Breaking of Goldstone modes in a two-component Bose-Einstein condensate

Year: 2019

Authors: Recati A., Piazza F.

Autors Affiliation: Univ Trento, INO CNR BEC Ctr, I-38123 Povo, Italy; Univ Trento, Dipartimento Fis, I-38123 Povo, Italy; INFN, Trento Inst Fundamental Phys & Applicat, I-38123 Trento, Italy; Max Planck Inst Phys Komplexer Syst, Nothnitzer Str 38, D-01187 Dresden, Germany; Univ Innsbruck, Inst Theoret Phys, A-6020 Innsbruck, Austria.

Abstract: We study the decay rate Gamma(k) of density excitations of two-component Bose-Einstein condensates at zero temperature. Those excitations, where the two components oscillate in phase, include the Goldstone mode resulting from condensation. While within Bogoliubov approximation the density sector and the spin (out-ofphase) sector are independent, they couple at the three-phonon level. For a Bose-Bose mixture we find that the Belyaev decay is slightly modified due to the coupling with the gapless spin mode. At the phase separation point the decay rate changes instead from the standard k(5) to a k(5/2) behavior due to the parabolic nature of the spin mode. If instead a coherent coupling between the two components is present, the spin sector is gapped and, away from the ferromagnetic-like phase transition point, the decay of the density mode is not affected. On the other hand, at the transition point, when the spin fluctuations become critical, the Goldstone mode is not well defined anymore since Gamma(k) proportional to k. As a consequence, we show that the friction induced by a moving impurity is enhanced-a feature which could be experimentally tested. Our results apply to every nonlinear 2-component quantum hydrodynamic Hamiltonian which is time-reversal invariant and possesses an U(1) x Z(2) symmetry.

Journal/Review: PHYSICAL REVIEW B

Volume: 99 (6)      Pages from: 064505-1  to: 064505-6

More Information: We thank Franco Dalfovo, Markus Oberthaler, and Wilhelm Zwerger for useful discussions. A.R. acknowledges the hospitality of the Theoretical Nanophysics and of the Quantum Matter Groups at the LMU. F.P. has been supported by the Austrian Programme for Advanced Research and Technology (APART) of the Austrian Academy of Sciences.
KeyWords: EXCITATIONS; STATES
DOI: 10.1103/PhysRevB.99.064505

Citations: 12
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