We study the Bose-Einstein condensate (BEC) with a fixed number of particles. On the basis of conventional statistical mechanics we introduce the, so called, Maxwell's demon ensemble, where only particle transfer (without energy exchange) is allowed. We show that this new ensemble can be used for the microcanonical description of the system. We apply our formalism to the case of BEC in a harmonic trap and give the analytic expressions for the ground state fluctuations. We compare these expressions with the exact numerical results obtained for relatively small condensates. [S0031-9007(97)03974-4]
Potential scattering of free-electron wave packets is considered in the framework of non-stationary quantum-mechanical theory. The general expression for the average angle of scattering is obtained. The traditional quantum-mechanical plane-wave approximation and classical results are shown to be incorporated in the results derived.
Potential scattering of free-electron wave packets is considered in the framework of the nonstationary quantum-mechanical theory. To characterize the efficiency of this process, the average angle of scattering is calculated in the lowest ͑first and second͒ orders of perturbation theory in the potential. In the approximation of a well-localized, narrow, and nonspreading wave packet, the first-order term is shown to agree with the classical angle of scattering, linear in the potential. This term is shown to vanish at a very large size of the initial wave packet ͑much larger than the size of a target͒. An analogy between the potential scattering of wave packets and electron-light ponderomotive scattering is discussed.
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