Quantum entanglement and fixed-point bifurcations

How does the classical phase space structure for a composite system relate to the entanglement characteristics of the corresponding quantum system? We demonstrate how the entanglement in nonlinear bipartite systems can be associated with a fixed point bifurcation in the classical dynamics. Using the example of coupled giant spins we show that when a fixed point undergoes a supercritical pitchfork bifurcation, the corresponding quantum state - the ground state - achieves its maximum amount of entanglement near the critical point. We conjecture that this will be a generic feature of systems whose classical limit exhibits such a bifurcation.Comment: v2: Structure of the paper changed for clarity, reduced length, now 9 pages with 6 figure

Phase diagram of the one-dimensional Holstein model of spinless fermions

The one-dimensional Holstein model of spinless fermions interacting with dispersionless phonons is studied using a new variant of the density matrix renormalisation group. By examining various low-energy excitations of finite chains, the metal-insulator phase boundary is determined precisely and agrees with the predictions of strong coupling theory in the anti-adiabatic regime and is consistent with renormalisation group arguments in the adiabatic regime. The Luttinger liquid parameters, determined by finite-size scaling, are consistent with a Kosterlitz-Thouless transition.Comment: Minor changes. 4 pages, 4 figures. To appear in Physical Review Letters 80 (1998) 560

Temperature dependence of the interlayer magnetoresistance of quasi-one-dimensional Fermi liquids at the magic angles

The interlayer magnetoresistance of a quasi-one-dimensional Fermi liquid is considered for the case of a magnetic field that is rotated within the plane perpendicular to the most-conducting direction. Within semi-classical transport theory dips in the magnetoresistance occur at integer amgic angles only when the electronic dispersion parallel to the chains is nonlinear. If the field direction is fixed at one of the magic angles and the temperature is varied the resulting variation of the scattering rate can lead to a non-monotonic variation of the interlayer magnetoresistance with temperature. Although the model considered here gives a good description of some of the properties of the Bechgaard salts, (TMTSF)2PF6 for pressures less than 8kbar and (TMTSF)2ClO4 it gives a poor description of their properties when the field is parallel to the layers and of the intralayer transport.Comment: 10pages, RevTeX + epsf, 3 figure

Observations of the structure and evolution of solar flares with a soft X-ray telescope

Soft X ray flare events were observed with the S-056 X-ray telescope that was part of the ATM complement of instruments aboard SKYLAB. Analyses of these data are reported. The observations are summarized and a detailed discussion of the X-ray flare structures is presented. The data indicated that soft X-ray emitted by a flare come primarily from an intense well-defined core surrounded by a region of fainter, more diffuse emission. An analysis of flare evolution indicates evidence for preliminary heating and energy release prior to the main phase of the flare. Core features are found to be remarkably stable and retain their shape throughout a flare. Most changes in the overall configuration seem to be result of the appearance, disappearance or change in brightness of individual features, rather than the restructuring or reorientation of these features. Brief comparisons with several theories are presented

Ferromagnetism, paramagnetism and a Curie-Weiss metal in an electron doped Hubbard model on a triangular lattice

Motivated by the unconventional properties and rich phase diagram of NaxCoO2 we consider the electronic and magnetic properties of a two-dimensional Hubbard model on an isotropic triangular lattice doped with electrons away from half-filling. Dynamical mean-field theory (DMFT) calculations predict that for negative inter-site hopping amplitudes (t<0) and an on-site Coulomb repulsion, U, comparable to the bandwidth, the system displays properties typical of a weakly correlated metal. In contrast, for t>0 a large enhancement of the effective mass, ferromagnetism and a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. Our observation of Nagaoka ferromagnetism is consistent with the A-type antiferromagnetism (i.e. ferromagnetic layers stacked antiferromagnetically) observed in neutron scattering experiments on NaxCoO2. We propose that `Curie-Weiss metal' phase observed in NaxCoO2 is a consequence of the crossover from ``bad metal'' with incoherent quasiparticles at temperatures T>T* and Fermi liquid behavior with enhanced parameters below T*, where T* is a low energy coherence scale induced by strong local Coulomb electron correlations. We propose a model which contains the charge ordering phenomena observed in the system which, we propose, drives the system close to the Mott insulating phase even at large dopings.Comment: 24 pages, 15 figures; accepted for publication in Phys. Rev.

Temperature Dependence of the Magnetic Susceptibility for Triangular-Lattice Antiferromagnets with spatially anisotropic exchange constants

We present the temperature dependence of the uniform susceptibility of spin-half quantum antiferromagnets on spatially anisotropic triangular-lattices, using high temperature series expansions. We consider a model with two exchange constants, $J_1$ and $J_2$ on a lattice that interpolates between the limits of a square-lattice ($J_1=0$), a triangular-lattice ($J_2=J_1$), and decoupled linear chains ($J_2=0$). In all cases, the susceptibility which has a Curie-Weiss behavior at high temperatures, rolls over and begins to decrease below a peak temperature, $T_p$. Scaling the exchange constants to get the same peak temperature, shows that the susceptibilities for the square-lattice and linear chain limits have similar magnitudes near the peak. Maximum deviation arises near the triangular-lattice limit, where frustration leads to much smaller susceptibility and with a flatter temperature dependence. We compare our results to the inorganic materials Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ and to a number of organic molecular crystals. We find that the former (Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$) are weakly frustrated and their exchange parameters determined through the temperature dependence of the susceptibility are in agreement with neutron-scattering measurements. In contrast, the organic materials are strongly frustrated with exchange parameters near the isotropic triangular-lattice limit.Comment: 10 pages, 9 figures and 1 table, revised versio