Shin Miyahara

Ferroelectricity induced by spin-dependent metal-ligand hybridization in Ba₂CoGe₂O₇.

We have investigated the variation of induced ferroelectric polarization under a magnetic field with various directions and magnitudes in a staggered antiferromagnet Ba₂CoGe₂O₇. While the ferroelectric polarization cannot be explained by the well-accepted spin current model nor the exchange striction mechanism, we have shown that it is induced by the spin-dependent p-d hybridization between the transition metal (Co) and ligand (O) via the spin-orbit interaction. On the basis of the correspondence between the direction of electric polarization and the magnetic state, we have also demonstrated the electrical control of the magnetization direction.

Theory of the orthogonal dimer Heisenberg spin model for SrCu2 (BO3)2

The magnetic properties of SrCu2 (BO3)2 are reviewed from a theoretical point of view. SrCu2 (BO3)2 is a new two-dimensional spin gap system and its magnetic properties are well described by a spin-1/2 antiferromagnetic Heisenberg model of the orthogonal dimer lattice. The model has a dimer singlet ground state whose exactness was proven by Shastry and Sutherland for a topologically equivalent model more than 20 years ago. The exactness of the ground state is maintained even if interlayer couplings are introduced for SrCu2 (BO3)2. In the two-dimensional model, quantum phase transitions take place between different ground states for which three phases are expected: a gapped dimer singlet state, a plaquette resonating valence bond state and a gapless magnetic ordered state. Analysis of the experimental data shows that the dimer singlet ground state is realized in SrCu2 (BO3)2. The orthogonality of the dimer bonds, which is the underlying symmetry of the exactness of the ground state, also leads to an unusual property of elementary excitations, namely the almost localized nature of the triplet excitations. Application of an external magnetic field changes the density of the triplet excitations. In general, there is competition between kinetic energies and interaction energies between triplets. The almost localized nature of the triplets makes it easy to form regular lattices. In fact, at certain densities, where the commensurability energy is significant, the triplet excitations form superstructures and plateaux appear at 1/2, 1/3, 1/4 and 1/8 in the magnetization curve. In high-magnetic-field experiments, magnetic plateaux at magnetizations of 1/3, 1/4 and 1/8 have been observed. Translational symmetry of the lattice is spontaneously broken at the plateaux, except for the 1/2 plateau. The 1/3 and 1/4 plateaux are expected to have magnetic superstructures of stripe form while the 1/2 plateau has a square unit cell and the 1/8 plateau a rhomboid cell. Especially at the 1/8 plateau, nuclear magnetic resonance experiments indicate the presence of at least 11 distinct Cu sites with different spin polarizations, which is the first evidence of breaking of the translational symmetry at the plateau phase. The spin texture calculated on the basis of a Heisenberg model with adiabatic spin–phonon coupling is consistent with the experimental results.

Chirality of matter shows up via spin excitations

Chirality is usually manifested by differences in a material’s response to left- and right-circularly polarized light. This difference is the result of the specific distribution of charge within chiral materials. A similar response has now been found to result from the chiral spin structure of an antiferromagnet.

Theory of Magnetoelectric Resonance in Two-Dimensional S ¼ 3=2 Antiferromagnet Ba2CoGe2O7 via Spin-Dependent Metal-Ligand Hybridization Mechanism

We investigate magnetic excitations in Ba 2 CoGe 2 O 7 . In terahertz absorption experiments of the compound, novel magnetic excitations, i.e., conventional magnetic resonance at 2 meV as well as both electric- and magnetic-active excitation at 4 meV, have been observed. These magnetic excitations can be explained naturally in an S =3/2 Heisenberg model on a square lattice with uniaxial anisotropy and Dzyaloshinsky–Moriya terms. We also indicate that, via the spin-dependent metal-ligand hybridization mechanism, the 4 meV excitation is electric-active because of the coupling between the spin and electric dipole. Moreover, at the 4 meV excitation, an interference between magnetic and electric responses emerges as a cross-correlated effect. Such cross-correlation effects explain the nonreciprocal linear directional dichroism observed in Ba 2 CoGe 2 O 7 .

Low-lying magnetic excitation of the Shastry-Sutherland model.

By using perturbation calculation and numerical diagonalization, the low-energy spin dynamics of the Shastry-Sutherland model is investigated with particular attention to the two-particle coherent motion. In addition to spin-singlet- and triplet-bound states, we find novel branches of coherent motion of a bound quintet pair, which are usually unstable because of repulsion. Unusual dispersion observed in neutron-scattering measurements is explained by the present theory. The importance of the effects of phonons is also pointed out.

Quantum Monte Carlo Simulation of the Trellis Lattice Heisenberg Model for SrCu2O3 and CaV2O5.

We study the spin-1/2 trellis lattice Heisenberg model, a coupled spin ladder system, both by perturbation around the dimer limit and by quantum Monte Carlo simulations. We discuss the influence of the inter-ladder coupling on the spin gap and the dispersion, and present results for the temperature dependence of the uniform susceptibility. The latter was found to be parameterized well by a mean-field type scaling ansatz. Finally we discuss fits of experimental measurements on SrCu 2 O 3 and CaV 2 O 5 to our results.

Nonreciprocal Directional Dichroism and Toroidalmagnons in Helical Magnets

We investigate a dynamical magnetoelectric effect due to a magnetic resonance in helical spin structures through the coupling between magnetization and electric polarization via a spin current mechanism. We show that the magnon has both the dynamical magnetic moment Δ M ω and the electric moment Δ P ω (⊥Δ M ω ), i.e., a dynamical toroidal moment, under external magnetic fields, and thus it is named the toroidalmagnon . The toroidalmagnon exists in most conical spin structures owing to the generality of the spin current mechanism. In the absorption of electromagnetic waves, the toroidalmagnon excitation process generally induces a nonreciprocal directional dichroism as a consequence of an interference of the magnetic and electric responses.

Orbital Ordering in Ferromagnetic Lu2V2O7

We have observed the orbital ordering in the ferromagnetic Mott-insulator Lu 2 V 2 O 7 by the polarized neutron diffraction technique. The orbital ordering pattern determined from the observed magnetic form factors can be explained in terms of a linear combination of wave functions | y z >, | z x > and | x y >;

Spin Dodecamer Formation in the Double-Exchange Spin Ice Model

We investigated the double-exchange spin ice (DESI) model on a kagome lattice by Monte Carlo simulation to study the effects of a geometrical frustration, and the mechanism that generates an ordered state in a metallic system. The DESI model on the kagome lattice is a frustrated metallic system due to an effective ferromagnetic interaction between localized spins caused by the double-exchange (DE) mechanism and a uniaxial anisotropy for the localized spins. A dodecagonal spin cluster (named dodecamer), which consists of twelve localized spins, appears at low temperature when the number of particles per site n ≃1/3 – 1/2. Such a dodecamer order is driven by both the kinetic energy gain due to the DE mechanism and the geometrical frustration. We discuss that cluster orders, in general, may be a common feature in itinerant electron systems coupled with frustrated adiabatic fields.

Electromagnons in the multiferroic state of perovskite manganites with symmetric exchange striction

We have investigated electrically active magnetic excitations (electromagnons) in perovskite manganites with the