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    Nonadiabatic charge pumping across two superconductors connected through a normal metal region by periodically driven potentials
    ( 2020-08-26) Soori, Abhiram ; Sivakumar, M.
    Periodically driven systems exhibit resonance when the difference between an excited state energy and the ground state energy is an integer multiple of ℏ times the driving frequency. On the other hand, when a superconducting phase difference is maintained between two superconductors, subgap states appear which carry a Josephson current. A driven Josephson junction therefore opens up an interesting avenue where the excitations due to applied driving affect the current flowing from one superconductor to the other. Motivated by this, we study charge transport in a superconductor-normal metal-superconductor junction where oscillating potentials are applied to the normal metal region. We find that for small amplitudes of the oscillating potential, driving at one site reverses the direction of current at the superconducting phase differences when difference between the subgap eigenenergies of the undriven Hamiltonian is integer multiple of ℏ times the driving frequency. For larger amplitudes of oscillating potential, driving at one site exhibits richer features. We show that even when the two superconductors are maintained at same superconducting phase, a current can be driven by applying oscillating potentials to two sites in the normal metal differing by a phase. We find that when there is a nonzero Josephson current in the undriven system, the local peaks and valleys in current of the system driven with an amplitude of oscillating potential smaller than the superconducting gap indicates sharp excitations in the system. In the adiabatic limit, we find that charge transferred in one time period diverges as a powerlaw with pumping frequency when a Josephson current flows in the undriven system. Our calculations are exact and can be applied to finite systems. We discuss possible experimental setups where our predictions can be tested.
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    Bose-Einstein condensation and the Casimir effect for an ideal Bose gas confined between two slabs
    ( 2007-08-17) Biswas, Shyamal
    We study the Casimir effect for a 3D system of ideal Bose gas in a slab geometry with a Dirichlet boundary condition. We calculate the temperature (T) dependence of the Casimir force below and above the Bose-Einstein condensation temperature (Tc). At T ≤ Tc the Casimir force vanishes as . For T ≳ Tc it weakly depends on temperature. For T ≫ Tc it vanishes exponentially. At finite temperatures this force for thermalized photons in between two plates has a classical expression which is independent of . At finite temperatures the Casimir force for our system depends on . © 2007 IOP Publishing Ltd.
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    More accurate theory for Bose-Einstein condensation fraction
    ( 2008-03-03) Biswas, Shyamal
    Bose-Einstein statistics is derived in the thermodynamic limit when the ratio of system size to thermal de Broglie wavelength goes to infinity. However, according to the experimental setup of Bose-Einstein condensation of harmonically trapped Bose gas of alkali atoms, the ratio near the condensation temperature (To) is 30-50. And, at ultralow temperatures well below To, this ratio becomes comparable to 1. We argue that finite size as well as the ultralow temperature induces corrections to Bose-Einstein statistics. From the corrected statistics we plot condensation fraction versus temperature graph. This theoretical plot satisfies well with the experimental plot [A. Griesmaier et al., Phys. Rev. Lett. 94 (2005) 160401]. © 2007 Elsevier B.V. All rights reserved.
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    Finite temperature scaling theory for the collapse of Bose-Einstein condensate
    ( 2009-01-01) Biswas, Shyamal
    We show how to apply the scaling theory in an inhomogeneous system like harmonically trapped Bose condensate at finite temperature. We calculate the temperature dependence of the critical number of particles by a scaling theory within the Hartree-Fock approximation and find that there is a dramatic increase in the critical number of particles as the condensation point is approached. © 2009 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
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    Casimir force on an interacting Bose-Einstein condensate
    ( 2010-04-19) Biswas, Shyamal ; Bhattacharjee, J. K. ; Majumder, Dwipesh ; Saha, Kush ; Chakravarty, Nabajit
    We have presented an analytic theory for the Casimir force on a Bose-Einstein condensate which is confined between two parallel plates. We have considered Dirichlet boundary conditions for the condensate wavefunction as well as for the phonon field. We have shown that the condensate wavefunction (which obeys the Gross-Pitaevskii equation) is responsible for the mean field part of the Casimir force, which usually dominates over the quantum (fluctuations) part of the Casimir force. © 2010 IOP Publishing Ltd.