arXiv subjects

Takashi Naka

Publications and source records attributed to Takashi Naka.

8 recordsLinked to original sources

arXiv2026

Effect of pressure on the superconducting properties of Au substituted PdTe$_2$ with the CdI$_2$-type structure

Transition metal ditellurides with the CdI2-type structure are materials with intriguing superconducting and electronic properties as demonstrated by PdTe2. Gold substituted PdTe2, AuxPd1-xTe2, adopts the CdI2-type structure for a Pd content larger than 43 at.% at room temperature, and in this range enhanced superconductivity with a critical temperature (Tc) above 4 K has been reported (Kudo et al., PRB 93, 140505, 2016). Here we present the effect of pressure on the structural and superconducting properties of AuxPd1-xTe2 for x=0.15, 0.25 and 0.35 with Tc =2.7, 4.1, and 4.6 K at 1 atm, respectively. Synchrotron radiation x-ray diffraction shows that the CdI2-type structure remains stable up to 8 GPa for all three compositions and that they have almost the same volume compressibility. Heat capacity measurements show that Au substituted PdTe2 exhibits type-II superconductivity, that evolves from weak-coupling BCS for x = 0.15 to strong-coupling for x = 0.25 and 0.35. Electrical resistivity measurements up to a pressure of 2.5 GPa show that Tc(P) for x = 0.25 and 0.35 passes through a shallow maximum of 4.2 and 4.7 K at P ~ 0.3 and 0.7 GPa, respectively, compared to the monotonic decrease for x = 0.15. Furthermore, the pressure variation of the superconducting H - T phase diagram at each composition indicates that the superconducting properties remain essentially unchanged with pressure. The composition dependence of $T_{\rm c}$ is discussed by comparing the experimental results of AuxPd1-xTe2 to those of undoped PdTe2.

cond-mat.supr-con↗

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↗

arXiv2021

Superconducting and structural properties of the type-I superconductor PdTe2 under high pressure

The transition metal dichalcogenide PdTe$_2$ has attractive features based on its classification as a type-II Dirac semimetal and the occurrence of type-I superconductivity, providing a platform for discussion of a topological superconductor. Our recent work revealed that type-I superconductivity persists up to pressures of $\sim2.5$ GPa and the superconducting transition temperature $T_{\rm c}$ reaches a maximum at around 1 GPa, which is inconsistent with the theoretical prediction. To understand its non-monotonic variation and investigate superconductivity at higher pressures, we performed structural analysis by x-ray diffraction at room temperature below 8 GPa and electrical resistivity measurements at low temperatures from 1 to 8 GPa. With regard to the superconductivity beyond 1 GPa, the monotonic decrease in $T_{\rm c}$ is reproduced without any noticeable anomalies; $T_{\rm c}$ changes from 1.8 K at 1 GPa to 0.82 K at 5.5 GPa with $dT_{\rm c}/dP\sim-0.22$ K/GPa. The crystal structure with spacegroup $P$\={3}$m$1 is stable in the pressure range we examined. On the other hand, the normalized pressure-strain analysis (finite strain analysis) indicates that the compressibility changes around 1 GPa, suggesting that a Lifshitz transition occurs. We here discuss the effect of pressure on the superconducting and structural properties based on the comparison of these experimental results.

cond-mat.supr-con↗

arXiv2018

Inversion-induced orbital and exchange disorders in antiferromagnetic A-site spinel CoAl2O4

The inversion and volume effects on magnetism in a spinel-type magnetically frustrated compound, CoAl2O4, and its gallium-substituted system, CoAl2-xGaxO4, were investigated. Magnetically frustrated Co2+ with spin S = 3/2 on the tetrahedral site formed a diamond lattice in CoAl2O4 located in the vicinity of the magnetic phase boundary between Neel and spin-spiral states. In the Ga-substituted system, the number of Co ions, the so-called inversion h dominating the octahedral site, increased with increasing x. From comprehensive crystallographic, magnetic, and thermal measurements, increments of both volume and inversion strongly reduced the Neel point, while the latter also induced a spin-glass state above the critical value of hc = 0.09. In the spin glass state, h > hc, the orbital degree of freedom of Co2+ ions in the octahedral site appeared in the magnetic entropy, which couples strongly with that of spin, even above the magnetic transitions. Above h ~ hc, the field-induced quenched magnetic moment appeared above the transitions. Therefore, a short range ordered state emerged among the paramagnetic, antiferromganetic, and spin-glass states in the magnetic phase diagram.

cond-mat.str-el↗

arXiv2016

Composition induced metal-insulator quantum phase transition in the Heusler type Fe2VAl

We report the magnetism and transport properties of the Heusler compound Fe2+xV1-xAl at -0.10 < x < 0.10 under pressure and a magnetic field. A metal-insulator quantum phase transition occurred at x = -0.05. Application of pressure or a magnetic field facilitated the emergence of finite zero-temperature conductivity around the critical point, which scaled approximately according to the power law. At x < -0.05, a localized paramagnetic spin appeared, whereas above the ferromagnetic quantum critical point at x = 0.05, itinerant ferromagnetism was established. At the quantum critical points at x = -0.05 and 0.05, the resistivity and specific heat exhibited singularities characteristic of a Griffiths phase appearing as an inhomogeneous electronic state.

cond-mat.str-el↗

arXiv2015

Multistage ordering and critical singularities in Co1-xZnxAl2O4 (0 < x < 1):Dilution and pressure effects in a magnetically frustrated system

We report comprehensive studies of the crystallographic,magnetic, and thermal properties of a spinel-type magnetically frustrated compound, CoAl2O4, and a magnetically diluted system,Co1-xZnxAl2O4. These studies revealed the effects of dilution and disorder when the tetrahedral magnetic Co ion was replaced by the nonmagnetic Zn ion. Low-temperature anomalies were observed in magnetic susceptibility at x < 0.6. A multicritical point was apparent at T = 3.4 K and x = 0.12, where the antiferromagnetic, spin glass-like, and paramagnetic phases met. At that point, the quenched ferromagnetic component induced by a magnetic field during cooling was sharply enhanced and was observable below x = 0.6. Around x = 0.6, magnetic susceptibility and specific heat were described by temperature power laws, chi ~ C/T ~ T-d, in accord with the site percolation threshold of the diamond lattice. This behavior is reminiscent of a quantum critical singularity. We propose an x-temperature phase diagram in the range 0 < x < 1 for Co1-xZnxAl2O4. The transition temperature of CoAl2O4 determined from magnetic susceptibility measured under hydrostatic pressure increased with increasing pressure.

cond-mat.str-el↗

arXiv2013

Superconducting properties of the ternary transition-metal silicide Zr2Ru3Si4

Superconducting properties of the polycrystalline Zr2Ru3Si4 were investigated by the electrical resistivity, magnetization and specific heat. By these measurements, bulk superconductivity with transition temperature Tc = 5.5 K was confirmed. Moreover, Zr2Ru3Si4 was found to be a type-II and intermediate-coupling superconductor. Interestingly, the electronic specific heat shows a deviation from a one-gap s-wave model and Hc2(T) shows unusual positive curvature in the vicinity of Tc. The first principle's calculation shows the existence of plural anisotropic Fermi surfaces. These results suggest that Zr2Ru3Si4 is not an isotropic single-gap superconductor, but possibly a multi-gap or an anisotropic gap superconductor.

cond-mat.supr-con↗

arXiv2007

Physical properties of a new cuprate superconductor Pr_2Ba_4Cu_7O_{15-δ}

We present studies of the thermal, magnetic and electrical transport properties of reduced polycrystalline Pr_2Ba_4Cu_7O_{15-δ} (Pr247) showing a superconducting transition at Tc = 10 - 16 K and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T2 dependence up to approximately 150 K. A clear specific heat anomaly was observed at Tc for Pr247 reduced in a vacuum for 24 hrs, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature TN of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13 mB. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^a dependence (a = 0.08 - 0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.

cond-mat.supr-con↗