M. Kohgi

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.

Crystal-structure modulation in the anomalous low-temperature phase of PrFe4P12

Abstract Nature of mysterious phase transition of the filled-skutterudite PrFe4P12 has been studied by X-ray diffraction experiments. Superlattice reflections characterized by a wave vector q =(1, 0, 0) were observed below the transition temperature of 6.5 K at zero magnetic field. The observed pattern in the reciprocal space is explained by the formation of modulated structure with dominant Fe-ion displacements by about 0.02% of the lattice constant. This result is consistent with the theoretically predicted nesting of Fermi surface producing a gap formation in electronic structure.

Neutron-diffraction study of the unusual ordered phases of Ce0.75La0.25B6

Abstract Ce0.75La0.25B6 shows paramagnetic phase (phase I), 4f-electron quadrupolar ordered phase (II), magnetically ordered phase (III), and an unknown phase IV. In order to investigate order parameters in phases III and IV, neutron diffraction experiments were performed down to 74 mK . Magnetic diffraction pattern measured in phase III is reproduced by a model in which spin arrangement is modified from the previously suggested structure for CeB6. No magnetic reflections were observed in phase IV. Thus, the order parameter in phase IV is expected to be 4f-electron multipoles rather than magnetic dipoles.

Commensurate-incommensurate magnetic phase transitions of CePtAl

The magnetic properties of CePtAl with orthorhombic TiNiSi-type chemical structure were investigated. Single-crystal experiments of electrical resistivity, specific heat and magnetic susceptibility reveal the existence of three magnetic phase transitions at T1=5.9 K (second order), T2=4.3 K (first order) and T3=2.2 K (first order). Powder neutron diffraction measurements detected complex magnetic structures consisting of two coexisting, partly incommensurate magnetic propagation vectors: k1=0 and k3=(0, 0.463(8), 0) for T2<or=T<or=T1; and k1=0 and k2=(0, 1/2, 0) for T<or=T2. The predominantly ferromagnetic k1 component and the antiferromagnetic k2 component of the magnetic low-temperature structure of CePtAl were refined at 1.5 K. Magnetism in CePtAl is dominated by a ferromagnetic component (1.35(4) mu B at 1.5 K) oriented parallel to the magnetically easy a axis. The complexity of magnetic structures in CePtAl appears to be caused by competing anisotropic magnetic exchange interactions in a network, where Ce-Ce nearest neighbours (3.56 AA) form chains parallel to the b axis and Ce-Ce second-nearest neighbours (3.77 AA) form chains along the a axis.

Low-energy excitations of the semimetallic one-dimensional S=1/2 antiferromagnet Yb4As3 ☆

We investigate a new route to quasi-one-dimensional spin-chain systems which originates from the charge-ordering transition out of a homogeneous mixed-valence state. We present evidence that Yb4As3 and P, Sb doped mixed crystals are well described by this mechanism. Many thermodynamic and low-temperature transport heavy-fermion-like properties can be explained by the existence of low-lying quasi-one-dimensional spin excitations in the Yb3+-chains. The observation of soliton excitations in a transverse external field gives further support to the existence of spin chains in Yb4As3. We also present recent results on spin-glass behavior at very low temperature caused by the interchain coupling and disorder. In addition, the yet unexplained magnetotransport effects are discussed.

X-ray diffraction studies of lattice properties in CeX (X = P and As) and Yb4As3

Abstract X-ray diffraction experiments were performed in order to study the lattice properties of the low-carrier-density strongly correlated electron systems CeX ( X = P and As) and Yb 4 As 3 . The lattice constants of CeP and CeAs take minima at 120 and 90 K, respectively. These results are explained by the model in which each crystal-field state is supposed to have its own effective ionic radius. The cell angle of Yb 4 As 3 shows the first-order-like increase from 90.0° to 90.7° accompanied with the charge ordering at T C = 288K. It gradually increases to 90.8° with decreasing temperature and is almost independent of temperature below 200 K. The temperature dependence of the cell angle is discussed in connection with the process of the charge ordering.

Lattice dynamics of the filled skutterudite CeOs4Sb12

Inelastic neutron scattering was performed on both poly- and single-crystalline samples to study the lattice dynamics of the Ce-filled antimonide skutterudite. For powder samples, no well-defined extra phonon excitation peak, which was reported to indicate a local Einstein oscillating mode, was detected in the low-energy region of the vibrational density of states within the experimental accuracy. However, a coherent phonon was clearly observed in single crystals at different temperatures. The dispersion curves were measured along two symmetry axes[0 1 1] and [1 0 0] at room temperature and compared with theoretical calculation by Feldman et al. [Phys. Rev. B 73, 014306 (2006)] using the model of local density approximation.

Resonant X-ray scattering study on the filled skutterudite PrFe4P12

Abstract Resonant X-ray scattering study was carried out to investigate an anomalous ordered state ( T A =6.5 K ) in the filled skutterudite PrFe4P12. At the Pr-LIII absorption edge, we observed resonant features in h+k+l=odd reflections, which are forbidden in the BCC structure above TA. Because these reflections contain the difference of anomalous scattering factor between two Pr atoms in the BCC unit cell, and the ordered state is attributed to the ordering of two different electronic states of Pr.

Phonon dispersion curves in CeOs4Sb12

Inelastic neutron scattering measurements at different temperatures were performed on single crystals of CeOs4Sb12. The room temperature phonon dispersion curves along principal axes [0 1 1] and [1 0 0] were obtained.

Magnetic form factor of UGe2 under high pressure

Abstract The magnetic form factor in the pressure-induced ferromagnetic superconductor UGe2 has been measured with polarized neutrons. The experiments were performed on single crystal samples at 2 K by applying a magnetic field of 2 T along the easy magnetization axis a under pressures of 0 and 1.4 GPa . In spite of the itinerant nature of the 5f electrons, the shapes of the observed magnetic form factors at both 0 and 1.4 GPa are very close to the theoretical 5f form factor of the free U3+ or U4+ ions calculated by the dipole approximation. The effect of pressure on the 5f form factor seems to be small in spite of the reduction of the magnetic moment.