Different models of gravitating Dirac fermions in optical lattices

In this paper I construct the naive lattice Dirac Hamiltonian describing the propagation of fermions in a generic 2D optical metric for different lattice and flux-lattice geometries. First, I apply a top-down constructive approach that we first proposed in [Boada {\it et al.,New J. Phys.} {\bf 13} 035002 (2011)] to the honeycomb and to the brickwall lattices. I carefully discuss how gauge transformations that generalize momentum (and Dirac cone) shifts in the Brillouin zone in the Minkowski homogeneous case can be used in order to change the phases of the hopping. In particular, I show that lattice Dirac Hamiltonian for Rindler spacetime in the honeycomb and brickwall lattices can be realized by considering real and isotropic (but properly position dependent) tunneling terms. For completeness, I also discuss a suitable formulation of Rindler Dirac Hamiltonian in semi-synthetic brickwall and $\pi$-flux square lattices (where one of the dimension is implemented by using internal spin states of atoms as we originally proposed in [Boada {\it et al.,Phys. Rev. Lett. } {\bf 108} 133001 (2012)] and [Celi {\it et al.,Phys. Rev. Lett. } {\bf 112} 043001 (2012)]).Comment: 14 pages, Submitted to EPJ Special Topics for the special issue on "Quantum Gases and Quantum Coherence"; v2: minor changes, figures and references added, similar to the published version, 21 pages, 3 figure

Estimation of the Shear Viscosity from 3FD Simulations of Au+Au Collisions at sNN=\sqrt{s_{NN}}= 3.3--39 GeV

An effective shear viscosity in central Au+Au collisions is estimated in the range of incident energies 3.3 GeV $\le \sqrt{s_{NN}}\le$ 39 GeV. The simulations are performed within a three-fluid model employing three different equations of state with and without the deconfinement transition. In order to estimate this effective viscosity, we consider the entropy produced in the 3FD simulations as if it is generated within the conventional one-fluid viscous hydrodynamics. It is found that the effective viscosity within different considered scenarios is very similar at the expansion stage of the collision: as a function of temperature ($T$) the viscosity-to-entropy ratio behaves as $\eta/s \sim 1/T^4$; as a function of net-baryon density ($n_B$), $\eta/s \sim 1/s$, i.e. it is mainly determined by the density dependence of the entropy density. The above dependencies take place along the dynamical trajectories of Au+Au collisions. At the final stages of the expansion the $\eta/s$ values are ranged from $\sim$0.05 at highest considered energies to $\sim$0.5 at the lowest ones.Comment: 4 pages, 3 figures, to be published in Eur. Phys. Journ.

Assessing the accuracy of Hartree-Fock-Bogoliubov calculations by use of mass relations

The accuracy of three different sets of Hartree-Fock-Bogoliubov calculations of nuclear binding energies is systematically evaluated. To emphasize minor fluctuations, a second order, four-point mass relation, which almost completely eliminates smooth aspects of the binding energy, is introduced. Applying this mass relation yields more scattered results for the calculated binding energies. By examining the Gaussian distributions of the non-smooth aspects which remain, structural differences can be detected between measured and calculated binding energies. Substructures in regions of rapidly changing deformation, specifically around $(N,Z)=(60,40)$ and $(90,60)$, are clearly seen for the measured values, but are missing from the calculations. A similar three-point mass relation is used to emphasize odd-even effects. A clear decrease with neutron excess is seen continuing outside the experimentally known region for the calculations.Comment: 13 pages, 9 figures, published versio

Twisting and buckling: a new undulation mechanism for artificial swimmers

We present an artificial swimmer consisting in a long cylinder of ferrogel which is polarized transversely and in opposite directions at each extremity. When it is placed on a water film and submitted to a transverse oscillating magnetic field, this artificial worm undulates and swims. Whereas symmetry breaking is due to the field gradient, the undulations of the worm result from a torsional buckling instability as the polarized ends tend to align with the applied magnetic field. The critical magnetic field above which buckling and subsequent swimming is observed may be predicted using elasticity equations including the effect of a magnetic torque. As the length of the worm is varied, several undulation modes are observed which are in good agreement with the bending modes of an elastic rod with free ends

Exact solution of the 1D Hubbard model in the atomic limit with inter-site magnetic coupling

In this paper we present for the first time the exact solution in the narrow-band limit of the 1D extended Hubbard model with nearest-neighbour spin-spin interactions described by an exchange constant J. An external magnetic field h is also taken into account. This result has been obtained in the framework of the Green's functions formalism, using the Composite Operator Method. By means of this theoretical background, we have studied some relevant features such as double occupancy, magnetization, spin-spin and charge-charge correlation functions and derived a phase diagram for both ferro (J>0) and anti-ferro (J<0) coupling in the limit of zero temperature. We also report a study on density of states, specific heat, charge and spin susceptibilities. In the limit of zero temperature, we show that the model exhibits a very rich phase diagram characterized by different magnetic orders and by the coexistence of charge and spin orderings at commensurate filling. Moreover, our analysis at finite temperature of density of states and response functions shows the presence of low-temperature charge and spin excitations near the phase boundaries.Comment: 20 pages, 32 figure

Double Exchange model for nanoscopic clusters

We solve the double exchange model on nanoscopic clusters exactly, and specifically consider a six-site benzene-like nanocluster. This simple model is an ideal testbed for studying magnetism in nanoclusters and for validating approximations such as the dynamical mean field theory (DMFT). Non-local correlations arise between neighboring localized spins due to the Hund's rule coupling, favoring a short-range magnetic order of ferro- or antiferromagnetic type. For a geometry with more neighboring sites or a sufficiently strong hybridization between leads and the nanocluster, these non-local correlations are less relevant, and DMFT can be applied reliably.Comment: 9 pages, 9 figures, 1 tabl

Geometry and linearly polarized cavity photon effects on the charge and spin currents of spin-orbit interacting electrons in a quantum ring

We calculate the persistent spin current inside a quantum ring as a function of the strength of the Rashba or Dresselhaus spin-orbit interaction. We provide analytical results for the spin current of a one-dimensional (1D) ring of non-interacting electrons for comparison. Furthermore, we calculate the time evolution in the transient regime of a two-dimensional (2D) quantum ring connected to electrically biased semi-infinite leads using a time-convolutionless non-Markovian generalized master equation. In the latter case, the electrons are correlated via the Coulomb interaction and the ring can be embedded in a photon cavity with a single mode of linearly polarized photon field. The electron-electron and electron-photon interactions are described by exact numerical diagonalization. The photon field can be polarized perpendicular or parallel to the charge transport. We find a pronounced charge current dip associated with many-electron level crossings at the Aharonov-Casher phase $\Delta\Phi=\pi$, which can be disguised by linearly polarized light. Qualitative agreement is found for the spin currents of the 1D and 2D ring. Quantatively, however, the spin currents are weaker in the more realistic 2D ring, especially for weak spin-orbit interaction, but can be considerably enhanced with the aid of a linearly polarized electromagnetic field. Specific spin current symmetries relating the Dresselhaus spin-orbit interaction case to the Rashba one are found to hold for the 2D ring in the photon cavity

Comparison of Tsallis statistics with the Tsallis-factorized statistics in the ultrarelativistic pppp collisions

The Tsallis statistics was applied to describe the experimental data on the transverse momentum distributions of hadrons. We considered the energy dependence of the parameters of the Tsallis-factorized statistics, which is now widely used for the description of the experimental transverse momentum distributions of hadrons, and the Tsallis statistics for the charged pions produced in $pp$ collisions at high energies. We found that the results of the Tsallis-factorized statistics deviate from the results of the Tsallis statistics only at low NA61/SHINE energies when the value of the entropic parameter is close to unity. At higher energies, when the value of the entropic parameter deviates essentially from unity, the Tsallis-factorized statistics satisfactorily recovers the results of the Tsallis statistics.Comment: 8 figures. arXiv admin note: text overlap with arXiv:1607.0195

Instability conditions for some periodic BGK waves in the Vlasov-Poisson system

A one-dimensional, collisionless plasma given by the Vlasov-Poisson system is considered and the stability properties of periodic steady state solutions known as Bernstein-Greene-Kruskal (BGK) waves are investigated. Sufficient conditions are determined under which BGK waves are linearly unstable under perturbations that share the same period as the equilibria. It is also shown that such solutions cannot support a monotonically decreasing particle distribution function.Comment: 8 pages; PACS codes 52.25.Dg, 02.30.Jr, 52.35.-

Description of Charged Particle Pseudorapidity Distributions in Pb+Pb Collisions with Tsallis Thermodynamics

The centrality dependence of pseudorapidity distributions for charged particles produced in Au+Au collisions at $\sqrt{s_{NN}}=130$ GeV and 200 GeV at RHIC, and in Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV at LHC are investigated in the fireball model, assuming that the rapidity axis is populated with fireballs following one distribution function. We assume that the particles in the fireball fulfill the Tsallis distribution. The theoretical results are compared with the experimental measurements and a good agreement is found. Using these results, the pseudorapidity distributions of charged particles produced in Pb+Pb central collisions at $\sqrt{s_{NN}}=5.02$ TeV and 10 TeV are predicted.Comment: 9 pages, 8 figure