arXiv subjects

Atsushi Miyake

Publications and source records attributed to Atsushi Miyake.

64 records · Page 4Linked to original sources

arXiv2015

Superconductivity in Room Temperature Stable electride and high-pressure phases of alkali metals

S-band metals such as alkali and alkaline earth metals do not undergo a superconducting transition (SCT) at an ambient pressure, but their high-pressure phases do. In contrast, room temperature stable electride electride (C12A7:e-) in which anionic electrons in the crystallographic sub-nanometer-size cages have high s-character exhibits SCT at 0.2-0.4K at an ambient pressure. In this paper we report that crystal and electronic structure of C12A7:e- are close to those of the high pressure superconducting phase of alkali and alkaline earth metals and the SCT of both materials is induced when electron nature at Fermi energy (EF) switches from s- to sd-hybridized state.

cond-mat.supr-con↗

arXiv2014

Anisotropic upper critical field of BiS2-based superconductor LaO0.5F0.5BiS2

Superconducting properties under high magnetic fields for the novel BiS2-based layered superconductor LaO0.5F0.5BiS2 (polycrystalline sample prepared using the high-pressure synthesis method) have been investigated. The anisotropy of the upper critical field was discussed by analyzing the temperature dependence of the temperature derivative of resistivity. The temperature dependence of upper critical field showed an anomalous behavior with a characteristic magnetic field of 8 T. We revealed that the anomalous behavior was caused by the existence of three kinds of upper critical field, HC2max, HC2mid and HC2min in LaO0.5F0.5BiS2. The lowest upper critical field HC2min is regarded as the Hc2 where the superconducting states of the grains with an orientation of H // c are suppressed. The HC2max and HC2mid are regarded as the Hc2 where the superconducting states of the grains with an orientation of H // ab are suppressed. The difference between the HC2max and HC2mid could be explained by the anisotropy of superconducting states within the Bi-S plane. The estimated anisotropy parameter of the upper critical fields was about 7.4 for the high-pressure-synthesized LaO0.5F0.5BiS2 polycrystalline sample. Since the value was clearly lower than the anisotropy parameter of over 30 observed in the previous report on the LaO0.5F0.5BiS2 single crystals grown under ambient pressure, we concluded that a larger anisotropy of superconductivity was not essential for the enhancement of Tc, and lower anisotropy might be important for a higher Tc in LaO1-xFxBiS2.

cond-mat.supr-con↗

arXiv2013

Interplay between Charge and Magnetic Ordering in YbPd

The pressure-temperature phase diagram of an intermediate-valence compound YbPd was revealed via simultaneous ac-calorimetry and electrical resistivity measurements. Two successive structural phase transition temperatures, $T_1$ = 125 K and $T_2$ = 110 K, are found to decrease with increasing pressure. The magnetic ordering transition at $T_{\rm N} ~sim$ 1.9 K decreases monotonically up to 1.7 GPa and disappears discontinuously at $P_{\rm c} sim$ 1.9 GPa, where the structural phase transition at $T_2$ is also suppressed. At $P_{\rm c}$, enhancement of the effective mass and residual resistivity are observed. Another probable magnetic phase transition, however, is found to take place at $T_{\rm ML} sim$0.3 K and smoothly varies even when crossing $P_{\rm c}$. Structural phase transitions at ambient pressure were also studied by a single crystal X-ray diffraction measurement. Below $T_2$, a tetragonal lattice distortion with doubling of the unit cell along the c-axis is observed, and thus there are two inequivalent Yb-sites below $P_{\rm c}$. These results indicate that one Yb-site, having larger valence configuration and thus smaller Kondo temperature, orders at $T_{\rm N}$. The other phase transition at $T_{\rm ML}$ seems to couple to the crystal structure for $T_2 < T < T_1$. YbPd is a unique system exhibiting magnetic orderings in metallic charge-ordered Yb systems.

cond-mat.str-el↗

arXiv2013

Charge ordering in the intermediate valence magnet YbPd

An x-ray diffraction study reveals the charge ordering structure in an intermediate valence magnet YbPd with a CsCl-structure. The valence of the Yb ions forms an incommensurate structure, characterized by the wavevector (+-0.07 +-0.07 1/2) below 130 K. At 105 K, the incommensurate structure turns into a commensurate structure, characterized by the wavevector (0 0 1/2). Based on the resonant x-ray diffraction spectra of the superlattice reflections, the valences of the Yb ions below 105 K are found to be 3+ and 2.6+. The origin of the long wavelength modulation is discussed with the aid of an Ising model having the second nearest neighbor interaction.

cond-mat.str-el↗

arXiv2012

Field-induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

We review our recent studies on ferromagnetic superconductors, UGe2, URhGe and UCoGe, where the spin-triplet state with the so-called equal spin pairing is realized. We focus on experimental results of URhGe and UCoGe in which the superconductivity occurs already at ambient pressure. The huge upper critical field Hc2 on UCoGe for the field along the hard magnetization axis (b-axis) is confirmed by the AC susceptibility measurements by the fine tuning of field angle. Contrary to the huge Hc2 along the hard-magnetization axis, Hc2 along the easy-magnetization axis (c-axis) is relatively small in value. However, the initial slope of Hc2, namely dHc2/dT (H -> 0) both in UCoGe and in URhGe indicates the large value, which can be explained by the magnetic domain effect detected in the magnetization measurements. The specific heat measurements using a high quality single crystal of UCoGe demonstrate the bulk superconductivity, which is extended under magnetic field for the field along c-axis.

cond-mat.str-el↗

arXiv2011

Properties of Ferromagnetic Superconductors

Thanks to the discovery in the last decade of three uranium ferromagnetic superconductors, UGe2, URhGe and UCoGe, the fascinating aspects of the interplay between the triplet state of Cooper pairing and ferromagnetism have emerged. Furthermore, as the ferromagnetic properties in the normal state are quite different with respect to the proximity of the ferromagnetic-paramagnetic instabilities, the feedback with the coexistence of superconductivity gives rise to quite different boundaries in pressure and magnetic field. Special attention is given on the location of the materials with respect to the tricriticality and on the reinforcement of superconductivity in a transverse field response with respect to the direction of the ferromagnetic sublattice magnetization. The other facts of the interplay between ferromagnetism and superconductivity is briefly mentioned.

cond-mat.str-el↗

arXiv2009

Pressure Evolution of the Ferromagnetic and Field Re-entrant Superconductivity in URhGe

Fine pressure ($P$) and magnetic field ($H$) tuning on the ferromagnetic superconductor URhGe are reported in order to clarify the interplay between the mass enhancement, low field superconductivity (SC) and field reentrant superconductivity (RSC) by electrical resistivity measurements. With increasing $P$, the transition temperature and the upper critical field of the low field SC decrease slightly, while the RSC dome drastically shifts to higher fields and shrinks. The spin reorientation field $H_{\rm R}$ also increases. At a pressure $P\sim 1.8$ GPa, the RSC has collapsed while the low field SC persists and may disappear only above 4 GPa. Via careful $(P, H)$ studies of the inelastic $T^2$ resistivity term, it is demonstrated that this drastic change is directly related with the $P$ dependence of the effective mass which determines the critical field of the low field SC and RSC on the basis of triplet SC without Pauli limiting field.

cond-mat.str-el↗

arXiv2009

Field Reentrance of the Hidden Order State of URu2Si2 under Pressure

Combination of neutron scattering and thermal expansion measurements under pressure shows that the so-called hidden order phase of URu2Si2 reenters in magnetic field when antiferromagnetism (AF) collapses at H_AF (T). Macroscopic pressure studies of the HO-AF boundaries were realized at different pressures via thermal expansion measurements under magnetic field using a strain gauge. Microscopic proof at a given pressure is the reappearance of the resonance at Q_0=(1,0,0) under field which is correlated with the collapse of the AF Bragg reflections at Q_0.

cond-mat.str-el↗

arXiv2009

Pressure Collapse of the Magnetic Ordering in MnSi via Thermal Expansion

The itinerant quasi-ferromagnetic metal MnSi has been studied by detailed thermal expansion measurements under pressures and magnetic fields. A sudden decrease of the volume at the critical pressure Pc ~1.6 GPa has been observed and is in good agreement with the pressure variation of the volume fraction of the spiral magnetic ordering. This confirms that the magnetic order disappears by a first order phase transition. The energy change estimated by the volume discontinuity on crossing Pc is of similar order as the Zeeman energy of the transition from the spiral ground state to a polarized paramagnetic one under magnetic field. In contrast to the strong pressure dependence of the transition temperature, the characteristic fields are weakly pressure dependent, indicating that the strength of the ferromagnetic and the Dzyaloshinskii-Moriya interactions do not change drastically around Pc. The evaluated results of the thermal expansion coefficient and the magnetostriction are analyzed thermodynamically. The Sommerfeld coefficient of the linear temperature term of the specific heat is enhanced just below Pc. The magnetic field-temperature phase diagrams in the ordered and paramagnetic phases are also compared. Comparison is made with other heavy fermion compounds with first order phase transition at 0 K.

cond-mat.str-el↗

arXiv2008

Field Re-entrant Superconductivity Induced by the Enhancement of Effective Mass in URhGe

High quality single crystals of a ferromagnetic superconductor URhGe were successfully grown. The electrical resistivity was measured for the field along b-axis in the orthorhombic crystal structure in order to study precisely its field re-entrant superconductivity which occurs in the vicinity of the field $H_{\rm R}$, where the easy magnetization switches from $c$ to b-axis. The field re-entrant superconductivity of URhGe is analyzed with special focus on its dependence with the value of residual resistivity $ρ_0$ and the coefficient $A$ of $T^2$ term of the resistivity. The experimental results are well explained by a crude model related with the field dependence of the effective mass $m^\ast$, where the corresponding critical temperature $T_{\mathrm{sc}}(m^\ast)$ and the upper critical field $H_{\rm c2}$ are strongly enhanced. Discussion is made on the interplay between magnetic and superconducting phase diagram as well as the link between $T_{\mathrm{sc}}$ and $A$ in other heavy fermion superconductors.

cond-mat.str-el↗