arXiv subjects

Yosuke Goto

Publications and source records attributed to Yosuke Goto.

At least 19 recordsLinked to original sources

arXiv2022

Glassy atomic vibrations and blurry electronic structures created by local structural disorders in high-entropy metal telluride superconductors

The motivation of this work is our recent observation of the robustness of superconductivity in a High-entropy (HE) superconductor Ag0.2In0.2Sn0.2Pb0.2Bi0.2Te (CsCl-type) to external pressure. The superconducting transition temperature (Tc) of Ag0.2In0.2Sn0.2Pb0.2Bi0.2Te is almost constant with pressure, described as robustness of superconductivity to pressure, whereas the PbTe with zero configurational entropy of mixing exhibits a clear decrease in Tc with pressure. Here, we investigated the atomic displacement parameters (Uiso), the atomic-vibration characteristics, and the electronic states of metal tellurides (MTe) with various configurational entropy of mixing (DSmix) at the M site. The Uiso for the M site is clearly increased by M-site alloying with DSmix > 1.1R, which is the evidence of local disorder introduced by the increase in DSmix via the solution of three or more M elements. The revealed vibrational density of states (DOS) shows a remarkable broadening with DSmix > 1.1R, which indicates glassy characteristics of atomic vibration in HE MTe with a NaCl-type structure (low-pressure phase). On the electronic states of the CsCl-type (high-pressure) phases, where the robustness of Tc is observed, blurry electronic band structure appears with increasing DSmix, which indicates the evolution of blurry (glassy) electronic states in HE MTe with the CsCl-type structure. The estimated electronic DOS at Fermi energy cannot explain the changes in Tc for HE MTe when assuming conventional electron-phonon superconductivity, but the conventional explanation seems to work for PbTe. Therefore, the pairing mechanisms in MTe with DSmix > 1.1R are affected by glassy phonon and/or blurry electronic states in MTe, and the robustness of superconductivity would be originating from unique electron-phonon coupling.

cond-mat.supr-con↗

arXiv2022

Lattice Anharmonicity in BiS2-Based Layered Superconductor RE(O,F)BiS2 (RE = La, Ce, Pr, Nd)

We studied 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 substituting the RE elements. With increasing in-plane chemical pressure, bulk modulus remarkably increases, and a high γG is observed for RE = Nd. The dependence of γG on in-plane chemical pressure exhibits a good correlation with Tc, and a higher Tc is achieved when γG is large for RE = Nd. In addition, γG shows a slight decrease by a decrease of F concentration (x) in REO1-xFxBiS2. Our results show that the anharmonic vibration of Bi along the c-axis is present in REO1-xFxBiS2, and the enhancement of the anharmonicity is positively linked to superconducting Tc and pairing mechanisms.

cond-mat.supr-con↗

arXiv2022

The upper critical field in the BiCh2-based superconductors CeOBiS1.7Se0.3 and PrO0.5F0.5BiS2-xSex (x = 0, 0.3)

We report the upper critical field (Bc2) in the BiCh2-based superconductors CeOBiS1.7Se0.3 and PrO0.5F0.5BiS2-xSex (x = 0, 0.3). Single crystals of CeOBiS1.7Se0.3 and PrO0.5F0.5BiS2-xSex (x = 0, 0.3) were grown using the flux method. Single-crystal structural analysis revealed that the crystal structure at room temperature is tetragonal (P4/nmm). Through electrical resistivity and magnetization measurements, bulk superconductivity was observed in all samples. For CeOBiS1.7Se0.3, the in-plane Bc2 is smaller than the conventional orbital limit and Pauli limit, suggesting that ferromagnetic ordering, which has been observed in a related Ce-containing BiCh2-based compound, affects Bc2 and superconductivity. In contrast, high in-plane Bc2 was observed for PrO0.5F0.5BiS2-xSex (x = 0, 0.3). We propose that the in-plane Bc2 is enhanced by antisymmetric spin-orbit coupling, which arises from the lack of local inversion symmetry.

cond-mat.supr-con↗

arXiv2022

Extremely high upper critical field in BiCh$_2$-based (Ch: S and Se) layered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2-x}$Se$_x$ ($x$ = 0.22 and 0.69)

Centrosymmetric compounds with local inversion symmetry breaking have tremendously interesting and intriguing physical properties. In this study, we focus on a BiCh$_2$-based (Ch: S, Se) layered superconductor, as a system with local inversion asymmetry, because spin polarisation based on the Rashba-Dresselhaus-type spin-orbit coupling has been observed in centrosymmetric BiCh$_2$-based LaOBiS$_2$ systems, while the BiCh$_2$ layer lacks local inversion symmetry. Herein, we report the existence of extremely high in-plane upper critical fields in the BiCh$_2$-based system LaO$_{0.5}$F$_{0.5}$BiS$_{2-x}$Se$_x$ ($x$ = 0.22 and 0.69). The superconducting states are not completely suppressed by the applied magnetic fields with strengths up to 55 T. Thus, we consider that the in-plane upper critical field is enhanced by the local inversion symmetry breaking and its layered structure. Our study will open a new pathway for the discovery of superconductors that exhibit a high upper critical field by focusing on the local inversion symmetry breaking.

cond-mat.supr-con↗

arXiv2021

Robustness of superconductivity to external pressure in high-entropy-alloy-type metal telluride AgInSnPbBiTe5

High-entropy-alloy (HEA) superconductors are a new class of disordered superconductors. In this study, we investigate the robustness of superconducting states in HEA-type metal telluride (MTe; M = Ag, In, Sn, Pb, Bi) under high pressure. PbTe exhibits a structural transition from a NaCl-type to an orthorhombic Pnma structure at low pressures, and further transitions to a CsCl-type structure at high pressures. When the superconductivity of the CsCl-type PbTe is observed, it is found that its superconducting transition temperature (Tc) decreases with pressure. However, in the HEA-type AgInSnPbBiTe5, Tc is almost independent of pressure, for pressures ranging from 13.0 to 35.1 GPa. This trend is quite similar to that observed in an HEA superconductor (TaNb)0.67(HfZrTi)0.33, which shows that the robustness of superconductivity to external pressure is a universal feature in HEA-type superconductors. To clarify the effects of the modification of the configurational entropy of mixing on the crystal structure, superconducting states, and electronic structure of MTe, electrical resistance measurements, synchrotron X-ray diffraction, and synchrotron X-ray absorption spectroscopy with partial fluorescence mode (PFY-XAS) for three MTe polycrystalline samples of PbTe, AgPbBiTe3, and AgInSnPbBiTe5 with different configurational entropies of mixing at the M site were performed.

cond-mat.supr-con↗

arXiv2021

Improvement of upper critical field by the compositionally-complex-alloy concept in A15 superconductor V3X (X: Al, Si, Ga, Ge, Sn)

Compositionally complex alloys (CCAs) are a new category of high-entropy materials containing more than two high-entropy phases in an alloy sample. We applied the concept of CCA to a A15-type compound superconductor V3X where X site is occupied by Al, Si, Ga, Ge, and Sn. The V3X sample shows bulk superconductivity with a transition temperature (Tc) of 6.4 K, which is close to the Tc of V3Ge. The upper critical field (Bc2(0)) of V3X estimated from magnetic susceptibility is 8.8 T, which is ~50% higher than that of V3Ge (Bc2(0) = 5.8 T). The presence of at least five different A15 phases with high configurational entropy of mixing were revealed. We consider that the examined V3X sample is an example system of CCA-type superconducting intermetallic compound.

cond-mat.supr-con↗

arXiv2021

Superconductivity in In-doped AgSnBiTe3 with possible band inversion

We investigated the chemical pressure effects on structural and electronic properties of SnTe-based material using partial substitution of Sn by Ag0.5Bi0.5, which results in lattice shrinkage. For Sn1-2x(AgBi)xTe, single-phase polycrystalline samples were obtained with a wide range of x. On the basis of band calculations, we confirmed that the Sn1-2x(AgBi)xTe system is basically possessing band inversion and topologically preserved electronic states. To explore new superconducting phases related to the topological electronic states, we investigated the In-doping effects on structural and superconducting properties for x = 0.33 (AgSnBiTe3). For (AgSnBi)(1-y)/3InyTe, single-phase polycrystalline samples were obtained for y = 0-0.5 by high-pressure synthesis. Superconductivity was observed for y = 0.2-0.5. For y = 0.4, the transition temperature estimated from zero-resistivity state was 2.4 K, and the specific heat investigation confirmed the emergence of bulk superconductivity. Because the presence of band inversion was theoretically predicted, and the parameters obtained from specific heat analyses were comparable to In-doped SnTe, we expect that the (AgSnBi)(1-y)/3InyTe and other (Ag,In,Sn,Bi)Te phases are candidate systems for studying topological superconductivity.

cond-mat.supr-con↗

arXiv2021

Axis-dependent carrier polarity in polycrystalline NaSn$_2$As$_2$

Transverse thermoelectric devices consist of only one thermoelectric material, unlike conventional longitudinal thermoelectric devices that require two types of thermoelectric materials with p- and n-type polarities. However, scalable synthesis of materials that demonstrate axis-dependent carrier polarity, which is a prospective component to demonstrate the transverse thermoelectric device, is challenging. This paper reports that polycrystalline NaSn$_2$As$_2$, which was prepared by using uniaxial hot pressing, displayed axis-dependent carrier polarity. The preferred orientation of the sample was confirmed through X-ray diffraction measurements. Seebeck coefficient measurements indicate that carrier polarity depends on the measurement direction, which is consistent with recently reported results on single crystals of NaSn$_2$As$_2$. Given that our sample preparation procedure is readily scalable, the present work shows the possibility for preparing transverse thermoelectric devices using polycrystalline NaSn$_2$As$_2$ with a preferred orientation.

physics.app-ph↗

arXiv2021

Anomalous broadening of specific heat jump at Tc in high-entropy-alloy-type superconductor TrZr2

A high-entropy-alloy-type (HEA-type) superconductor is new category of highly disordered superconductors. Therefore, finding brand-new superconducting characteristics in the HEA-type superconductors would open new avenue to investigate the relationship between structural disorder and superconductivity. Here, we report on the remarkable broadening of specific heat jump near a superconducting transition tempreature (Tc) in transition-metal zirconides (TrZr2) with different mixing entropy (ΔSmix) at the Tr site. With increasing ΔSmix, the superconducting transition seen in specific heat became broader, whereas those seen in magnetization were commonly sharp. Therefore the broadening of specific heat jump would be related to the microscopic inhomogeneity of the formation of Cooper pairs behind the emergence of bulk superconductivity states.

cond-mat.supr-con↗

arXiv2020

Possible pairing mechanism switching driven by structural symmetry breaking in BiS2-based layered superconductors

Investigation of isotope effects on superconducting transition temperature (Tc) is one of the useful methods to examine whether electron-phonon interaction is essential for pairing mechanisms. The layered BiCh2-based (Ch: S, Se) superconductor family is a candidate for unconventional superconductors, because unconventional isotope effects have previously been observed in La(O,F)BiSSe and Bi4O4S3. In this study, we investigated the isotope effects of 32S and 34S in the high-pressure phase of (Sr,La)FBiS2, which has a monoclinic crystal structure and a higher Tc of 10 K under high pressures, and observed conventional-type isotope shifts in Tc. The conventional-type isotope effects in the monoclinic phase of (Sr,La)FBiS2 are different from the unconventional isotope effects observed in La(O,F)BiSSe and Bi4O4S3, which have a tetragonal structure. The obtained results suggest that the pairing mechanisms of BiCh2-based superconductors could be switched by a structural-symmetry change in the superconducting layers induced by pressure effects.

cond-mat.supr-con↗

arXiv2020

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 La2O2Bi3Ag0.6Sn0.4S6, 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 temperature Tc of ~ 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 a Tc ~ 8 K were observed. Our results suggest the possible commonality on the factor essential for Tc in Bi-based superconductors with two-layer-type and four-layer-type conducting layers.

cond-mat.supr-con↗

arXiv2020

Investigation of superconducting properties and possible nematic superconductivity in self-doped BiCh2-based superconductor CeOBiS1.7Se0.3

We investigate the superconducting properties and possible nematic superconductivity of self-doped BiCh2-based (Ch: S, Se) superconductor CeOBiS1.7Se0.3 through the measurements of in-plane anisotropy of magnetoresistance. Single crystals of CeOBiS1.7Se0.3 were grown using a flux method. Single-crystal structural analysis revealed that the crystal structure at room temperature is tetragonal (P4/nmm). Bulk superconductivity with a transition temperature of 3.3 K was observed through electrical resistivity and magnetization measurements. Investigation of anisotropy of upper critical field suggested relatively low anisotropy in the crystal as compared to other BiCh2-based superconductors. In the superconducting states of CeOBiS1.7Se0.3, two-fold symmetric in-plane anisotropy of magnetoresistance was observed, which indicates the in-plane rotational symmetry breaking in the tetragonal structure and hence the possibility of nematic superconductivity in CeOBiS1.7Se0.3.

cond-mat.supr-con↗

arXiv2020

Synthesis of high-entropy-alloy-type superconductors (Fe,Co,Ni,Rh,Ir)Zr2 with tunable transition temperature

We report on the synthesis and superconductivity of high-entropy-alloy-type (HEA-type) compounds TrZr2 (Tr = Fe, Co, Ni, Rh, Ir), in which the Tr site satisfies the criterion of HEA. Polycrystalline samples of HEA-type TrZr2 with four different compositions at the Tr site were synthesized by arc melting method. The phase purity and crystal structure were examined by Rietveld refinement of X-ray diffraction profile. It has been confirmed that the obtained samples have a CuAl2-type tetragonal structure. From analyses of elemental composition and mixing entropy at the Tr site, the HEA state for the Tr site was confirmed. The physical properties of obtained samples were characterized by electrical resistivity and magnetization measurements. All the samples show bulk superconductivity with various transition temperature (Tc). The Tc varied according to the compositions and showed correlations with the lattice constant c and Tr-Zr bond lengths. Introduction of an HEA site in TrZr2 is useful to achieve systematic tuning of Tc with a wide temperature range, which would be a merit for superconductivity application.

cond-mat.supr-con↗

arXiv2020

Structural phase diagram of LaO1-xFxBiSSe: suppression of the structural phase transition by partial F substitutions

We have investigated low-temperature crystal structure of BiCh2-based compounds LaO1-xFxBiSSe (x = 0, 0.01, 0.02, 0.03, and 0.5), in which anomalous two-fold-symmetric in-plane anisotropy of superconducting states has been observed for x = 0.5. From synchrotron X-ray diffraction experiments, a structural transition from tetragonal to monoclinic was observed for x = 0 and 0.01 at 340 and 240 K, respectively. For x = 0.03, a structural transition and broadening of the diffraction peak were not observed down to 100 K. These facts suggest that the structural transition could be suppressed by 3% F substitution in LaO1-xFxBiSSe. Furthermore, the crystal structure for x = 0.5 at 4 K was examined by low-temperature (laboratory) X-ray diffraction, which confirmed that the tetragonal structure is maintained at 4 K for x = 0.5. Our results suggest that the two-fold-symmetric in-plane anisotropy of superconducting states observed for LaO0.5F0.5BiSSe was not originated from structural symmetry lowering.

cond-mat.supr-con↗

arXiv2020

Investigation of in-plane anisotropy of c-axis magnetoresistance for BiCh2-based layered superconductor NdO0.7F0.3BiS2

We have investigated the in-plane anisotropy of the c-axis magnetoresistance (MR) in both superconducting and normal states of single crystals of NdO0.7F0.3BiS2 under in-plane magnetic fields. In the superconducting states of NdO0.7F0.3BiS2, four-fold-symmetric in-plane anisotropy of the c-axis MR was observed below the superconducting transition temperature. Since the crystal structure of NdO0.7F0.3BiS2 is tetragonal, the rotational symmetry in the superconducting state is preserved in the present compound. This result is clearly different from the previous report observed in LaO0.5F0.5BiSSe single crystals, where the in-plane MR in the superconducting state shows two-fold symmetry. On the other hand, in the normal states of NdO0.7F0.3BiS2, two-fold symmetric MR with a small amplitude was observed. The possible origin of the two-fold-symmetric behavior was discussed with the presence of local structural disorder in the conducting plane of BiCh2-based compounds.

cond-mat.supr-con↗

arXiv2020

Two-fold symmetry of in-plane magnetoresistance anisotropy in the superconducting states of BiCh2-based LaO0.9F0.1BiSSe single crystal

Recently, two-fold symmetric in-plane anisotropy of the superconducting properties have been observed in a single crystal of BiCh2-based (Ch: S, Se) layered superconductor LaO0.5F0.5BiSSe having a tetragonal (four-fold-symmetric) in-plane structure; the phenomena are very similar to those observed in nematic superconductors. To explore the origin of the two-fold symmetric anisotropy in the BiCh2-based system, we have investigated the electron-doping dependence on the anisotropy by examining the in-plane anisotropy of the magnetoresistance in the superconducting states for a single crystal of LaO0.9F0.1BiSSe under high magnetic fields up to 15 T. We observed a two-fold symmetry of in-plane anisotropy of magnetoresistance for LaO0.9F0.1BiSSe. The results obtained for LaO0.9F0.1BiSSe are quite similar to those observed for LaO0.5F0.5BiSSe, which has a higher electron doping concentration than LaO0.9F0.1BiSSe. Our present finding suggests that the emergence of the in-plane symmetry breaking in the superconducting state is robust to the carrier concentration in the series of LaO1-xFxBiSSe.

cond-mat.supr-con↗

arXiv2020

Evolution of two bulk-superconducting phases in Sr0.5RE0.5FBiS2 (RE: La, Ce, Pr, Nd, Sm) by external hydrostatic pressure effect

Polycrystalline samples of Sr1-xRExFBiS2 (RE: La, Ce, Pr, Nd, and Sm) were synthesized via the solid-state reaction and characterized using synchrotron X-ray diffraction. Although all the Sr0.5RE0.5FBiS2 samples exhibited superconductivity at transition temperatures (Tc) within the range of 2.1-2.7 K under ambient pressure, the estimated superconducting volume fraction was small. This indicated the non-bulk nature of superconductivity in these samples under ambient pressure. A dramatic evolution of the bulk superconducting phases was achieved on applying an external hydrostatic pressure. Near pressures below 1 GPa, bulk superconductivity was induced with a Tc of 2.5-2.8 K, which is termed as the low-P phase. Moreover, the high-P phase (Tc = 10.0-10.8 K) featuring bulk characteristics was observed at higher pressures. Pressure-Tc phase diagrams indicated that the critical pressure for the emergence of the high-P phase tends to increase with decreasing ionic radius of the doped RE ions. According to the high-pressure X-ray diffraction measurements of Sr0.5La0.5FBiS2, a structural phase transition from tetragonal to monoclinic also occurred at approximately 1.1 GPa. Thus, this phase transition indicates a pressure-induced superconducting-superconducting transition similar to the transition in LaO0.5F0.5BiS2. Bulk superconducting phases in Sr0.5RE0.5FBiS2 induced by the external hydrostatic pressure effect are expected to be useful for evaluating the mechanisms of superconductivity in BiCh2-based superconductors.

cond-mat.supr-con↗

arXiv2020

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 < 0.7), 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 W/mK 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.

cond-mat.mtrl-sci↗