30 research outputs found
Number of closed-channel molecules in the BEC-BCS crossover
Using a two-channel model, we show that the number of closed channel
molecules in a two-component Fermi gas close to a Feshbach resonance is
directly related to the derivative of the energy of the gas with respect to the
inverse scattering length. We extract this quantity from the fixed node
Monte-Carlo equation of state and we compare to the number of closed channel
molecules measured in the Rice experiment with lithium [Partridge et al., Phys.
Rev. Lett. 95, 020404 (2005)]. We also discuss the effect of a difference
between the trapping potentials seen by a closed-channel molecule and by an
open-channel pair of atoms in terms of an effective position-dependent
scattering length.Comment: published online in Eur. Phys. J.
Measuring Chern numbers in Hofstadter strips
Topologically non-trivial Hamiltonians with periodic boundary conditions are
characterized by strictly quantized invariants. Open questions and fundamental
challenges concern their existence, and the possibility of measuring them in
systems with open boundary conditions and limited spatial extension. Here, we
consider transport in Hofstadter strips, that is, two-dimensional lattices
pierced by a uniform magnetic flux which extend over few sites in one of the
spatial dimensions. As we show, an atomic wavepacket exhibits a transverse
displacement under the action of a weak constant force. After one Bloch
oscillation, this displacement approaches the quantized Chern number of the
periodic system in the limit of vanishing tunneling along the transverse
direction. We further demonstrate that this scheme is able to map out the Chern
number of ground and excited bands, and we investigate the robustness of the
method in presence of both disorder and harmonic trapping. Our results prove
that topological invariants can be measured in Hofstadter strips with open
boundary conditions and as few as three sites along one direction.Comment: v1: 17 pages, 10 figures; v2: minor changes, reference added, SciPost
style, 26 pages, 10 figures; v3: published versio
Three-dimensional droplets of swirling superfluids
A new method for the creation of 3D solitary topological modes, corresponding to vortical droplets of a two-component dilute superfluid, is presented. We use the recently introduced system of nonlinearly coupled Gross-Pitaevskii equations, which include contact attraction between the components, and quartic repulsion stemming from the Lee-Huang-Yang correction to the mean-field energy. Self-trapped vortex tori, carrying the topological charges m1=m2=1 or m1=m2=2 in their components, are constructed by means of numerical and approximate analytical methods. The analysis reveals stability regions for the vortex droplets (in broad and relatively narrow parameter regions for m1=m2=1 and m1=m2=2, respectively). The results provide the first example of stable 3D self-trapped states with the double vorticity, in any physical setting. The stable modes are shaped as flat-top ones, with the space between the inner hole, induced by the vorticity, and the outer boundary filled by a nearly constant density. On the other hand, all modes with hidden vorticity, i.e., topological charges of the two components m1=-m2=1, are unstable. The stability of the droplets with m1=m2=1 against splitting (which is the main scenario of possible instability) is explained by estimating analytically the energy of the split and un-split states. The predicted results may be implemented, exploiting dilute quantum droplets in mixtures of Bose-Einstein condensates.Peer ReviewedPostprint (author's final draft
Frustrated magnets without geometrical frustration in bosonic flux ladders
We propose a scheme to realize the frustrated spin-1/2 quantum XX model with
ultracold bosonic atoms in optical lattices. Our approach is based on a square
ladder of magnetic flux close to with one real and one synthetic spin
dimension. Although this system does not have geometrical frustration, we show
that at low energies it maps into an effective triangular ladder with staggered
fluxes for specific values of the synthetic tunneling. We numerically
investigate its rich phase diagram and show that it contains bond-ordered-wave
and chiral superfluid phases. Our scheme gives access to minimal instances of
frustrated magnets without the need for real geometrical frustration, in a
setup of minimal experimental complexity.Comment: Main text: 5 pages + references, 3 figures; supplemental material: 9
pages, 2 figure
Interacting bosonic flux ladders with a synthetic dimension: Ground-state phases and quantum quench dynamics
Flux ladders constitute the minimal setup enabling a systematic understanding
of the rich physics of interacting particles subjected simultaneously to strong
magnetic fields and a lattice potential. In this paper, the ground-state phase
diagram of a flux-ladder model is mapped out using extensive density-matrix
renormalization-group simulations. The emphasis is put on parameters which can
be experimentally realized exploiting the internal states of potassium atoms as
a synthetic dimension. The focus is on accessible observables such as the
chiral current and the leg-population imbalance. Considering a particle filling
of one boson per rung, we report the existence of a Mott-insulating Meissner
phase as well as biased-ladder phases on top of superfluids and Mott
insulators. Furthermore, we demonstrate that quantum quenches from suitably
chosen initial states can be used to probe the equilibrium properties in the
transient dynamics. Concretely, we consider the instantaneous turning on of
hopping matrix elements along the rungs or legs in the synthetic flux-ladder
model, with different initial particle distributions. We show that clear
signatures of the biased-ladder phase can be observed in the transient
dynamics. Moreover, the behavior of the chiral current in the transient
dynamics is discussed. The results presented in this paper provide guidelines
for future implementations of flux ladders in experimental setups exploiting a
synthetic dimension.Comment: as published, with plotted data in json forma
Simulating twistronics without a twist
We propose a scheme to emulate the essence of twisted bilayer graphene by
exploiting ultracold atoms in an optical lattice. In our scheme, no bilayer nor
twist are directly realized. Instead, two synthetic layers are produced
exploiting coherently-coupled internal atomic states, and a supercell structure
is generated via a spatially-dependent Raman coupling. We show that this system
displays a band structure similar to that of magic angle twisted bilayer
graphene, and explain its origin by deriving underlying effective Hamiltonians
via perturbative approaches. Our proposal can be implemented using
state-of-the-art experimental techniques, and opens the route towards the
controlled study of strongly-correlated flat band accompanied by hybridization
physics akin to magic angle bilayer graphene in cold atom quantum simulators.Comment: 9 pages, 8 figures, includes supplementary materia
Lifetime of double occupancies in the Fermi-Hubbard model
We investigate the decay of artificially created double occupancies in a
repulsive Fermi-Hubbard system in the strongly interacting limit using
diagrammatic many-body theory and experiments with ultracold fermions on
optical lattices. The lifetime of the doublons is found to scale exponentially
with the ratio of the on-site repulsion to the bandwidth. We show that the
dominant decay process in presence of background holes is the excitation of a
large number of particle hole pairs to absorb the energy of the doublon. We
also show that the strongly interacting nature of the background state is
crucial in obtaining the correct estimate of the doublon lifetime in these
systems. The theoretical estimates and the experimental data are in fair
quantitative agreement
Double transfer through Dirac points in a tunable honeycomb optical lattice
We report on Bloch-Zener oscillations of an ultracold Fermi gas in a tunable honeycomb lattice. The quasi-momentum distribution of the atoms is measured after sequentially passing through two Dirac points. We observe a double-peak feature in the transferred fraction to the second band, both as a function of the band gap at the Dirac points and the quasi-momentum of the trajectory. Our results are in good agreement with a simple analytical model based on two successive Landau-Zener transitions. Owing to the variation of the potential gradient over the cloud size, coherent StĂĽckelberg oscillations are not visible in our measurements. This effect of the harmonic confinement is confirmed by a numerical simulation of the dynamics of a trapped 2D syste