptibility, and electrical resistivity of the BiS2-based superconductor LaO0.5F0.5Bi(S1-xSex)2 (x = 0 - 0.7). With expanding lattice volume by Se substitution, bulk superconducti
vity was induced for x > 0.2, and the highest Tc of 3.8 K was observed in x = 0.5 (LaO0.5F0.5BiSSe). Metallic conductivity was observed for x > 0.3 in the resistivity measurement, whereas semiconducting-like behavior was observed for x < 0.2. The induction of bulk superconductivity by the partial substitution of S by Se in the LaO0.5F0.5BiS2 superconductor should be positively linked to the enhancement of metallic conductivity.
We have systematically investigated the crystal structure and the magnetic properties of BiS2-based superconductor Ce1-xNdxO1-yFyBiS2 (x = 0 - 1.0, y = 0.3, 0.5 and 0.7) and Nd1-zSmzO1-yFyBiS2 (x = 0 - 0.8, y = 0.3, 0.5 and 0.7). In the REOBiS2 syste
m, both crystal structure and physical properties are tunable by mixing the RE (RE = rare earth) site with Ce, Nd and Sm due to the difference of ionic radius of RE. In the Ce1-xNdxO1-yFyBiS2 system, bulk superconductivity is observed for x = 1.0 with y = 0.3 and x = 0.5 - 1.0 with y = 0.5. The transition temperature (Tc) increases with increasing Nd concentration. The highest Tc is 4.8 K for x = 1.0 with y = 0.5 in the Ce1-xNdxO1-yFyBiS2 system. By the Nd substitution for Ce, lattice contraction along the a axis is generated while the c axis does not show a remarkable dependence on Nd concentration. The lattice constant of c decreases with increasing F concentration. Furthermore, we found that the Nd site can be replaced by smaller Sm ions up to z = 0.8 in Nd1-zSmzO1-yFyBiS2. Bulk superconductivity is observed within z = 0 - 0.8 for y = 0.3 and z = 0 - 0.6 for y = 0.5. The Tc increases with increasing Sm concentration. The highest Tc is 5.6 K for z = 0.8 with y = 0.3. With increasing Sm concentration, the lattice constant of a decreases while the lattice constants of c does not show a remarkable dependence on Sm concentration. We found that the chemical pressure generated by systematic solution of the RE site in the blocking layer commonly induces lattice contraction along the a axis in Ce1-xNdxO1-yFyBiS2 and Nd1-zSmzO1-yFyBiS2. The obtained results indicate that both optimal F concentration and uniaxial lattice contraction along the a axis generated by chemical pressure are essential for the inducement of bulk superconductivity in the REO1-yFyBiS2 system.
High-quality polycrystalline samples of LaO0.5F0.5BiS2 were obtained using high-pressure synthesis technique. The LaO0.5F0.5BiS2 sample prepared by heating at 700 C under 2 GPa showed superconductivity with superconducting transition temperatures (Tc
) of Tconset = 11.1 and Tczero = 8.5 K in the electrical resistivity measurements and Tconset = 11.5 and Tcirr = 9.4 K in the magnetic susceptibility measurements, which are obviously higher than those of the LaO0.5F0.5BiS2 polycrystalline samples obtained using conventional solid-state reaction. It was found that the high-Tc phase can be stabilized under high pressure and relatively-low annealing temperature. X-ray diffraction analysis revealed that the high-Tc phase possessed a small ratio of lattice constants of a and c, c/a.
We investigated the crystal structure and superconducting properties of As-grown and high-pressure-annealed PrO0.5F0.5BiS2. We found that the high-pressure annealing generates uniaxial lattice contraction along the c axis. Both As-grown and high-pres
sure-annealed PrO0.5F0.5BiS2 show bulk superconductivity. The Tc of PrO0.5F0.5BiS2 is clearly enhanced from Tczero = 3.6 K to Tczero = 5.5 K by high-pressure annealing. Unexpectedly, the semiconducting characteristics is relatively enhanced by high-pressure annealing. Namely, we assume that the enhancement of Tc can not be understood by an increase of electron carriers. Having considered these facts, we conclude that the enhancement of Tc correlates with uniaxial lattice contraction along the c axis in PrO0.5F0.5BiS2.
Correlation between crystal structure and superconducting properties of the BiS2-based superconductor LaO0.5F0.5BiS2 was investigated. We have prepared LaO0.5F0.5BiS2 polycrystalline samples with various lattice constants. It was found that the annea
ling the sample under high pressure generated uniaxial strain along the c axis. Further, the highly-strained sample showed higher superconducting properties. We concluded that the uniaxial strain along the c axis was positively linked with the enhancement of superconductivity in the LaO1-xFxBiS2 system.
