Takehide Koyama

Evidence for Appearance of an Internal Field in the Ordered State of CeRu2Al10 by µ+SR

We report zero-field µ + SR measurements made at J-PARC on CeRu 2 Al 10 . Below the phase transition temperature T 0 ∼27 K, a clear modulation of µ + polarization due to an internal field is observed, indicating that magnetic ordering takes place. Compared with the internal field expected from the effective magnetic moment in the disordered state, the observed internal field at the µ + site is quite small. This indicates that the ordered Ce magnetic moment is small (∼10 -2 µ B ) and/or the nearest pair of Ce magnetic moments are mutually anti-parallel. The T -dependence of the internal field is somewhat peculiar, suggesting that another ordering is taking place as well.

Frustration-induced valence bond crystal and its melting in Mo3Sb7.

(121/123)Sb nuclear quadrupole resonance and muon spin relaxation experiments of Mo_3Sb_7 revealed symmetry breakdown to a nonmagnetic state below the transition recently found at T_S approximately 50 K. The transition is characterized by a distinct lattice dynamics suggested from narrowing of nuclear fields. We point out that the Mo sublattice is a unique three-dimensional frustrated lattice where nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions compete, and propose that tetragonal distortion to release the frustration stabilizes long-range order of spin-singlet dimers, i.e., valence bond crystal, which is thermally excited to the dynamic state with cubic symmetry.

Pressure-Induced Localization of 4f Electrons in the Intermediate Valence Compound SmB6

We report high-pressure studies of X-ray diffraction and 11 B-nuclear magnetic resonance (NMR) in the intermediate- valence compound SmB6. The pressure dependence of the lattice constant was precisely determined and no anomaly was observed up to 9.1 GPa. The temperature dependence of the nuclear quadrupole resonance frequencyQ, obtained from the 11 B-NMR measurements, is predominantly contributed by on-site charge distribution. Using the relationship betweenQ and Sm valence at ambient pressure reported previously, we estimate the pressure dependence of the Sm valence up to 6 GPa as well. The increase in the Sm valence accelerates with pressure and reaches an increase of about 10% at 6 GPa. The pressure-induced localization of Sm 4f -holes may be responsible for the long-range magnetic order under pressure.

Quasi-One-Dimensional Itinerant Electron System (La–Y)Mn4Al8 with Soft Spin Gap

(La 1- x Y x )Mn 4 Al 8 (0 ≤ x ≤1) is a very unique itinerant electron system, in which the spin pseudogap can be controlled continuously and nearly uniformly in a wide range of the gap width from Δ≃250 to 500 K by the anisotropic volume shrinkage induced by the chemical pressure. The hydrostatic pressure on LaMn 4 Al 8 also increases Δ but tends to fill the gap. The strong and anisotropic volume dependence of Δ strongly supports the idea that the origin of the gap formation is associated with the one-dimensional geometry of the Mn spin arrangement.

An evidence for a spin gap in copper–thiospinel

Abstract The electronic and magnetic properties of a copper–thiospinel metal CuTi 2 S 4 have been investigated with magnetic susceptibility, χ , and 47,49 Ti and 63 Cu NMR. At temperatures above ∼5 K , the temperature-dependent component of both χ and the Knight shift K of 47,49 Ti exhibits a Curie–Weiss-type behavior with θ=−24 K . K of 63 Cu is, on the other hand, is positive and depends weakly on the temperature. Then, we conclude that the magnetism originates mainly from the d bands of Ti at the B-site, and Cu at the A-site is close to d 10 . Below ∼5 K , the temperature-dependent components of all the χ, K and relaxation rate, (T 1 T) −1 , exhibit rapid decreases, indicating that the system transforms into a spin singlet state.

Magnetic properties of copper cluster-spins in BaCuO2+x (x = 0 and 0.14), investigated by magnetization and nuclear magnetic resonance measurements

To clarify the combined magnetic behaviour of BaCuO2+x, which contains in the body-centred-cubic unit cell two Cu18O24 spherical clusters, eight Cu6O12 ring clusters, and six CuO4 lone units, we made measurements of magnetization, 63,65Cu nuclear magnetic resonance (NMR) near 6 T, and pure quadrupole resonance (PQR) in zero field on samples with well-defined oxygen contents x = 0 and 0.14. Quantitative analyses of Curie–Weiss-like susceptibility data for BaCuO2.00 indicate that Cu spins within the Cu6 clusters go into a ferromagnetically (FM) ordered ground state with a maximum spin Sr = 3 below K, and those within the Cu18 clusters into an FM-like ground state below K with net spin that is less than the maximum possible spin Ss = 9. For BaCuO2.14 the Cu spins within the Cu6 clusters go into an FM ordered ground state at a lower temperature of K, and these clusters exhibit no signature of long-range magnetic ordering down to 2 K. The NMR spectra for BaCuO2.00 observed below K have a trapezoidal structure that is known to be a characteristic of long-range magnetic ordering, and is consistent with the antiferromagnetic ordering of the Cu6 clusters below TN = 15 K predicted previously. On the other hand, the spectrum for BaCuO2.14 observed in a temperature range between 1.2 and K has an electric quadrupole-split powder pattern structure. This provides microscopic evidence that there is no long-range magnetic ordering in BaCuO2.14. The addition of 0.14 moles of oxygen atoms results in a large suppression of some magnetic entities. We find, however, some anomalies in the NMR and PQR data for BaCuO2.14 near K, which could be associated with short-range ordering of the paramagnetic Cu18 clusters.

Magnetic and Electronic Properties of URu2Si2 Revealed by Comparison with Nonmagnetic References ThRu2Si2 and LaRu2Si2

We have carried out nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on ThRu2Si2 and LaRu2Si2, which are the nonmagnetic references of the intriguing heavy fermion URu2Si2. The comparisons of URu2Si2 with the reference materials allow us to analyze the already known NMR and NQR data on URu2Si2 phenomenologically and semiquantitatively. The study of 101Ru-NQR frequency suggests the relatively close electronic configuration of URu2Si2, including the valence of the actinide ion, to that of the tetravalent ThRu2Si2 at high temperatures, as well as the delocalization of 5f electrons at low temperatures. Ising-like spin fluctuations along the c-axis were brought to light by 29Si-NMR data in the so-called hidden order phase of URu2Si2. The unique magnetic property is plausibly associated with the mechanism of the unconventional superconductivity.

Investigation of Local Symmetries in the Hidden-Order Phase of URu2Si2

The local symmetry at the Ru(\(4d\)) and Si(\(4e\)) sites in URu2Si2 was investigated above and below the hidden-order temperature \(T_{\text{O}}=17.5\) K by measuring 101Ru-nuclear quadrupole resonance (NQR) and 99Ru-nuclear magnetic resonance (NMR) in single-crystalline samples and 73Ge-NMR in a 10%-73Ge-substituted polycrystalline sample. For both sites, no evidence of symmetry change below \(T_{\text{O}}\) was observed. The asymmetry parameter η of the electric field gradient, which reflects the local rotation symmetry, was estimated to be zero with accuracies of 0.0010–0.0015 and 0.05 for the \(4d\) and \(4e\) sites, respectively. The results suggest that the symmetry change at \(T_{\text{O}}\) is extremely small or intrinsically invisible by the present method.

Mechanism of Field Induced Fermi Liquid State in Yb-Based Heavy-Fermion Compound: X-ray Absorption Spectroscopy and Nuclear Magnetic Resonance Studies of YbCo2Zn20

We have investigated the field dependences of Yb valence as well as of the magnetic properties of the heavy-fermion (HF) compound YbCo 2 Zn 20 by X-ray absorption spectroscopy, 59 Co nuclear magnet...

NMR study of heavy fermion superconductor with no inversion symmetry CePt3Si

We report Pt195 and Si29 NMR studies of an antiferromagnetic heavy fermion superconductor CePt3Si with no inversion symmetry in the crystal structure. The temperature dependence of spin-lattice relaxation rates (1∕T1) for Pt and Si nuclei measured between 2 and 300K might be explained by the contributions from a low-lying crystalline-electric-field level and a quasiparticle due to a hybridization between the ground state and conduction electrons. Just below the superconducting transition temperature (Tc) no remarkable enhancement in 1∕T1 was observed within an experimental error. No significant decrease in Si29 Knight shift was observed for parallel direction to the c axis below Tc. This indicates that spin triplet pairing is probably realized for the superconductivity.