The magnetic excitations of the double-exchange (DE) model are usually discussed in terms of an equivalent ferromagnetic Heisenberg model. However this equivalence is valid only at a quasi-classical level—we show that both quantum and thermal corrections to the magnetic properties of the DE model differ from any effective Heisenberg model because its spin excitations interact only indirectly, t...

ABSTRACT: We investigate the mechanism of spin gap formation in a two-dimensional model relevant to Mott insulators such as CaV4O9. From the perturbation expansion and quantum Monte Carlo calculations, the origin of the spin gap is ascribed to the four-site plaquette singlet in contrast to the dimer gap established in the generalized dimerized Heisenberg model. We investigate the mechanism of s...

in the Spin-2 J1-J2 Square Heisenberg Model Shou-Shu Gong, Wei Zhu, D. N. Sheng, Olexei I. Motrunich, and Matthew P. A. Fisher Department of Physics and Astronomy, California State University, Northridge, California 91330, USA Department of Physics, California Institute of Technology, Pasadena, California 91125, USA Department of Physics, University of California, Santa Barbara, California 9310...

We have studied the Heisenberg model on the triangular lattice using quantum Monte Carlo techniques (up to 144 sites) and exact diagonalization (up to 36 sites). By studying the spin gap as a function of the system size we have obtained robust evidence for a gapless spectrum, confirming the existence of long-range Néel order. Our best estimate is that in the thermodynamic limit the order parame...

Custodial SU(2) breaking due to dynamical fermion masses is studied in a rather general context and it is shown how some well known limiting cases are correctly described. The type of “gap equation” which can systematically lead to extra negative contributions to the so–called ρ–parameter is emphasized. Furthermore general model independent features are discussed and it is shown how electro–wea...

A new 2-parameter quadratic deformation of the quantum oscillator algebra and its 1-parameter deformed Heisenberg subalgebra are considered. An infinite dimensional Fock module representation is presented which at roots of unity contains null vectors and so is reducible to a finite dimensional representation. The cyclic, nilpotent and unitary representations are discussed. Witten’s deformation ...

We investigate the behavior of the photon-photon scattering near the threshold ! = m e , by comparing the exact result with the approximation obtained using the Euler-Heisenberg eeective Lagrangian supplemented with higher-order derivative corrections. We also examine the ! 2 =m 2 e behavior predicted by including one-loop corrections to the Euler-Heisenberg eeective Lagrangian together with th...

By using the concept of negativity, we investigate entanglement in (1/2,1) mixed-spin Heisenberg systems. We obtain the analytical results of entanglement in small isotropic Heisenberg clusters with only nearest-neighbor (NN) interactions up to four spins and in the four-spin Heisenberg model with both NN and next-nearest-neighbor (NNN) interactions. For more spins, we numerically study effects...