Interlayer Superfluidity in Bilayer Systems of Fermionic Polar Molecules

Abstract

We consider fermionic polar molecules in a bilayer geometry where they are oriented perpendicularly to the layers, which permits both low inelastic losses and superfluid pairing. The dipole-dipole interaction between molecules of different layers leads to the emergence of interlayer superfluids. The superfluid regimes range from BCS-like fermionic superfluidity with a high Tc to Bose-Einstein (quasi-)condensation of interlayer dimers, thus exhibiting a peculiar BCS-BEC crossover. We show that one can cover the entire crossover regime under current experimental conditions. PACS numbers: 03.75.Ss, Ultracold gases of dipolar particles attract great interest because the dipole-dipole interaction drastically changes the nature of quantum degenerate regimes compared to ordinary short-range interacting gases [1,2]. This has been demonstrated in experiments with Bosecondensed chromium atoms which have a magnetic moment of 6µ B equivalent to an electric dipole moment of 0.05 D [3][4][5]. The recent experiments on creating polar molecules in the ground ro-vibrational state [6,7] and cooling them towards quantum degeneracy [6] have made a breakthrough in the field. For such molecules polarized by an electric field the dipole-dipole interaction is several orders of magnitude larger than for atomic magnetic dipoles. This opens fascinating prospects for the observation of new quantum phases [1,2,[8][9][10][11][12]. The main obstacle is the decay of the system due to ultracold chemical reactions, such as KRb+KRb⇒K 2 +Rb 2 found in JILA experiments [13]. These reactions are expected to be suppressed by the intermolecular repulsion in 2D geometries where the molecules are oriented perpendicularly to the plane of their translational motion [14].In this Letter we consider fermionic polar molecules in a bilayer geometry where the dipoles are oriented perpendicularly to the layers (Fig. 1), which leads to low inelastic losses and allows for the possibility of superfluid pairing. The interaction between dipoles of different layers may lead to the emergence of an interlayer superfluid, that is a superfluid 2D gas where Cooper pairs are formed by fermionic molecules of different layers. We show that the interlayer dipole-dipole interaction provides a higher superfluid transition temperature than that for 2D spin-1/2 fermions with attractive short-range interaction.Interestingly, an increase in the interlayer dipole-dipole coupling leads to a novel BCS-BEC crossover resembling that studied for atomic fermions near a Feshbach resonance [15,16]. The reason is that two dipoles belonging to different layers can always form a bound state [17]. As long as the binding energy ǫ b is much smaller than the Fermi energy E F , or equivalently the size of the interlayer two-body bound state greatly exceeds the intermolecular spacing in the {x, y} plane, the ground state of the system is the BCS-paired interlayer superfluid. Once a reduction of the interlayer spacing λ or an increase of the molecular dipole moment d by an electric field make ǫ b >> E...

Collective modes, stability, and superfluid transition of a quasi-two-dimensional dipolar Fermi gas

“…Then we have T c ≈ 0.04 ε (0) F ≈ 4 nK, which is comparable to the values of T c that are to be expected in bilayer systems (see Refs. [14][15][16][48][49][50]). In order to solve Eqs.…”

“…With heteronuclear molecules, in particular, rotational degrees of freedom can be excited in a controlled way by applying external electric fields, and are associated with large electric dipole moments [4][5][6][7]. This possibility of inducing strong and anisotropic dipole-dipole interactions between the molecules opens fascinating prospects for the observation of various many-body effects and novel quantum phases [4,[6][7][8][9][10][11][12][13][14][15][16].…”

New frontiers for quantum gases of polar molecules

“…In bulk gases of bosonic magnetic atoms, the competition between contact and dipolar interactions has led to the observation of droplets, which are stabilized by quantum fluctuations [99,100]. For fermionic molecules, the dipolar interactions can contribute an energy comparable to the Fermi energy [40], and can give rise to topological superfluid phases [15,16] and superfluid pairing between layers of an optical lattice [17]. The field of ultracold polar molecules shows no signs of slowing down, and there should be many fruitful experiments in the next few years.…”

“…Second, and perhaps more importantly, the anisotropic and long-range nature of the interaction allows for the realization of novel quantum phases, especially when the spatial dimensions of the system can be varied with optical lattices. Some of these engineered systems may find relevance to outstanding problems in condensed-matter physics [13,14]; moreover, qualitatively new types of physics can emerge in the presence of long-range interactions [15,16,17,18,19,20,21]. As a specific example of a unique feature of long-range interactions, a spin-1/2 Hamiltonian can be encoded based on a pair of oppositeparity rotational states where dipolar interactions give rise to a direct spin exchange coupling between molecules [22,23].…”

Dipoles on a two-leg ladder

“…These problems can be overcome in low-dimensional geometries where the dipolar particles are confined to either twodimensional (2D) planes or one-dimensional (1D) tubes with and without the presence of lattice potentials. A number of interesting predictions have been made for the phases of system with dipolar interactions, including exotic superfluids [15][16][17][18][19] , Luttinger liquids [20][21][22][23][24][25] , Mott insulators 26,27 , interlayer pairing [28][29][30][31] , non-trivial quantum critical points 32,33 , modified confinement-induced resonances [34][35][36][37] , roton modes and stripe instabilities [38][39][40][41][42][43][44][45] , and crystallization [46][47][48][49][50][51][52][53][54] , as well as formation of chain complexes [55][56][57][58][59][60]…”