The Einstein-de Haas effect is a phenomenon in which angular momentum is transferred from microscopic spins to mechanical rotation of a macroscopic rigid body. We report an observation of the Einstein-de Haas effect in a spinor-dipolar Bose-Einstein condensate, in which the intrinsic magnetic dipole-dipole interaction mediates coherent transfer of angular momentum from atomic spins to collective circulation of a quantum fluid. The depolarized spinor components displayed ring-shaped density distributions that were confirmed as quantized vortices through matter-wave interferometry, revealing a coherent conversion between spin and orbital angular momentum. This observation opens a pathway to exploring ground-state phases with broken chiral symmetry, spin textures, and mass circulation, as well as the Barnett effect in dipolar quantum gases.
Motivated by the recent experimental realization of a Bose-Einstein condensate (BEC) of europium atoms, we investigate the self-bound droplet state for a dipolar BEC with spin degrees of freedom. Under a sufficiently weak magnetic field, the droplet has a torus shape with circulating spin vectors, which is referred to as a magnetic vortex. The ground state transforms from a torus to a cigar shape through bistability with increasing magnetic field. Dynamical change of the magnetic field causes the torus to rotate due to the Einstein–de Haas effect. The magnetic vortices form a supersolid in a confined system. Published by the American Physical Society 2024
Creation of stable intrinsically anisotropic self-bound states with embedded vorticity is a challenging issue. Previously, no such states in Bose–Einstein condensates (BECs) or other physical settings were known. Dipolar BEC suggests a unique possibility to predict stable two dimensional anisotropic vortex quantum droplets (2D-AVQDs). We demonstrate that they can be created with the vortex axis oriented perpendicular to the polarization of dipoles. The stability area and characteristics of the 2D-AVQDs in the parameter space are revealed by means of analytical and numerical methods. Further, the rotation of the polarizing magnetic field is considered, and the largest angular velocities, up to which spinning 2D-AVQDs can follow the rotation in clockwise and anti-clockwise directions, are found. Collisions between moving 2D-AVQDs are studied too, demonstrating formation of bound states with a vortex-antivortex-vortex structure. A stability domain for such stationary bound states is identified. Unstable dipolar states, that can be readily implemented by means of phase imprinting, quickly transform into robust 2D-AVQDs, which suggests a straightforward possibility for the creation of these states in the experiment.
We propose an experiment which proves the possibility of spinning gaseous media via dipolar interactions in the spirit of the famous Einstein-de Haas effect for ferromagnets. The main idea is to utilize resonances that we find in spinor condensates of alkali atoms while these systems are placed in an oscillating magnetic field. A significant transfer of angular momentum from spin to motional degrees of freedom observed on resonance is a spectacular manifestation of dipolar effects in spinor condensates.
The interacting Bose-Einstein condensates are mechanically stabilized by an external confinement such as magnetic, or optical traps. When the confinement is removed, they exhibit an expansion or collapse behavior depending on the interaction being repulsive, or attractive, respectively. The Gross-Pitaevskii mean-field theories are generally sufficient to provide a qualitative understanding of the condensate equilibrium properties and dynamics, therefore, beyond mean-field fluctuations are considered as minor quantitative corrections. The quantum droplet phases are self-stabilized phases of the condensate, in which the attractive mean-field interaction is stabilized by the repulsive Lee-Huang-Yang fluctuations without any external trap. The droplets are studied theoretically for binary Bose mixture and dipolar condensates, and both are recently realized in the ultracold atomic gas experiments. Here we propose the spin-1 condensate as another candidate to exhibit a quantum droplet phase. We argue that the spin-1 condensates in the polar and anti-ferromagnetic order parameters can exhibit a similar self-stabilizing mechanism. We also realize an analogy between the polarized spin-1 and Rabi-coupled binary mixture condensates. The Rabi frequency
ωR and quadratic Zeeman energy
q can tune the Lee-Huang-Yang energy for the Rabi-coupled mixture and spin-1 gas, respectively. Furthermore, the detuning
δ and linear Zeeman energy
p are external parameters to tune the polarization in Rabi-coupled mixture, and spin-1 gas, respectively, which can significantly change the mean-field energy. We create the phase diagram for the mechanical stability of the Rabi-coupled mixture and spin-1 gas on the parameter planes of
ωR−δ and
q−p, respectively. We finally address how the Bose mixture quantum droplets respond to the rotation. We find that the strongly confined quantum droplets exhibit triangular vortex lattices under rapid rotation. The properties of the vortex lattices differ from the repulsive condensates in three points. First, the change in the droplet size is only fractional at the rapid rotation limit, unlike the diverging radius of the repulsive condensate. This property can lead the way to realizing the lowest Landau level states and strongly correlated phases in the rotating ultracold gases. Secondly, the change in the core size throughout the vortex lattice is more tractable due to the finite density at the edge of the condensate. Finally, the vortex lattices are slightly distorted, which agrees with the expected vortex density profile due to the nonuniform superfluid density. In the rapid rotation limit, the droplet approaches the flat-top density profile, in which the vortex lattice is closest to a perfect periodicity.
We show that an analogue to the classical Einstein-de Haas effect can appear in ultracold dipolar Fermi gases. The anisotropic nature of dipole-dipole interactions can lead to a transfer of magnetization into orbital angular momentum. Remarkably, distinct from a Bose-Einstein condensate, this transfer is accompanied by twisting motion, where individual spin components rotate in opposite directions with larger orbital angular momenta than the full system, possibly leading to easier experimental observation of the effect. This feature is induced by the deformation of the Fermi surface and the direction of the twisting motion can be controlled by an s-wave scattering length or external magnetic field, possibly providing a method of measuring scattering lengths of strongly dipolar atomic species.