Hikaru Kawamura

Phase Transitions of Anisotropic Heisenberg Antiferromagnets on the Triangular Lattice

Phase transitions in anisotropic Heisenberg antiferromagnets with nearest neighbour coupling constants ( J x , J y , J z ) on the triangular lattice have been investigated. In particular, for the case of Heisenberg-Ising model, | J z |>| J x |=| J y |>0, successive phase transitions due to orderings of z components and of x y components have been found. A non-trivial degeneracy is found both in the ground state and in harmonic excitations. Divergent-like behaviour of the uniform susceptibility in the x y plane at low temperatures is found, which is supposed to be caused by the non-trivial degeneracy.

Chirality-Driven Anomalous Hall Effect in Weak Coupling Regime

Anomalous Hall effect arising from non-trivial spin configuration (chirality) is studied treating perturbatively the exchange coupling to localized spins. Scattering by normal impurities is included. Chirality is shown to drive locally Hall current and leads to overall Hall effect if there is a finite uniform chirality. This contribution is independent of the conventional spin–orbit contribution and shows distinct low-temperature behavior. In mesoscopic spin glasses, the chirality-induced anomalous Hall effect is expected below the spin–glass transition temperature. Measurement of the Hall coefficient would be useful in experimentally confirming the chirality ordering.

Phase Transition of the Heisenberg Antiferromagnet on the Triangular Lattice in a Magnetic Field

Phase transitions of the two-dimensional classical Heisenberg antiferromagnet on the triangular lattice are studied in the presence of magnetic fields. Three types of ordered phase and successive phase transitions among them are found. Properties of each phase are investigated by means of a Monte Carlo similation.

New Critical Behavior I–Heisenberg Antiferromagnet on the Layered-Triangular Lattice

Phase transition of frustrated Heisenberg antiferromagnet on the three-dimensional layered-triangular lattice is studied via a symmetry argument and Monte Carlo simulations. A symmetry argument shows that the order-parameter space of this model is isomorphic to the three-dimensional rotation group SO(3) or the projective space P 3 , different from those of the corresponding nonfrustrated systems. The possibility is suggested that the system exhibits a novel type of phase transition characterized by its SO(3) symmetry. Extensive Monte Carlo simulations indicate the occurrence of a single continuous transition. By applying the finite-size scaling, respective exponents are estimated as α≃0.4, β≃0.25, γ≃1.1 and ν≃0.53. These values differ considerably from those for the standard nonfrustrated systems strongly suggesting a phase transition belonging to a new universality class. Several systems, including the superfluid A phase of helium 3, certain types of helical or canted magnets, are predicted to belong to ...

Chiral-glass transition and replica symmetry breaking of a three-dimensional Heisenberg spin glass

Extensive equilibrium Monte Carlo simulations are performed for a three-dimensional Heisenberg spin glass with the nearest-neighbor Gaussian coupling to investigate its spin-glass and chiral-glass orderings. The occurrence of a finite-temperature chiral-glass transition without the conventional spin-glass order is established. Critical exponents characterizing the transition are different from those of the standard Ising spin glass. The calculated overlap distribution suggests the appearance of a peculiar type of replica-symmetry breaking in the chiral-glass ordered state.

Chiral Glass: A New Equilibrium Phase of Ceramic High- T c Superconductors

Possible occurrence of an equilibrium thermodynamic phase with a spontaneously broken time-reversal symmetry is studied in a model ceramic superconductor with anisotropic pairing symmetry. It is shown by Monte Carlo simulations that such a “chiral-glass” phase is truly stable even under the influence of screening. Existence of frustration in zero external field, arising from the d -wave pairing symmetry of high- T c superconductors, is essential to realize this phase. Via a finite-size scaling analysis, critical exponents associated with the chiral-glass transition are estimated to be ν CG =1.3±0.2 and η CG =-0.2±0.2. These values turn out to be close to those of the Ising spin glass. Phase diagram of the model is constructed and the implications to experiments are discussed.

DYNAMICAL SIMULATION OF SPIN-GLASS AND CHIRAL-GLASS ORDERINGS IN THREE-DIMENSIONAL HEISENBERG SPIN GLASSES

Spin-glass and chiral-glass orderings in three-dimensional Heisenberg spin glasses are studied with and without randaom magnetic anisotropy by dynamical Monte Carlo simulations. In isotropic case, clear evidence of a finite-temperature chiral-glass transition is presented. While the spin autocorrelation exhibits only an interrupted aging, the chirality autocorrelation persists to exhibit a pronounced aging effect reminisecnt of the one observed in the mean-field model. In anisotropic case, asymptotic mixing of the spin and the chirality is observed in the off-equilibrium dynamics.

Nature of orbital-glass transition in d-wave ceramic superconductors

A close connection is pointed out between the orbital-glass state recently proposed for high- T c ceramic superconductors and the chiral-glass state identified in X Y spin glasses. Based on this analogy, the orbital-glass state is argued to be an equilibrium phase with broken reflection (charge-conjugation) symmetry, but not a true superconductor on long length and time scales. The transition to the normal state lies in the universality class of the 3 d Ising spin glass. It is predicted that the nonlinear susceptibility in the zero-field limit exhibits a negative divergence for d -wave ceramics and a positive divergence for s -wave ceramics.

Ground-state phase diagrams of frustrated spin- S XXZ chains: Chiral ordered phases

The ground-state phase diagram of the frustrated spin-S XXZ chain with the competing nearest- and next-nearest-neighbor antiferromagnetic couplings is studied numerically by using the density-matrix renormalization-group method for the cases of

New critical behavior. II: XY antiferromagnet on the layered-triangular lattice

S=1/2,