38 research outputs found
(3+1) Massive Dirac Fermions with Ultracold Atoms in Optical Lattices
We propose the experimental realization of (3+1) relativistic Dirac fermions
using ultracold atoms in a rotating optical lattice or, alternatively, in a
synthetic magnetic field. This approach has the advantage to give mass to the
Dirac fermions by coupling the ultracold atoms to a Bragg pulse. A dimensional
crossover from (3+1) to (2+1) Dirac fermions can be obtained by varying the
anisotropy of the lattice. We also discuss under which conditions the
interatomic potentials give rise to relativistically invariant interactions
among the Dirac fermions
Spontaneous formation and relaxation of spin domains in antiferromagnetic spin-1 quasi-condensates
Quantum systems of many interacting particles at low temperatures generally
organize themselves into ordered phases of matter, whose nature and symmetries
are captured by an order parameter. In the simplest cases, this order parameter
is spatially uniform. For example, a system of localized spins with
ferromagnetic interactions align themselves to a common direction and build up
a macroscopic magnetization on large distances. However, non-uniform situations
also exist in nature, for instance in antiferromagnetism where the
magnetization alternates in space. The situation becomes even richer when the
spin-carrying particles are mobile, for instance in the so-called stripe phases
emerging for itinerant electrons in strongly-correlated materials.
Understanding such inhomogeneously ordered states is of central importance in
many-body physics. In this work, we study experimentally the magnetic ordering
of itinerant spin-1 bosons in inhomegeneous spin domains at nano-Kelvin
temperatures. We demonstrate that spin domains form spontaneously after a phase
separation transition, \textit{i.e.} in the absence of external magnetic force,
purely because of the antiferromagnetic interactions between the atoms.
Furthermore, we explore how the equilibrium domain configuration emerges from
an initial state prepared far-from-equilibrium.Comment: Supplementary material available as ancillary fil
Optical Flux Lattices for Two-Photon Dressed States
We describe a simple scheme by which "optical flux lattices" can be
implemented in ultracold atomic gases using two-photon dressed states. This
scheme can be applied, for example, to the ground state hyperfine levels of
commonly used atomic species. The resulting flux lattices simulate a magnetic
field with high mean flux density, and have low energy bands analogous to the
lowest Landau level. We show that in practical cases the atomic motion
significantly deviates from the adiabatic following of one dressed state, and
that this can lead to significant interactions even for fermions occupying a
single band. Our scheme allows experiments on cold atomic gases to explore
strong correlation phenomena related to the fractional quantum Hall effect,
both for fermions and bosons.Comment: 6 page
Topological superfluids on a lattice with non-Abelian gauge fields
Two-component fermionic superfluids on a lattice with an external non-Abelian
gauge field give access to a variety of topological phases in presence of a
sufficiently large spin imbalance. We address here the important issue of
superfluidity breakdown induced by spin imbalance by a self-consistent
calculation of the pairing gap, showing which of the predicted phases will be
experimentally accessible. We present the full topological phase diagram, and
we analyze the connection between Chern numbers and the existence of
topologically protected and non-protected edge modes. The Chern numbers are
calculated via a very efficient and simple method.Comment: 6 pages, 5 figures to be published in Europhysics Letter
Flux lattices reformulated
We theoretically explore the optical flux lattices produced for ultra-cold
atoms subject to laser fields where both the atom-light coupling and the
effective detuning are spatially periodic. We analyze the geometric vector
potential and the magnetic flux it generates, as well as the accompanying
geometric scalar potential. We show how to understand the gauge-dependent
Aharonov-Bohm singularities in the vector potential, and calculate the
continuous magnetic flux through the elementary cell in terms of these
singularities. The analysis is illustrated with a square optical flux lattice.
We conclude with an explicit laser configuration yielding such a lattice using
a set of five properly chosen beams with two counterpropagating pairs (one
along the x axes and the other y axes), together with a single beam along the z
axis. We show that this lattice is not phase-stable, and identify the one
phase-difference that affects the magnetic flux. Thus armed with realistic
laser setup, we directly compute the Chern number of the lowest Bloch band to
identify the region where the non- zero magnetic flux produces a topologically
non-trivial band structure.Comment: 22 pages, 7 figure
Precision measurement of spin-dependent interaction strengths for spin-1 and spin-2 87Rb atoms
We report on precision measurements of spin-dependent interaction-strengths
in the 87Rb spin-1 and spin-2 hyperfine ground states. Our method is based on
the recent observation of coherence in the collisionally driven spin-dynamics
of ultracold atom pairs trapped in optical lattices. Analysis of the Rabi-type
oscillations between two spin states of an atom pair allows a direct
determination of the coupling parameters in the interaction hamiltonian. We
deduce differences in scattering lengths from our data that can directly be
compared to theoretical predictions in order to test interatomic potentials.
Our measurements agree with the predictions within 20%. The knowledge of these
coupling parameters allows one to determine the nature of the magnetic ground
state. Our data imply a ferromagnetic ground state for 87Rb in the f=1
manifold, in agreement with earlier experiments performed without the optical
lattice. For 87Rb in the f=2 manifold the data points towards an
antiferromagnetic ground state, however our error bars do not exclude a
possible cyclic phase.Comment: 11 pages, 5 figure
Geometric Phases generated by the non-trivial spatial topology of static vector fields coupled to a neutral spin-endowed particle. Application to 171Yb atoms trapped in a 2D optical lattice
We have constructed the geometric phases emerging from the non-trivial
topology of a space-dependent magnetic field, interacting with the spin
magnetic moment of a neutral particle. Our basic tool is the local unitary
transformation which recasts the magnetic spin interaction under a diagonal
form. Rewriting the kinetic term in the "rotated" frame requires the
introduction of non-Abelian covariant derivatives, involving the gradients of
the Euler angles which define the orientation of the local field. Within the
rotated frame, we have built a perturbation scheme,assuming that the
longitudinal non-Abelian field component dominates the transverse ones, to be
evaluated to second-order. The geometry embedded in the longitudinal gauge
vector field and its curl, the geometric magnetic field, is described by the
associated Aharonov-Bohm phase. As an illustration, we study the physics of
cold 171Yb atoms dressed by two sets of circularly polarized beams, forming
square or triangular 2D optical lattices. The geometric field is computed
explicitly from the Euler angles. The magnitude of 2nd-order corrections due to
transverse fields can be reduced to the percent level by a choice of light
intensity which keeps the dressed atom loss rate below 5 s^{-1}. An auxiliary
optical lattice confines the atoms within 2D domains where the geometric field
is pointing upward.Comment: 12 pages, 4 figures. Comments and one figure added about the effect
of the additional scalar potential (sec. V.B). To be published in J. Phys.
A:Math. Theo
Production of Sodium Bose--Einstein condensates in an optical dimple trap
We report on the realization of a sodium Bose--Einstein condensate (BEC) in a
combined red-detuned optical dipole trap, formed by two beams crossing in a
horizontal plane and a third, tightly focused dimple trap propagating
vertically. We produce a BEC in three main steps: loading of the crossed dipole
trap from laser-cooled atoms, an intermediate evaporative cooling stage which
results in efficient loading of the auxiliary dimple trap, and a final
evaporative cooling stage in the dimple trap. Our protocol is implemented in a
compact setup and allows us to reach quantum degeneracy even with relatively
modest initial atom numbers and available laser power
Particles in non-Abelian gauge potentials - Landau problem and insertion of non-Abelian flux
We study charged spin-1/2 particles in two dimensions, subject to a
perpendicular non-Abelian magnetic field. Specializing to a choice of vector
potential that is spatially constant but non-Abelian, we investigate the Landau
level spectrum in planar and spherical geometry, paying particular attention to
the role of the total angular momentum J = L +S. After this we show that the
adiabatic insertion of non-Abelian flux in a spin-polarized quantum Hall state
leads to the formation of charged spin-textures, which in the simplest cases
can be identified with quantum Hall Skyrmions.Comment: 24 pages, 10 figures (with corrected legends