Observation of Two Sound Modes in a Binary Superfluid Gas

Abstract

We study the propagation of sound waves in a binary superfluid gas with two symmetric components. The binary superfluid is constituted using a Bose-Einstein condensate of 23 Na in an equal mixture of two hyperfine ground states. Sound waves are excited in the condensate by applying a local spin-dependent perturbation with a focused laser beam. We identify two distinct sound modes, referred to as density sound and spin sound, where the densities of the two spin components oscillate in phase and out of phase, respectively. The observed sound propagation is explained well by the two-fluid hydrodynamics of the binary superfluid. The ratio of the two sound velocities is precisely measured with no need for absolute density calibration, and we find it in quantitatively good agreement with known interaction properties of the binary system.

Crossover from weak to strong quench in a spinor Bose-Einstein condensate

“…tally verified in our recent measurement of the speed of spin sound [21]. Calculating the quench strength as q = q f /(c 2 n), the range of q in our experiment is given by 0.12 < q < 4.4, covering from the weak to strong regime.…”

“…In our experiment, the value of q f /h is controlled within the range of 2.6 to 82 Hz. The spin interaction energy is c 2 n 0 = h × 30.7 Hz for the peak atomic density n 0 of the condensate [19][20][21][22] 1 . Because the sample thickness R z is smaller than the spin healing length ξ s,0 = / √ 2mc 2 n 0 ≈ 2.7 µm, the spin dynamics in the highly oblate sample is effectively 2D, and the magnon dispersion for the 2D spin dynamics is determined by the effective density n = 2 3 n 0 which is obtained by averaging the parabolic TF density profile along the tightly confining axial direction under the assumption of hydrodynamic equilibrium [23,24].…”

“…From [19], as = 1.88 a 0 with a 0 being the Bohr radius, which was experimentally confirmed in Refs. [20,21]. Note that in our previous studies [5,7], we used a value of as = 0.823a 0 from [22].…”