134 research outputs found
Phase separation in fermionic systems with particle-hole asymmetry
We determine the ground-state phase-diagram of a Hubbard Hamiltonian with
correlated hopping, which is asymmetric under particle-hole transform. By
lowering the repulsive Coulomb interaction U at appropriate filling and
interaction parameters, the ground state separates into a hole and an electron
conducting phases: two different wave vectors characterize the system and
charge-charge correlations become incommensurate. By further decreasing U
another transition occurs at which the hole conducting region becomes
insulating, and conventional phase separation takes place. Finally, for
negative U the whole system eventually becomes a paired insulator. It is
speculated that such behavior could be at the origin of the incommensurate
superconducting phase recently discovered in the 1D Hirsch model. The exact
phase boundaries are calculated in one dimension.Comment: 4 pages, 2 figure
Detecting the tunneling rates for strongly interacting fermions on optical lattices
Strongly interacting fermionic atoms on optical lattices are studied through
a Hubbard-like model Hamiltonian, in which tunneling rates of atoms and
molecules between neighboring sites are assumed to be different. In the limit
of large onsite repulsion U, the model is shown to reproduce the t-J
Hamiltonian, in which the J coefficient of the Heisenberg term depends on the
particle-assisted tunneling rate g: explicitly, . At half-filling, g
drives a crossover from a Brinkman-Rice paramagnetic insulator of fully
localized atoms (g=0) to the antiferromagnetic Mott insulator of the standard
Hubbard case (g=t). This is observed already at the intermediate coupling
regime in the number of doubly occupied sites, thus providing a criterion to
extract from measurements the effective value of g.Comment: 5 pages, 3 figure
Entanglement in extended Hubbard models and quantum phase transitions
The role of two-point and multipartite entanglement at quantum phase
transitions (QPTs) in correlated electron systems is investigated. We consider
a bond-charge extended Hubbard model exactly solvable in one dimension which
displays various QPTs, with two (qubit) as well as more (qudit) on-site degrees
of freedom involved. The analysis is carried out by means of appropriate
measures of bipartite/multipartite quantum correlations. It is found that all
transitions ascribed to two-point correlations are characterized by an
entanglement range which diverges at the transition points. The exponent
coincides with that of the correlation length at the transitions. We introduce
the correlation ratio, namely, the ratio of quantum mutual information and
single-site entanglement. We show that at T=0, it captures the relative role of
two-point and multipartite quantum correlations at transition points,
generalizing to qudit systems the entanglement ratio. Moreover, a finite value
of quantum mutual information between infinitely distant sites is seen to
quantify the presence of off-diagonal long-range order induced by multipartite
entanglement.Comment: 14 pages, 8 figures, 2 table
Recent results on integrable electronic models
We review the approach of generalized permutator to produce a class of
integrable quantum Hamiltonians, as well as the technique of Sutherland species
(SS) to map a subclass of it into solvable spinless fermions models. In
particular, we apply the above scheme to construct integrable interacting
electron Hamiltonians: first we review the extended Hubbard case, discussing
both ground state and thermodynamics; then we pass to constrained fermion
models, generating 56 integrable cases, among which both supersymmetric t-J
model and infinite U Hubbard model are obtained, as well as other physically
interesting cases, such as a particular t-V model. For the latter we describe
how the complete spectrum can be gained by means of SS technique. Finally we
speculate about possible applications to spin S models.Comment: Review article; 12 pages, 4 figures. Appeared on Recent Research
Developements in Physics 5, 513-534 (Transworld Research Network, India,
2004
Bipartite entanglement of quantum states in a pair basis
The unambiguous detection and quantification of entanglement is a hot topic
of scientific research, though it is limited to low dimensions or specific
classes of states. Here we identify an additional class of quantum states, for
which bipartite entanglement measures can be efficiently computed, providing
new rigorous results. Such states are written in arbitrary
dimensions, where each basis state in the subsystem A is paired with only one
state in B. This new class, that we refer to as pair basis states, is
remarkably relevant in many physical situations, including quantum optics. We
find that negativity is a necessary and sufficient measure of entanglement for
mixtures of states written in the same pair basis. We also provide analytical
expressions for a tight lower-bound estimation of the entanglement of
formation, a central quantity in quantum information.Comment: 8 pages, 10 figure
Low energy quantum regimes of 1D dipolar Hubbard model with correlated hopping
We apply the bosonization technique to derive the phase diagram of a balanced
unit density two-component dipolar Fermi gas in a one dimensional lattice
geometry. The considered interaction processes are of the usual contact and
dipolar long-range density-density type together with peculiar correlated
hopping terms which can be generated dynamically. Rigorous bounds for the
transition lines are obtained in the weak coupling regime. In addition to the
standard bosonization description, we derive the low energy phase diagram
taking place when part of the interaction is embodied non-perturbatively in the
single component Hamiltonians. In this case the Luttinger liquid regime is
shown to become unstable with respect to the opening of further gapped phases,
among which insulating bond ordered wave and Haldane phases, the latter with
degenerate edge modes.Comment: 6 pages, 1 figur
Hidden XY structure of the bond-charge Hubbard model
The repulsive one-dimensional Hubbard model with bond-charge interaction
(HBC) in the superconducting regime is mapped onto the spin-1/2 XY model with
transverse field. We calculate correlations and phase boundaries, realizing an
excellent agreement with numerical results. The critical line for the
superconducting transition is shown to coincide with the analytical
factorization line identifying the commensurate-incommensurate transition in
the XY model.Comment: 4 pages, 3 figure
Brane parity orders in the insulating state of Hubbard ladders
The Mott insulating state of the Hubbard model at half-filling could be
depicted as a spin liquid of singly occupied sites with holon-doublon quantum
fluctuations localized in pairs. In one dimension the behavior is captured by a
finite value of the charge parity string correlator, which fails to remain
finite when generalized to higher dimensions. We recover a definition of parity
brane correlator which may remain nonvanishing in presence of interchain
coupling, by assigning an appropriate fractional phase to the parity breaking
fluctuations. In case of Hubbard ladders at half-filling, we find that the
charge parity brane is non-zero at any repulsive value of interaction. The spin
parity brane instead becomes nonvanishing in the even-leg case, in
correspondence to the onset of the spin gapped D-Mott phase, which is absent in
the odd-leg case. The behavior of the parity correlators is also analyzed by
means of a numerical DMRG analysis of the one- and two-leg ladder.Comment: Main article: 5 pages, 1 figure. Supplementary information: 4 pages,
8 figure
Symmetry protected topological phases of 1D interacting fermions with spin-charge separation
The low energy behavior of a huge variety of one-dimensional interacting
spinful fermionic systems exhibits spin-charge separation, described in the
continuum limit by two sine-Gordon models decoupled in the charge and spin
channels. Interaction is known to induce, besides the gapless Luttinger liquid
phase, eight possible gapped phases, among which are the Mott, Haldane,
charge-/spin-density, and bond-ordered wave insulators, and the Luther Emery
liquid. Here we prove that some of these physically distinct phases have
nontrivial topological properties, notably the presence of degenerate protected
edge modes with fractionalized charge/spin. Moreover, we show that the eight
gapped phases are in one-to-one correspondence with the symmetry-protected
topological (SPT) phases classified by group cohomology theory in the presence
of particle-hole symmetry P. The latter result is also exploited to
characterize SPT phases by measurable nonlocal order parameters which follow
the system evolution to the quantum phase transition. The implications on the
appearance of exotic orders in the class of microscopic Hubbard Hamiltonians,
possibly without P symmetry at higher energies, are discussed.Comment: latest version: 8 pages, 1 Tabl
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