arXiv subjects

Fumihiro Ishikawa

Publications and source records attributed to Fumihiro Ishikawa.

11 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 CdI$_2$-type structure are materials with intriguing superconducting and electronic properties as demonstrated by PdTe$_2$. Gold substituted PdTe$_2$, Au$_x$Pd$_{{\rm 1}-x}$Te$_2$, adopts the CdI$_2$-type structure for a Pd content larger than 43 at.\% at room temperature, and in this range enhanced superconductivity with a critical temperature ($T_{\rm c}$) above 4 K has been reported (Kudo \it{et al}., PRB \bf{93}, 140505, 2016). Here we present the effect of pressure on the structural and superconducting properties of Au$_x$Pd$_{{\rm 1}-x}$Te$_2$ for $x=0.15$, 0.25 and 0.35 with $T_{\rm c} =2.7$, 4.1, and 4.6 K at 1 atm, respectively. Synchrotron radiation x-ray diffraction shows that the CdI$_2$-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 PdTe$_2$ exhibits type-II superconductivity, that evolves from weak-coupling BCS for $x=0.15$ to intermediate coupling for $x=0.25$ and 0.35. Electrical resistivity measurements up to a pressure of 2.5 GPa reveal that $T_{\rm c}(P)$ for $x=0.25$ and 0.35 has a shallow maximum with $T_{\rm c}^{\rm max}$ is 4.2 and 4.7 K at $P\sim 0.3$ and 0.7 GPa, respectively, whereas a monotonic decrease is observed for $x=0.15$. Using the McMillan equation, with the Debye temperature ${\it Θ}_{\rm R}$ extracted from the resistance data under pressure as input, we find a smooth variation of $T_{\rm c}$ with the electron-phonon coupling strength $λ_{\rm ep}$ spanning the entire pressure and concentration range, from PdTe$_2$ to 35 at.\% Au substitution. We conclude superconductivity across the Au$_x$Pd$_{{\rm 1}-x}$Te$_2$ series under pressure is governed by a delicate balance between lattice stiffening and the strength of the electron phonon coupling.

cond-mat.supr-con↗

arXiv2021

Neural Network Approach to Construction of Classical Integrable Systems

Integrable systems have provided various insights into physical phenomena and mathematics. The way of constructing many-body integrable systems is limited to few ansatzes for the Lax pair, except for highly inventive findings of conserved quantities. Machine learning techniques have recently been applied to broad physics fields and proven powerful for building non-trivial transformations and potential functions. We here propose a machine learning approach to a systematic construction of classical integrable systems. Given the Hamiltonian or samples in latent space, our neural network simultaneously learns the corresponding natural Hamiltonian in real space and the canonical transformation between the latent space and the real space variables. We also propose a loss function for building integrable systems and demonstrate successful unsupervised learning for the Toda lattice. Our approach enables exploring new integrable systems without any prior knowledge about the canonical transformation or any ansatz for the Lax pair.

nlin.SI↗

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

Localized Mode and Nonergodicity of a Harmonic Oscillator Chain

We present a simple and microscopic physical model that breaks the ergodicity. Our model consists of coupled classical harmonic oscillators, and the motion of the tagged particle obeys the generalized Langevin equation satisfying the second fluctuation dissipation theorem. It is found that although the nonergodicity strength, which is expected to detect the ergodicity breaking, for this model vanishes, the velocity auto correlation function of the tagged particle asymptotically oscillates. We analyze the model by using the molecular dynamics and the exact diagonalization as well as the rigorous mapping to the generalized Langevin equation. Our analysis reveals that the asymptotic oscillation is caused by a localized mode with an isolated frequency from the continuous phonon spectrum.

cond-mat.stat-mech↗

arXiv2016

Pressure-induced topological phase transition in polar semiconductor BiTeBr

We performed X-ray diffraction and electrical resistivity measurement up to pressures of 5 GPa and the first-principles calculations utilizing experimental structural parameters to investigate the pressure-induced topological phase transition in BiTeBr having a noncentrosymmetric layered structure (space group P3m1). The P3m1 structure remains stable up to pressures of 5 GPa; the ratio of lattice constants, c/a, has a minimum at pressures of 2.5 - 3 GPa. In the same range, the temperature dependence of resistivity changes from metallic to semiconducting at 3 GPa and has a plateau region between 50 and 150 K in the semiconducting state. Meanwhile, the pressure variation of band structure shows that the bulk band-gap energy closes at 2.9 GPa and re-opens at higher pressures. Furthermore, according to the Wilson loop analysis, the topological nature of electronic states in noncentrosymmetric BiTeBr at 0 and 5 GPa are explicitly revealed to be trivial and non-trivial, respectively. These results strongly suggest that pressure-induced topological phase transition in BiTeBr occurs at the pressures of 2.9 GPa.

cond-mat.mtrl-sci↗

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↗

arXiv2014

Collapse of CuO Double Chains and Suppression of Superconductivity in High-Pressure Phase of YBa$_2$Cu$_4$O$_8$

The crystal structure and electrical resistivity of YBa$_2$Cu$_4$O$_8$ (Y124) were studied under high pressure up to 18 GPa using diamond-anvil cells, respectively, in order to clarify its conduction mechanism. Y124 causes the first-order phase-transition into the orthorhombic Immm at pressure around 11 GPa. The high-pressure phase (HPP) also shows the superconductivity, while the manner of temperature dependence of electrical resistance and the pressure dependence of transition temperature, Tc, drastically change above 11 GPa. The CuO$_2$ plane persists in HPP but the CuO double chains collapse with the phase transition and transform into three-dimensional Cu-O network, resulting in the renewal of conduction system.

cond-mat.supr-con↗

arXiv2013

Synthesis of New Layered Oxypnictides Sr2CrO2(FeAs)2

We successfully synthesized new layered iron oxypnictide Sr2CrO2(FeAs)2 by solid-state-reaction techniques. This is the first report on the Fe-substituted Sr2Mn3As2O2-type oxypnictides to our knowledge. A pellet of stoichiometric mixtures of SrO, Cr and FeAs wrapped with tantalum foil was sintered at 900°C for 96 h. Results of x-ray diffraction measurement and Rietveld refinements show that the structure of the sample belongs to the space group of I4/mmm with the lattice constants of a = 3.9948(1) Å and c = 18.447(1) Å. Temperature dependence of the resistivity for Sr2CrO2(FeAs)2 shows semiconductor-like behavior. Additionally, the sample does not exhibit superconducting transition in measurements of resistivity and magnetization even in the low temperature down to 3 K.

cond-mat.supr-con↗

arXiv2010

Pressure-induced phase transition of Bi2Te3 into the bcc structure

The pressure-induced phase transition of bismuth telluride, Bi2Te3, has been studied by synchrotron x-ray diffraction measurements at room temperature using a diamond-anvil cell (DAC) with loading pressures up to 29.8 GPa. We found a high-pressure body-centered cubic (bcc) phase in Bi2Te3 at 25.2 GPa, which is denoted as phase IV, and this phase apperars above 14.5 GPa. Upon releasing the pressure from 29.8 GPa, the diffraction pattern changes with pressure hysteresis. The original rhombohedral phase is recovered at 2.43 GPa. The bcc structure can explain the phase IV peaks. We assumed that the structural model of phase IV is analogous to a substitutional binary alloy; the Bi and Te atoms are distributed in the bcc-lattice sites with space group Im-3m. The results of Rietveld analysis based on this model agree well with both the experimental data and calculated results. Therefore, the structure of phase IV in Bi2Te3 can be explained by a solid solution with a bcc lattice in the Bi-Te (60 atomic% tellurium) binary system.

cond-mat.mtrl-sci↗

arXiv2009

Pressure-induced superconductivity in Eu$_{0.5}$Ca$_{0.5}$Fe$_2$As$_2$ : FeAs-based superconductivity hidden by antiferromagnetism of Eu sublattice

To clarify superconductivity in EuFe2As2 hidden by antiferromagnetism of Eu2+, we investigated a Ca-substituted sample, Eu0.5Ca0.5Fe2As2, under high pressure. For ambient pressure, the sample exhibits a spin-density-wave (SDW) transition at TSDW = 191 K and antiferromagnetic order at TN = 4 K, but no evidence of superconductivity down to 2 K. The Ca-substitution certainly weakens the antiferromagnetism. With increasing pressure, TSDW shifts to lower temperature and becomes more unclear. Above 1.27 GPa, pressure-induced superconductivity with zero resistivity is observed at around Tc = 20 K. At 2.14 GPa, Tc reaches a maximum value of 24 K and the superconducting transition becomes the sharpest. These features of emergence of the superconductivity are qualitatively similar to those observed in AFe2As2 (A = Ba, Ca).

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↗