Toshiya Inami

Ferroelectricity from iron valence ordering in the charge-frustrated system LuFe2O4.

Ferroelectric materials are widely used in modern electric devices such as memory elements, filtering devices and high-performance insulators. Ferroelectric crystals have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles (specifically, a polar displacement of anions and cations). First-principles calculations and electron density analysis of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the orbital hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, an alternative model—electronic ferroelectricity—has been proposed in which the electric dipole depends on electron correlations, rather than the covalency. This would offer the attractive possibility of ferroelectric materials that could be controlled by the charge, spin and orbital degrees of freedom of the electron. Here we report experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe2O4. Using resonant X-ray scattering measurements, we determine the ordering of the Fe2+ and Fe3+ ions. They form a superstructure that supports an electric polarization consisting of distributed electrons of polar symmetry. The polar ordering arises from the repulsive property of electrons—electron correlations—acting on a frustrated geometry.

High-Magnetic-Field X-ray Absorption Spectroscopy of Field-Induced Valence Transition in YbInCu4

The magnetic field-induced valence transition in YbInCu 4 has been studied by X-ray absorption spectroscopy at the Yb L III -edge up to 41 T. The field-induced valence transition is clearly observe...

New universality class of antiferromagnetic phase transition in CsMnBr3

The critical exponents γ and ν of an XY antiferromagnet on a layered-triangular lattice, CsMnBr 3 , have been measured by neutron scattering. They are γ=1.10±0.05 and ν=0.57±0.03 in the reduced temperature range 0.015< e < 0.1 above the Neel temperature. These and the previously measured β show that this phase transition belongs to the universality class characterized by the O(2) symmetry order-parameter in three dimensions, as calculated by Kawamura.

High Field X-ray Diffraction Study on a Magnetic-Field-Induced Valence Transition in YbInCu4

We report the first high-field x-ray diffraction experiment using synchrotron x-rays and pulsed magnetic fields exceeding 30 T. The lattice deformation due to a magnetic-field-induced valence transition in YbInCu 4 is studied. It has been found that the Bragg reflection profile at 32 K changes significantly at around 27 T due to the structural transition. In the vicinity of the transition field, the low field and high field phases alternate with the discontinuous change in the lattice constant. Thus, it is evident that the field-induced valence state transition is a first-order phase transition. The field dependence of the low-field-phase Bragg peak intensity is found to be roughly scaled with the magnetization.

Momentum Dependence of Charge Excitations in the Electron-Doped Superconductor Nd1:85Ce0:15CuO4: A Resonant Inelastic X-Ray Scattering Study

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-T(c) superconductor Nd1.85 Ce0.15 CuO4. The intraband and interband excitations across the Fermi energy are separated for the first time by tuning the experimental conditions properly to measure charge excitations at low energy. A dispersion relation with q-dependent width emerges clearly in the intraband excitation, while the intensity of the interband excitation is concentrated around 2 eV near the zone center. The experimental results are consistent with theoretical calculation of the RIXS spectra based on the Hubbard model.

Magnetization Process of the Triangular Lattice Antiferromagnets, RbFe(MoO4)2 and CsFe(SO4)2

High-field magnetization of two hexagonal layered compounds RbFe(MoO 4 ) 2 and CsFe(SO 4 ) 2 is measured. In the magnetization curves of both compounds, a plateau-like structure with one-third of the saturated moment is observed. This anomaly is analyzed in terms of the magnetization process of a two-dimensional triangular lattice Heisenberg antiferromagnet with weak Ising-type anisotropy.

Evidence of antiferroquadrupolar ordering of DyB2C2

The antiferroquadrupolar ordering in DyB2C2 has been investigated using the x-ray resonant scattering technique. The dependence of the intensities of two superlattice peaks on temperature, azimuthal angle, and energy of the incident x-rays was measured. The results show that the antiferroquadrupole ordering, which accompanies a structural phase transition, exists below TQ = 25 K. A structural model for the antiferroquadrupole ordering, for which the propagation vectors are (0,0,1/2) and (0,1,1/2), is presented.

High-Field Magnetization of One-Dimensional S=1/2 Trimer System 3CuCl2·2dioxane

The high-field magnetization process of a one-dimensional S =1/2 trimer system, 3CuCl 2 ·2dioxane, is studied on powder samples using a pulse magnet up to 30 T between 1.5 K and 9 K. The magnetization increases nonlinearly and saturates around 15 T, characteristic of low-dimensional Heisenberg antiferromagnets. The magnetization data are consistent with a previously proposed model in which ferromagnetic trimers composed of three S =1/2 spins are weakly coupled antiferromagnetically to form a S =3/2 antiferromagnetic chain. Based on this model, the antiferromagnetic intertrimer interaction is directly derived as J AF =-16.2 K. However, the observed magnetization curve suggests that a composite spin system consisting of three S =1/2 spins has more of a quantum nature than does a genuine S =3/2 system, because the ferromagnetic intratrimer interaction is not sufficiently strong.

New Tetracritical Behavior in CsMnBr3

The magnetic phase diagram of CsMnBr 3 has been determined from precise magnetization measurements. The single transition occurring in zero field splits into two successive transitions in a finite field applied in the c -plane. This indicates that the zero-field Neel point T N =8.32 K is a tetracritical point. The crossover exponents evaluated suggest that the tetracritical behavior is governed by the n =2 chiral universality.

Polarization-analyzed resonant inelastic x-ray scattering of the orbital excitations in KCuF 3

We report a Cu