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arXiv:1110.2336 · Physical Review A

Polar molecules in bilayers with high population imbalance

Michael Klawunn, Alessio Recati

Published Jul 26, 2013• 6 citations
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Abstract

We investigate a dilute Fermi gas of polar molecules confined into a bilayer setup with dipole moments polarized perpendicular to the layers. In particular, we consider the extreme case of population imbalance, where we have only one particle in one layer and many particles in the other one. The single molecule is attracted by the dilute Fermi gas through the interlayer dipole-dipole force, presenting an interesting impurity problem with long-range anisotropic interaction. We characterize the quasiparticle, calculating its chemical potential with a Brueckner-Hartree-Fock approach. Moreover, we determine the momentum relaxation rate for finite momentum and temperature. The latter is reminiscent of Coulomb drag in bilayer electronic systems. For a confined system we relate the results for the chemical potential with the measurement of the oscillation frequency of the impurity. The momentum relaxation rate provides instead an estimate on how quickly the oscillations are damped.

DOI
10.1103/physreva.88.013633

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We investigate a dilute Fermi gas of polar molecules confined into a bilayer setup with dipole moments polarized perpendicular to the layers. In particular, we consider the extreme case of population imbalance, where we have only one particle in one layer and many particles in the other one. The single molecule is attracted by the dilute Fermi gas through the interlayer dipole-dipole force, presenting an interesting impurity problem with long-range anisotropic interaction. We characterize the quasiparticle, calculating its chemical potential with a Brueckner-Hartree-Fock approach. Moreover, we determine the momentum relaxation rate for finite momentum and temperature. The latter is reminiscent of Coulomb drag in bilayer electronic systems. For a confined system we relate the results for the chemical potential with the measurement of the oscillation frequency of the impurity. The momentum relaxation rate provides instead an estimate on how quickly the oscillations are damped.
We study the Fermi polaron problem with dipolar fermions in a bilayer geometry, where a single dipolar particle in one layer interacts with a Fermi sea of dipolar fermions in the other layer. By evaluating the polaron spectrum, we obtain the appearance of a series of attractive branches when the distance between the layers diminishes. We relate these to the appearance of a series of bound two-dipole states when the interlayer dipolar interaction strength increases. By inspecting the orbital angular-momentum component of the polaron branches, we observe an interchange of orbital character when system parameters such as the gas density or the interlayer distance are varied. Furthermore, we study the possibility that the lowest energy two-body bound state spontaneously acquires a finite center-of-mass momentum when the density of fermions exceeds a critical value, and we determine the dominating orbital angular momenta that characterize the pairing. Finally, we propose to use the tunneling rate from and into an auxiliary layer as an experimental probe of the impurity spectral function.
We consider two-dimensional spin-polarized dipolar Fermi gases confined in a double-layer system and calculate the momentum transfer between the layers as a function of temperature to investigate the transport properties of the system. We use the Hubbard approximation to describe the correlation effects and the screening between the dipoles within a single layer. The effective interlayer interaction between the dipoles across the layers is obtained by the random-phase approximation. We calculate the interaction strength and the layer separation distance dependence of the drag rate, and we show that there is a critical distance below which the system is unstable. In addition, we calculate the typical behavior of the collective modes related to the density fluctuations.
We study the BCS superfluid transition in a single-component fermionic gas of dipolar particles loaded in a tight bilayer trap, with the electric dipole moments polarized perpendicular to the layers. Based on the detailed analysis of the interlayer scattering, we calculate the critical temperature of the interlayer superfluid pairing transition when the layer separation is both smaller (dilute regime) and on the order or larger (dense regime) than the mean interparticle separation in each layer. Our calculations go beyond the standard BCS approach and include the many-body contributions resulting in the mass renormalization, as well as additional contributions to the pairing interaction. We find that the many-body effects have a pronounced effect on the critical temperature and can either decrease (in the very dilute limit) or increase (in the dense and moderately dilute limits) the transition temperature as compared to the BCS approach.
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-Bose-Einstein condensation crossover. We show that one can cover the entire crossover regime under current experimental conditions.
We consider two parallel layers of two-dimensional spin-polarized dipolar Fermi gas without any tunneling between the layers. The effective interactions describing screening and correlation effects between the dipoles in a single layer (intra-layer) and across the layers (interlayer) are modeled within the Hubbard approximation. We calculate the rate of momentum transfer between the layers when the gas in one layer has a steady flow. The momentum transfer induces a steady flow in the second layer which is assumed initially at rest. This is the drag effect familiar from double-layer semiconductor and graphene structures. Our calculations show that the momentum relaxation time has temperature dependence similar to that in layers with charged particles which we think is related to the contributions from the collective modes of the system.
The observation of significant dipolar effects in gases of ultracold polar molecules typically demands a strong external electric field to polarize the molecules. We show that, even in the absence of a significant polarization, dipolar effects may play a crucial role in the physics of polar molecules in bilayers, provided that the molecules in each layer are initially prepared in a different rotational state. Then, interlayer dipolar interactions result in a nonlocal swap of the rotational state between molecules in different layers, even for weak applied electric fields. The interlayer scattering due to the dipole-dipole interaction leads to a nontrivial dependence of the swapping rate on density, temperature, interlayer spacing, and population imbalance. For reactive molecules such as KRb, chemical recombination immediately follows a nonlocal swap and dominates the losses even for temperatures well above quantum degeneracy, and hence could be observed under current experimental conditions.