Yukihiro F. Inoue

Superconductivity and Structural Phase Transitions in Caged Compounds RT2Zn20 (R = La, Pr, T = Ru, Ir)

Electrical resistivity ρ, specific heat C , magnetization M measurements are reported on four compounds, LaRu 2 Zn 20 , PrRu 2 Zn 20 , LaIr 2 Zn 20 , and PrIr 2 Zn 20 , which crystallize in a cubic CeCr 2 Al 20 -type structure. LaRu 2 Zn 20 , LaIr 2 Zn 20 , and PrIr 2 Zn 20 show superconducting transitions at T C = 0.2, 0.6, and ∼0.05 K, respectively, whereas PrRu 2 Zn 20 remains a normal state down to 0.04 K. This is the first observation of superconductivity in the family of RT 2 X 20 (T = transition metal; X = Al and Zn). Furthermore, structural phase transitions manifest themselves at T s = 150, 138, and 200 K for LaRu 2 Zn 20 , PrRu 2 Zn 20 , and LaIr 2 Zn 20 , respectively. No magnetic transition is found in PrRu 2 Zn 20 and PrIr 2 Zn 20 down to 1.8 K. On cooling PrIr 2 Zn 20 below 2 K, the specific heat divided by temperature, C / T , continuously increases and reaches 5 J/(K 2 ·mol) at 0.4 K, suggesting that Pr 4 f 2 electrons are involved in the heavy-fermion state, as observed in a related compo...

Nonmagnetic ground states and phase transitions in the caged compounds PrT2Zn20 (T?=?Ru, Rh and Ir)

We performed electrical resistivity ρ, magnetic susceptibility χ, specific heat C and electron diffraction measurements on single-crystalline samples of PrT2Zn20 (T = Ru, Rh and Ir). The three compounds show the Van Vleck paramagnetic behavior, indicating the nonmagnetic crystalline electric field (CEF) ground states. A Schottky-type peak appears at around 14 K, irrespective of the T element, which can be moderately reproduced by a doublet–triplet model. For T = Ru, a structural transition occurs at Ts = 138 K, below which no phase transition appears down to 0.04 K. On the other hand, for T = Ir, antiferroquadrupole (AFQ) ordering arising from the nonmagnetic Γ3 doublet takes place at TQ = 0.11 K. For T = Rh, despite a structural transition between 170 and 470 K, the CEF ground state is still the non-Kramers Γ3 doublet. However, no phase transition due to the Γ3 doublet was observed even down to 0.1 K.

Giant Uniaxial Anisotropy in the Magnetic and Transport Properties of CePd5Al2

Electrical resistivity � , magnetic susceptibility � , magnetization M, and specific heat measurements are reported on a singlecrystalline sample of CePd5Al2, showing successive antiferromagnetic orderings at TN1 ¼ 4:1 K and TN2 ¼ 2:9 K. The temperature dependence ofshows a Kondo metal behavior with large anisotropy, � c=� a ¼ 3:2 at 20 K, and opening of a superzone gap along the tetragonal c-direction below TN1. Both TN1 and TN2 gradually increase with applying pressure up to 2.5 GPa. The data of � ðTÞ and MðBÞ in the paramagnetic state were analyzed using a crystalline electric field (CEF) model. It led to a Kramers doublet ground state with wave functions consisting primarily of j� 5=2i, whose energy level is isolated from the excited states by 230 and 300 K. This CEF effect gives rise to the large anisotropy in the paramagnetic state. In the ordered state, the uniaxial magnetic anisotropy is manifested as Mc=Ma ¼ 20 in B ¼ 5 T and at 1.9 K, andc=� a ¼ 25 in B ¼ 0:1 T and at 4 K. In powder neutron diffraction experiments, magnetic reflections were observed owing to the antiferromagnetic ordered states below TN1, whereas no additional reflection was found below TN2.

NMR Evidence of Freezing of Rattling Motion in LaIr2Zn20 and LaRu2Zn20

139 La nuclear magnetic resonance (NMR) measurements have been carried out to investigate structural phase transition in LaIr 2 Zn 20 and LaRu 2 Zn 20 . The former compound undergoes second-order structural phase transition at T s =200 K, whereas the latter undergoes first-order transition at T s =150 K. For both compounds, narrow Lorentzian La-NMR spectra with no electric field gradient (EFG) were observed above T s , indicating that the local symmetry at the La site holds a cubic symmetry. Below T s , the La NMR spectrum of LaIr 2 Zn 20 splits into more than two lines with broad tails distributed over a frequency range of 0.8 MHz, while that of LaRu 2 Zn 20 broadens with broad tails over a frequency range of 1.6 MHz. The appearance of an EFG at the La site below T s is clear evidence that the cubic local symmetry at the La site lowers below T s . Both compounds show unusual increases in the nuclear spin-lattice relaxation rate at approximately T s , indicating the slowing down of the fluctuation of the ...

Elastic modulus of cage compound PrRu2Zn20

Elastic properties of the cage compound PrRu2Zn20 have been investigated by means of ultrasonic measurement. The transverse modulus (C11–C12)/2 shows elastic softening below 250 K and exhibits a discontinuous change in the slope at 150 K. An abrupt elastic hardening is observed below Ts = 138 K which is a non-magnetic phase transition temperature. The phase transition at Ts is accompanied by hysteresis indicating that the transition is of first-order. The softening above 150 K is well reproduced by Curie-Weiss-like formula for the co-operative Jahn-Teller effect with a negative quadrupole-quadrupole coupling constant. The phase transition at Ts is not caused by 4f-electrons of Pr ions. There is a possibility that the elastic hardening below 3 K originates from the rattling motion.

Sinusoidally modulated magnetic structure of a kondo lattice compound CePd5Al2

The magnetic structure of CePd5Al2, crystallizing in the tetragonal ZrNi2Al5-type structure, was studied by neutron diffraction technique using single crystals. This compound exhibits antiferromagnetic (AF) orderings at TN1 = 4.1 K and TN2 = 2.9 K. Below TN1 magnetic peaks were observed at Q = (0.235, 0.235, 0) and its equivalent positions. Our analysis of the Q-dependence of the peak intensities suggests a sinusoidally modulated magnetic structure where the magnetic moments of 2.0(1) μB/Ce align along the c-axis. Third-order harmonics were also observed at temperatures from 3 K to 0.8 K, suggesting that the magnetic moments square up in the ground state. Inelastic neutron scattering measurements revealed well-defined crystalline electric field (CEF) excitations at 21.3 and 22.4 meV. The derived wave functions of the CEF ground state consist primarily of |±5/2, which reproduce the anisotropic behavior of the magnetic susceptibility and isothermal magnetization in the paramagnetic state.