arXiv subjects

Ryuji Higashinaka

Publications and source records attributed to Ryuji Higashinaka.

29 records · Page 2Linked to original sources

arXiv2016

Bulk Superconductivity Induced by In-plane Chemical Pressure Effect in Eu0.5La0.5FBiS2-xSex

We have investigated Se substitution effect to superconductivity of an optimally-doped BiS2-based superconductor Eu0.5La0.5FBiS2. Eu0.5La0.5FBiS2-xSex samples with x = 0-1 were synthesized. With increasing x, in-plane chemical pressure is enhanced. For x = 0.6, 0.8, and 1, superconducting transitions with a large shielding volume fraction are observed in magnetic susceptibility measurements, and the highest Tc is 3.8 K for x = 0.8. From low-temperature electrical resistivity measurements, a zero-resistivity state is observed for all the samples, and the highest Tc is observed for x = 0.8. With increasing Se concentration, characteristics of electrical resistivity changes from semiconducting-like to metallic, suggesting that the emergence of bulk superconductivity is linked with the enhanced metallicity. A superconductivity phase diagram of the Eu0.5La0.5FBiS2-xSex superconductor is established. Temperature dependences of electrical resistivity show an anomalous two-step transition under high magnetic fields. Hence, the resistivity data are analyzed with assuming in-plane anisotropy of upper critical field.

cond-mat.supr-con↗

arXiv2016

Specific Heat and Electrical Transport Properties of Sn0.8Ag0.2Te Superconductor

Sn0.8Ag0.2Te is a new superconductor with Tc ~ 2.4 K. The superconducting properties of Sn0.8Ag0.2Te have been investigated by specific heat measurements under magnetic fields. Bulk nature of superconductivity was confirmed from the amplitude of the specific heat jump at the superconducting transition, and the amplitude is consistent with fully-gapped superconductivity. Upper critical field was estimated from specific heat and electrical resistivity measurements under magnetic fields. The Hall coefficient was positive, suggesting that the Ag acts as a p-type dopant in Sn0.8Ag0.2Te.

cond-mat.supr-con↗

arXiv2015

Pronounced - Log T Divergence in Specific Heat of Nonmetallic CeOBiS$_2$: A Mother Phase of BiS$_2$-Based Superconductor

The low-temperature properties of CeOBiS$_2$ single crystals are studied by electrical resistivity, magnetization, and specific heat measurements. Ce 4f -electrons are found to be in a well-localized state split by crystalline-electric-field (CEF) effects. The CEF ground state is a pure $J_z$ = $\pm$ 1/2 doublet, and excited doublets are located far above. At low temperatures in zero field, we observe pronounced $-\log T$ divergence in the specific heat, revealing the presence of quantum critical fluctuations of 4f magnetic moments near a quantum critical point (QCP). Considering that CeOBiS$_2$ is a nonmetal, this phenomenon cannot be attributed to the competition between Kondo and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions as in numerous f-electron-based strongly correlated metals, indicating an unconventional mechanism. We suggest that CeOBiS$_2$ is the first material found to be located at a QCP among geometrically frustrated nonmetallic magnets.

cond-mat.str-el↗

arXiv2014

Possible Existence of Partially Disordered Sm Ions in Magnetically Ordered State of Ising Magnet SmPt2Si2: a Single Crystal Study

We have succeeded in growing single crystals of SmPt2Si2 and have performed electrical resistivity, magnetization and specific heat measurements. The magnetic susceptibility shows that SmPt2Si2 is an Ising magnet with the crystalline-electric-field ground state of Jz=+-3/2. We have found the existence of two magnetically ordered states, i.e., an antiferromagnetically (AFM) ordered phase (I) setting in at TN=5.1 K and a field-induced magnetization plateau phase (II). In the phase I, unlike the usual AFM states, a pronounced Curie-Weiss contribution remains in magnetic susceptibility, indicating the existence of "partially disordered" Sm ions. Largely enhanced Sommerfeld-coefficient (350 mJ/K2mol) in the phase I can be attributed to formation of Kondo sublattice with heavy quasiparticles in the partially disordered Sm ions. These findings may reflect substantial magnetic frustrations inherent in SmPt2Si2.

cond-mat.str-el↗

arXiv2013

AC Susceptibility of the Dipolar Spin Ice Dy2Ti2O7: Experiments and Monte Carlo Simulations

Experimental data of the frequency-dependent ac susceptibility for the dipolar spin ice Dy2Ti2O7 has been analyzed by Monte Carlo simulations on the basis of the single-spin-flip Metropolis algorithm. We have directly evaluated the ac susceptibility by applying an ac magnetic field. We found that the simulated behaviors reproduces the experimental behavior of the ac susceptibility in the temperature range from 0.6 to 1.0 K, where dilute magnetic monopoles diffusively move. The conversion factor from simulation time to real time, i.e., the rate of hopping of monopoles to nearest-neighbor sites, strongly depends on temperature.

cond-mat.str-el↗

arXiv2011

Unusual Field-Insensitive Phase Transition and Kondo Behavior in SmTi$_2$Al$_{20}$

Magnetization, electrical resistivity and specific heat measurements were performed on high-quality single crystalline SmTi$_2$Al$_{20}$ (residual resistivity ratio $\sim$ 40) grown by Al self-flux method. A Kondo-like $\log T$ dependence in the resistivity is observed below 50 K. We discovered a field-insensitive phase transition at $T_{x}$ = 6.5 K and a field-insensitive heavy fermion behavior with the electronic specific heat coefficient $γ$ = 150 mJ/(K$^{2}$ mol). Specific heat analysis reveals that the ground state is a $Γ_{8}$ quartet state and the Sm magnetic dipole moment $m_{\rm Sm}$ ($\sim 0.5 μ_{\rm B}$ at $T \simeq$ 0) orders below $T_{x}$ in spite of the field-insensitive behavior. Possible reasons for the field insensitiveness will be discussed.

cond-mat.str-el↗

arXiv2006

Field-induced confinement in (TMTSF)2ClO4 under accurately aligned magnetic fields

We present transport measurements along the least conducting c direction of the organic superconductor (TMTSF)2ClO4, performed under an accurately aligned magnetic field in the low temperature regime. The experimental results reveal a two-dimensional confinement of the carriers in the (a,b) planes which is governed by the magnetic field component along the b' direction. This 2-D confinement is accompanied by a metal-insulator transition for the c axis resistivity. These data are supported by a quantum mechanical calculation of the transverse transport taking into account in self consistent treatment the effect of the field on the interplane Green function and on the intraplane scattering time.

cond-mat.supr-con↗

arXiv2005

Field-Induced Transition on Triangular Plane in the Spin Ice Compound Dy2Ti2O7

The origin of the lowest-temperature anomaly reported several years ago using a polycrystalline sample of the spin ice compound Dy2Ti2O7 had remained unresolved. Here we finally clarify its origin by susceptibility measurements down to 65 mK using single crystals under accurate control of the magnetic fields in two independent directions. We demonstrate that the transition is induced under a subtle field combination that precisely cancel the nearest-neighbor spin interactions acting on the spins on the triangular lattice within the pyrochlore structure. Contrary to the other two fieldinduced transitions, this transition is driven only by the interactions beyond the nearest neighbors. Our observation thus provides the first qualitative evidence for the essential importance of the dipolar interaction beyond the nearest neighbors in the spin ice.

cond-mat.str-el↗

arXiv2005

Magnetodielectric Response of the Spin-Ice Dy2Ti2O7

We report the magneto-dielectric response of single crystals of the spin-ice compound Dy2Ti2O7 down to 0.26 K. The dielectric constant under zero magnetic field exhibits a clear decrease reflecting the development of the local two-spins-in, two-spins-out structure below about 1.2 K. Both the real and imaginary parts of the dielectric constant under magnetic fields sensitively respond to various changes in the spin structures. We found that the real part can be described in terms of local spin correlations among the moments of tetrahedra, rather than among individual Dy3+ moments. Using the peaks in the imaginary part, we have constructed a precise field-temperature phase diagram in the [111] field direction. We thus demonstrate that the magneto-dielectric response can be a high-sensitivity local probe of the spin state of geometrically frustrated systems.

cond-mat.str-el↗

arXiv2004

Low Temperature Specific Heat of Dy$_2$Ti$_2$O$_7$ in the Kagome Ice State

We report the specific heat of single crystals of the spin ice compound Dy$_2$Ti$_2$O$_7$ at temperatures down to 100 mK in the so-called Kagome ice state. In our previous paper, we showed the anisotropic release of residual entropy in different magnetic field directions and reported new residual entropy associated with spin frustration in the Kagome slab for field in the [111] direction. In this paper, we confirm the first-order phase transition line in the field-temperature phase diagram and the presence of a critical point at (0.98 T, 400 mK), previously reported from the magnetization and specific-heat data. We newly found another peak in the specific heat at 1.25 T below 0.3 K. One possible explanation for the state between 1 T and 1.25 T is the coexistence of states with different spin configurations including the 2-in 2-out one (Kagome ice state), the 1-in 3-out state (ordered state) and paramagnetic one (free-spin state).

cond-mat.str-el↗

arXiv2003

Anisotropic Release of the Residual Zero-point Entropy in the Spin Ice Compound Dy2Ti2O7: Kagome-ice Behavior

We report the specific heat and entropy of single crystals of the spin ice compound Dy2Ti2O7 at temperatures down to 0.35 K. We apply magnetic fields along the four characteristic directions: [100], [110], [111] and [112]. Because of Ising anisotropy, we observe anisotropic release of the residual zero-point entropy, attributable to the difference in frustration dimensionality. In the high magnetic field along these four directions, the residual entropy is almost fully released and the activation entropy reaches Rln2. However, in the intermediate field region, the entropy in fields along the [111] direction is different from those for the other three field directions. For the [111] direction the frustration structure changes from that of three-dimensional(3D) pyrochlore to that of two-dimensional(2D) Kagome-like lattice with constraint due to the ice rule, leading to different values of zero-point entropy.

cond-mat.str-el↗