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Journal of Physics Conference Series

Superfluid-insulator transition of two-band fermionic atom systems in optical lattices

Takuji Higashiyama, Kensuke Inaba, Sei-ichiro Suga

Published Feb 1, 2009• 0 citations
Publisher

Abstract

We study the multiband effects on ultracold fermionic atoms in optical lattices by two-site dynamical mean-field theory. We find that the density wave state becomes stable for a small orbital splitting region. As the orbital splitting is increased, the transition from the density wave state to the superfluid state occurs. By systematically changing the orbital splitting and the attractive interaction, we obtain the phase diagram at half-filling.

DOI
10.1088/1742-6596/150/3/032098

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We study the multiband effects on ultracold fermionic atoms in optical lattices by two-site dynamical mean-field theory. We find that the density wave state becomes stable for a small orbital splitting region. As the orbital splitting is increased, the transition from the density wave state to the superfluid state occurs. By systematically changing the orbital splitting and the attractive interaction, we obtain the phase diagram at half-filling.
We study two-band effects on ultracold fermionic atoms in optical lattices by means of dynamical mean-field theory. We find that at half filling an atomic-density-wave (ADW) state emerges owing to the two-band effects in the attractive interaction region, while an antiferromagnetic state appears in the repulsive interaction region. As the orbital splitting is increased, quantum phase transitions from the ADW state to the superfluid state and from the antiferromagnetic state to the metallic state occur in the corresponding regions. By systematically changing the orbital splitting and the interaction, we obtain the phase diagram at half filling. The results are discussed using the effective boson model derived for strong attractive interaction.
The superfluid-insulator transition of fermionic atoms in optical lattices is investigated by two-site dynamical mean-field theory. It is shown that the Mott transition occurs as a result of multiband effects. The quasiparticle weight in the superfluid state decreases significantly as the system approaches the Mott transition point. By changing the interaction and the orbital splitting, we obtain the phase diagram at half filling. The numerical results are discussed in comparison with the effective boson model.
We study two models realized by two-component Fermi gases loaded in optical lattices. We clarify that multiband effects inevitably caused by the optical lattices generate a rich structure, when the systems crossover from the region of weakly bound molecular bosons to the region of strongly bound atomic bosons. Here the crossover can be controlled by attractive fermion interaction. One of the present models is a case with attractive fermion interaction, where an insulator-superfluid transition takes place. The transition is characterized as the transition between a band insulator and a Bose-Einstein condensate superfluid state. Differing from the conventional Bardeen-Cooper-Schrieffer (BCS) superfluid transition, this transition shows unconventional properties. In contrast to the one-particle excitation gap scaled by the superfluid order parameter in the conventional BCS transition, because of the multiband effects, a large gap of one-particle density of states is retained all through the transition, although the superfluid order grows continuously from zero. A re-entrant transition with lowering temperature is another unconventionality. The other model is the case with coexisting attractive and repulsive interactions. Within a mean-field treatment, we find a new insulating state, an orbital ordered insulator. This insulator is one candidate for the Mott insulator of molecular bosons and is the first example that the orbital internal degrees of freedom of molecular bosons appears explicitly. Besides the emergence of a new phase, a coexisting phase also appears where superfluidity and an orbital order coexist just by doping holes or particles. The insulating and superfluid particles show differentiation in momentum space as in the high-Tc{T}_{\text{c}}Tc​ cuprate superconductors.
We study ultracold fermionic atoms trapped in an optical lattice with harmonic confinement by means of the dynamical mean-field approximation. It is demonstrated that a supersolid state, where an s-wave superfluid coexists with a density-wave state with a checkerboard pattern, is stabilized by attractive onsite interactions on a square lattice. Our new finding here is that a confining potential plays an invaluable role in stabilizing the supersolid state. We establish a rich phase diagram at low temperatures, which clearly shows how the insulator, the density wave and the superfluid compete with each other to produce an intriguing domain structure. Our results shed light on the possibility of the supersolid state in fermionic optical lattice systems.
We study the multiband superfluid (SF) and the SF to band-insulator transition of strongly interacting fermionic atoms in an optical lattice at a filling of two fermions per well. We present physical arguments to show that a consistent mean-field description of this problem is obtained by retaining only intraband pairing between the fermions. Using this approach we obtain a reasonable account of the experimentally observed critical lattice depth for the SF-BI transition and the modulated components of the condensate density and make predictions for the lattice depth dependence of the quasiparticle gap which can be tested in future experiments. We also highlight some interesting features unique to cold atom superfluids within this intraband pairing approximation; for instance, the pair field is forced to be uniform in space and the Hartree field vanishes identically. These arise as a result of the fact that while the pairing interaction is cut off at the scale of the Debye frequency in conventional superconductors or at the lattice scale in tight-binding model Hamiltonians, such a cutoff is absent for cold Fermi gases.