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arXiv:cond-mat/0305290 · Low Temperature Physics

Drag of superfluid current in bilayer Bose systems

D. V. Fil, S. I. Shevchenko

Published Oct 1, 2004• 14 citations
arXivHTMLPDF

Abstract

An effect of nondissipative drag of a superfluid flow in a system of two Bose gases confined in two parallel quasi-two-dimensional traps is studied. Using an approach based on the introduction of density and phase operators, we compute the drag current at zero and finite temperatures for arbitrary ratio of particle densities in the adjacent layers. We demonstrate that in a system of two ring-shaped traps the “drag force” influences the drag trap in the same way as an external magnetic flux influences a superconducting ring. This allows one to use the drag effect to control persistent current states in superfluids and opens up the possibility of implementing a Bose analog of the superconducting Josephson flux qubit.

DOI
10.1063/1.1808194

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Low Temperature Physics•Jul 1, 1999•8 citations
An effect of nondissipative drag of a superfluid flow in a system of two Bose gases confined in two parallel quasi-two-dimensional traps is studied. Using an approach based on the introduction of density and phase operators, we compute the drag current at zero and finite temperatures for arbitrary ratio of particle densities in the adjacent layers. We demonstrate that in a system of two ring-shaped traps the “drag force” influences the drag trap in the same way as an external magnetic flux influences a superconducting ring. This allows one to use the drag effect to control persistent current states in superfluids and opens up the possibility of implementing a Bose analog of the superconducting Josephson flux qubit.
A microscopic theory of a nondissipative drag in a two-component superfluid Bose gas is developed. The expression for the drag current in the system with the components of different atomic masses, densities, and scattering lengths is derived. It is shown that the drag current is proportional to the square root of the gas parameter. The temperature dependence of the drag current is studied and it is shown that at temperature of order or smaller than the interaction energy the temperature reduction of the drag current is rather small. A possible way of measuring the drag factor is proposed. A toroidal system with the drag component confined in two half-ring wells separated by two Josephson barriers is considered. Under certain condition such a system can be treated as a Bose-Einstein counterpart of the Josephson charge qubit in an external magnetic field. It is shown that the measurement of the difference of number of atoms in two wells under a controlled evolution of the state of the qubit allows one to determine the drag factor.
A microscopic theory of superfluid drag in a two-component Bose gas is developed. The drag factor is shown to be proportional to the square root of the gas parameter. Basing on the similarity between the drag force and vector potential of magnetic field we propose how the superfluid drag can be used to detect a Berry phase in a Bose counterpart of the Josephson charge qubit. We consider a system in which the drag component, situated inside the drive component, is confined in two half-ring traps separated by two Josephson barriers. Under cyclic adiabatic change of Josephson coupling parameters and an asymmetry of two traps a Berry phase is generated. This phase can be observed though measurements of relative number of atoms in two traps. The Berry phase depends on the drag factor and its detection can be used for determining the value of the drag.
Superfluid properties of electron-hole pairs in a quantum Hall four-layer system are investigated. The system is considered as a solid state realization of a two-component superfluid Bose gas with dipole-dipole interaction. One superfluid component is formed in the top bilayer and the other component - in the bottom one. We obtain the dispersion equation for the collective mode spectrum and compute the critical parameters (the critical interlayer distance and the critical currents) versus the filling factor. We find that the critical currents of the components depend on each other. The maximum critical current of a given component can be reached if the current of the other component is equal to zero. The non-dissipative drag effect between the components is studied. It is shown that in the system considered the drag factor is very large. Under appropriate conditions it can be about 10 per sent, that is at least in three order larder than one predicted for two-component atomic Bose gases.
Two-component systems consisting of mutually interacting particles can demonstrate both intracomponent transport effects and intercomponent entrainment (or drag) effects. In the presence of superfluidity, the intracomponent transport is characterized by dissipative conductivity and superfluid weight in the framework of two-fluid model, and intercomponent entrainment gives rise to normal and nondissipative drag effects. We present unified treatment of all these effects for spatially homogeneous two-component atomic Bose-Einstein condensates based on the Bogoliubov theory, focusing specifically on the drag effects. Calculating finite-frequency intra- and intercomponent conductivities with taking into account quasiparticle damping, we derive and numerically check analytical Drude-like approximations applicable at low frequencies and Lorentz-like approximations applicable at higher frequencies in the vicinity of the resonant energy of spin-to-density Bogoliubov quasiparticle conversion. As possible physical realizations of two-component atomic systems, we consider three-dimensional Bose-Bose mixtures and closely spaced two-layered systems of magnetic dipolar atoms.