Amitsuka Hiroshi

Anomalous Uniaxial Pressure Effect on the Phase IV Ordering in CexLa1-xB6

We have examined the magnetization M of Ce 0.75 La 0.25 B 6 under uniaxial pressures applied along the [001] direction, focusing our attention on the mysterious phase IV ordering. Magnetization of phase IV, which is nearly isotropic at an ambient pressure, becomes strongly anisotropic at a relatively weak uniaxial pressure of 60 MPa. On entering phase IV from the paramagnetic phase I, M ( T ) increases (decreases) for the field direction H ∥[001] ([110]), suggesting that the order parameter is intrinsically anisotropic. Interestingly, M ( T ) for H ∥[001] exhibits an upward-bending at the ordering temperature T I–IV , and T I–IV ( H ) seems to increase in magnetic field unlike the case of ordinary antiferromagnetic orderings. The results are discussed in view of a spontaneous ordering of an octupole moment in phase IV.

Magnetization Study on the History-Dependent Peak Effect in the Superconducting Mixed State of CeRu2.

A huge peak of irreversible magnetization in CeRu 2 single crystal, together with the equilibrium magnetization, was investigated by means of a static magnetization measurement using a Faraday force magnetometer. The residual resistance ratio of the used crystal exceeds 250, indicating that it is a clean superconductor. Contrary to the previous reports, there is no pinning-free region in the magnetization curves below the upper critical field H c2 . The detailed minor-hysteresis-loop measurement has revealed that the peak structure in the increasing-field magnetization curve around the onset field of the peak strongly depends on previous field-history, resulting in an apparent asymmetry of the peak structure. No trace of paramagnetic suppression of superconductivity in the vortex-penetrated state is observed from the superconducting equilibrium magnetization curves. The peak position, although highly anisotropic, can be interrelated to the peak strength, regardless of the applied field direction. It is li...

Superconducting Properties of Noncentrosymmetric Superconductor LaPt3Si Studied by Muon Spin Spectroscopy

The superconducting properties of the noncentrosymmetric superconductor LaPt3Si were investigated by \(\mu\)SR spectroscopy. Within experimental accuracy, spontaneous internal fields were not observed in the superconducting phase by zero-field \(\mu\)SR experiments. Transverse-field \(\mu\)SR experiments revealed the coexistence of the intermediate and mixed states in the superconducting phase, which are characteristic features of type-I and type-II superconductors, respectively. We found that the observed anomalous field distribution is consistent with the results of a recent numerical calculation for type-1.5 superconductors.

Effects of Uniaxial Stress on Antiferromagnetic Moment in the Heavy Electron Compound URu2Si2

We have performed the elastic neutron scattering experiments under uniaxial stress σ along the tetragonal [100], [110] and [001] directions for URu 2 Si 2 . For σ∥[100] and [110], the antiferromagnetic moment µ o is strongly enhanced from 0.02 µ B /U (σ=0) to 0.22 µ B /U (σ=0.25 GPa) at 1.5 K. The rate of increase ∂µ o /∂σ is roughly estimated to be ∼1.0 µ B /GPa, which is much larger than that for the hydrostatic pressure (∼0.25 µ B /GPa). Above 0.25 GPa, µ o shows a tendency to saturate similar to the behavior in the hydrostatic pressure. For σ∥[001], on the other hand, µ o shows only a slight increase to 0.028 µ B /U (σ=0.46 GPa) with a rate of ∼0.02 µ B /GPa. The observed anisotropy suggests that the competition between the hidden order and the antiferromagnetic state in URu 2 Si 2 is strongly coupled with the tetragonal four-fold symmetry and the c / a ratio, or both.We have performed the elastic neutron scattering experiments under uniaxial stress \sigma along the tetragonal [100], [110] and [001] directions for URu2Si2. For \sigma // [100] and [110], the antiferromagnetic moment \mu_o is strongly enhanced from 0.02 \mu_B (\sigma=0) to 0.22 \mu_B (\sigma=2.5 kbar) at 1.5 K. The rate of increase d\mu_o/d\sigma is roughly estimated to be ~ 0.1 \mu_B/kbar, which is much larger than that for the hydrostatic pressure (~ 0.025 \mu_B/kbar). Above 2.5 kbar, \mu_o shows a tendency to saturate similar to the behavior in the hydrostatic pressure. For \sigma // [001], on the other hand, \mu_o shows only a slight increase to 0.028 \mu_B (\sigma = 4.6 kbar) with a rate of ~ 0.002 \mu_B/kbar. The observed anisotropy suggests that the competition between the hidden order and the antiferromagnetic state in URu2Si2 is strongly coupled with the tetragonal four-fold symmetry and the c/a ratio, or both.

Observation of Low-Energy Einstein Phonon and Superconductivity in Single-Crystalline LaBe13

The thermal and electrical transport properties of single-crystalline LaBe13 have been investigated by specific-heat (C) and electrical-resistivity (ρ) measurements. The specific-heat measurements in a wide temperature range revealed the presence of a hump anomaly near 40 K in the C(T)/T curve, indicating that LaBe13 has a low-energy Einstein-like-phonon mode with a characteristic temperature of ∼177 K. In addition, a superconducting transition was observed in the ρ measurements at the transition temperature of 0.53 K, which is higher than the value of 0.27 K reported previously by Bonville et al. Furthermore, an unusual T3 dependence was found in ρ(T) below ∼50 K, in contrast to the behavior expected from the electron–electron scattering or the electron–Debye phonon scattering.

Magnetically Ordered State and Crystalline-Electric-Field Effects in SmBe13

The physical properties of single-crystalline SmBe13 with a NaZn13-type cubic structure have been studied by electrical resistivity (ρ), specific heat (C), and magnetization (M) measurements in mag...

Evidence of a New Current-Induced Magnetoelectric Effect in a Toroidal Magnetic Ordered State of UNi4B

Magnetization measurements under direct electric current were performed in a toroidal magnetic ordered state of UNi4B to test a recent theoretical prediction of current-induced magnetization in a metallic system lacking local-inversion symmetry. We found that electric current parallel to \([2\bar{1}\bar{1}0]\) and [0001] in the hexagonal 4-index notation induces a uniform magnetization along the \([01\bar{1}0]\) direction. The observed behavior of the induced magnetization is essentially consistent with the theoretical prediction; however, it also shows an inconsistency suggesting that the antiferromagnetic state of UNi4B could not be simply regarded as a uniform toroidal order in the ideal honeycomb layered structure.Magnetization measurements under direct electric currents were performed for toroidal magnetic ordered state of UNi4B to test a recent theoretical prediction of current-induced magnetization in a metallic system lacking local inversion symmetry. We found that each of the electric currents parallel to [21̄1̄0] and [0001] in the hexagonal 4-index notation induces uniform magnetization in the direction of [011̄0]. The observed behavior of the induced magnetization is essentially consistent with the theoretical prediction; however it also shows an inconsistency suggesting that the antiferromagnetic state of UNi4B could not simply be regarded as a uniform toroidal order in the ideal honeycomb layered structure.

Pressure-Induced Quantum Critical Point in Ferromagnet U4Ru7Ge6

The electrical resistivity ρ of the ferromagnet U 4 Ru 7 Ge 6 has been studied under high pressure up to ∼4.5 GPa by using an indenter-type high-pressure cell. The Curie temperature T C = 11.2 K at ambient pressure decreases monotonously with increasing pressure, and seems to disappear at ∼2.6 GPa, where an enhancement of the coefficient A assuming Fermi liquid regime is observed. This result indicates a presence of a quantum critical point (QCP) of U 4 Ru 7 Ge 6 at around 2.6 GPa. In addition, non-Fermi liquid behavior (ρ∼ T 5/3 ) predicted by the SCR theory of three dimensional ferromagnet is also observed near the QCP.

73Ge-NMR Investigation of the Hidden Order in URu2Si2

In order to investigate the local symmetry at the Si(4 e )-site above and below hidden order temperature T O =17.5 K in URu 2 Si 2 , we have carried out 73 Ge-nuclear magnetic resonance (NMR) measurement on 10% 73 Ge-substituted sample URu 2 (Si 0.9 Ge 0.1 ) 2 . The specific heat measurement on the present sample shows a clear second-order transition, indicating that the hidden order is still a bulk property. Besides, equally split 73 Ge-NMR lines due to the nuclear quadrupole interaction gives evidence for fairly uniform electric-field gradient at the Ge-substituted sites, Although this is the first attempt to investigate the local symmetry at the 4 e -site by proving electric interactions, no evidence for symmetry lowering below T O has been obtained at present.

Heterogeneous Metallic State in URu2Si2

A review of recent attempts to understand the pressure-temperature phase diagram of URu 2 Si 2 is given. At ambient (and weak) pressures, the low- T properties of this system are dominated by an unidentified ordered state (hidden order: HO) and non-trivial superconductivity, while by applying hydrostatic (or uniaxial or chemical) pressures, a change to a regime dominated by antiferromagnetic order (AF) is observed. In between these two regimes, an unusual heterogeneous metallic state is found. This change in regime is clearly demonstrated by neutron scattering, NMR and µSR measurements. The experimental results and strain analyses show that the low- T state of this system is extremely sensitive to the shape effects, and a slight increase in the axial strain [δ( c / a )]/( c / a ) (≡\hatη) can easily change the ground state from HO to AF. The critical value is estimated to be very small (\hatη c ∼10 -3 ), suggesting that a strain distribution in a crystal is responsible for the observed heterogeneity. We w...