[Paper Review] Two-component spin mixtures

Giacomo Lamporesi|arXiv (Cornell University)|Apr 7, 2023

Cold Atom Physics and Bose-Einstein Condensates4 citations

TL;DR

This paper investigates two-component spin mixtures of ultracold sodium atoms, focusing on ground-state properties, collective excitations, and coherent coupling between spin states. Using the Gross-Pitaevskii and Bogoljubov frameworks, it demonstrates that coherent coupling induces spin dynamics and Faraday spin waves, enabling the observation of dissipationless spin currents and interface-driven spin wave generation in inhomogeneous systems.

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.

Motivation & Objective

• To understand the ground-state behavior and collective excitations of binary bosonic superfluids in two-component spin mixtures.
• To investigate the effects of coherent coupling between internal spin states using a variable external field.
• To explore the emergence of spin currents and interface-driven spin wave generation in inhomogeneous, miscible superfluid mixtures.
• To establish connections between ultracold atomic systems and phenomena in high-energy physics and cosmology, such as quark confinement and ergoregion physics.
• To provide a theoretical and experimental platform for studying dissipationless magnetic heterostructures and para-ferromagnetic phase transitions.

Proposed method

• Formulates the system using the Gross-Pitaevskii equation (GPE) for two-component Bose-Einstein condensates with contact interactions.
• Applies the Thomas-Fermi approximation to derive the density profile in harmonic traps, assuming $ N \gg 1 $ and negligible kinetic energy.
• Uses Bogoljubov theory to describe elementary excitations, including phonon-like modes and spin waves.
• Introduces a coherent coupling via two-photon Rabi coupling between spin states, modeled as a spin-orbit-like term in the Hamiltonian.
• Analyzes spin dynamics using the Landau-Lifshitz equations in the far-from-equilibrium regime, particularly at interfaces between magnetically distinct regions.
• Validates theoretical predictions with GPE simulations and experimental data from Faraday spin wave measurements.

Experimental results

Research questions

• RQ1 How do ground-state properties and collective excitations of a two-component bosonic superfluid change under coherent coupling between spin states?
• RQ2 What are the conditions under which spin currents and spin wave generation emerge in inhomogeneous, miscible superfluid mixtures?
• RQ3 How does the interplay between local interactions and external coupling fields lead to the formation of magnetic heterostructures in ultracold atomic systems?
• RQ4 In what ways can coherent spin mixtures serve as analogs for high-energy physics phenomena such as quark confinement or ergoregion effects?
• RQ5 What role does the spin current play in the non-equilibrium dynamics of extended superfluid systems with spatially varying coupling?

Key findings

• In the presence of coherent coupling, the system exhibits Rabi oscillations in regions where the coupling field dominates over interactions, while remaining polarized in high-density central regions.
• Spin wave generation is observed at interfaces between para- and ferromagnetic-like domains, driven by strong magnetization gradients and spin currents.
• Faraday spin waves are experimentally measured and reproduced via GPE simulations, confirming the predicted Bogoljubov spectrum under coherent coupling.
• The spin dynamics in extended systems are well described by the dissipationless Landau-Lifshitz equations due to the large separation between spin and density energy scales.
• The system supports a dissipationless 1D magnetic heterostructure with alternating para- and ferromagnetic-like phases, enabling the study of topological spin textures.
• Theoretical analysis predicts confinement of half-quantum vortices in coherently coupled mixtures, with strong analogies to quark confinement in quantum chromodynamics.