71 research outputs found
Enhancement of critical temperatures in disordered bipartite lattices
We study the strong enhancement, induced by random hopping, of the critical
temperatures characterizing the transitions to superconductivity,
charge-density wave and antiferromagnetism, which can occur in bipartite
lattice models at half-filling, like graphene, by means of an extended
Finkel'stein non-linear -model renormalization group approach. We show
that, if Cooper channel interaction dominates, superconducting critical
temperature can be enhanced at will, since superconductivity cannot be broken
by entering any Anderson insulating phase. If, instead, staggered interactions
are relevant, antiferromagnetic order is generated by disorder at a temperature
well above that expected for a clean system.Comment: 7 pages, 4 figures, final versio
Extended Kitaev chain with longer-range hopping and pairing
We consider the Kitaev chain model with finite and infinite range in the
hopping and pairing parameters, looking in particular at the appearance of
Majorana zero energy modes and massive edge modes. We study the system both in
the presence and in the absence of time reversal symmetry, by means of
topological invariants and exact diagonalization, disclosing very rich phase
diagrams. In particular, for extended hopping and pairing terms, we can get as
many Majorana modes at each end of the chain as the neighbors involved in the
couplings. Finally we generalize the transfer matrix approach useful to
calculate the zero-energy Majorana modes at the edges for a generic number of
coupled neighbors.Comment: 14 pages, 16 figure
Spreading of correlations and Loschmidt echo after quantum quenches of a Bose gas in the Aubry-Andr\'e potential
We study the spreading of density-density correlations and the Loschmidt
echo, after different sudden quenches in an interacting one dimensional Bose
gas on a lattice, also in the presence of a superimposed aperiodic potential.
We use a time dependent Bogoliubov approach to calculate the evolution of the
correlation functions and employ the linked cluster expansion to derive the
Loschmidt echo.Comment: 10 pages, 14 figures, a section on momentum distribution function is
include
Long-distance entanglement in Motzkin and Fredkin spin chains
We derive some entanglement properties of the ground states of two classes of
quantum spin chains described by the Fredkin model, for half-integer spins, and
the Motzkin model, for integer ones. Since the ground states of the two models
are known analytically, we can calculate the entanglement entropy, the
negativity and the quantum mutual information exactly. We show, in particular,
that these systems exhibit long-distance entanglement, namely two disjoint
regions of the chains remain entangled even when the separation is sent to
infinity, i.e. these systems are not affected by decoherence. This strongly
entangled behavior, occurring both for colorful versions of the models (with
spin larger than 1/2 or 1, respectively) and for colorless cases (spin 1/2 and
1), is consistent with the violation of the cluster decomposition property.
Moreover we show that this behavior involves disjoint segments located both at
the edges and in the bulk of the chains.Comment: 36 pages, 13 figure
Quasiparticle conductivities in disordered d-wave superconductors
We study the quasiparticle transport coefficients in disordered d-wave
superconductors. We find that spin and charge excitations are generally
localized unless magnetic impurities are present. If the system is close to a
nesting point in the impurity-scattering unitary limit, the tendency towards
localization is reduced while the quasiparticle density of states gets enhanced
by disorder. We also show that the residual repulsive interaction among
quasiparticles has a delocalizing effect and increases the density of states.Comment: 13 pages, no figure
Using magnetic stripes to stabilize superfluidity in electron-hole double monolayer graphene
Experiments have confirmed that double monolayer graphene cannot generate
finite temperature electron-hole superfluidity. This has been shown to be due
to very strong screening of the electron-hole pairing attraction. The linear
dispersing energy bands in monolayer graphene prevent attempts to reduce the
strength of the screening. We propose a new hybrid device in which the two
sheets of monolayer graphene are placed in a modulated periodic perpendicular
magnetic field. Such a magnetic field preserves the isotropic Dirac cones of
the original monolayers but it reduces the slope of the cones so that the
monolayer Fermi velocity is smaller. We show that with current
experimental techniques, this reduction in can sufficiently weaken the
screening to permit electron-hole superfluidity at measurable temperatures.Comment: Revised version. MultiSuper collaboration: http://www.multisuper.or
Anomalous universal adiabatic dynamics: The case of the Fredkin model
When a system is driven across a second-order quantum phase transition, the
number of defects which are produced scales with the speed of the variation of
the tuning parameter according to a universal law described by the Kibble-Zurek
mechanism. We study a possible breakdown of this prediction proving that the
number of defects can exhibit another universal scaling law which is still
related only to the critical exponents and , but differs from the
Kibble-Zurek result. Finally we provide an example, the deformed Fredkin spin
chain, where this violation of the standard adiabatic dynamics can occur.Comment: 5 pages, 5 figure
Fluctuation theorems and expected utility hypothesis
The expected utility hypothesis is a popular concept in economics that is
useful for making decisions when the payoff is uncertain. In this paper, we
investigate the implications of a fluctuation theorem in the theory of expected
utility. In particular, we wonder whether entropy could serve as a guideline
for gambling. We prove the existence of a bound involving the certainty
equivalent which depends on the entropy produced. Then, we examine the
dependence of the certainty equivalent on the entropy by looking at specific
situations, for instance, the work extraction from a non-equilibrium initial
state.Comment: 4 pages, 3 figures, comments welcom
Quasiprobability distribution of work in the quantum Ising model
A complete understanding of the statistics of the work done by quenching a
parameter of a quantum many-body system is still lacking in the presence of an
initial quantum coherence in the energy basis. In this case, the work can be
represented by a class of quasiprobability distributions. Here, we try to
clarify the genuinely quantum features of the process by studying the work
quasiprobability for an Ising model in a transverse field. We consider both a
global and a local quench, by focusing mainly on the thermodynamic limit. We
find that, while for a global quench there is a symmetric non-contextual
representation with a Gaussian probability distribution of work, for a local
quench we can get quantum contextuality as signaled by a negative fourth moment
of the work. Furthermore, we examine the critical features related to a quantum
phase transition and the role of the initial quantum coherence as useful
resource.Comment: 15 pages, 4 figures. Comments welcom
- âŠ