Two-component spin mixtures

TL;DR

Two-component spin mixtures in ultracold bosonic gases offer a controlled platform for exploring coupled superfluidity, miscibility, and spin dynamics. The paper synthesizes ground-state properties, density and spin excitations, topological defects, and the impact of coherent coupling, supported by Sodium-based experiments that quantify static polarizability, spin-dipole modes, and Bogoliubov spectra. It draws deep connections to magnetic analogies, phase diagrams, and dissipationless spin transport, with implications for spintronics and cross-disciplinary physics. Through coherent coupling and non-equilibrium dynamics, the work highlights rich regimes including para- to ferromagnetic transitions and massive spin excitations, illustrating the versatility of spinor quantum fluids as quantum simulators.

Abstract

The high degree of control on ultracold gases allows us to precisely manipulate their internal state. When the gas is made of atoms in two different internal states, it can be considered as a two-component spin mixture. Below a critical temperature, the gas becomes a superfluid mixture, never realized before with any other platform, and therefore interesting to study per se, but it also constitutes a promising and versatile platform for applications in spintronic devices or to study phenomena belonging to very different fields, such as magnetism, high-energy physics or gravitation. Here, I will revisit ground-state properties and excitations of a binary bosonic superfluid, and then introduce a coherent coupling between the states and treat the global state of the atoms as a spin in the presence of a variable external field.

Figures (9)

• Figure 1: Pictorial illustration of the ground-state spatial arrangement of a two-component mixture in a flat-box trap in the miscible (left) or immiscible (right) case. In the first case, both components occupy the whole volume $V$, whereas in the second case $a$-particles (blue) are restricted in a volume $V_a$ and the remaining $V_b=V-V_a$ is occupied by $b$-particles.
• Figure 2: a) GPE simulation of the ground state spatial arrangement of a two-component balanced mixture in a harmonic potential. The shaded yellow and violet regions show the density of each component for different combinations of interaction constants. The black line is the total density. All simulations include a (realistic) tiny magnetic field gradient, added to break the left-right symmetry. b) Experimental observation of a miscible (left) or immiscible (right) Sodium spin mixture, after Stern-Gerlach vertical separation in time of flight.
• Figure 3: a) Center of mass displacement $d$ for different separation of the harmonic trap minima (2$x_0$) of the two components, showing the static dipole polarizability of the system $\mathcal{P}=d/2x_0$. b) Spin-dipole oscillations for different amplitudes $x_0$. Reprinted figure with permission from T. Bienaimé et al., Phys. Rev. A 94, 063652 (2016). Copyright (2016) by the American Physical Society.
• Figure 4: Relative position of the two BECs centers of mass (blue) and of the thermal components ones (red) below (a) and above (b) $T_c$, in the collisionless regime. c-d) Same as (a-b), but in the collisional regime. Reprinted figure with permission from E. Fava et al., Phys. Rev. Lett. 120, 170401 (2018). Copyright (2018) by the American Physical Society.
• Figure 5: a) Sketch of the elongated mixture in the optical trap with a modulation of the radial trap frequency. b-c) Total density (red) and relative density (blue) resulting by modulating at small ($\omega_1$) or higher ($\omega_2$) frequency the radial trapping confinement of the miscible Sodium spin mixture. d) Decay mechanism of an excitation of energy $E=\hbar \omega_M$ in the density (red) or spin (blue) channel. e-f) Comparison between predicted (lines) and measured (colorplot) Bogoljubov spectra for the density (red) and spin (blue) channels. The spin spectrum is also reported in the density one, where residual crosstalk is present. Reprinted figure with permission from R. Cominotti et al., Phys. Rev. Lett. 128, 210401 (2022). Copyright (2022) by the American Physical Society.