arXiv subjects

Yoshikazu Mizuguchi

Publications and source records attributed to Yoshikazu Mizuguchi.

At least 55 records · Page 3Linked to original sources

arXiv2023

Evaluation of insulating behavior by Pb Substitution and Pressure-Induced Superconductivity in La2O2Bi3-xPb1+xS6

We report synthesis, characterization, and physical properties of layered bismuth-sulfide compounds La2O2Bi3-xPb1+xS6. We synthesized a new La2O2Bi3PbS6 compound, whose crystal structure is similar to those of the La2O2Bi3AgS6 superconductor or La2O2Bi2Pb2S6 (insulator) with a four-layer-type conducting layer. The crystal structure of La2O2Bi3PbS6 is characterized in a tetragonal P4/nmm space group, and the lattice parameters are a = 4.084(1) Å and c = 19.71(2) Å. The obtained La2O2Bi3PbS6 sample exhibits narrow gap semiconductor (metallic-like) transport behavior with an anomaly near the 160 K. A metallic-like to insulator transition has been observed through Pb substitution, with increasing x in La2O2Bi3-xPb1+xS6. Furthermore, we applied external pressure for x = 0 and observed pressure-induced superconductivity. The onset of superconductivity appeared at 0.93 GPa, and the highest transition temperature was 3.1 K at 2.21 GPa.

cond-mat.supr-con↗

arXiv2023

Nonvolatile Magneto-Thermal Switching in MgB2

Ongoing research explores thermal switching materials to control heat flow. Specifically, there has been interest in magneto-thermal switching (MTS) materials based on superconductors, which only exhibited switching behavior when a magnetic field was applied. However, a recent report highlighted nonvolatile MTS in commercial Sn-Pb solders, attributed to magnetic flux trapping. In this study, we focused on flux trapping in a type-II superconductor MgB2. Magnetization and thermal conductivity measurements under magnetic fields were conducted on polycrystalline MgB2. We confirmed that magnetic flux was indeed trapped in MgB2 even after demagnetization. Additionally, we observed nonvolatile MTS in MgB2 as well as Sn-Pb solders. These results suggest that the nonvolatile MTS may be a widespread characteristic of superconducting materials with flux trapping.

cond-mat.supr-con↗

arXiv2023

Metallurgy, superconductivity, and hardness of a new high-entropy alloy superconductor Ti-Hf-Nb-Ta-Re

We explored quinary body-centered cubic (bcc) high-entropy alloy (HEA) superconductors with valence electron concentrations (VECs) ranging from 4.6 to 5.0, a domain that has received limited attention in prior research. Our search has led to the discovery of new bcc Ti-Hf-Nb-Ta-Re superconducting alloys, which exhibit an interesting phenomenon of phase segregation into two bcc phases with slightly different chemical compositions, as the VEC increases. The enthalpy of the formation of each binary compound explains the phase segregation. All the alloys investigated were categorized as type-II superconductors, with superconducting critical temperatures ($T_\mathrm{c}$) ranging from 3.25 K to 4.38 K. We measured the Vickers microhardness, which positively correlated with the Debye temperature, and compared it with the hardness values of other bcc HEA superconductors. Our results indicate that $T_\mathrm{c}$ systematically decreases with an increase in hardness beyond a threshold of approximately 350 HV. Additionally, we plotted $T_\mathrm{c}$ vs. VEC for representative quinary bcc HEAs. The plot revealed the asymmetric VEC dependence. The correlation between the hardness and $T_\mathrm{c}$, as well as the asymmetric dependence of $T_\mathrm{c}$ on VEC can be attributed to the simultaneous effects of the electronic density of states at the Fermi level and electron-phonon coupling under the uncertainty principle, especially in the higher VEC region.

cond-mat.supr-con↗

arXiv2023

Superconductivity in In-doped AgPbBiTe3 compounds synthesized by high-pressure synthesis

NaCl-type metal tellurides (MTe) have been widely studied due to unique physical properties. We investigated the In-doping effects on structural and physical properties of Na-Cl type (AgPbBi)(1-x)/3InxTe and the superconducting properties of the In-doped samples. Polycrystalline samples with x = 0-0.5 were synthesized by utilizing high-pressure synthesis. For x = 0.2-0.5, superconductivity was observed in magnetization measurements, where the highest transition temperature (Tc) was 2.8 K for x = 0.4. We measured specific heat for x = 0.4 and confirmed the bulk nature of the superconductivity. The evolution of the Seebeck coefficient and lattice constant by In doping suggests that In valence state is In+3, and the In doping generates electron carriers in the (AgPbBi)(1-x)/3InxTe system.

cond-mat.supr-con↗

arXiv2023

Large magneto-thermal-switching ratio in superconducting Pb wires

Thermal switching by magnetic fields is one of the important functionalities in thermal management technologies. In low-temperature devices, superconducting states can be used as a magneto-thermal-switching (MTS) component, because carrier thermal conductivity (\k{appa}) is strongly suppressed in superconducting states. Recently, we demonstrated that the MTS ratio (MTSR) of pure Nb reached 650% at a temperature (T) of 2.5 K under a magnetic field (H) of 4.0 kOe [M. Yoshida et al., Appl. Phys. Express 16, 033002 (2023)]. In this study, to enrich knowledge on MTS of superconductors, the MTSRs of pure Pb wires with 5N and 3N purities were investigated by measuring the temperature or magnetic field dependences of \k{appa}. For 5N-Pb, a large MTSR exceeding 1000% was observed below 3.6 K under H > 600 Oe. Although higher MTSRs were expected at lower temperatures under H > 600 Oe, the obtained data under those conditions were accompanied by large errors due to magnetic-field-induced huge \k{appa} at low temperatures. In contrast, the \k{appa} for 3N-Pb were observed to be clearly lower than that for 5N-Pb. Although the magnetic-field-induced change in \k{appa} was small, the MTSR at T = 2.5 K was 300%. These results suggest that Pb is a promising material for low-temperature magneto-thermal switching because of wide-range \k{appa} tunable by magnetic field and the purity.

cond-mat.mtrl-sci↗

arXiv2023

Effects of equivalent composition on superconducting properties of high-entropy REOBiS$_2$ (RE = La, Ce, Pr, Nd, Sm, Gd) single crystals

Superconductors are influenced by high-entropy alloys (HEAs); these have been investigated in various functional materials. REOBiS$_2$ (RE = La, Ce, Pr, Nd, Sm, and Gd in different combinations) single crystals with HEAs at the RE-site were successfully grown using the flux method. The obtained crystals were plate-shaped (1 mm$^2$) with a well-developed c-plane. Ce was present in both trivalent (Ce$^{3+}$) and tetravalent (Ce$^{4+}$) electronic configurations; the concentration of Ce$^{4+}$ at the RE-site was approximately 10 at% in all single crystals. The single crystals showed superconducting transition temperature with zero resistivity within 1.2-4.2 K. The superconducting transition temperature, superconducting anisotropy, electronic specific heat coefficient, and Debye temperature of the crystals were not correlated with the mixed entropy at the RE-site. Except for the electronic specific heat coefficient, the variation of these parameters as a function of mixed entropy showed different trends for equivalent and non-equivalent RE element compositions. Thus, the configuration of RE elements influences the superconducting properties of REOBiS$_2$ single crystals, alluding to a method of modulating transition temperatures.

cond-mat.supr-con↗

arXiv2023

Magneto-thermal-switching properties of superconducting Nb

Recently, thermal switching has been extensively studied because it is a key component for thermal management in electronic devices. Here, we show a huge magneto-thermal-switching ratio (MTSR) in pure Nb at temperatures lower than its superconducting transition temperature (Tc = 9.2 K). The MTSR increases with decreasing temperature, and MTSR of 650% was observed at T = 2.5 K under H = 4.0 kOe. The thermal switching in superconductors with the huge MTSR will be useful for improvement of the performance of low-temperature electronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Axis thermal expansion switching in transition-metal zirconides TrZr2 by tuning the c/a ratio

This study examines the temperature-dependent evolution of the lattice constants for various CuAl2-type compounds, including NiZr2, (Co,Rh,Ir)Zr2, (Fe,Co,Rh,Ir)Zr2, and (Co,Ni,Cu,Rh,Ir)Zr2, in the pursuit of negative or zero thermal expansion. Results reveal that NiZr2 has positive thermal expansion, while the other compounds exhibit uniaxial negative thermal expansion along the c-axis contraction. The study suggests that the c-axis thermal expansion can be controlled by manipulating the c/a ratio through Tr-site substitution, providing a design principle for achieving negative thermal expansion of the c-axis and potentially zero thermal expansion in a single compound in TrZr2 compounds.

cond-mat.mtrl-sci↗

arXiv2022

Uniaxial negative thermal expansion in a tetragonal V3S-type superconductor IrZr3

Recently, uniaxial negative thermal expansion (NTE) has been observed in transition-metal zirconides including tetragonal (CuAl2-type) CoZr2 and orthorhombic (Re3B-type) CoZr3. Here, we report the observation of uniaxial NTE in tetragonal (V3S-type) IrZr3. With increasing temperature, the lattice parameter for the a-axis and volume of IrZr3 increase, but the lattice parameter for the c-axis decreases. IrZr3 is a superconductor with a transition temperature of 2.3 K. The present result suggests that uniaxial NTE and related anomalous axis thermal expansion occur in various transition-metal zirconides, which will be important for designing superconducting device robust to temperature cycles using zero-thermal-expansion superconductors.

cond-mat.mtrl-sci↗

arXiv2022

Estimation of Grüneisen Parameter of Layered Superconductor LaO0.5F0.5BiS2-xSex (x = 0.2, 0.4, 0.6, 0.8, 1.0)

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 greater than 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.

cond-mat.supr-con↗

arXiv2022

Uniaxial negative thermal expansion in an orthorhombic superconductor CoZr3

We investigated the temperature evolution of crystal structure of orthorhombic CoZr3, which is a superconductor with a transition temperature of 4.3 K, by synchrotron and laboratory (CuKα) X-ray diffraction. Uniaxial negative thermal expansion along the c-axis, which is similar to that observed in tetragonal CoZr2, has been observed at a wide temperature range of T = 90-800 K in CoZr3, while a-and b-axis exhibit positive thermal expansion.

cond-mat.mtrl-sci↗

arXiv2022

Weak antilocalization induced by Se substitution in layered BiCh$_2$-based (Ch = S, Se) superconductors LaO$_{1-x}$F$_x$BiS$_{2-y}$Se$_y$

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 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 indicates the existence of carrier localization. The heavily Se-substituted compounds show metallic behavior in the entire-temperature region. Sign changes of the Hall coefficients are observed for the x = 0.2 samples, which possibly is related to a charge-density wave (CDW). 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 WL emerges around the moderately F-doped and Se-free LaO0.8F0.2BiS2. The change of the resistivity behavior by the F and Se substitution clearly correlates to the difference of the magnetoconductance. Moreover, the localization regions of the WAL-WL crossover and weak WAL states are possibly associated with the CDW. We propose that the BiCh2-based system is a good platform for studying relationship between WAL, superconductivity, and electronic ordering because those states are tunable by element substitutions with bulk single crystals.

cond-mat.supr-con↗

arXiv2022

Sign change in c-axis thermal expansion and lattice collapse by Ni substitution in Co1-xNixZr2 superconductors

We investigated the structural, electronic, and superconducting properties of Co1-xNixZr2, in which c-axis thermal expansion is systematically controlled. At x (smaller than) 0.3, c-axis negative thermal expansion (NTE) was observed, and the thermal expansion constant αc approached zero with increasing x. At x = 0.4-0.6, zero thermal expansion was observed, and positive thermal expansion (PTE) appeared for x (greater than) 0.7. By analyzing the c/a ratio, we observed a possible collapsed transition in the tetragonal lattice at around x = 0.6-0.8. The lattice collapse results in c-axis PTE and the suppression of bulk superconductivity.

cond-mat.supr-con↗

arXiv2022

Anomalous thermal expansion in a CuAl2-type superconductor CoZr2

In this work, we show that the CuAl2-type superconductor CoZr2 and alloyed systems exhibit anomalous thermal expansion in a wide temperature range. We performed neutron powder diffraction and X-ray powder diffraction on CoZr2 and observed remarkably anisotropic thermal expansion with a c-axis negative thermal expansion constant of αc < -15 μK-1 in a wide temperature range of T = 50-573 K. With decreasing temperature, the lattice constant a decreases, while the lattice constant c continuously increases in CoZr2. The origin of the anisotropic shrinkage/expansion of the a-axis/c-axis by cooling is explained by the small change in the Co-Zr bond and the systematic decrease in the Zr-Co-Zr angle. Similar thermal expansion was observed in alloyed systems, (Fe,Co,Ni)Zr2 and (Fe,Co,Ni,Rh,Ir)Zr2, which suggests that the phenomenon is common feature in the TrZr2 system. We propose that zero-thermal expansion metals would be achieved by optimizing the contrasting thermal expansion of a-axis and c-axis in TrZr2.

cond-mat.mtrl-sci↗

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

Superconductivity and hardness of the equiatomic high-entropy alloy HfMoNbTiZr

We have found that body-centered cubic (bcc) HfMoNbTiZr is a type-II BCS high-entropy alloy (HEA) superconductor with a superconducting critical temperature of $T_\mathrm{c}$=4.1 K. By employing a Debye temperature $θ_\mathrm{D}$ of 263 K and $T_\mathrm{c}$, the electron-phonon coupling constant $λ_\mathrm{e-p}$ is calculated to be 0.63. The electronic structure calculation revealed band broadening with energy uncertainties due to atomic disorders. The superconducting properties are compared among equiatomic quinary bcc HEA superconductors. $T_\mathrm{c}$ decreases with decreasing $λ_\mathrm{e-p}$, which is negatively correlated with $θ_{D}$. The negative correlation between $λ_\mathrm{e-p}$ and $θ_{D}$ would be caused by a shorter phonon lifetime at a higher $θ_{D}$, which is based on phonon broadening due to atomic disorder and the uncertainty principle. The Vickers microhardness was measured for several bcc HEA superconductors. $T_\mathrm{c}$ is exceptionally low in a superconductor with relatively high hardness, because the hardness generally increases with increasing $θ_\mathrm{D}$, and $θ_\mathrm{D}$ is negatively correlated with $T_\mathrm{c}$ in the high-entropy state.

cond-mat.supr-con↗

arXiv2022

Superconductivity of high-entropy-alloy-type transition-metal zirconide (Fe,Co,Ni,Cu,Ga)Zr2

We synthesized a new high-entropy-alloy-type (HEA-type) superconductor (Fe,Co,Ni,Cu,Ga)Zr2 with a Tc of 2.9 K. The EDX analyses revealed that the actual composition of the transition-metal site (Tr-site) is Tr = Fe0.18Co0.18Ni0.16Cu0.25Ga0.23, which gives the configurational entropy of mixing ΔSmix = 1.60R for the Tr site. Neutron powder diffraction revealed that the sample has a tetragonal CuAl2-type (space group: #140). The lattice constant of a monotonically decreases with decreasing temperature, but the lattice constant of c does not exhibit a clear shrinkage. Isotropic displacement parameter for both the Tr and Zr sites are large, which is probably caused by the HEA-type Tr site. The small temperature dependences of Uiso for both sites also indicate the presence of the local structural disorder in (Fe,Co,Ni,Cu,Ga)Zr2. From electrical resistivity, magnetic susceptibility, and specific heat measurements, bulk superconductivity was confirmed.

cond-mat.supr-con↗

arXiv2022

Tuning of Carrier Concentration and Superconductivity in High-Entropy-Alloy-Type Metal Telluride (AgSnPbBi)(1-x)/4InxTe

High-entropy-alloy-type (HEA-type) compound superconductors have been drawing much attention as a new class of exotic superconductors with local structural inhomogeneity. NaCl-type (Ag,In,Sn,Pb,Bi)Te is a typical HEA-type superconductor, but the carrier doping mechanism had been unclear. In this study, we synthesized (Ag,In,Sn,Pb,Bi)Te with various In concentration using high-pressure synthesis: the studied system is (AgSnPbBi)(1-x)/4InxTe (x = 0-0.4). Single-phase samples were obtained for x = 0-0.3. A semiconductor-like temperature dependence of resistivity was observed for x = 0, while superconductivity appeared for the In-doped samples. The highest transition temperature (Tc) was 3.0 K for x = 0.3. The Seebeck coefficient decreases with increase of x, which suggests that In3+ generates electron carriers in (AgSnPbBi)(1-x)/4InxTe. Tuning of carrier concentration and superconducting properties of (Ag,In,Sn,Pb,Bi)Te would be useful for further investigation of exotic superconductivity in the HEA-type compound.

cond-mat.supr-con↗