Yanagisawa Tatsuya

Rattling in Clathrate Compounds of ROs4Sb12 and R3Pd20Ge6

We have performed ultrasonic experiments on clathrate compounds ROs 4 Sb 12 and R 3 Pd 20 Ge 6 to explore rattling and tunneling of a guest rattlers motion. We have found frequency dependence of the elastic constants and ultrasonic attenuations, which is called as ultrasonic dispersion, due to thermally activated rattling. At low temperatures, Curie-type elastic softening obeying 1/ T with antiferro-type intersite interaction, which corresponds to quantum tunneling, appears and continues even to the lowest 20 mK in LaOs 4 Sb 12 and La 3 Pd 20 Ge 6 . The understanding of the local charge fluctuation, rattling and tunneling, being coupled to conduction electrons in metals is a basic issue in the aspect of electron–phonon interaction in strongly correlated phenomena such as heavy fermion state, superconductivity and multi-channel Kondo screening effect.

Off-Center Rattling and Tunneling in Filled Skutterudite LaOs4Sb12

Off-center rattling of La-ion in the filled skutterudite compound LaOs 4 Sb 12 has been investigated by means of ultrasonic measurements. An elastic constant C 11 shows a remarkable frequency dependence in elastic constant and ultrasonic attenuation at around 25 K. This frequency dependent ultrasonic dispersion reveals thermally activated Γ 23 -type off-center rattling of La-ion. An attempt time τ 0 = 5.0×10 -11 sec and an activation energy E = 127 K were obtained from the analysis.

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...

Quadrupole Susceptibility of HoB2C2

We have investigated the elastic constants of the ternary rare earth compound HoB 2 C 2 with a tetragonal structure by means of the ultrasonic method. The elastic constants of ( C 11 - C 12 )/2, C 44 and C 66 , corresponding to the transverse ultrasonic modes exhibit the softening proportional to reciprocal temperature below about 40 K. This softening is commonly enhanced in the ordered phase IV below T C 1 =5.9 K. An abrupt increase of elastic constants is found in the ordered phase III below T C 2 =5.0 K characterized by the simultaneous antiferro-type ordering of electric quadrupole and magnetic dipole moments. The quadrupole susceptibility based on a model consisting of a ground state E-doublet and an excited singlet with a low energy in HoB 2 C 2 is employed for the analysis of the softening above T C 1 =5.9 K. The enhancement of the elastic softening in the phase IV of HoB 2 C 2 resembles to the soft C 44 mode in the phase IV in Ce 0.75 La 0.25 B 6 .

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.

Ultrasonic Measurements on the Cage-Structured Clathrate Compound U3Pd20Si6

Ultrasonic measurements of U 3 Pd 20 Si 6 were performed in order to investigate elastic properties of the cage-structured compound U 3 Pd 20 Si 6 and also examine a presence of ultrasonic dispersion (UD) due to off-center rattling, which has been observed in the elastic constant C 44 of similar systems R 3 Pd 20 Ge 6 ( R = La-Nd). At temperatures over antiferromagnetic ordering at T N = 19 K, neither an up-turn anomaly nor frequency dependence of the elastic constants, regarding the UD, was found. On the other hand, the elastic constant C 44 shows pronounced softening due to crystalline electric field effect of U 4+ ions Γ 5 triplet ground state for both 4 a and 8 c sites. Below T N , precipitous elastic softening accompanied by a sudden depression of ultrasonic echo was observed in each ultrasonic mode. A drastic change of Δ C 44 / C 44 ∼30% at T N , in particular, implies a strong magneto-elastic coupling of 5 f -electrons or a possible contribution of a ferroquadrupole interaction to the antiferromag...

Specific Heat and Magnetization Studies of the Superconducting Mixed State of UBe13

Low-temperature specific heat and static dc magnetizetion measurements have been performed for the superconducting state of the heavy-fermion compound UBe 13 . The superconducting phase diagram B c2 – T ( B ∥[110]) has been obtained by our specific heat and magnetization measurements. From the slope of the upper critical field and specific heat jumps at superconducting transition temperatures in magnetic fields, we have also obtained a temperature dependence of Ginzburg–Landau parameter κ 2 along [110]. It has been revealed that κ 2 decreases with decreasing temperature down to 0.5 K, which indicates that a paramagnetic effect may be present at least down to 0.5 K for [110].

Quadrupole Susceptibility of Γ3 Pseudo-Doublet Ground State in Rare-Earth Garnet Tb3Ga5O12

We have investigated elastic properties of a garnet-type single crystal Tb 3 Ga 5 O 12 by ultrasonic measurements. Temperature dependence of the transverse elastic constants ( C 11 – C 12 )/2 and C 44 shows strong Curie-type softening below 150 K with minima around 30–50 K and also below 5 K. These effects are due to quadrupole susceptibilities for the crystalline electric field system with a pseudo-doublet Γ 3 ground state and an excited triplet Γ 5 at 57.3 K. Magnetic field dependence of the ( C 11 – C 12 )/2 mode in H ∥[110] shows step-like anomaly due to a level crossing at around 9.5 T.