arXiv subjects

Akira Miura

Publications and source records attributed to Akira Miura.

47 records · Page 3Linked to original sources

arXiv2017

Synthesis, Crystal Structure, and Physical Properties of New Layered Oxychalcogenide La2O2Bi3AgS6

We have synthesized a new layered oxychalcogenide La2O2Bi3AgS6. From synchrotron X-ray diffraction and Rietveld refinement, the crystal structure of La2O2Bi3AgS6 was refined using a model of the P4/nmm space group with a = 4.0644(1) Å and c = 19.412(1) Å, which is similar to the related compound LaOBiPbS3, while the interlayer bonds (M2-S1 bonds) are apparently shorter in La2O2Bi3AgS6. The tunneling electron microscopy (TEM) image confirmed the lattice constant derived from Rietveld refinement (c ~ 20 Å). The electrical resistivity and Seebeck coefficient suggested that the electronic states of La2O2Bi3AgS6 are more metallic than those of LaOBiS2 and LaOBiPbS3. The insertion of a rock-salt-type chalcogenide into the van der Waals gap of BiS2-based layered compounds, such as LaOBiS2, will be a useful strategy for designing new layered functional materials in the layered chalcogenide family.

cond-mat.mtrl-sci↗

arXiv2017

Evolution of Anisotropic Displacement Parameters and Superconductivity with Chemical Pressure in BiS2-Based REO0.5F0.5BiS2 (RE = La, Ce, Pr, and Nd)

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 smaller-radius 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-xSex. 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.

cond-mat.supr-con↗

arXiv2017

Intrinsic Phase Diagram of Superconductivity in the BiCh2-based System Without In-plane Disorder

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.

cond-mat.supr-con↗

arXiv2017

Bi Substitution Effects on Superconductivity of Valence-Skip superconductor AgSnSe2

We have synthesized AgSn1-xBixSe2 polycrystalline samples to investigate the effect of partial substitution of mixed-valence Sn by Bi3+ to the superconductivity of the valence-skip superconductor AgSnSe2. The Bi-substituted AgSn1-xBixSe2 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 4.5 K (x = 0) to 5.0 K (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.

cond-mat.supr-con↗

arXiv2016

Superconductivity in CeOBiS2 with cerium valence fluctuation

Resistivities of single-crystalline as well as poly-crystalline samples of CeOBiS2 without fluorine doping were measured at temperatures down to 0.13 K, and were compared with those of poly-crystalline LaOBiS2 and PrOBiS2. Both poly-crystalline and single-crystalline CeOBiS2 exhibited zero resistivity below 1.2 K while poly-crystalline LaOBiS2 and PrOBiS2 did not show zero resistivity down to 0.13 K. Superconducting transition temperature of CeOBiS2 was reduced by increasing the applied current density. The superconductivity of CeOBiS2 without chemical doping is likely triggered by the carriers induced by the valence fluctuation between Ce3+ and Ce4+.

cond-mat.supr-con↗

arXiv2015

Structures and Optical Absorption of Bi2OS2 and LaOBiS2

The band gaps of isostructural Bi2OS2 and LaOBiS2 were examined using optical absorption and discussed with the band structures calculated based on the crystal structures determined using synchrotron X-ray diffraction. The Bi 6p and S 3p orbitals in the Bi-S plane were computationally predicted to constitute the bands near the Fermi level. The optical reflectance spectra of Bi2OS2 and LaOBiS2 showed optical band gaps of ca. 1.0 eV, which were close to the computationally calculated direct band gaps of ca. 0.8 eV. Our results show that Bi2OS2 and LaOBiS2 are semiconductors containing direct band gaps of 0.8-1.0 eV, and they are suggested to be candidates for optoelectronic materials in the near-infrared region without highly toxic elements.

cond-mat.mtrl-sci↗

arXiv2015

C-axis electrical resistivity of PrO$_{1-a}$F$_a$BiS$_2$ single crystals

The high anisotropy in RO1-aFaBiS2 (R denotes a rare-earth element) superconductors demonstrates their potential use as intrinsic Josephson junctions, considering the weak coupling among BiS2-PrO(F)-BiS2 (superconducting-normal-superconducting) layers along the c-axis. We grew PrO1-aFaBiS2 single crystals using CsCl/KCl flux. The superconducting anisotropies of the grown single crystals were estimated to be approximately 40-50 from the effective mass model. The c-axis transport properties were characterized using single-crystal s-shaped intrinsic Josephson junctions with a focused ion beam. Along the c-axis, the crystals showed zero resistivity at 2.7 K and a critical current density of 1.33*10^3 A/cm2 at 2.0 K. The current-voltage curve along the c-axis displayed hysteresis. The c-axis transport measurements under a magnetic field parallel to the ab-plane revealed a "lock-in" state due to the Josephson vortex flow, indicating that BiS2 superconductors are promising candidates for intrinsic Josephson junctions.

cond-mat.supr-con↗

arXiv2015

In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure

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 structure 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.

cond-mat.supr-con↗

arXiv2014

Crystal structures of LaO1-xFxBiS2 (x~0.23, 0.46): effect of F doping on distortion of Bi-S plane

The crystal structures of superconducting LaO1-xFxBiS2 (x ~ 0.23, 0.46) were determined by single-crystal X-ray diffraction analysis. Their space group was P4/nmm. Distortion of the Bi-S plane changed when the F content was increased from 0.23 to 0.46, and a nearly flat Bi-S plane was formed at x ~ 0.46. Computational calculations supported this effect of F doping on distortion of Bi-S plane. LaO1-xFxBiS2 with higher F contents were computationally predicted to be thermodynamically more unstable under ambient pressure. We discussed the bonding, conductivities, and synthetic routes of LaO1-xFxBiS2.

cond-mat.supr-con↗

arXiv2013

Growth and superconducting properties of F-substituted ROBiS2 (R = La, Ce, Nd) single crystals

F-substituted ROBiS2 (R = La, Ce, Nd) superconducting single crystals with different F concentration were grown successfully using CsCl/KCl flux. All the obtained single crystals had a plate-like shape with a well-developed ab-plane of 1-2 mm in size. The flux components of Cs, K, and Cl were not detected in the obtained single crystals by electron probe microanalysis. The grown single crystals of F-substituted LaOBiS2 and CeOBiS2 showed superconducting at about 3 K while the Tc of the F-substituted NdOBiS2 exhibited approximately 5 K. The superconducting anisotropy of the single crystals of F-substituted LaOBiS2 and NdOBiS2 was estimated to be 30-45 according to the effective mass model whereas those values were 13-21 for the F-substituted CeOBiS2 single crystals. The F-substituted CeOBiS2 single crystals exhibited magnetic order at about 7 K that apparently coexisted with superconductivity below around 3 K.

cond-mat.supr-con↗

arXiv2013

Structural Analysis and Superconducting Properties of F-Substituted NdOBiS2 Single Crystals

F-substituted NdOBiS2 superconducting single crystals were grown using CsCl/KCl flux. This is the first example of the single-crystal growth of a BiS2-based superconductor. The obtained single crystals had a plate-like shape with a size of 1-2 mm and a well-developed ab-plane. The crystal structure of the grown crystals was determined by single-crystal X-ray diffraction analysis to be the tetragonal space group P4/nmm (#129) with a = 3.996(3) A and c = 13.464(6) A. The chemical formula of the grown crystals was approximately Nd0.98(0.06)O0.7(0.1)F0.3(0.1)Bi0.98(0.04)S2, and Cs, K, and Cl were not detected in the grown crystals by electron probe microanalysis. The grown crystals had a critical temperature of approximately 5 K. The superconducting anisotropy of the single crystals was estimated to be about 30 from the effective mass model and the upper critical field.

cond-mat.supr-con↗