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We have reported the X-ray diffraction patterns and resistivity measurements of the BiS2-based superconductor EuSr2Bi2S2Se2F4 up to 15 GPa. High-pressure apparatuses include diamond anvil cells, piston cylinder cells, and cubic anvil cells. With applying pressure, peaks of Tc = 3.26 K (at 0.8 GPa) and Tc = 2.35 K (at 4 GPa) were observed by appear new superconducting phase. A new superconducting phase appeared owing to the structural phase transitions. At 7 GPa, superconductivity was not observed down to 2 K but above 8 GPa, superconductivity was observed near 3 K. Besides, we have also confirmed that superconductivity above 8.0 GPa also appeared with a structural phase transition (at 8.35 GPa). There are four types of superconducting phases below 12 GPa. Therefore, EuSr2Bi2S2Se2F4 has complex superconducting phases, which can be attributed to the structural phase transitions. [ABSTRACT FROM AUTHOR]

We examined the effect of Pb substitution in the layered superconductor LaO0.5F0.5Bi1−xPbxS2 (x = 0–0.15) through the measurements of the resistivity, thermal expansion, specific heat, and Seebeck coefficient. These transport and thermal properties show anomalies at certain temperatures (T*) for x ≥ 0.08. The large thermal expansion anomalies, specific heat anomalies, and the existence of hystereses in the above measurements indicate a first-order structural phase transition at T*. Additionally, the Seebeck coefficient indicates that the anomalies at T* are related not only to the lattice system, but also to the electronic system. Superconductivity is not observed above 2 K at x = 0.08, which is around the phase boundary where T* vanishes. The suppression of superconductivity around the structural phase boundary suggests a close relationship between the lattice and superconductivity. [ABSTRACT FROM AUTHOR]

Layered BiS2-based superconductor La(O,F)BiS2 can exhibit superconductivity via carrier doping, and its super-conducting properties can be changed by applying hydrostatic or chemical pressure through elemental substitution. Single-crystal LaO0.5F0.5BiS2 exhibits filamentary superconductivity with Tc of approximately 3 K. We found that the partial substitution of Bi with Sn in LaO0.5F0.5BiS2 induces a structural phase transition from the P4=nmm to P21=m structure. Bulk superconductivity with Tc of approximately 6 K emerged near this structural phase transition. We also observed filamentary superconductivity with Tc of approximately 8 K, which was approximately three times higher than that of the unsubstituted sample. This finding provides a new route for further increasing the superconducting transition temperature of this system. [ABSTRACT FROM AUTHOR]

BiS2-based superconductor Eu2SrBi2S2.5Se1.5F4 has been investigated to clarify between Tc and crystal structure under high-pressure. The electrical resistivity by Cubic anvil cell and XRD by Diamond anvil cell were measured by generating hydrostatic pressure. In electrical resistivity measurements, Tc increased from 2.05 to 3.22 K at 1.5 GPa and decreased below 2.62 K at 3 GPa. However, under higher-pressure, Tc exhibited a peak again at 4 GPa. These experimental results suggest the existence of pressure-induced multiple superconducting phases in Eu2SrBi2S2.5Se1.5F4. By XRD measurements, we can find that pressure-induced structural phase transitions occur near the pressure in the observation of Tc discontinuities. These results indicate the emergence of new superconducting phases through structural phase transitions. Eu2SrBi2S2.5Se1.5F4 was found to have three superconducting phases up to 12 GPa. [ABSTRACT FROM AUTHOR]

The powder X-ray diffraction pattern of the BiS2-based compound Eu3Bi2S4F4, acquired under high pressure, reveals a structural phase transition at 3.55 GPa. Reportedly, Eu3Bi2S4F4 exhibits a metal-semiconductor transition and bulk superconductivity at pressures close to 4 GPa. These transitions are induced by structural phase transition. [ABSTRACT FROM AUTHOR]

Sb- and Se-substituted CeOBiS2 single crystals have been successfully grown using CsCl/KCl flux. Sb and Se substitution dependence of the superconductivity on CeOBiS2 was investigated and the superconducting phase diagram at above 0.3 K was described. The non-linear boundary between superconductivity and non-superconductivity was revealed. [ABSTRACT FROM AUTHOR]

Recently, robustness of superconductivity (transition temperature, Tc) under high pressures has been observed in high-entropy alloy (HEA), bcc-type Ti-Zr-Hf-Nb-Ta, and HEA-type compounds (Ag, In, Sn, Pb, Bi)Te with a NaCl-type structure. Since those materials have three-dimensional crystal structure, investigation on the pressure dependence of Tc of low-dimensional materials is needed to understand the phenomena. Here, we investigated the superconducting properties and the crystal structure of BiS2-based layered system REO0.5F0.5BiS2. Although the robustness of Tc was induced in MTe with increasing M-site configurational entropy, the increase in RE-site configurational entropy does not induce robustness of Tc under high pressures in REO0.5F0.5BiS2. The crystal structure of HEA-type REO0.5F0.5BiS2 was confirmed as monoclinic P21=m, which is the same space group as the zero-entropy counterpart LaO0.5F0.5BiS2. The results suggest that an increase in configurational entropy at blocking layers do not affect crystal structure and superconducting properties under high pressures in the BiS2-based layered system. [ABSTRACT FROM AUTHOR]

We report transport properties for layered BiCh2-based (Ch = S, Se) superconductors LaO1−xFxBiS2−ySey (x = 0.2, 0.5, y = 0–1.05) and the observation of possible weak antilocalization (WAL). Electrical resistivity and Hall coefficients for the Se-poor samples increase with decreasing temperature. The increase becomes less pronounced with increasing Se concentration, indicating a loss of insulating behavior. Interestingly, the moderately Se-substituted samples exhibit metallic behavior in the high-temperature region and a weak increase in the resistivity in the low-temperature regions, which implies the existence of carrier localization. The heavily Se-substituted compounds show metallic behavior in the entire-temperature region. Magnetoresistance measurements indicate that WAL is realized in the heavily Se-substituted systems. The WAL behavior is weakened by the changes in F and Se concentrations. A crossover state of the WAL and weak localization (WL) emerges around the moderately F-doped and Se-free LaO0.8F0.2BiS2. The change of the resistivity and Hall coefficient by the F and Se substitution clearly correlates to the difference of the magnetoconductance. We propose that the BiCh2-based system is a good platform for studying the relationship between localization and superconductivity because those states are tunable by element substitutions with bulk single crystals. [ABSTRACT FROM AUTHOR]

We performed photoelectron and Auger-electron holography to study the local structure of La(O,F)BiS2. We found that the previously reported surface-dominant Bi 4f component exhibits a hologram pattern derived from the forward focusing peaks. This provides spectroscopic evidence of the existence of two bulk Bi chemical sites. A comparison of the experimental holograms with the simulated holograms using the tetragonal and charge density wave (CDW) models, as well as with a monoclinic structure model proposed for the high-pressure phase exhibiting the highest superconducting transition temperature among BiS2-based superconductors, showed that the monoclinic structure model does not reproduce the experimental hologram. This suggests that it may not be a dominant local structure for as-grown La(O,F)BiS2. The experimental hologram of the F KLL Auger electron is similar to that of the O KLL, providing direct evidence that F atoms are incorporated into the O sites. These results are valuable for understanding the physical properties of La(O,F)BiS2. [ABSTRACT FROM AUTHOR]

The superconducting transition temperatures, Tc, of La4Ni3O10+δ (δ = 0.04 and −0.01) were determined under various pressures up to 124.9 GPa by electrical resistance measurements with a diamond anvil cell. Tc exhibits a strong dependence on oxygen content within the pressure range of approximately 20 and 80 GPa. At 48.0 GPa, Tc of La4Ni3O10.04 peaks at 36 K, marking the highest Tc reported thus far. [ABSTRACT FROM AUTHOR]

The superconducting properties and structural parameters, including the Grüneisen parameter (γG), of the BiCh2-based (Ch: S, Se) layered superconductor LaO0.5F0.5BiS2−xSex were investigated. The superconducting transition temperature (Tc) increased with increasing Se concentration (x), and bulk superconductivity was induced by Se substitution. γG exhibited a larger value when x ≥ 0.4, where the superconducting properties were improved. These results suggest that the increase of lattice anharmonicity is preferable for superconductivity in LaO0.5F0.5BiS2−xSex. The discussion includes data for related superconductors REO0.5F0.5BiS2 (RE = Pr, Nd), where a positive correlation between γG and Tc is revealed for Se-poor samples. In addition, specific heat analyses suggest the presence of a low-energy optical phonon mode for LaO0.5F0.5BiS2−xSex. [ABSTRACT FROM AUTHOR]

We estimated Grüneisen parameter (γG) to investigate lattice anharmonicity in a layered BiS2-based superconductor system REO1-xFxBiS2 (RE = La, Ce, Pr, Nd), where in-plane chemical pressure was tuned by the ionic radii of the RE elements, and the carrier concentration was tuned by F concentration. With increasing in-plane chemical pressure, bulk modulus remarkably increases, and a larger γG is observed for RE = Nd. The dependence of γG on in-plane chemical pressure exhibits a good correlation with Tc, suggesting that a larger γG results in a higher Tc in the system. In addition, γG shows a slight decrease by a decrease of F concentration (x) in REO1-xFxBiS2. Our results suggest that the enhancement of anharmonicity in REO1-xFxBiS2 is achieved by the suppression of in-plane disorder by in-plane chemical pressure and carrier doping. From analogy to thermoelectric LaOBi(S,Se)2, in which anharmonic vibration of Bi along the c-axis was enhanced by in-plane chemical pressure, we propose that the anharmonic vibration of Bi along the c-axis is present and in REO1-xFxBiS2, and the enhancement of the anharmonicity is positively linked to superconducting Tc and pairing mechanisms. [ABSTRACT FROM AUTHOR]

BiS2-based superconductors have attracted considerable attention owing to the possible occurrence of exotic superconductivity. Recently, the superconducting critical temperature (Tc) of La(O,F)BiS2 has been reported to be enhanced with Pb or Sn doping. In this study, we have performed photoelectron holography (PEH) and core-level photoelectron spectroscopy (PES) to clarify the substituted site and valence state of Sn atoms in Sn-substituted La(O,F)BiS2. The PEH study revealed that the Sn atoms are incorporated into the Bi sites. A core-level PES study suggested that the valence state of the substituted Sn atoms is close to that of Sn2+, indicating that the Sn dopants provide hole carriers that compensate the electron carriers introduced by the F doping. These results are fundamental information for understanding the impact of Sn doping on this compound. [ABSTRACT FROM AUTHOR]

We report a synchrotron radiation X-ray diffraction study of the BiS2-layered superconductor LaO0.5F0.5BiS2. We observed superlattice reflections with the propagation vector \(\boldsymbol{q}_{\text{lat}} = (\zeta ,\zeta ,0.500(1))\) (ζ = 0.2075(1) at 30 K) below T* ≈ 260 K. The reflection conditions indicate that the superlattice structure corresponds to in-plane transverse-type modulation. This finding along with the results of bond valence sum analysis suggests charge density wave formation with qcdw = 2qlat induced by Peierls-type instability. [ABSTRACT FROM AUTHOR]

We report the Se substitution effects on the crystal structure, superconducting properties, and valence states of self-doped BiCh2-based compound CeOBiS2−xSex. Polycrystalline CeOBiS2−xSex samples with x = 0–1.0 were synthesized. For x = 0.4 and 0.6, bulk superconducting transitions with a large shielding volume fraction were observed in magnetic susceptibility measurements; the highest transition temperature (Tc) was 3.0 K for x = 0.6. A superconductivity phase diagram of CeOBiS2−xSex was established based on Tc estimated from the electrical resistivity and magnetization measurements. The emergence of superconductivity in CeOBiS2−xSex was explained with two essential parameters of in-plane chemical pressure and carrier concentration, which systematically changed with increasing Se concentration. [ABSTRACT FROM AUTHOR]

LaOBiPbS3 is a kind of pnictogen-dichalcogenide layered compounds, which have recently been experimentally investigated as thermoelectric materials owing to their low thermal conductivity and high controllability of constituent elements. However, thermoelectric performance of LaOBiPbS3 is at present not very high and that of its analogous compounds remains to be unknown. In this study, we theoretically investigate thermoelectric properties of 24 possible variations of the constituent elements in LaOBiPbS3 from the viewpoint of the electronic structure. We find that some compounds can have much better thermoelectric performance than LaOBiPbS3; in particular, LaOSbPbSe3 is predicted to have a power factor five times as large as that of LaOBiPbS3. Here, the choice of the pnictogen atom (As, Sb, and Bi), of which the low-energy conduction bands mainly consist, correlates with the calculated power factor and the dimensionless figure of merit, ZT. Such correlation comes from the fact that the low-dimensionality of the electronic structure, which enhances the density of states near the band edge, strongly depends on the pnictogen atom through, e.g., the strength of the spin–orbit coupling. Moreover, hybridization of the wave functions in the pnictogen-dichalcogenide layer and those in the rock-salt layer plays a key role in gap opening, and thus is important for achieving high thermoelectric performance. In LaOSbPbSe3, such hybridization also pushes up the conduction band bottom, which enhances the density of states near the band edge and thus the power factor. [ABSTRACT FROM AUTHOR]

A polycrystalline sample of the high-entropy-alloy-type telluride AgInSnPbBiTe5 was synthesized using high-pressure synthesis. Superconductivity with a transition temperature of 2.6 K was observed in AgInSnPbBiTe5. Elemental and structural analyses revealed that five metals are mixed in a metal site of an NaCl-type structure. Since AgInSnPbBiTe5 has a cation site (Ag, In, Sn, Pb, and Bi) and an anion site (Te), this is the first example of a high-entropy-alloy (HEA) pseudo-binary superconductor. [ABSTRACT FROM AUTHOR]

Superconductivity is achieved in Nd1−xCexOBiS2 via electron doping using mixed-valence Ce ions. Single crystals with x = 0.2, 0.3, 0.4, and 0.5 are generated using a CsCl flux method. Plate-like single crystals with dimensions of 0.8 × 0.8 × 0.2mm³ were obtained. The magnetic susceptibility χ(T) indicates large diamagnetism, and the electrical resistivity ρ(T) indicates zero resistivity. The maximum value of Tc is observed at 4.7K in Nd0.8Ce0.2OBiS2 from χ(T). From the ρ(T) measurements taken in several magnetic fields, the upper critical field μ0Hc2(0) is estimated to be ∼12 and ∼0.34 T for the ab- and c-planes, respectively. We redetermined μ0Hc2(0) of NdO0.7F0.3BiS2, as ∼35 and ∼0.78 T for the ab- and c-planes, respectively. The anisotropic parameter Γ is estimated to be ∼35 for Nd0.7Ce0.3OBiS2 and ∼45 for NdO0.7F0.3BiS2. The μ0Hc2(0) of Nd0.7Ce0.3OBiS2 is approximately two times smaller than that of NdO0.7F0.3BiS2, although the difference of Γ is approximately 10. [ABSTRACT FROM AUTHOR]

We have investigated the in-plane anisotropy of the c-axis magnetoresistance for single crystals of a BiCh2-based superconductor LaO0.5F0.5BiSSe under in-plane magnetic fields. We observed two-fold symmetry in the c-axis magnetoresistance in the ab-plane of LaO0.5F0.5BiSSe while the crystal possessed a tetragonal square plane with fourfold symmetry. The observed symmetry lowering in magnetoresistance from the structural symmetry may be related to the nematic states, which have been observed in the superconducting states of several unconventional superconductors. [ABSTRACT FROM AUTHOR]

In BiS2-based layered superconductors, the existence of gap nodes on Fermi-surface curves has been suggested from angle-resolved photoemission spectroscopy measurements, whereas the conventional s-wave gap has been proposed from measurements of superfluid density and thermal conductivity. To reconcile these two distinct experimental results of the gap node, we investigate nonmagnetic impurity effects in the superconductor with a disconnected pocketlike Fermi-surface structure. Then, we claim that the seemingly contradictory situation concerning the gap node is resolved by a concept of dirty nodal extended s-wave superconductivity. Provided that it is unnecessary to consider the nodes of the gap, at first glance, the conventional s-wave gap seems to be a unique solution, but in the pocketlike Fermi-surface topology, a nontrivial possibility of a nodeless d-wave superconductor is pointed out. To clarify the gap symmetry, we propose to perform experiments on nuclear magnetic relaxation rate T-1 1 in BiS2-based layered superconductors. [ABSTRACT FROM AUTHOR]

In order to understand the mechanisms behind the emergence of superconductivity by the chemical pressure effect in REO0.5F0.5BiS2 (RE = La, Ce, Pr, and Nd), where bulk superconductivity is induced by the substitutions with a smallerradius RE, we performed synchrotron powder X-ray diffraction, and analyzed the crystal structure and anisotropic displacement parameters. With the decrease of the RE3+ ionic radius, the in-plane disorder of the S1 sites significantly decreased, very similar to the trend observed in the Se-substituted systems: LaO0.5F0.5BiS2-xSex and Eu0.5La0.5FBiS2-x-Sex. Therefore, the emergence of bulk superconductivity upon the suppression of the in-plane disorder at the chalcogen sites is a universal scenario for the BiCh2-based superconductors. In addition, we indicated that the amplitude of vibration along the c-axis of the in-plane chalcogen sites may be related to the Tc in the BiCh2-based superconductors. [ABSTRACT FROM AUTHOR]

We measured the electrical resistivity of the BiS2-based compound EuFBiS2 under high pressure. Polycrystalline EuFBiS2 shows insulator-metal transition and pressure-induced superconductivity above 0.7 GPa. The superconducting transition temperature increases with increasing applied pressure and shows a maximum value around 8.6K at 1.8 GPa. [ABSTRACT FROM AUTHOR]

A pressure-induced phase transition in LaO0.5F0.5BiSeS is demonstrated by performing resistance measurements with a diamond anvil cell. The superconducting transition temperature (Tc) of LaO0.5F0.5BiSeS changes from a low-Tc phase (4.9 K) to a high-Tc phase (7 K) owing to the application of pressure up to 5.1 GPa. [ABSTRACT FROM AUTHOR]

Superconducting behavior with exotic characteristics is often observed in materials with a layered two-dimensional crystal structure. Low dimensionality affects the electronic structure of these materials, potentially leading to high transition temperatures (Tc) and/or unconventional pairing mechanisms. In this letter, we report on superconductivity in layered tin arsenide NaSn2As2. The crystal structure consists of Sn2As2 bilayers, bound by the van der Waals forces and separated by Na+ ions. Measurements of electrical resistivity and specific heat confirm the bulk nature of superconductivity of NaSn2As2, with Tc of 1.3 K. Our results suggest that layered SnAs is a basic structure, offering another universality class in the family of layered superconductors. The results provide a new platform for the studies of physics and chemistry of low-dimensional superconductors with lone pair electrons. [ABSTRACT FROM AUTHOR]

We report the effect of Se substitution on the crystal structure and superconductivity of BiCh2-based (Ch: S, Se) layered compounds Eu0.5Ce0.5FBiS2-xSex (x = 0-1). Crystal structure analysis showed that both lattice constants, a and c, increased with increasing x, which is different from the related La-doped system Eu0.5La0.5FBiS2-xSex. This is due to Se substitution at both in-plane and out-of-plane Ch sites in the present Ce-doped system. Zero resistivity was observed for x = 0.2-1 above 2K. The superconducting properties of Eu0.5Ce0.5FBiS2-xSex were investigated by magnetic susceptibility measurement, and the highest superconducting transition temperature of 3.5K was obtained for x = 0.6 with a large shielding volume fraction. The emergence of bulk superconductivity and metallic conductivity can be qualitatively described in terms of the increased in-plane chemical pressure effect. A magnetic anomaly below 8K, probably because of the ferromagnetic order of the magnetic moment of Ce3+ ions, coexists with bulk superconductivity in the BiCh2 layer. Since the effect of Se substitution on the magnetic transition temperature is ignorable, we suggest that the coupling between the magnetic order at the (Eu,Ce)F layer and the superconductivity at the Bi(S,Se)2 layer is weak. [ABSTRACT FROM AUTHOR]

We have investigated the crystal structure and physical properties of LaO1-xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1-xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2-0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder. [ABSTRACT FROM AUTHOR]

We have synthesized AgSn1-xBixSe2 polycrystalline samples to investigate the effect of partial substitution of mixedvalence Sn by Bi3+ to the superconductivity of the valence-skip superconductor AgSnSe2. The Bi-substituted AgSn1-x- BixSe2 were obtained up to x = 0.2, but an insulating phase Ag2SnBi2Se5 with a NaCl-type structure showed up above x = 0.3. The superconducting transition temperature increased from 4.5K (x = 0) to 5.0K (x = 0.1) by Bi substitution. The enhancement of superconductivity by the suppression of the valence-skip states of Sn suggests that the valence-skip states of Sn are not positively linked to the pairing mechanisms of superconductivity in the AgSnSe2 system. [ABSTRACT FROM AUTHOR]

We perform first-principles band structure calculations for the tetragonal and monoclinic structures of LaO0.5F0.5BiS2. We find that the Bi 6px,y bands on two BiS2 layers exhibit a sizable splitting at the X = (π, 0, 0) and several other kpoints for the monoclinic structure. We show that this feature originates from the inter-BiS2 layer coupling strongly enhanced by the symmetry breaking of the crystal structure. The Fermi surface also shows a large splitting and becomes anisotropic with respect to the kx- and ky-directions in the monoclinic structure, whereas it remains almost flat with respect to the kz-direction. [ABSTRACT FROM AUTHOR]

To study the superconducting gap structure of BiS2-based layered compound NdO0.71F0.29BiS2 (Tc = 5 K), we measured the thermal conductivity κ, which is a sensitive probe of low-energy quasiparticle spectrum. In the absence of a magnetic field, residual linear term in the thermal conductivity κ0=T at T → 0 is vanishingly small, indicating that the residual normal fluid, which is expected for nodal superconductors, is absent. Moreover, the applied magnetic field hardly affects thermal conductivity in wide range of the vortex state, indicating the absence of Doppler shifted quasiparticles. These results provide evidence that NdO0.71F0.29BiS2 is a fully gapped superconductor. The obtained gap structure, along with the robustness of the superconductivity against the impurity, suggest a conventional s-wave superconducting state in NdO0.71F0.29BiS2. [ABSTRACT FROM AUTHOR]

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 ρ(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 x 105 and 5.07 x 105 A/cm² (2 K) for x = 0.2 and 0.8 respectively, using the Bean's model. The thermal activation energy (U0=kB) is 61K for x = 0.2 and 140K 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. [ABSTRACT FROM AUTHOR]

We report X-ray diffraction studies of single-crystal samples of LaOBiS2 and CeOBiS2, which are mother materials for BiS2-based layered superconductors. The structural symmetry in LaOBiS2 is lowered from tetragonal to monoclinic, whereas no such symmetry lowering occurs in CeOBiS2. Crystal structure analysis demonstrates that the space group of LaOBiS2 is monoclinic P21m. The in-plane S atoms uniformly shift along the a-axis in each BiS2 layer. Because the directions of the shift are opposite in the two adjacent BiS2 layers, the spatial inversion symmetry remains. The distortion is attributed to the in-plane lattice mismatch between the BiS2 layers and the block layers. [ABSTRACT FROM AUTHOR]

Pressure effects on the recently discovered BiS2-based superconductor Bi2(O,F)S2 (Tc = 5.1 K) were examined by two different methods: high-pressure resistivity measurement and high-pressure annealing. The effects of these two methods on the superconducting properties of Bi2(O,F)S2 were significantly different, although in both methods, hydrostatic pressure is applied to the sample using a cubic-anvil-type apparatus. In high-pressure resistivity measurement, Tc linearly decreased at a rate of -1.2K GPa-1. In contrast, the Tc of 5.1K is maintained after high-pressure annealing under 2 GPa and 470 °C of an optimally doped sample despite a significant change in lattice parameters. In addition, superconductivity was observed in fluorine-free Bi2OS2 after high-pressure annealing. These results suggest that high-pressure annealing has a unique effect on the physical properties of layered compounds. [ABSTRACT FROM AUTHOR]

We have studied crystal structure and physical properties of REO1-yFyBiS2 (Ce1-xNdxO1-yFyBiS2 and Nd1-zSmzO1-yFyBiS2) with three different F concentration (y = 0.7, 0.5, and 0.3) to investigate relationship between the emergence of superconductivity and crystal structure in the REO1-yFyBiS2 series. REO1-yFyBiS2 is suitable for discussing chemical pressure effect on physical properties because the RE site at the blocking layer can be fully or partially substituted by various RE3+ ions, which could systematically tune the lattice volume. With increasing chemical pressure (with decreasing lattice volume), lattice constant of a-axis systematically decreases while lattice constant of c-axis does not show a remarkable change, indicating that the RE site substitution basically affect the lattice constant of a-axis. On the other hand, lattice constant of c-axis can be tuned by F concentration. On the basis of magnetic susceptibility measurements, we have obtained three kinds of superconductivity phase diagram with y = 0.7, 0.5, and 0.3 as a function of RE concentration (chemical pressure). For all the systems with y = 0.7, 0.5, and 0.3, chemical pressure basically increases superconducting transition temperature (Tc) with increasing chemical pressure. Having compared these three phase diagrams, we have suggested that there are at least two important structure parameters, (1) lattice contraction along a-axis, (2) optimal lattice contraction ratio (c/a), are essential for the emergence of superconductivity and increase in Tc in REO1-yFyBiS2. [ABSTRACT FROM AUTHOR]

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 superconductivity 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. [ABSTRACT FROM AUTHOR]

We show the observation of the coexistence of bulk superconductivity and magnetic ordering in CeO1-xFxBiS2 (x = 0-1.0) prepared by annealing under high pressure. In the CeO1-xFxBiS2 system, both superconductivity and two types of ferromagnetic-like ordering with magnetic transition temperatures of 4.5 and 7.5 K are induced upon systematic F substitution. This fact suggests that carriers generated by the substitution of O by F are supplied to not only the BiS2 superconducting layers but also the CeO blocking layers. Furthermore, the highest superconducting transition temperature is observed when the ferromagnetic-like transition temperature increases, which implies that superconductivity and this magnetic ordering are linked to each other in the CeO1-xFxBiS2 system. [ABSTRACT FROM AUTHOR]

The low-temperature properties of CeOBiS² single crystals are studied by electrical resistivity, magnetization, and specific heat measurements. Ce 4f-electrons are found to be in a well-localized state split by crystalline-electric-field (CEF) effects. The CEF ground state is a pure Jz = ±1/2 doublet, and excited doublets are located far above. At low temperatures in zero field, we observe pronounced--log T divergence in the specific heat, revealing the presence of quantum critical fluctuations of 4 f magnetic moments near a quantum critical point (QCP). Considering that CeOBiS² is a nonmetal, this phenomenon cannot be attributed to the competition between Kondo and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions as in numerous f-electron-based strongly correlated metals, indicating an unconventional mechanism. We suggest that CeOBiS² is the first material found to be located at a QCP among geometrically frustrated nonmetallic magnets. [ABSTRACT FROM AUTHOR]

We present scanning tunneling microscopy measurements on a cleaved surface of the recently discovered superconductor NdO0.7F0.3BiS2 with a transition temperature (Tc) of 5.1 K. Tunneling spectra at 4.2 K (below Tc) and 22 K (above Tc) show a large spectroscopic gap (~40 mV), which is inconsistent with the metallic nature demonstrated in bulk measurements. Moreover, we find two interesting real-space electronic features. The first feature is a "checkerboard stripe" electronic state characterized by an alternating arrangement of two types of nanocluster. In one cluster, one-dimensional electronic stripes run along one Bi-Bi direction, whereas, in the other cluster, the stripes run along the other Bi-Bi direction. The second feature is a nanoscale electronic inhomogeneity whose microscopic source seems to be atomic defects on the cleaved surface or dopant F atoms. [ABSTRACT FROM AUTHOR]

BiS2-based LaO1-xFxBiS2 (x = 0.5) becomes superconductive at Tc = 2.5 K. Electrical resistivity and magnetization measurements are performed under pressure to determine the pressure dependence of the superconducting transition temperature Tc up to 18 GPa. We observe that Tc abruptly increases from 2.5 to 10.7K at a pressure of 0.7 GPa. According to high-pressure X-ray diffraction measurements of LaO1-xFxBiS2 (x = 0 and 0.5), a structural phase transition from tetragonal to monoclinic also occurs at ~0.8 GPa. We consider that the pressure-induced enhancement of superconductivity for LaO1-xFxBiS2 is caused by the structural phase transition. [ABSTRACT FROM AUTHOR]

We report the impact of hydrostatic pressure (0-1.97 GPa) on superconductivity of recently discovered 2K superconductor Sr0.5La0.5FBiS2. Resistivity under pressure measurements are performed by using HPC-33 Piston type pressure cell with Quantum design DC resistivity Option. The superconducting transition temperature (Tc) is increased by 5 fold to around 10K with just above 1 GPa pressure, which remains nearly unaltered for studied higher pressures of up to 1.97 GPa. The fivefold increase in Tc accompanied with decrease in normal state resistivity of Sr0.5La0.5FBiS2 with just above 1 GPa pressure calls for the attention of solid state physics community. [ABSTRACT FROM AUTHOR]

We use core level and valence band soft X-ray photoemission spectroscopy (SXPES) to investigate the electronic structure of new BiS2 layered superconductor LaO1-xFxBiS2. The core level spectra of doped samples show a new spectral feature on the lower binding energy side of the Bi 4f main peak, which may be explained by core-hole screening with metallic states near the Fermi level (EF). The experimental electronic structure and its x dependence (a higher binding energy shift of the valence band as well as appearance of new states near EF having the dominant Bi 6p character) were found to be consistent with the predictions of band structure calculations in general. Noticeable deviation of the spectral shape of the states near EF from that of calculations might provide insight into interesting physical properties. This study provides the first experimental electronic structure of the new BiS2 layered superconductors. [ABSTRACT FROM AUTHOR]

We have examined the electrical transport properties of densified LaOBiS2-xSex, which constitutes a new family of thermoelectric materials. The power factor increases with increasing concentration of Se, i.e., Se substitution leads to an enhanced electrical conductivity, without suppression of the Seebeck coefficient. Hall measurements reveal that the carrier mobility increases with decreasing carrier concentration as Se doping, which is responsible for the low electrical resistivity and large absolute values of the Seebeck coefficient in the system. [ABSTRACT FROM AUTHOR]

We report the superconducting properties of Y-doped semimetallic Bi3O2S3 superconductors. From electrical resistivity measurements, the Tc,zero of the optimally Y-doped Bi3O2S3 was higher than that of pure Bi3O2S3. This Y content dependence of the superconducting properties in Bi3O2S3 is explained by the chemical pressure effect. [ABSTRACT FROM AUTHOR]

We demonstrate the electrical and thermal transport of the layered bismuth-based sulfide EuBiS2F from 300 to 623 K. Although significant hybridization between Eu 4f and Bi 6p electrons was reported previously, the carrier transport of the compound is similar to that of F-doped LaBiS2O, at least above 300 K. The lattice thermal conductivity is lower than that of isostructural SrBiS2F, which is attributed to the heavier atomic mass of Eu ions than that of Sr ions. [ABSTRACT FROM AUTHOR]