David Gottlieb

Singlet excitations in the anisotropic S=12 antiferromagnetic Heisenberg model

Abstract We generalize to arbitrary exchange anisotropy an analytic approach to the ground state and excitations of the S= 1 2 Heisenberg antiferromagnet previously formulated for exchange anisotropy toward the Ising limit. This way we obtain a theory of antiferromagnetism that takes in from the Ising to the XY extreme models, yields more precision than linear spinwave theory and, in opposition to the latter, is expressed in terms of excitations carrying no spin.

General solution for the antiferromagnetic Heisenberg model with exchange anisotropy toward the Ising limit

Abstract The Heisenberg antiferromagnetic model for spin S and no external field is approximately solved for anisotropy, dimension, or spin size, assuring high antiferromagnetic order. Explicit closed-form expressions for the ground state, ground state energy, excitations and correlation functions are obtained. The approximate solution converges rapidly to the exact one as S or the lattice dimensionality increases, or when the anisotropy approaches the Ising limit. In particular, for two and three dimensional lattices it remains very accurate all the way from the Ising to the isotropic Heisenberg models. The proposed new solution gives more accuracy than linear spin waves theory and is expressed in terms of a set of excitations carrying no spin, in opposition to the spin-one excitations of spin waves theory.

Structural instability of a boson system in one dimension

Abstract In this work we report the occurrence of an anomalous phase transition due to the presence of libron-phonon interaction. In particular, we find that the system becomes deformed above the critical temperature T c . This is contrary to the case of the classical Peierls transition for which the deformation occurs below T c , but it can be understood by the increase in entropy of the deformed phase.

Interplay of holon and spinon dynamics in doped anisotropic Heisenberg chains

Abstract The role played by doping in one-dimensional quantum antiferromagnets is investigated using a decoupling of the charge and spin degrees of freedom that allows for magnetic phase changes when the concentration of holes increases. Our approach predicts quasi-continuous as well as discontinuous metal-insulator transitions, depending on the nature of the corresponding magnetic transformation. We also provide a clue for the observed behavior of the Hall coefficient, with a sign change caused by doping, suggesting that the change in the character of carriers is related to different magnetic correlations.

Approximate solution of the heisenberg antiferromagnet in a triangular lattice

Abstract The anisotropic antiferromagnetic Heisenberg model in a triangular lattice is solved approximately using the pair non-magnetic excitation theory (PNME) which was generalized in order to apply to non-bipartite lattices. The ground energy state Eg, eigenfunction and excitations energies are obtained for arbitrary spin and anisotropy. In particular, for the isotropic case and spin 1 2 , Eg = −0.172. This value corresponds to a 6% error when compared with the exact value obtained numerically from small cluster calculations and compares successfully with energies obtained by other methods. A parametric phase transition for ordered - disordered ground state is obtained as function of the spin and anisotropy.

Variational solution for the spin 12 Heisenberg antiferromagnet with nearest and next nearest neighbor coupling

Abstract The anisotropic spin 1 2 Heisenberg hamiltonian with competing nearest and next nearest neighbors interaction is studied by a variational version of the paired non magnetic excitation theory in one and two dimensions. The results are compared with numerical calculations obtained using the Lanczos approximation in a cluster of 12 atoms and with the Oliveira variational approach which was generalized in order to study the anisotropic case in one and two dimension. A single expression permits the exploration of all the parameter space obtaining a generalization of the Neel solution along the z direction, the collinear state (2-2), and the transversal Neel state.

Transversal Effect in a Metal-to-Metal Interface

It is shown that a transversal potential appears when a current flows along a film with a transversal impurity gradient. The potential is evaluated in the relaxation time approximation for the Boltzmann-Bloch equation, being quadratic in the applied field. Semi-classical Monte Carlo simulations have verified those results.

Structural instability in a boson system

Abstract We have found a boson system (libron interacting with phonons in a lattice) which is unstable in such a way that at high temperature (H.T.) it becomes deformed while at low Temperatures is homogeneous. The stable periodicity at H.T. occurs at a finite wavelength. This a first order phase transition.

Transversal magnetoresistance in La2CuO4

Abstract It is shown that in compounds whose Fermi Surface (F.S.) is a square based cylinder, the magnetoresistance is temperature independent (from 4 to 300 K) for a broad range of magnetic field ( B = 0 to 200 kG). From this kind of experiment the sharpness of the corners of the F.S. for the compound La 2 CuO 4 and similar superconductors with the layer-like structure of K 2 NiF 4 can be estimated.