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Eu0.5La0.5FBiS2-xSex is a new BiS2-based superconductor system. In Eu0.5La0.5FBiS2-xSex, electron carriers are doped to the BiS2 layer by the substitution of Eu by La. Bulk superconductivity in this system is induced by increasing the in-plane chemical pressure, which is controlled by the Se concentration (x). In this study, we have analysed the crystal structure of Eu0.5La0.5FBiS2-xSex using synchrotron powder diffraction and the Rietveld refinement. The precise determination of the structural parameters and thermal factors suggest that the emergence of bulk superconductivity in Eu0.5La0.5FBiS2-xSex is achieved by the enhanced in-plane chemical pressure and the decrease in in-plane disorder., Comment: 5 pages, 2 figures, to appear in proceedings of ISS2016

We examined the crystal structure of the new thermoelectric material LaOBiS2-xSex, whose thermoelectric performance is enhanced by Se substitution, by using powder synchrotron X-ray diffraction and Rietveld refinement. The emergence of metallic conductivity and enhancement of the thermoelectric power factor of LaOBiS2-xSex can be explained with the higher in-plane chemical pressure caused by the increase of Se concentration at the in-plane Ch1 site (Ch = S, Se). High-temperature X-ray diffraction measurements for optimally substituted LaOBiSSe revealed anomalously large atomic displacement parameters (Uiso) for Bi and Ch atoms in the BiCh2 conduction layers. The anisotropic analysis of the atomic displacement parameters (U11 and U33) for the in-plane Bi and Ch1 sites suggested that Bi atoms exhibit large atomic displacement along the c-axis direction above 300 K, which could be the origin of the low thermal conductivity in LaOBiSSe. The large Bi vibration along the c-axis direction could be related to in-plane rattling, which is a new strategy for attaining low thermal conductivity and phonon-glass-electron-crystal states., Comment: 25 pages, 5 figures, 3 supplemental materials, version accepted in JAP

The electronic structure of nearly optimally-doped novel superconductor LaO$_{1-x}$F$_x$BiS$_2$ (${\it x}$ = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (${\it E}_{\rm F}$), which are well reproduced by first principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near ${\it E}_{\rm F}$ shows a square-like distribution around the $\Gamma$(Z) point in addition to electronlike Fermi surface (FS) sheets around the X(R) point, indicating that FS of LaO$_{0.54}$F$_{0.46}$BiS$_2$ is in close proximity to the theoretically-predicted topological change., Comment: 6 pages, 3 figures, + supplemental material

Bulk magnetization, transport and neutron scattering measurements were performed to investigate the electronic and magnetic properties of a polycrystalline sample of the newly discovered ferromagnetic superconductor, CeO$_{0.3}$F$_{0.7}$BiS$_{2}$. Ferromagnetism develops below T$_{FM}$ = 6.54(8) K and superconductivity is found to coexist with the ferromagnetic state below T$_{SC}$ ~ 4.5 K. Inelastic neutron scattering measurements reveal a very weakly dispersive magnetic excitation at 1.8 meV that can be explained by an Ising-like spin Hamiltonian. Under application of an external magnetic field, the direction of the magnetic moment changes from the c-axis to the ab-plane and the 1.8 meV excitation splits into two modes. A possible mechanism for the unusual magnetism and its relation to superconductivity is discussed.

We study normal state electrical, thermoelectrical and thermal transport in polycrystalline BiS2-based compounds, which become superconducting by F doping on the O site. In particular we explore undoped LaOBiS2 and doped LaO0.5F0.5BiS2 samples, prepared either with or without high pressure annealing, in order to evidence the roles of doping and preparation conditions. The high pressure annealed sample exhibits room temperature values of resistivity ro around 5 mohmcm, Seebeck coefficient S around -20 microV/K and thermal conductivity k around 1.5 W/Km, while the Hall resistance RH is negative at all temperatures and its value is -10-8 m3/C at low temperature. The sample prepared at ambient pressure exhibits RH positive in sign and five times larger in magnitude, and S negative in sign and slightly smaller in magnitude. These results reveal a complex multiband evolution brought about by high pressure annealing. In particular, the sign inversion and magnitude suppression of RH, indicating increased electron-type carrier density in the high pressure sample, may be closely related to previous findings about change in lattice parameters and enhancement of superconducting Tc by high pressure annealing. As for the undoped sample, it exhibits the 10 times larger resistivity, 10 times larger |S| and 10 times larger |RH| than its doped counterpart, consistently with its insulating nature. Our results point out the dramatic effect of preparation conditions in affecting charge carrier density as well as structural, band and electronic parameters in these systems., Comment: 9 pages, 5 figures

We report on the magnetic and superconducting properties of LaO0.5F0.5BiS2 by means of zero- (ZF) and transverse-field (TF) muon-spin spectroscopy measurements (uSR). Contrary to previous results on iron-based superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TF-uSR data give access to the superfluid density, which shows a marked 2D character with a dominant s-wave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, and its nonsuperconducting parent compound LaOBiS$_{2}$. The crystal structures and vibrational modes have been examined. Upon F-doping, while the lattice contracts significantly along $c$ and expands slightly along $a$, the buckling of the BiS$_2$ plane remains almost the same. In the inelastic measurements, a large difference in the high energy phonon modes was observed upon F substitution. Alternatively, the low energy modes remain almost unchanged between non-superconducting and superconducting states either by F-doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes, and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material., Comment: 5 pages, 4 figures, 1 table

Novel BiS2-based superconductors LaO1-xFxBiS2 prepared by the high pressure synthesis technique were systematically studied. It was found that the high pressure annealing strongly the lattice as compared to the LaO1-xFxBiS2 samples prepared by conventional solid state reaction at ambient pressure. Bulk superconductivity was observed within a wide F-concentration range of x = 0.2 ~ 0.7. On the basis of those results, we have established a phase diagram of LaO1-xFxBiS2., Comment: 11 pages, 6 figures