Anomalous enhancement of quasiparticle current near a potential barrier in a Bose-Einstein condensate

Abstract

We investigate tunneling properties of Bogoliubov phonons in a Bose-Einstein condensate. We find the anomalous enhancement of the quasiparticle current Jq carried by Bogoliubov phonons near a potential barrier, due to the supply of the excess current from the condensate. This effect leads to the increase of quasiparticle transmission probability in the low energy region found by Kovrizhin et al.. We also show that the quasiparticle current twists the phase of the condensate wavefunction across the barrier, leading to a finite Josephson supercurrent Js through the barrier. This induced supercurrent flows in the opposite direction to the quasiparticle current so as to cancel out the enhancement of Jq and conserve the total current J = Jq + Js.PACS numbers: 03.75. Kk,03.75.Lm,67.85.De Phonon in a superfluid system is an example of Goldstone mode which appears in various fields of physics associated with spontaneous broken symmetry [1]. It is a manifestation of broken (global) U (1) symmetry which underlies the macroscopic quantum nature of the system, and a key to understand the low energy properties of superfluids. In particular, in a Bose superfluid, it plays fundamental roles for the superfluidity [2]. Phonon has been observed in various systems such as superfluid 4 He [3], superconducting films [4], atomic superfluid fermi gases [5], as well as Bose-Einstein condensation (BEC) of cold atomic gases [6]. Due to the high degree of controllability, BEC of cold atomic gases offers an opportunity to study novel properties of phonons in the superfluid phase.Bogoliubov phonon [7] in a BEC has been a longstanding issue of investigation in cold atomic gases [8].In the last few years, quantum tunneling of Bogoliubov phonon has attracted much attention [9,10,11,12,13,14]. Since Bogoliubov phonon is a collective excitation of a BEC, its tunneling property has specific features which are quite different from that of free particles. In fact, an anomalous tunneling property of Bogoliubov phonon has been predicted in [9,10]. It has been shown that the transmission probability of Bogoliubov phonon through a potential barrier increases at low energies and always unity in the zero-energy limit, irrespective of the height of the barrier [9,10]. Although several mechanisms were proposed in [10,11], the underlying physics of this anomalous tunneling has not been understood yet.In this paper, we report alternative anomalous tunneling properties of Bogoliubov phonons. We find that the quasiparticle current is not conserved, but greatly enhanced near the potential barrier at low incident energies, due to the excess current supplied from the condensate. This anomalous enhancement of the quasiparticle current increases the transmission probability of Bogoliubov phonon in the low energy region, which is consistent with the tunneling property in [9,10]. In addition, we show that the quasiparticle current twists the phase of the condensate wavefunction, leading to a Josephson supercurrent through the barrier. The excess part of the quasip...

Tunneling properties of Bogoliubov mode and spin-wave modes in supercurrent states of a spin-1 ferromagnetic spinor Bose-Einstein condensate

“…The perfect transmission of the transverse spin wave in the current-flowing state provides a much simpler example of anomalous tunneling compared to earlier cases. In the earlier cases (the Bogoliubov mode [1][2][3][5][6][7][8][9][10][11]13] and the transverse spin wave in the absence of supercurrent [12,[14][15][16]), the anomalous tunneling always occurs in the limit where the wave number k approaches zero. In the present case, on the other hand, anomalous tunneling occurs at a finite wave number.…”

“…Recently, an anomalous tunneling phenomenon has been extensively discussed in the field of Bose gas superfluids [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. In this phenomenon, the Bogoliubov mode tunnels through a barrier without reflection in the low-energy limit.…”

Transmission of excitations in a spin-1 Bose-Einstein condensate through a barrier

“…Since a potential barrier generally inhibits transmission of a particle and leads to total reflection in the low-momentum limit, this perfect transmission in the BEC, termed anomalous tunneling [2], has attracted much attention and has been studied extensively and intensively [2][3][4][5][6][7][8][9][10][11][12][13][14][15].…”

Tunneling, diffusion, and dissociation of Feshbach molecules in optical lattices

“…The tunneling, diffusion, and dissociation properties of composite particles in periodic lattices is a subject of increasing interest [14][15][16]. The combined quantum tunneling, interference and dissociation can be used as a tool to prepare and exploit the properties of the intrinsic molecular structure and the population of bound and continuum states through the controlled diffusion of molecules in periodic potentials.…”

“…In references [14][15][16], a treatment of molecular diffusion in one-dimensional optical lattices has been carried out. Instead of a "prepared" initial state, an exact bound state solution is obtained for the same Hamiltonian used in our work.…”