Instability of Rotationally Tuned Dipolar Bose-Einstein Condensates

Abstract

The possibility of effectively inverting the sign of the dipole-dipole interaction, by fast rotation of the dipole polarization, is examined within a harmonically trapped dipolar Bose-Einstein condensate. Our analysis is based on the stationary states in the Thomas-Fermi limit, in the corotating frame, as well as direct numerical simulations in the Thomas-Fermi regime, explicitly accounting for the rotating polarization. The condensate is found to be inherently unstable due to the dynamical instability of collective modes. This ultimately prevents the realization of robust and long-lived rotationally tuned states. Our findings have major implications for experimentally accessing this regime.

Vortex lattice formation in dipolar Bose-Einstein condensates via rotation of the polarization

“…While Branch III terminates at Ω = ω ⊥ with α → −ω ⊥ as Ω → ω ⊥ , the overcritical Branch II persists for Ω → ∞. Though we do not consider this limit, it has been found that Branch II monotonically approaches the limit α → 0 − as Ω → ∞ [62]. In Fig.…”

“…In Ref. [62], it was found that a bifurcation exists in this system at a given rotation frequency Ω b , a quantity that is dependent on dd and γ, at which the number of stationary solutions increases from 1 to 3 [62]. When dd = 0, the bifurcation diagram is symmetric about the Ω axis and two additional symmetric branches emerge when Ω = ω ⊥ √ 2.…”

“…In this system the properties of Branch II (α < 0 ∀ Ω ∈ [Ω b , ∞)) have been studied in previous investigations into the effective rotational tuning of dd [62,64]. Instead we are interested in Branch I of the stationary solutions (α ≥ 0) in the presence of a spherically-symmetric trap, that is, γ = 1.…”

Rotational tuning of the dipole-dipole interaction in a Bose gas of magnetic atoms

“…We have examined the properties of the resulting state in dynamical simulations to verify the effects of the tuned interactions, and the ramp time scale needed to avoid excessively exciting collective modes. We observe the dynamic instabilities predicted by Prasad et al [12], and find that the kinetic energy provides a useful observable of the instability and allows us to identify an instability time. Our results indicate that the time scale of the instabilities is sensitive to the rotation frequency of the magnetic field, and we find that in many cases the instability can be delayed for sufficiently high frequency.…”

“…Initially E K displays oscillatory dynamics at a frequency comparable to the trap frequency, arising from the collective modes excited. However, at later times E K suddenly starts rapid growth, corresponding to the instability identified in [12] and marking where the condensate begins to heat. For the Ω/2π = 2 kHz case, corresponding to the same simulation shown in Fig.…”