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We study the flow of a weakly-interacting Bose-Einstein condensate around an obstacle by numerical solution of the Gross-Pitaevskii equation. We observe vortex emission and the formation of bow waves leading to pressure drag. We compare the drag law with that of an ideal Bose gas, and show that interactions reduce the drag force. This reduction can be explained in terms of a 'collisional screening' of the obstacle.
Temperature dependent impedance spectroscopy enables the many contributions to the dielectric and resistive properties of condensed matter to be deconvoluted and characterized separately. We have achieved this for multiferroic epitaxial thin films of BiFeO 3 ͑BFO͒ and BiMnO 3 ͑BMO͒, key examples of materials with strong magnetoelectric coupling. We demonstrate that the true film capacitance of the epitaxial layers is similar to that of the electrode interface, making analysis of capacitance as a function of film thickness necessary to achieve deconvolution. We modeled non-Debye impedance response using Gaussian distributions of relaxation times and reveal that conventional resistivity measurements on multiferroic layers may be dominated by interface effects. Thermally activated charge transport models yielded activation energies of 0.60± 0.05 eV ͑BFO͒ and 0.25± 0.03 eV ͑BMO͒, which is consistent with conduction dominated by oxygen vacancies ͑BFO͒ and electron hopping ͑BMO͒. The intrinsic film dielectric constants were determined to be 320± 75 ͑BFO͒ and 450± 100 ͑BMO͒.
Above a critical velocity, the dominant mechanism of energy transfer between a moving object and a dilute Bose-Einstein condensate is vortex formation. In this paper, we discuss the critical velocity for vortex formation and the link between vortex shedding and drag in both homogeneous and inhomogeneous condensates. We find that at supersonic velocities sound radiation also contributes significantly to the drag force.
We simulate the motion of a massive object through a dilute Bose-Einstein condensate by numerical solution of the non-linear Schrödinger equation coupled to an equation of motion for the object. Under a constant applied force, the object accelerates up to a maximum velocity where a vortex ring is formed which slows the object down. If the applied force is less than a critical value, the object becomes trapped within the vortex core. We show that the motion can be described using the time-independent quantum states, and use these states to predict the conditions required for vortex scattering.Typeset using EURO-T E X
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