Heat transport in the XXZXXZ spin chain: from ballistic to diffusive regimes and dephasing enhancement

In this work we study the heat transport in an XXZ spin-1/2 Heisenberg chain with homogeneous magnetic field, incoherently driven out of equilibrium by reservoirs at the boundaries. We focus on the effect of bulk dephasing (energy-dissipative) processes in different parameter regimes of the system. The non-equilibrium steady state of the chain is obtained by simulating its evolution under the corresponding Lindblad master equation, using the time evolving block decimation method. In the absence of dephasing, the heat transport is ballistic for weak interactions, while being diffusive in the strongly-interacting regime, as evidenced by the heat-current scaling with the system size. When bulk dephasing takes place in the system, diffusive transport is induced in the weakly-interacting regime, with the heat current monotonically decreasing with the dephasing rate. In contrast, in the strongly-interacting regime, the heat current can be significantly enhanced by dephasing for systems of small size

Transport enhancement from incoherent coupling between one-dimensional quantum conductors

We study the non-equilibrium transport properties of a highly anisotropic two-dimensional lattice of spin-1/2 particles governed by a Heisenberg XXZ Hamiltonian. The anisotropy of the lattice allows us to approximate the system at finite temperature as an array of incoherently coupled one-dimensional chains. We show that in the regime of strong intrachain interactions, the weak interchain coupling considerably boosts spin transport in the driven system. Interestingly, we show that this enhancement increases with the length of the chains, which is related to superdiffusive spin transport. We describe the mechanism behind this effect, compare it to a similar phenomenon in single chains induced by dephasing, and explain why the former is much stronger

Beyond mean-field bistability in driven-dissipative lattices: bunching-antibunching transition and quantum simulation

In the present work we investigate the existence of multiple nonequilibrium steady states in a coherently driven XY lattice of dissipative two-level systems. A commonly used mean-field ansatz, in which spatial correlations are neglected, predicts a bistable behavior with a sharp shift between low- and high-density states. In contrast one-dimensional matrix product methods reveal these effects to be artifacts of the mean-field approach, with both disappearing once correlations are taken fully into account. Instead, a bunching-antibunching transition emerges. This indicates that alternative approaches should be considered for higher spatial dimensions, where classical simulations are currently infeasible. Thus we propose a circuit QED quantum simulator implementable with current technology to enable an experimental investigation of the model considered

Modelling diffusion of innovations in a social network

A new simple model of diffusion of innovations in a social network with upgrading costs is introduced. Agents are characterized by a single real variable, their technological level. According to local information agents decide whether to upgrade their level or not balancing their possible benefit with the upgrading cost. A critical point where technological avalanches display a power-law behavior is also found. This critical point is characterized by a macroscopic observable that turns out to optimize technological growth in the stationary state. Analytical results supporting our findings are found for the globally coupled case.Comment: 4 pages, 5 figures. Final version accepted in PR

Communication in networks with hierarchical branching

We present a simple model of communication in networks with hierarchical branching. We analyze the behavior of the model from the viewpoint of critical systems under different situations. For certain values of the parameters, a continuous phase transition between a sparse and a congested regime is observed and accurately described by an order parameter and the power spectra. At the critical point the behavior of the model is totally independent of the number of hierarchical levels. Also scaling properties are observed when the size of the system varies. The presence of noise in the communication is shown to break the transition. Despite the simplicity of the model, the analytical results are a useful guide to forecast the main features of real networks.Comment: 4 pages, 3 figures. Final version accepted in PR

Magnetodielectric coupling of infrared phonons in single crystal Cu2_{2}OSeO3_{3}

Reflection and transmission as a function of temperature have been measured on a single crystal of the magnetoelectric ferrimagnetic compound Cu$_{2}$OSeO$_{3}$ utilizing light spanning the far infrared to the visible portions of the electromagnetic spectrum. The complex dielectric function and optical properties were obtained via Kramers-Kronig analysis and by fits to a Drude-Lortentz model. The fits of the infrared phonons show a magnetodielectric effect near the transition temperature ($T_{c}\sim 60$~K). Assignments to strong far infrared phonon modes have been made, especially those exhibiting anomalous behavior around the transition temperature

Coexistence of energy diffusion and local thermalization in nonequilibrium XXZ spin chains with integrability breaking

In this work we analyze the simultaneous emergence of diffusive energy transport and local thermalization in a nonequilibrium one-dimensional quantum system, as a result of integrability breaking. Specifically, we discuss the local properties of the steady state induced by thermal boundary driving in a XXZ spin chain with staggered magnetic field. By means of efficient large-scale matrix product simulations of the equation of motion of the system, we calculate its steady state in the long-time limit.We start by discussing the energy transport supported by the system, finding it to be ballistic in the integrable limit and diffusive when the staggered field is finite. Subsequently, we examine the reduced density operators of neighboring sites and find that for large systems they are well approximated by local thermal states of the underlying Hamiltonian in the nonintegrable regime, even for weak staggered fields. In the integrable limit, on the other hand, this behavior is lost, and the identification of local temperatures is no longer possible. Our results agree with the intuitive connection between energy diffusion and thermalization

The Space Interferometry Mission Astrometric Grid Giant-Star Survey. I. Stellar Parameters and Radial Velocity Variability

We present results from a campaign of multiple epoch echelle spectroscopy of relatively faint (V = 9.5-13.5 mag) red giants observed as potential astrometric grid stars for the Space Interferometry Mission (SIM PlanetQuest). Data are analyzed for 775 stars selected from the Grid Giant Star Survey spanning a wide range of effective temperatures (Teff), gravities and metallicities. The spectra are used to determine these stellar parameters and to monitor radial velocity (RV) variability at the 100 m/s level. The degree of RV variation measured for 489 stars observed two or more times is explored as a function of the inferred stellar parameters. The percentage of radial velocity unstable stars is found to be very high -- about 2/3 of our sample. It is found that the fraction of RV-stable red giants (at the 100 m/s level) is higher among stars with Teff \sim 4500 K, corresponding to the calibration-independent range of infrared colors 0.59 < (J-K_s)_0 < 0.73. A higher percentage of RV-stable stars is found if the additional constraints of surface gravity and metallicity ranges 2.3< log g < 3.2 and -0.5 < [Fe/H] < -0.1, respectively, are applied. Selection of stars based on only photometric values of effective temperature (4300 K < Teff < 4700 K) is a simple and effective way to increase the fraction of RV-stable stars. The optimal selection of RV-stable stars, especially in the case when the Washington photometry is unavailable, can rely effectively on 2MASS colors constraint 0.59 < (J-K_s)_0 < 0.73. These results have important ramifications for the use of giant stars as astrometric references for the SIM PlanetQuest.Comment: Astronomical Journal, in press, 22 pages, 11 Postscript figures, uses aastex.cl