Determination of local atomic displacements in CeO(1-x)F(x)BiS2 system

High-pressure effects on superconducting properties and crystal structure of Bi-based layered superconductor La2O2Bi3Ag0.6Sn0.4S6

The effects of pressure on the superconducting properties of a Bi-based layered superconductor La₂O₂Bi₃Ag_0.6Sn_0.4S₆, which possesses a four-layer-type conducting layer, have been studied through the electrical resistance and magnetic susceptibility measurements. The crystal structure under pressure was examined using synchrotron x-ray diffraction at SPring-8. In the low-pressure regime, bulk superconductivity with a transition temperatureT_cof ∼4.5 K was induced by pressure, which was achieved by in-plane chemical pressure effect owing to the compression of the tetragonal structure. In the high-pressure regime above 6.4 GPa, a structural symmetry lowering was observed, and superconducting transitions with aT_c∼ 8 K were observed. Our results suggest the possible commonality on the factor essential forT_cin Bi-based superconductors with two-layer-type and four-layer-type conducting layers.

Crystal Structure and Thermoelectric Transport Properties of As-Doped Layered Pnictogen Oxyselenides NdO0.8F0.2Sb1-xAsxSe2

We report the synthesis and thermoelectric transport properties of As-doped layered pnictogen oxyselenides NdO0.8F0.2Sb1-xAsxSe2 (x ≤ 0.6), which are predicted to show high-performance thermoelectric properties based on first-principles calculation. The crystal structure of these compounds belongs to the tetragonal P4/nmm space group (No. 129) at room temperature. The lattice parameter c decreases with increasing x, while a remains almost unchanged among the samples. Despite isovalent substitution of As for Sb, electrical resistivity significantly rises with increasing x. Very low thermal conductivity of less than 0.8 Wm-1K-1 is observed at temperatures between 300 and 673 K for all the examined samples. For As-doped samples, the thermal conductivity further decreases above 600 K. Temperature-dependent synchrotron X-ray diffraction indicates that an anomaly also occurs in the c-axis length at around 600 K, which may relate to the thermal transport properties.

Superconductivity in La2O2M4S6 -Type Bi-based Compounds: A Review on Element Substitution Effects

Since 2012, layered compounds containing Bi-Ch (Ch: S and Se) layers have been extensively studied in the field of superconductivity. The most-studied system is BiS2-based superconductors with two-layer-type conducting layers. Recently, superconductivity was observed in La2O2M2S6 (M = metals), which contains four-layer-type conducting layers. The four-layer-type Bi-based superconductors are new systems in the family of Bi-based superconductors; we can expect further development of Bi-based layered superconductors. In this review article, we summarize the progress of synthesis, structural analysis, investigations on superconducting properties, and material design of the four-layer-type Bi-based superconductors. In-plane chemical pressure is the factor essential for the emergence of bulk superconductivity in the system. The highest Tc of 4.1 K was observed in Rare Earth elements (RE) substituted La2-xRExO2Bi3Ag0.6Sn0.4S6.

Suppression of structural instability in LaOBiS2-x Se x by Se substitution

Isovalent substitution of S by Se in LaOBiS_2-x Se _x has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS₂ structural instability, we have studied the local structure of LaOBiS_2-x Se _x ([Formula: see text]) using temperature dependent Bi L₃-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS₂ sub-lattice is largely distorted in LaOBiS₂ (x  =  0.0), with two in-plane Bi-S1 distances separated by  ∼0.4 Å instead LaOBiSSe (x  =  1.0) showing much smaller local disorder with two in-plane Bi-Se distances in the plane being separated by  ∼0.2 Å. Temperature dependent study shows that the two Bi-S1 distances are characterized by different bond strength in LaOBiS₂ (x  =  0.0) while it is similar for the Bi-Se distances in LaOBiSSe (x  =  1.0). The out of plane Bi-S2 bond is harder in LaOBiSSe indicating that the structural instability of BiS₂ layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS_2-x Se _x is an important factor to describe differing electronic and thermal transport of the two compounds.

Crystal Structure and Superconductivity of Tetragonal and Monoclinic Ce1- xPr xOBiS2

Ce_{1- x}Pr _xOBiS₂ powders and Ce_0.5Pr_0.5OBiS₂ single crystals were synthesized and their structure and superconductive properties were examined by X-ray diffraction, X-ray absorption, electronic resistivity, and magnetization. While PrOBiS₂ was found to be in a monoclinic phase with one-dimensional Bi-S zigzag chains showing no superconductive transition above 0.1 K, CeOBiS₂ was in a tetragonal phase with two-dimensional Bi-S planes showing zero resistivity below 1.3 K. In the range x = 0.3-0.9 in Ce_{1- x}Pr _xOBiS₂, both monoclinic and tetragonal phases were formed together with zero resistivity up to a maximum temperature of 2.2 K. A Ce_0.5Pr_0.5OBiS₂ single crystal, which showed both zero resistivity and a decrease in magnetization at ∼2.4 K, presented a tetragonal structure. Short Bi-S bonding in flat two-dimensional Bi-S planes and mixed Ce³⁺/Ce⁴⁺ were characteristic features of the Ce_0.5Pr_0.5OBiS₂ single crystal, which presumably triggered its superconductivity.

Metallic phase in stoichiometric CeOBiS2 revealed by space-resolved ARPES

Recently CeOBiS₂ system without any fluorine doping is found to show superconductivity posing question on its origin. Using space resolved ARPES we have found a metallic phase embedded in the morphological defects and at the sample edges of stoichiometric CeOBiS₂. While bulk of the sample is semiconducting, the embedded metallic phase is characterized by the usual electron pocket at X point, similar to the Fermi surface of doped BiS₂-based superconductors. Typical size of the observed metallic domain is larger than the superconducting correlation length of the system suggesting that the observed superconductivity in undoped CeOBiS₂ might be due to this embedded metallic phase at the defects. The results also suggest a possible way to develop new systems by manipulation of the defects in these chalcogenides with structural instability.

Role of the local structure in superconductivity of LaO0.5F0.5BiS2-x Se x system

We have studied the local structure of LaO_0.5F_0.5BiS_2-x Se _x by Bi L₁-edge extended x-ray absorption fine structure (EXAFS). We find a significant effect of Se substitution on the local atomic correlations with a gradual elongation of average in-plane Bi-S bondlength. The associated mean square relative displacement, measuring average local distortions in the BiS₂ plane, hardly shows any change for small Se substitution, but decreases significantly for [Formula: see text]. The Se substitution appears to suppress the local distortions within the BiS₂ plane that may optimize in-plane orbital hybridization and hence the superconductivity. The results suggest that the local structure of the BiS₂-layer is one of the key ingredients to control the physical properties of the BiS₂-based dichalcogenides.

Role of valence changes and nanoscale atomic displacements in BiS2-based superconductors

Superconductivity within layered crystal structures has attracted sustained interest among condensed matter community, primarily due to their exotic superconducting properties. EuBiS₂F is a newly discovered member in the BiS₂-based superconducting family, which shows superconductivity at 0.3 K without extrinsic doping. With 50 at.% Ce substitution for Eu, superconductivity is enhanced with Tc increased up to 2.2 K. However, the mechanisms for the T_c enhancement have not yet been elucidated. In this study, the Ce-doping effect on the self-electron-doped superconductor EuBiS₂F was investigated by X-ray absorption spectroscopy (XAS). We have established a relationship between Ce-doping and the T_c enhancement in terms of Eu valence changes and nanoscale atomic displacements. The new finding sheds light on the interplay among superconductivity, charge and local structure in BiS₂-based superconductors.

Electronic and magnetic properties of superconducting LnO1-x F x BiS2 (Ln = La, Ce, Pr, and Nd) from first principles

A density functional theory study of the BiS2 superconductors containing rare-earths: LnO1-x F x BiS2 (Ln  =  La, Ce, Pr, and Nd) is presented. We find that CeO0.5F0.5BiS2 has competing ferromagnetic and weak antiferromagnetic tendencies, the first one corresponding to experimental results. We show that PrO0.5F0.5BiS2 has a strong tendency for magnetic order, which can be ferromagnetic or antiferromagnetic depending on subtle differences in 4f orbital occupations. We demonstrate that NdO0.5F0.5BiS2 has a stable magnetic ground state with weak tendency to order. Finally, we show that the change of rare earth does not affect the Fermi surface, and predict that CeOBiS2 should display a pressure induced phase transition to a metallic, if not superconducting, phase under pressure.

Coexistence of superconductivity and complex 4 f magnetism in Eu0.5Ce0.5BiS2F

EuBiS2F is a self-doped superconductor due to the mixed valence of Eu. Here we report that, with the Ce substitution for Eu by 50 at.%, the material exhibits ferromagnetic ordering at 8 K for the Ce-4 f moment, superconductivity at 2.2 K in the BiS2 layers and possibly antiferromagnetic ordering at 2.1 K for the Eu-4 f spins. The Eu valence is essentially divalent with the Ce incorporation. We tentatively interpret the coexistence of ferromagnetism and superconductivity by considering different Bi-6p orbitals that are responsible for the superconductivity itself and for mediating the ferromagnetic interaction, respectively. We argue that the antiferromagnetic ordering of the Eu-4 f spins is most likely due to a magnetic dipole-dipole interaction.