167 research outputs found
Phase-induced transport in atomic gases: from superfluid to Mott insulator
Recent experimental realizations of artificial gauge fields for cold atoms
are promising for generating steady states carrying a mass current in strongly
correlated systems, such as the Bose-Hubbard model. Moreover, a homogeneous
condensate confined by hard-wall potentials from laser sheets has been
demonstrated, which provides opportunities for probing the intrinsic transport
properties of isolated quantum systems. Using the time-dependent Density Matrix
Renormalization Group (TDMRG), we analyze the effect of the lattice and
interaction strength on the current generated by a quench in the artificial
vector potential when the density varies from low values (continuum limit) up
to integer filling in the Mott-insulator regime. There is no observable mass
current deep in the Mott-insulator state as one may expect. Other observable
quantities used to characterize the quasi-steady state in the bulk of the
system are the Drude weight and entanglement entropy production rate. The
latter in particular provides a striking signature of the superfluid-Mott
insulator transition. Furthermore, an interesting property of the superfluid
state is the formation of shock and rarefaction waves at the boundaries due to
the hard-wall confining potentials. We provide results for the height and the
speed of the shock front that propagates from the boundary toward the center of
the lattice. Our results should be verifiable with current experimental
capabilities.Comment: 11 pages, 7 figures, final published versio
Superfluid weight in the isolated band limit within the generalized random phase approximation
The superfluid weight of a generic lattice model with attractive Hubbard
interaction is computed analytically in the isolated band limit within the
generalized random phase approximation. Time-reversal symmetry, spin rotational
symmetry, and the uniform pairing condition are assumed. It is found that the
relation obtained in [https://link.aps.org/doi/10.1103/PhysRevB.106.014518]
between the superfluid weight in the flat band limit and the so-called minimal
quantum metric is valid even at the level of the generalized random phase
approximation. For an isolated, but not necessarily flat, band it is found that
the correction to the superfluid weight obtained from the generalized random
phase approximation is
also the sum of a conventional contribution and a geometric
contribution , as in the case of the known mean-field result
, in which the geometric
term is a weighted average of the quantum metric. The
conventional contribution is geometry independent, that is independent of the
orbital positions, while it is possible to find a preferred, or natural, set of
orbital positions such that . Useful analytic expressions
are derived for both the natural orbital positions and the minimal quantum
metric, including its extension to bands that are not necessarily flat.
Finally, using some simple examples, it is argued that the natural orbital
positions may lead to a more refined classification of the topological
properties of the band structure.Comment: 28 pages, 3 figure
Flat band induced non-Fermi liquid behavior of multicomponent fermions
We investigate multicomponent fermions in a flat band and predict
experimental signatures of non-Fermi liquid behavior. We use dynamical
mean-field theory to obtain the density, double occupancy and entropy in a Lieb
lattice for and components. We derive a
mean-field scaling relation between the results for different values of
, and study its breakdown due to beyond-mean field effects. The
predicted signatures occur at temperatures above the N\'eel temperature and
persist in presence of a harmonic trapping potential, thus they are observable
with current ultracold gas experiments.Comment: 6 pages, 5 figures and and a supplementary materia
The XYZ chain with Dzyaloshinsky-Moriya interactions: from spin-orbit-coupled lattice bosons to interacting Kitaev chains
Using the density-matrix renormalization-group algorithm (DMRG) and a
finite-size scaling analysis, we study the properties of the one-dimensional
completely-anisotropic spin-1/2 XYZ model with Dzyaloshinsky-Moriya (DM)
interactions. The model shows a rich phase diagram: depending on the value of
the coupling constants, the system can display different kinds of ferromagnetic
order and Luttinger-liquid behavior. Transitions from ferromagnetic to
Luttinger-liquid phases are first order. We thoroughly discuss the transition
between different ferromagnetic phases, which, in the absence of DM
interactions, belongs to the XX universality class. We provide evidence that
the DM exchange term turns out to split this critical line into two separated
Ising-like transitions and that in between a disordered phase may appear. Our
study sheds light on the general problem of strongly-interacting
spin-orbit-coupled bosonic gases trapped in an optical lattice and can be used
to characterize the topological properties of superconducting nanowires in the
presence of an imposed supercurrent and of interactions.Comment: 18 pages, 8 figure
Signatures of many-body localization of quasiparticles in a flat band superconductor
We construct a class of exact eigenstates of the Hamiltonian obtained by
projecting the Hubbard interaction term onto the flat band subspace of a
generic lattice model. These exact eigenstates are many body states in which an
arbitrary number of localized fermionic particles coexist with a sea of mobile
Cooper pairs with zero momentum. By considering the dice lattice as an example,
we provide evidence that these exact eigenstates are in fact manifestation of
local integrals of motions of the projected Hamiltonian. In particular the spin
and particle densities retain memory of the initial state for a very long time,
if localized unpaired particles are present at the beginning of the time
evolution. This shows that many-body localization of quasiparticles and
superfluidity can coexist even in generic two-dimensional lattice models with
flat bands, for which it is not known how to construct local conserved
quantities. Our results open new perspectives on the old condensed matter
problem of the interplay between superconductivity and localization.Comment: 20 Pages, 9 figure
Quantum geometry in superfluidity and superconductivity
We review the theoretical description of the role of quantum geometry in
superfluidity and superconductivity of multiband systems, with focus on flat
bands where quantum geometry is wholly responsible for supercurrents. This
review differs from previous ones in that it is based on the most recent
understanding of the theory: the dependence of the self-consistent order
parameter on the supercurrent is properly taken into account, and the
superfluid weight in a flat band becomes proportional to the minimal quantum
metric. We provide a recap of basic quantum geometric quantities and the
concept of superfluid density. The geometric contribution of superconductivity
is introduced via considering the two-body problem. The superfluid weight of a
multiband system is derived within mean-field theory, leading to a topological
bound of flat band superconductivity. The physical interpretation of the flat
band supercurrent in terms of Wannier function overlaps is discussed.Comment: 32 pages, 1 figure, Lecture notes for the Proceedings of the
International School of Physics "Enrico Fermi" Course 211 "Quantum Mixtures
with Ultra-Cold Atoms" (Varenna, Italy, 2022
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