
Quantumtorqueinduced breaking of magnetic domain walls in ultracold gases
A rich variety of physical effects in spin dynamics arise at the interface between different magnetic materials1. Engineered systems with interlaced magnetic structures have been used to implement spin transistors, memories and other spintronic devices2,3. However, experiments in solidstate systems can be difficult to interpret because of disorder and losses. Here we realize analogues of magnetic junctions using a coherently coupled mixture of ultracold bosonic gases. The spatial inhomogeneity of the atomic gas makes the system change its behaviour from regions with oscillating magnetization—resembling a magnetic material in the presence of an external transverse field—to regions with a defined magnetization, similar to magnetic materials with ferromagnetic anisotropy stronger than external fields. Starting from a farfromequilibrium fully polarized state, magnetic interfaces rapidly form. At the interfaces, we observe the formation of shortwavelength magnetic waves. They are generated by a quantum torque contribution to the spin current and produce strong spatial anticorrelations in the magnetization. Our results establish ultracold gases as a platform for the study of farfromequilibrium spin dynamics in regimes that are not easily accessible in solidstate systems.
A. Farolfi, A. Zenesini, D. Trypogeorgos, C. Mordini, A. Gallemí, A. Roy, A. Recati, G. Lamporesi and G. Ferrari, Nature Phys. 17, 1359 (2021)


Linear Response Study of Collisionless Spin Drag
In this work we are concerned with the understanding of the collisionless drag or entrainment between two superfluids, also called AndreevBashkin effect, in terms of current response functions. The drag density is shown to be proportional to the cross transverse currentcurrent response function, playing the role of a normal component for the single species superfluid density. We can in this way link the existence of finite entrainment with the exhaustion of the energyweighted sum rule in the spin channel. The formalism is then used to reproduce some known results for a weakly interacting BoseBose mixture. Finally we include the drag effect to determine the beyond meanfield correction on the speed of sound and on the spin dipole excitations for a homogeneous and trapped gas, respectively.
D. Romito, C. Lobo, A. Recati, Phys. Rev. Research 3, 023196 (2021)


Collisionless drag for a onedimensional twocomponent BoseHubbard model
We theoretically investigate the elusive AndreevBashkin collisionless drag for a twocomponent onedimensional BoseHubbard model on a ring. By means of tensor network algorithms, we calculate the superfluid stiffness matrix as a function of intra and interspecies interactions and of the lattice filling. We then focus on the most promising region close to the socalled pairsuperfluid phase, where we observe that the drag can become comparable with the total superfluid density. We elucidate the importance of the drag in determining the longrange behavior of the correlation functions and the spin speed of sound. In this way, we are able to provide an expression for the spin Luttinger parameter K_S in terms of drag and the spin susceptibility. Our results are promising in view of implementing the system by using ultracold Bose mixtures trapped in deep optical lattices, where the size of the sample is of the same order of the number of particles we simulate. Importantly, the mesoscopicity of the system, far from being detrimental, appears to favor a large drag, avoiding the BerezinskiiKosterlitzThouless jump at the transition to the pairsuperfluid phase which would reduce the region where a large drag can be observed.
Daniele Contessi, Donato Romito, Matteo Rizzi, Alessio Recati,
Phys. Rev. Research 3, L022017 (2021)


Quantized vortices in dipolar supersolid BoseEinstein condensed gases
We have characterized quantized vortices in a dipolar BoseEinstein condensed, focusing on the supersolid regime. We have found in particular that (i) the angular momentum per particle associated with the vortex line is smaller than ℏ, reflecting the reduction of the global superfluidity; (ii) the nucleation in a rotating trap is triggered  as for a standard condensate  by the softening of the quadrupole mode; (iii) many vortices can be arranged into a honeycomb structure, which coexists with the triangular geometry of the supersolid lattice and persists during the free expansion of the atomic cloud.
A. Gallemì, S. M. Roccuzzo, S. Stringari, A. Recati, Phys. Rev. A 102, 023322 (2020)


A quantum Gutzwiller approach for the BoseHubbard model
We have developed the equivalent of Bogoliubov theory (for weakly interacting Bose gases) for the BoseHubbard model, but starting from the Gutzwiller ansatz.
We find that the approach is extremely well suited to calculate the correlations of the systems. The model is benchmarked against the available QMC result.
The approach is very promising in order to calculate dynamical properties of the BH model.
F. Caleffi, M. Capone, C. Menotti, I. Carusotto, A. Recati, Phys. Rev. Research 2, 033276 (2020)


Rotating a Supersolid
A supersolid shows both solid and superfluid properties. By rotating
it you can clearly see that the moment of inertia is strongly reduced due to the superfluid irrotational flow. Even when the global superfluid behaviour diseapper the moment of inertia show reduction due to the single droplet (local) superfluidity.
S. M. Roccuzzo, A. Gallemì, A. Recati, S. Stringari, Phys. Rev. Lett. 124, 045702 (2020)
