IRIS Università degli Studi di Padovahttps://www.research.unipd.itIl sistema di repository digitale IRIS acquisisce, archivia, indicizza, conserva e rende accessibili prodotti digitali della ricerca.Thu, 22 Oct 2020 18:49:40 GMT2020-10-22T18:49:40Z10611Analytical approach to the two-site Bose-Hubbard model: From Fock states to Schrödinger cat states and entanglement entropyhttp://hdl.handle.net/11577/2495390Titolo: Analytical approach to the two-site Bose-Hubbard model: From Fock states to Schrödinger cat states and entanglement entropy
Abstract: We study the interpolation from occupation number Fock states to Schrödinger cat states on systems modeled by a two-mode Bose-Hubbard Hamiltonian, like, for instance, bosons in a double well or superconducting Cooper pair boxes. In the repulsive interaction regime, by a simplified single particle description, we calculate analytically energy, number fluctuations, stability under coupling to a heat bath, entanglement entropy, and Fisher information, all in terms of hypergeometric polynomials of the single particle overlap parameter. Our approach allows us to find how those quantities scale with the number of bosons. In the attractive interaction regime we calculate the same physical quantities in terms of the imbalance parameter, and find that the spontaneous symmetry breaking, occurring at interaction Uc, predicted by a semiclassical approximation, is valid only in the limit of infinite number of bosons. For a large but finite number we determine a characteristic strength of interaction Uc*, which can be promoted as the crossover point from coherent to incoherent regimes and can be identified as the threshold of fragility of the cat state. Moreover, we find that the Fisher information is always in direct ratio to the variance of on-site number of bosons, for both positive and negative interactions. We finally show that the entanglement entropy is maximum close to Uc* and exceeds its coherent value within the whole range of interaction between 2Uc and zero.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/11577/24953902012-01-01T00:00:00ZJosephson physics of spin-orbit-coupled elongated Bose-Einstein condensateshttp://hdl.handle.net/11577/2856709Titolo: Josephson physics of spin-orbit-coupled elongated Bose-Einstein condensates
Abstract: We consider an ultracold bosonic binary mixture confined in a quasi-one-dimensional double-well trap. The two bosonic components are assumed to be two hyperfine internal states of the same atom. We suppose that these two components are spin-orbit coupled to each other. We employ the two-mode approximation starting from two coupled Gross-Pitaevskii equations and derive a system of ordinary differential equations governing the temporal evolution of the interwell population imbalance of each component and of the polarization, which is the imbalance of the total populations of the two species. From this set of equations we disentangle the different macroscopic quantum tunneling and self-trapping scenarios occurring for both population imbalances and the polarization in terms of the interplay between the interatomic interactions and the other relevant energies in the problem, like the spin-orbit coupling or the conventional tunneling term. We find a rich dynamics in all three variables and discuss the experimental feasibility of such a system.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/11577/28567092014-01-01T00:00:00ZOne-dimensional repulsive Fermi gas in a tunable periodic potentialhttp://hdl.handle.net/11577/3249138Titolo: One-dimensional repulsive Fermi gas in a tunable periodic potential
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11577/32491382017-01-01T00:00:00ZFermi surface fluctuations and breakdown of Fermi liquid behaviorhttp://hdl.handle.net/11577/156874Titolo: Fermi surface fluctuations and breakdown of Fermi liquid behavior
Abstract: Electron-electron interactions can induce Fermi surface deformations which break the point-group symmetry of the crystal structure of the system. In the vicinity of such a "Pomeranchuk instability" the Fermi surface is easily deformed by anisotropic perturbations, and exhibits enhanced collective fluctuations. We analyze Fermi surface fluctuation effects in a two-dimensional electron system on a square lattice in the vicinity of a Pomeranchuk instability with d-wave symmetry. At a quantum critical point d-wave density correlations and the dynamical forward scattering interaction diverge with a dynamical exponent z = 3. The singular forward scattering leads to large self-energy corrections, which destroy Fermi liquid behavior over the whole Fermi surface except near the Brillouin zone diagonal. The contribution from classical fluctuations to the self-energy spoils omega/T scaling in the quantum critical regime. We discuss to what extent d-wave Fermi surface fluctuations may play a role in cuprate superconductors.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11577/1568742007-01-01T00:00:00ZThe role of the impurity-potential range in disordered d-wave superconductorshttp://hdl.handle.net/11577/156875Titolo: The role of the impurity-potential range in disordered d-wave superconductors
Abstract: We analyse how the range of disorder affects the localization properties of quasiparticles in a two-dimensional d-wave superconductor within the standard non-linear σ-model approach to disordered systems. We show that for purely long range disorder, which only induces intra-node scattering processes, the approach is free from the ambiguities which often beset the disordered Dirac-fermion theories, and gives rise to a Wess–Zumino–Novikov–Witten action leading to vanishing density of states and finite conductivities. We also study the crossover induced by inter-node scattering due to a short range component of the disorder, thus providing a coherent non-linear σ-model description in agreement with all the various findings of different approaches.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11577/1568752007-01-01T00:00:00ZMagnetic barriers and confinement of Dirac-Weyl quasiparticles in graphenehttp://hdl.handle.net/11577/156873Titolo: Magnetic barriers and confinement of Dirac-Weyl quasiparticles in graphene
Abstract: We discuss the properties of the two-dimensional massless Dirac-Weyl quasiparticles realized in graphene monolayers in the presence of inhomogeneous magnetic fields. We show that in contrast to electrostatic barriers, appropriate magnetic barriers are able to confine these quasiparticles. This allows for a novel way of designing mesoscopic structures (e.g., quantum dots, quantum point contacts) in graphene.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/11577/1568732007-01-01T00:00:00ZMagnetic superlattice and finite-energy Dirac points in graphenehttp://hdl.handle.net/11577/156864Titolo: Magnetic superlattice and finite-energy Dirac points in graphene
Abstract: We study the band structure of graphene’s Dirac-Weyl quasiparticles in a one-dimensional magnetic superlattice formed by a periodic sequence of alternating magnetic barriers. The spectrum and the nature of the states strongly depend on the conserved longitudinal momentum and on the barrier width. At the center of the superlattice Brillouin zone we find new Dirac points at finite energies where the dispersion is highly anisotropic, in contrast to the dispersion close to the neutrality point which remains isotropic. This finding suggests the possibility of collimating Dirac-Weyl quasiparticles by tuning the doping.
Sat, 01 Jan 2011 00:00:00 GMThttp://hdl.handle.net/11577/1568642011-01-01T00:00:00ZNematic order and non-Fermi liquid behavior from a Pomeranchuk instability in a two-dimensional electron systemhttp://hdl.handle.net/11577/156863Titolo: Nematic order and non-Fermi liquid behavior from a Pomeranchuk instability in a two-dimensional electron system
Abstract: Interactions in Fermi systems can induce a "Pomeranchuk instability" leading to orientational symmetry breaking, that is, nematic order. In a metallic system close to such an instability the Fermi surface is easily deformed by anisotropic perturbations, and exhibits enhanced collective fluctuations. We discuss electrons on a square lattice near a Pomeranchuk instability with d-wave symmetry. The strong response of such a system to a small orthorhombic perturbation can explain naturally the large in-plane anisotropy of electronic and magnetic properties observed in detwinned YBCO crystals. Fluctuations in a quantum critical regime near the instability provide a mechanism for non-Fermi liquid behavior. They lead to a singular forward scattering interaction, which destroys fermionic quasi-particles on the whole Fermi surface except at "cold spots" on the Brillouin zone diagonal.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11577/1568632009-01-01T00:00:00ZMultiple magnetic barriers in graphenehttp://hdl.handle.net/11577/156868Titolo: Multiple magnetic barriers in graphene
Abstract: We study the behavior of charge carriers in graphene in inhomogeneous perpendicular magnetic fields. We consider two types of one-dimensional magnetic profiles, uniform in one direction: a sequence of N magnetic barriers and a sequence of alternating magnetic barriers and wells. In both cases, we compute the transmission coefficient of the magnetic structure by means of the transfer-matrix formalism and the associated conductance. In the first case the structure becomes increasingly transparent upon increasing N at fixed total magnetic flux. In the second case we find strong wave-vector filtering and resonant effects.
Thu, 01 Jan 2009 00:00:00 GMThttp://hdl.handle.net/11577/1568682009-01-01T00:00:00ZCritical temperature of non-interacting Bose gases on disordered latticeshttp://hdl.handle.net/11577/156869Titolo: Critical temperature of non-interacting Bose gases on disordered lattices
Abstract: For a non-interacting Bose gas on a lattice we compute the shift of the critical temperature for condensation when random-bond and on-site disorder are present. We evidence that the shift depends on the space dimensionality D and the filling fraction f. For D ->∞ (infinite range model), using results from the theory of random matrices, we show that the shift of the critical temperature is negative, depends on f, and vanishes only for large f. The connections with analogous results obtained for the spherical model are discussed. For D = 3 we find that, for large f, the critical temperature Tc is enhanced by disorder and that the relative shift does not appreciably depend on f; in contrast, for small f, Tc decreases, in agreement with the results obtained for a Bose gas in the continuum. We also provide numerical estimates for the shift of the critical temperature due to disorder induced in a non-interacting Bose gas by a bichromatic incommensurate potential.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/11577/1568692008-01-01T00:00:00Z