arXiv subjects

Kiyotaka Miyoshi

Publications and source records attributed to Kiyotaka Miyoshi.

6 recordsLinked to original sources

arXiv2025

Crossover of Superconductivity across the antiferromagnetic end point in FeSe$_{\rm 1-x}$S$_{\rm x}$ under pressure

Temperature-pressure ($T$-$P$) phase diagrams of FeSe$_{\rm 1-x}$S$_{\rm x}$ were investigated by the measurements of dc magnetization ($M$) and electrical resistivity ($ρ$) under pressure, using single crystal specimens with $x$=0.04, 0.08 and 0.13. For all specimens, the $M$($T$) curves under pressure near the end point of the antiferromagnetic (AFM) phase are found to show a two-step diamagnetic response, which can be described as the sum of two diamagnetic components $M_1$($T$) and $M_2$($T$), indicating that two superconducting (SC) phases with different $T_{\rm c}$ values coexist within a pressure range of $Δ$$P$$\sim$1 GPa. Moreover, the pressure dependence of the amplitudes of $M_1$($T$) and $M_2$($T$) indicates a continuous transfer of the volume fraction between the two SC phases. These behaviors suggest that a crossover of superconductivity occurs in conjunction with the emergence of AFM phase and imply that the SC phases inside and outside the AFM phase could have different origins.

cond-mat.supr-con↗

arXiv2021

Disappearance and Survival of Superconductivity in FeSe under High Pressure

Superconductivity in FeSe was investigated under high pressure through the measurements of DC magnetization by using a diamond anvil cell. We successfully observed that the disappearance of the superconductivity as a results of the appearance of the non-superconducting ortho I\hspace{-1pt}I phase above $\sim$7 GPa ($\sim$5 GPa), when Ar (glycerin) is used as the pressure transmitting media. Contrary to this, it has been found that the superconductivity even survives under pressure above 7 GPa, when the thickness ($t$) of a platelet-single crystal specimen is reduced. The survival of the superconductivity above 7 GPa is consistent with a previous observation under hydrostatic pressure by using a cubic anvil apparatus, suggesting that the hydrostaticity of the pressure is improved by reducing $t$. It is also inferred that the appearance of the ortho I\hspace{-1pt}I phase is due to uniaxial stress along the [001] direction.

cond-mat.supr-con↗

arXiv2020

Magnetoelectric Effect in the Antiferromagnetic Ordered State of Ce$_{3}$TiBi$_{5}$ with Ce Zig-Zag Chains

In this study, we investigate the magnetoelectric (ME) effect of the newly discovered antiferromagnetic (AFM) compound Ce$_{3}$TiBi$_{5}$ with a hexagonal structure (space group $P6_{3}$/$mcm$). In this system, Ce ions form zig-zag chains that extend along the $c$-axis. We focus on the lack of the local inversion symmetry at the Ce site, although the crystal structure has an inversion center. A theoretical study has predicted that magnetization can be induced by an electric current in the AFM ordering on the zig-zag chain. We conducted magnetization measurements on Ce$_{3}$TiBi$_{5}$ under an applied constant electric current and a static magnetic field around the AFM ordering temperature of $T_{\rm N}$ = 5.0 K. We successfully observed of the current-induced magnetization below $T_{\rm N}$. The magnitude of the current-induced magnetization has a linear electric current dependence and exhibits no magnetic field dependence. This behavior is consistent with the theoretical prediction of the ME effect in the ferrotoroidal ordered state.

cond-mat.str-el↗

arXiv2018

Novel electronic states realized by the interplay between Li diffusion and Co$^{\rm 3+}$$/$Co$^{\rm 4+}$ charge ordering in Li$_{\rm x}$CoO$_2$

Measurements of dc magnetization ($M$) and electrical resistivity ($ρ$) have been carried out as a function of temperature ($T$) for layered oxide Li$_{\rm x}$CoO$_2$ (0.51$\leq$$x$$\leq$1.0) using single crystal specimens. After slow cool of the specimens down to 10 K, both of the $M$($T$) and $ρ$($T$) curves are found to exhibit a clear anomaly due to the occurrence of Co$^{\rm 3+}$$/$Co$^{\rm 4+}$ charge ordering (CO) at $T_{\rm S}$$\sim$155 K for 0.6$\leq$$x$$\leq$0.98 (at $T_{\rm S}$$\sim$180$-$190 K for 0.5$\leq$$x$$\leq$0.55). After rapid cool of the specimens, additional anomalies are observed related to the onset of Li diffusion at $T_{\rm F1}$$\sim$370 K and$/$or $T_{\rm F2}$=120$-$130 K. Due to phase mixing with compositions of nearly LiCoO$_2$ and Li$_{\rm 2/3}$CoO$_2$, the specimens with 0.7$\lesssim$$x$$\lesssim$0.9 show anomlies both at $T_{\rm F1}$ and $T_{\rm F2}$. For 0.6$\lesssim$$x$$\lesssim$0.9, the resistivity measured after rapid cool is found to be fairly larger than that measured after slow cool below $T_{\rm S}$. The enhanced resistivity can be explained by the scenario that disordered Co$^{\rm 3+}$$/$Co$^{\rm 4+}$ arrangements, which have been observed and revealed to have an insulating electronic structure contrasting to the regular CO state in the previous scanning tunneling microscopy measurements [Phys. Rev. Lett. {\bf 111}, 126104 (2013)], are realized due to the formation of an amorphous-like structure of Li ions after rapid cool via the interlayer Coulomb coupling. An electronic phase diagram for 0.5$\leq$$x$$\leq$1.0 is proposed.

cond-mat.str-el↗

arXiv2013

Enhanced Superconductivity on the Tetragonal Lattice in FeSe under Hydrostatic Pressure

Superconductivity under pressure in FeSe ($T_{\rm c}$$\sim$7.5 K) has been investigated using single-crystal specimens through the measurements of DC magnetization and electrical resistivity. A characteristic three-step increase in $T_{\rm c}$ has been found under hydrostatic pressure up to $\sim$34 K above 7 GPa. The structural transition from a tetragonal phase to an orthorhombic phase ($T_{\rm s}$$\sim$87 K) is found to disappear at $P$$\sim$2.3 GPa, above which $T_{\rm c}$ increases rapidly, suggesting that the superconductivity is enhanced by the tetragonal environment. Under non-hydrostatic pressure, the increase in $T_{\rm c}$ is suppressed and the superconductive volume fraction is considerably reduced above 2 GPa, probably owing to the breaking of the tetragonal lattice symmetry by the uniaxial stress. The intimate correlation between the enhanced (suppressed) superconductivity and the tetragonality (orthorhombicity) in the phase diagram is a common feature of FeSe and other iron-pnictide superconductors.

cond-mat.supr-con↗

arXiv2013

Superconductivity under pressure in $R$FeAsO$_{\rm 1-x}$F$_{\rm x}$ ($R$=La, Ce$-$Sm) by dc magnetization measurements

Superconducting transition under pressure ($P$) has been investigated for optimum-doped $R$FeAsO$_{\rm 1-x}$F$_{\rm x}$ ($R$=La, Ce$-$Sm) by dc magnetization measurements. For $R$=La, $T_{\rm c}$ is found to be pressure independent up to $P$$\sim$3.0 GPa and then shows a monotonic decrease with increasing pressure. The plateau width decreases for the system with smaller lattice constants, and shrinks to almost zero for $R$=Sm. From the $T_{\rm c}$($P$) data, we construct the $T_{\rm c}$ evolution map on the height of the As atom from the Fe-plane $h_{\rm As}$ versus lattice constant $a$ or $c$ plane. It is shown that the characteristic $T_{\rm c}$ variations under pressure for all compounds and the rapid decrease in $T_{\rm c}$ induced by changing $R$-element from Sm to La can be described by a common $T_{\rm c}$($h_{\rm As}$, $a$ or $c$) surface, suggesting that $T_{\rm c}$ is determined by $h_{\rm As}$ and lattice constant. It is also suggested that there exists an upper limit of the lattice constant $a_{\rm ulm}$ (or $c_{\rm ulm}$), above which $dT_{\rm c}$$da$ (or $dT_{\rm c}$$dc$) changes the sign from positive to negative.

cond-mat.supr-con↗