arXiv:0805.3031 · Physical Review A

Proposed realization of itinerant ferromagnetism in optical lattices

Shizhong Zhang, Hsiang-Hsuan Hung, Congjun Wu

Published Nov 17, 2010• 85 citations

Abstract

We propose a scheme to realize the itinerant ferromagnetism of two-component cold fermionic atoms in the $p$-orbital bands in optical lattices. The band flatness in the two-dimensional honeycomb lattice dramatically amplifies the interaction effect, driving the ferromagnetic transition even with a relatively weak repulsive interaction. This scheme has the advantage that the stability of the system can be maintained without causing decay to the molecular state as one approaches the Feshbach resonance from the side with the positive scattering length. Experimental signatures and detections are also discussed.

DOI: 10.1103/physreva.82.053618

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arXiv:0805.3031 · Physical Review A

Proposed realization of itinerant ferromagnetism in optical lattices

Shizhong Zhang, Hsiang-Hsuan Hung, Congjun Wu

Published Nov 17, 2010• 85 citations

arXiv HTML PDF

Abstract

We propose a scheme to realize the itinerant ferromagnetism of two-component cold fermionic atoms in the

p

-orbital bands in optical lattices. The band flatness in the two-dimensional honeycomb lattice dramatically amplifies the interaction effect, driving the ferromagnetic transition even with a relatively weak repulsive interaction. This scheme has the advantage that the stability of the system can be maintained without causing decay to the molecular state as one approaches the Feshbach resonance from the side with the positive scattering length. Experimental signatures and detections are also discussed.

DOI: 10.1103/physreva.82.053618

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Abstract. We report on the realization of a strongly interacting quantum degenerate gas of fermionic atoms in a three-dimensional optical lattice. We prepare a bandinsulating state for a two-component Fermi gas with one atom per spin state per lattice site. Using a Feshbach resonance, we induce strong interactions between the atoms. When sweeping the magnetic field from the repulsive side towards the attractive side of the Feshbach resonance we induce a coupling between Bloch bands leading to a transfer of atoms from the lowest band into higher bands. Sweeping the magnetic field across the Feshbach resonance from the attractive towards the repulsive side leads to two-particle bound states and ultimately to the formation of molecules. From the fraction of formed molecules we determine the temperature of the atoms in the lattice.Abstract. We report on the realization of a strongly interacting quantum degenerate gas of fermionic atoms in a three-dimensional optical lattice. We prepare a bandinsulating state for a two-component Fermi gas with one atom per spin state per lattice site. Using a Feshbach resonance, we induce strong interactions between the atoms. When sweeping the magnetic field from the repulsive side towards the attractive side of the Feshbach resonance we induce a coupling between Bloch bands leading to a transfer of atoms from the lowest band into higher bands. Sweeping the magnetic field across the Feshbach resonance from the attractive towards the repulsive side leads to two-particle bound states and ultimately to the formation of molecules. From the fraction of formed molecules we determine the temperature of the atoms in the lattice.

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We propose a scheme to realize the itinerant ferromagnetism of two-component cold fermionic atoms in the

p

We study the ground-state properties of the interacting spinless fermions in the

{p}_{x,y}

-orbital bands in the two-dimensional honeycomb optical lattice, which exhibit different features from those in the

{p}_{z}

-orbital system of graphene. In addition to two dispersive bands with Dirac cones, the tight-binding band structure exhibits another two completely flat bands over the entire Brillouin zone. With the realistic sinusoidal optical potential, the flat bands acquire a finite but much smaller bandwidth compared to the dispersive bands. The band flatness dramatically enhanced interaction effects giving rise to various charge and bond ordered states at commensurate fillings of

n=\frac{i}{6}(i=1--6)

. At

n=\frac{1}{6}

, the many-body ground states can be exactly solved as the close-packed hexagon states which can be stabilized even in the weakly interacting regime. The dimerization of bonding strength occurs at both

n=\frac{1}{2}

and

\frac{5}{6}

, and the latter case is accompanied with the charge-density wave of holes. The trimerization of bonding strength and charge inhomogeneity appear at

n=\frac{1}{3},\frac{2}{3}

. These crystalline orders exhibit themselves in the noise correlations of the time-of-flight spectra.

We carry out ab initio study of a ground state phase diagram of spin-

1∕2

cold fermionic atoms within a twofold degenerate

p

band of an anisotropic optical lattice. Using the Gutzwiller variational approach, we show that a robust ferromagnetic phase exists for a vast range of band fillings and interacting strengths. The ground state crosses over from a spin density wave state to a spin-1 Neel state at half filling. An additional harmonic trap will induce spatial separation of various phases. We also discuss several relevant observable consequences and detection methods. An experimental test of the results reported here may shed some light on the long-standing issue of itinerant ferromagnetism.

f

-wave pairing superfluidity of spinless fermions in

{p}_{x,y}

-orbital bands of two-dimensional honeycomb optical lattices. The nontrivial orbital band structure, rather than strong correlation effects, gives rise to the unconventional pairing with the nodal lines of the

f

-wave symmetry. With a confining harmonic trap, zero-energy Andreev bound states appear around the circular boundary with a sixfold symmetry. The experimental realization and detection of this novel pairing state are feasible.