arXiv subjects

Hiroshi Amitsuka

Publications and source records attributed to Hiroshi Amitsuka.

25 records · Page 2Linked to original sources

arXiv2016

Crystalline Electric Field and Kondo Effect in SmOs4Sb12

Our ultrasound results obtained in pulsed magnetic fields show that the filled-skutterudite compound SmOs$_4$Sb$_{12}$ has the $Γ_{67}$ quartet crystalline-electric-field ground state. This fact suggests that the multipolar degrees of freedom of the $Γ_{67}$ quartet play an important role in the unusual physical properties of this material. On the other hand, the elastic response below $\approx$ 20 T cannot be explained using the localized 4$f$-electron model, which does not take into account the Kondo effect or ferromagnetic ordering. The analysis result suggests the presence of a Kondo-like screened state at low magnetic fields and its suppression at high magnetic fields above 20 T even at low temperatures.

cond-mat.str-el↗

arXiv2014

Field-Orientation Dependence of Low-Energy Quasiparticle Excitations in the Heavy-Electron Superconductor UBe13

Low-energy quasiparticle excitations in the superconducting (SC) state of UBe$_{13}$ were studied by means of specific-heat ($C$) measurements in a rotating field. Quite unexpectedly, the magnetic-field dependence of $C(H)$ is linear in $H$ with no angular dependence at low fields in the SC state, implying that the gap is fully open over the Fermi surfaces, in stark contrast to the previous expectation. In addition, a characteristic cubic anisotropy of $C(H)$ was observed above 2~T with a maximum (minimum) for $H$ $||$ $[001]$ ($[111]$) within the $(1\bar{1}0)$ plane, both in the normal as well as in the SC states. This oscillation possibly originates from the anisotropic response of the heavy quasiparticle bands, and might be a key to understand the unusual properties of UBe$_{13}$.

cond-mat.supr-con↗

arXiv2012

Γ3-type Lattice Instability and the Hidden Order of URu2Si2

We have performed ultrasonic measurements on single-crystalline URu2Si2 with pulsed magnetic fields, in order to check for possible lattice instabilities due to the hybridized state and the hidden-order state of this compound. The elastic constant (C11-C12)/2, which is associated with a response to the Γ3-type symmetry-breaking (orthorhombic) strain field, shows a three-step increase at H > 35 T for H || c at low temperatures, where successive meta-magnetic transitions are observed in the magnetization. We discovered a new fact that the absolute change of the softening of (C11-C12)/2 in the temperature dependence is quantitatively recovered at the suppression of hybridized-electronic state and the hidden order in high-magnetic field for H \perp c associated with the successive transitions. The present results suggest that the Γ3-type lattice instability, is related to both the emergence of the hybridized electronic state and the hidden-order parameter of URu2Si2. On the other hand, magnetic fields H || [100] and [110] enhance the softening of (C11-C12)/2 in the hidden order phase, while no step-like anomaly is observed up to 68.7 T. We discuss the limitation of the localized-electron picture for describing these features of URu2Si2 by examination of a crystalline electric field model in terms of mean-field theory.

cond-mat.str-el↗

arXiv2012

A Study of Ni-Substitution Effects on Heavy-Fermion CeCu2Si2 - Similarities between Ni Substitution and High-Pressure Effects -

The effects of Ni substitution on Ce(Cu1-xNix)2Si2 have been studied by specific heat and electrical resistivity measurements. The specific heat measurement has revealed that the enhanced quantum fluctuations around an antiferromagnetic quantum critical point are markedly suppressed by Ni substitution, and that the Fermi liquid state recovers in the Nirich region (x > 0.12). The characteristic T-linear dependence of the resistivity has been observed at approximately x ~ 0.10 together with a sign of superconductivity. The variation of n in the form of rho - rho0 = aT^n against Tmax^1, at which the resistivity peaks, coincides with the case of high-pressure experiments on pure CeCu2Si2. The anomalous T-linear behavior appears to occur in the crossover region from the Kondo regime to the valence fluctuation regime rather than in the conventional antiferromagnetic quantum critical region.

cond-mat.str-el↗

arXiv2011

Maki Parameter and Upper critical Field of the Heavy-Fermion Superconductor UBe13

We have performed low-temperature specific-heat measurements in magnetic fields for a single crystal UBe13. It has been observed that our sample exhibits a superconducting transition at an intermediate temperature between previously reported values for two variant samples called H type and L type. The specific heat C(T) of our sample shows a T^3 behavior in the temperature range below 0.7 Tc, which is similar to the behavior of the H-type sample, suggesting the existence of point nodes in the superconducting gap function. We have obtained the upper-critical-field curves Hc2 for the [001], [110], and [111] crystal axes, which show no anisotropy at least down to the lowest measured temperature of 0.5 K. We have also derived the Maki parameter kappa2, and it has been revealed that the kappa2 steeply decreases isotropically upon cooling just below Tc. Paramagnetic effects and the symmetry of Cooper pairing of UBe13 are discussed.

cond-mat.str-el↗

arXiv2011

Magnetic-Field-Independent Ultrasonic Dispersions in the Magnetically Robust Heavy Fermion System SmOs4Sb12

Elastic properties of the filled skutterudite compound SmOs$_4$Sb$_{12}$ have been investigated by ultrasonic measurements. The elastic constant $C_{11}(ω)$ shows two ultrasonic dispersions at $\sim$15 K and $\sim$53 K for frequencies $ω$ between 33 and 316 MHz, which follow a Debye-type formula with Arrhenius-type temperature-dependent relaxation times, and remain unchanged even with applied magnetic fields up to 10 T. The corresponding activation energies were estimated to be $E_2$ = 105 K and $E_1$ = 409 K, respectively. The latter, $E_1$, is the highest value reported so far in the Sb-based filled skutterudites. The presence of magnetically robust ultrasonic dispersions in SmOs$_4$Sb$_{12}$ implies a possibility that an emergence of a magnetically insensitive heavy fermion state in this system is associated with a novel local charge degree of freedom which causes the ultrasonic dispersion.

cond-mat.str-el↗

arXiv2010

Magnetization, Maxwell's Relations and the Local Physics of the Dilute System Th_{1-x}U_{x}Ru_{2}Si_{2}

The dilute Kondo compound, Th_{1-x}U_{x}Ru_{2}Si_{2} (TURS), displays non-Fermi liquid behavior but no zero-point entropy; it thus appears to elude description by known single-ion models. It may also provide a clue to the underlying local degrees of freedom in its dense counterpart, URu_{2}Si_{2}. Here we use high-resolution magnetization studies to cross-check the thermodynamic consistency of previous experiments. Measurements of the field-dependence of the temperature-scale, T_F(H), associated with Fermi liquid behavior probes the nature of the underlying impurity fixed point. We find that T_F(H) grows linearly with applied field, in contrast to the quadratic form expected for the two-channel Kondo model. We use a scaling argument to show that the observed behavior of T_F(H) is consistent with the absence of zero-point entropy, suggesting novel impurity behavior in this material. More generally, we suggest the field-magnetization as a probe of single-ion physics and make predictions for its behavior in other actinide compounds.

cond-mat.str-el↗