Immiscible binary spin mixtures in 2D Bose-Einstein condensates*

Immiscible Bose-Einstein condensates feature spin domains separated by nearly 1D domain walls; this offers a wide range of opportunities for studying nonlinear, thermodynamic, and out-of-equilibrium physics. Experimentally, we create a binary mixture of two hyperfine states in a quasi-2D ²³Na Bose-Einstein condensate confined in a 2D box potential. Controlled by a magnetic field gradient, colorful dynamics on the domain walls--the ripplon excitations in the stable configuration and the Rayleigh-Taylor instability in the unstable configuration--are observed and reported in [1]. The formation and relaxation of the domains offer insights into the spin turbulence [2]. The presence of "soft" interface modes also opens avenues to ultra-low temperature thermometry. Stabilizing unstable interface modes under a parametric drive would also serve as a parallel to what is observed in classical fluids [3].

[1] Y. Geng, J. Tao, M. Zhao, S. Mukherjee, S. Eckel, G. K. Campbell, and I. B. Spielman, The Rayleigh-Taylor Instability in a Binary Quantum Fluid, arXiv:2411.19807 [cond-mat.quant-gas].

2] T. Kadokura and H. Saito, Kolmogorov-Hinze Scales in Turbulent Superfluids, Phys. Rev. Lett. 133, (2024).

[3] B. Apffel, F. Novkoski, A. Eddi, and E. Fort, Floating under a levitating liquid, Nature 585, 48 (2020).