Keisuke T. Matsumoto

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...

Antiferro-Quadrupolar Ordering at the Lowest Temperature and Anisotropic Magnetic Field–Temperature Phase Diagram in the Cage Compound PrIr2Zn20

The cage compound PrIr 2 Zn 20 with the non-Kramers doublet ground state shows a phase transition at T Q ∼0.11 K. To investigate the origin of the phase transition, we have carried out ultrasonic m...

Quadrupole-driven non-Fermi-liquid and magnetic-field-induced heavy fermion states in a non-Kramers doublet system

Orbital degrees of freedom in condensed matters could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focussed on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at TQ = 0.11 K. We observed non-Fermi liquid (NFL) behaviors emerging in the electrical resistivity and the 4f contribution to the specific heat, C_4f, in the paramagnetic state at T > TQ. Moreover, in magnetic fields below 6 T, all data set of the electrical resistivity and C_4f(T) are well scaled with characteristic temperatures T0s. This is the first observation of the NFL state in the nonmagnetic quadrupole-active system, whose origin is intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons. Below 0.13 K, the electrical resistivity and C_4f(T) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.

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.

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 ...

Low-energy optical phonon modes in the caged compound LaRu2Zn20

The caged compound LaRu2Zn20 exhibits a structural transition at TS =150 K, whose driving mechanism remains elusive. We have investigated atomic dynamics by the measurements of specific heat C and inelastic X-ray scattering (IXS). The lattice part of the specific heat Clat divided by T3, Clat/T3, shows a broad peak at around 15 K, which is reproduced by two Einstein modes with characteristic temperatures of 35 K and 82 K, respectively. IXS measurements along the [111] and [110] directions reveal optical phonon modes at 3 meV (35 K) and 7 meV (80 K), respectively, whose values agree with the values of Einstein temperatures. The first principles calculation has assigned the phonon modes at 3 meV as the optical modes of Zn atoms located at the middle of two La atoms. The low-energy vibration of the Zn atom perpendicular to the there-fold axis is thought to lead the structural instability of LaRu2Zn20.

Anomalous Enhancement of Seebeck Coefficient in Pr-Based 1-2-20 System with Non-Kramers Doublet Ground States

Low-temperature Seebeck coefficient S/T measurements have been performed on Pr-based 1-2-20 system, PrTr2X20 (Tr = Ti, Ta, V, Ir, X = Al, Zn) with non-Kramers doublet ground states. For PrTr2X20 with X = Al, we find a large S/T, which amounts to those of heavy fermion metals. By contrast, S/T for PrIr2Zn20 is found to be considerably small as the same order of magnitude as those of ordinary metals, despite the commonly enhanced Sommerfeld coefficient γ throughout the system. A satisfactory of the quasi-universal relation between S/T and γ as well as the Kadowaki-Woods relation demonstrates that the mass enhancement is realized in PrTr2Al20 due to the hybridization between f-electrons and the conduction electrons. We also find that the small S/T of PrIr2Zn20 is enhanced at low temperatures under the fields on the verge of quadrupole ordered phase, and in the same regime, the electrical resistivity follows the quadratic temperature dependence with a steep slope as a characteristic of Fermi liquid. The results imply an emergence of a nontrivial coherent state with sizable mass enhancement associated with the quadrupole degree of freedom.

Nonmagnetic Γ3 doublet ground state in a caged compound PrRh2Zn20

We have prepared single crystals of PrRh2Zn20, which is isostructural and isoelectronic to PrIr2Zn20 where coexistence of superconductivity and antiferroquadrupolar order has been recently found. The electrical resistivity ρ, magnetic susceptibility χ, isothermal magnetization M, and specific heat C have been measured. Hysteretic behavior appears in ρ(T) for 170 K 2 T and a broad peak in C(T) at around 10 K are reproduced by the two crystal-field levels split by 32 K; a nonmagnetic Γ3 doublet ground state and a magnetic Γ4 triplet excited state.

Effect of La Substitution in PrIr 2 Zn 20 on the Superconductivity and Antiferro-Quadrupole Order

The electrical resistivity ρ and the specific heat C of Pr1−xLaxIr2Zn20 (0 ≤ x ≤ 1) were measured to study the effect of La substitution in PrIr2Zn20 on the superconducting transition at Tc = 0.05 K and the antiferro-quadrupolar (AFQ) order at TQ = 0.11 K, which occur in a nonmagnetic Γ3 doublet ground state. For x = 0.04, a cusp in C(T) due to the AFQ order was observed at TQ = 0.07 K, while the cusp disappeared for x ≥ 0.09, indicating that the AFQ order rapidly collapses with the dilution of the Pr lattice. On the other hand, in the whole range of 0 ≤ x ≤ 1, the superconducting transition to zero resistance was observed. With increasing x, Tc hardly changes up to x = 0.47, above which Tc increases linearly with x. The results indicate a rather weak interplay between Tc and TQ. We have measured the magnetic field variation of Tc to determine the upper critical field of the superconducting state. The estimated effective mass m* for x = 0 is twice larger than that for x = 1, suggesting that moderately str...

Interplay between low-energy optical phonon modes and structural transition in PrT2Zn20 (T=Ru and Ir)

Atomic dynamics of PrT2Zn20 for T=Ru with a structural transition at Ts=138 K and T=Ir without such a transition have been studied by inelastic X-ray scattering (IXS) measurements. The IXS spectra for T=Ru reveal an optical phonon excitation at 3 meV. We assign it to low-energy vibration of the Zn atom at the 16c site by taking account of the first principles calculation [Hasegawa et al. 2012 J. Phys.: Conf. Proc. 391 012016]. For T=Ir, on the other hand, the optical excitation at 3 meV was not observed. The contrasting results indicate that the low-energy optical phonon mode has a role in the structural transition in PrRu2Zn20 and isostructural La counterparts.