Naoto Metoki

Evidence for Magnetic-Field-Induced Quadrupolar Ordering in the Heavy-Fermion Superconductor PrOs4Sb12

Neutron diffraction experiments on the heavy-fermion superconductor PrOs 4 Sb 12 revealed that a small antiferromagnetic moment, parallel to [010] ( y -direction), is induced in the field-induced phase ( H ∥ z ). The analysis, based on the Γ 1 singlet ground state crystal field model, shows that antiferro-order of O y z -type quadrupole moments of Pr ions is formed in the field-induced phase. This strongly suggests that the crystal field ground-state of Pr ions in PrOs 4 Sb 12 is the Γ 1 singlet and the quadrupolar ordering is induced due to level crossing with an excited state under a magnetic field.

Direct observation of quadrupolar excitons in the heavy-fermion superconductor PrOs4Sb12

We report inelastic neutron scattering experiments performed to investigate the low energy magnetic excitations on single crystals of the heavy-fermion superconductor PrOs(4)Sb(12). The observed excitation clearly softens at a wave vector Q=(1,0,0), which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and its intensity at Q=(1,0,0) is smaller than that around the zone center. This result directly evidences that this excitonic behavior is derived mainly from nonmagnetic quadrupolar interactions. Furthermore, the narrowing of the linewidths of the excitations below the superconducting transition temperature indicates the close connection between the superconductivity and the excitons.

Magnetic Excitations in Heavy-Fermion Superconductor PrOs4Sb12

Magnetic excitations in a single crystal of heavy-fermion superconductor PrOs 4 Sb 12 have been studied by inelastic neutron scattering (INS) at low temperatures under high magnetic fields. The crystal field (CF) excitations at 0.7 meV observed in a previous INS study of a polycrystal sample were confirmed in this study on a single crystal. The observed excitations in the zero magnetic field soften around a wave vector (1,0,0), which is the modulation vector of the field-induced antiferro-quadrupolar ordering, suggesting the presence of quadrupolar instability. The observed field dependence of excitations is better reproduced in a singlet ground-state model rather than in a non-Kramers doublet ground-state model. In a high resolution experiment, furthermore, the change of the excitation spectra at (1,0,0) was observed on passing through the superconducting transition temperature, indicating that the excitation and the superconductivity are closely related in PrOs 4 Sb 12 .

Incommensurate Magnetic Ordering and Spin-Liquid-Like State in a Triangular Lattice BaVS3. Neutron Diffraction and Scattering Study.

We performed powder neutron diffraction and scattering experiments of the \(S = \frac{1}{2}\) triangular lattice system, BaVS 3 , and found magnetic reflections below T X ≃30 K. The propagation vector is determined to be incommensurate (0.226 0.226 0) in the hexagonal index. The ordered moment is roughly estimated to be ∼0.5 µ B /V. Although the elastic magnetic diffractions disappear above T X , low-energy inelastic scattering, which has an Q -ω dispersion similar to that below T X , was observed between T X and the metal-insulator transition point T MI ≃70 K, indicating the presence of a long-range and dynamic antiferromagnetic correlation, i.e., a spin-liquid-like state, between T X and T MI .

Magnetic Structure Determination of CeT2Al10 (T = Ru and Os): Single Crystal Neutron Diffraction Studies

Neutron diffraction studies of Ce T 2 Al 10 ( T = Ru and Os), which shows an anomalous transition at T 0 = 27 K for CeRu 2 Al 10 and at 29 K for CeOs 2 Al 10 , have been carried out using single crystal samples of the compounds. In the ordered state below T 0 , obvious superlattice reflections have been found, indicating the appearance of ordered magnetic moments. The magnetic structures of both compounds could be commonly explained with a propagation vector q = (0, 1, 0), where collinear antiferromagnetic moments with magnitudes of 0.42(1) µ B for CeRu 2 Al 10 and of 0.29(1) µ B for CeOs 2 Al 10 are aligned along the c -axis.

Coupling between magnetic and superconducting order parameters and evidence for the spin excitation gap in the superconducting state of a heavy fermion superconductor UPd2Al3

Neutron scattering experiments have been carried out in order to study the interplay between magnetism and superconductivity in a heavy fermion superconductor, UPd 2 Al 3 . We have observed 1% suppression of the (0 0 0.5) magnetic peak intensity below the superconducting transition temperature T c . This is direct evidence for the coupling of the magnetic order parameter with the superconducting one. Furthermore, we have observed a spin excitation gap associated with superconductivity. The gap energy Δ E g increases continuously from Δ E g = 0 to 0.4 meV with decreasing temperature from T c to 0.4 K. This gap energy corresponds to 2 k B T c , which is smaller than the superconducting gap expected from the BCS theory (3.5 k B T c ). These results are indicative of the strong interplay between magnetism and superconductivity.

Neutron Scattering Study of the Crystal and Magnetic Structures in Itinerant-5f Antiferromagnets UNiGa5 and UPtGa5

The crystal and magnetic structures of 5 f itinerant antiferromagnets UNiGa 5 and UPtGa 5 were studied by means of neutron scattering. The powder diffraction data were well explained by the HoCoGa 5 tetragonal structure with space group P4/mmm. In UNiGa 5 , uranium spins in the simple tetragonal lattice have a Neel-type structure with a relatively large moment of 0.9 µ B /U along the [0,0,1] direction. On the other hand, magnetic moments of uranium atoms in UPtGa 5 are aligned ferromagnetically in the (0,0,1) plane and are directed along the [0,0,1] direction in an antiferromagnetic sequence (↑↓). An ordered moment is 0.24 µ B /U. The nearest-neighbor interaction might be different in these isostructural compounds.

Interplay between magnetism and superconductivity in URu2Si2 studied by neutron scattering experiments

Neutron scattering experiments have been carried out for a heavy fermion superconductor URu 2 Si 2 . We found that the (1 0 0) magnetic Bragg peak intensity is reduced by 1–2% in magnitude below the superconducting transition temperature of 1.15 K. This is direct evidence of the interplay between magnetism and superconductivity in URu 2 Si 2 .

Rattling of Pr in Heavy-Fermion Superconductor PrOs4Sb12 Revealed by Neutron Powder Diffraction

Neutron powder diffraction experiments were carried out on the heavy-fermion superconductor PrOs 4 Sb 12 down to 7.7 K in order to reveal the thermal displacement of Pr and investigate the possible existence of off-center potential minima for Pr in the Sb cage. Rietveld analysis with the structure in which Pr is located at the center of the Sb cage well reproduces the observed patterns. A large thermal displacement was obtained for Pr of U Pr =0.0344 A 2 at 295.1 K, roughly corresponding to a mean thermal displacement of 0.18 A. This value is 5 times larger than those of other constituents, U Os and U Sb . U Pr remains large, U Pr =0.0062 A 2 , even at 7.7 K, which is twice as large as U Os and U Sb . Neither structural transition nor significant change of the Sb cage accompanied this change in U Pr . The large U Pr and its strong temperature dependence indicate the rattling of Pr under a shallow potential in the Sb cage. An equal distribution of Pr at 12 e sites ( x 0 0) was introduced to approximate the...

In situ neutron diffraction study of aligning of crystal orientation in diamagnetic ceramics under magnetic fields

We have first studied the orientation process of α-Al2O3 fine particles, using in situ neutron diffraction measurements on the particles in a liquid solvent under magnetic fields. The neutron diffraction intensity of 006 and 116 reflections is significantly reduced by the vertical magnetic field up to 100kOe, while the 113 and 204 reflections remain unchanged within the experimental accuracy. The energy balance between the magnetic anisotropic energy and the thermal fluctuation following the Boltzmann distribution is important for the crystal orientation of the α-Al2O3 particles in suspension. Comparing the experimental data with the theoretical equation, we find that the magnetic field, where the particles are fully oriented, is above 200kOe. Our obtained data prove that the in situ neutron diffraction measurement is a powerful tool for investigating the alignment of the crystal orientation under magnetic fields.