40 research outputs found
Phase Diagram of the spin S=1/2 Extended XY model
The quantum phase transition in the ground state of the Extended spin S=1/2
XY model is studied in detail. Using the exact solution of the model the low
temperature thermodynamics, as well as the ground state phase diagram of the
model in the presence of applied uniform and/or staggered magnetic field are
discussed.Comment: 12 pages, 12 figure
Supersolid Bose-Fermi Mixtures in Optical Lattices
We study a mixture of strongly interacting bosons and spinless fermions with
on-site repulsion in a three-dimensional optical lattice. For this purpose we
develop and apply a generalized DMFT scheme, which is exact in infinite
dimensions and reliably describes the full range from weak to strong coupling.
We restrict ourselves to half filling. For weak Bose-Fermi repulsion a
supersolid forms, in which bosonic superfluidity coexists with charge-density
wave order. For stronger interspecies repulsion the bosons become localized
while the charge density wave order persists. The system is unstable against
phase separation for weak repulsion among the bosons.Comment: 4 pages, 5 pictures, Published versio
Triplet superconductivity in a 1D itinerant electron system with transverse spin anisotropy
In this paper we study the ground state phase diagram of a one-dimensional
t-J-U model away from half-filling. In the large-bandwidth limit and for
ferromagnetic exchange with easy-plane anisotropy a phase with gapless charge
and massive spin excitations, characterized by the coexistence of triplet
superconducting and spin density wave instabilities is realized in the ground
state. With increasing ferromagnetic exchange transitions into a ferrometallic
and then a spin gapped triplet superconducting phase take place.Comment: 11 pages, 10 figures, accepted for publication in Eur. Phys. J.
N\'{e}el transition of lattice fermions in a harmonic trap: a real-space DMFT study
We study the magnetic ordering transition for a system of harmonically
trapped ultracold fermions with repulsive interactions in a cubic optical
lattice, within a real-space extension of dynamical mean-field theory (DMFT).
Using a quantum Monte Carlo impurity solver, we establish that
antiferromagnetic correlations are signaled, at strong coupling, by an enhanced
double occupancy. This signature is directly accessible experimentally and
should be observable well above the critical temperature for long-range order.
Dimensional aspects appear less relevant than naively expected.Comment: 4 pages, 4 figure
Generalized Dynamical Mean-Field Theory for Bose-Fermi Mixtures in Optical Lattices
We give a detailed discussion of the recently developed Generalized Dynamical
Mean-Field Theory (GDMFT) for a mixture of bosonic and fermionic particles. We
show that this method is non-perturbative and exact in infinite dimensions and
reliably describes the full range from weak to strong coupling. Like in
conventional Dynamical Mean-Field Theory, the small parameter is 1/z, where z
is the lattice coordination number. We apply the GDMFT scheme to a mixture of
spinless fermions and bosons in an optical lattice. We investigate the
ossibility of a supersolid phase, focussing on the case of 1/2 filling for the
fermions and 3/2 filling for the bosons.Comment: 12+ pages 6 figure
Magnetism and domain formation in SU(3)-symmetric multi-species Fermi mixtures
We study the phase diagram of an SU(3)-symmetric mixture of three-component
ultracold fermions with attractive interactions in an optical lattice,
including the additional effect on the mixture of an effective three-body
constraint induced by three-body losses. We address the properties of the
system in by using dynamical mean-field theory and variational Monte
Carlo techniques. The phase diagram of the model shows a strong interplay
between magnetism and superfluidity. In the absence of the three-body
constraint (no losses), the system undergoes a phase transition from a color
superfluid phase to a trionic phase, which shows additional particle density
modulations at half-filling. Away from the particle-hole symmetric point the
color superfluid phase is always spontaneously magnetized, leading to the
formation of different color superfluid domains in systems where the total
number of particles of each species is conserved. This can be seen as the SU(3)
symmetric realization of a more general tendency to phase-separation in
three-component Fermi mixtures. The three-body constraint strongly disfavors
the trionic phase, stabilizing a (fully magnetized) color superfluid also at
strong coupling. With increasing temperature we observe a transition to a
non-magnetized SU(3) Fermi liquid phase.Comment: 36 pages, 17 figures; Corrected typo
Resonant Superfluidity in an Optical Lattice
We study a system of ultracold fermionic Potassium (40K) atoms in a
three-dimensional optical lattice in the vicinity of an s-wave Feshbach
resonance. Close to resonance, the system is described by a multi-band
Bose-Fermi Hubbard Hamiltonian. We derive an effective lowest-band Hamiltonian
in which the effect of the higher bands is incorporated by a self-consistent
mean-field approximation. The resulting model is solved by means of Generalized
Dynamical Mean-Field Theory. In addition to the BEC/BCS crossover we find a
phase transition to a fermionic Mott insulator at half filling, induced by the
repulsive fermionic background scattering length. We also calculate the
critical temperature of the BEC/BCS-state and find it to be minimal at
resonance.Comment: 19 pages, 3 figure