Abstract
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 solid-state 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 far-from-equilibrium fully polarized state, magnetic interfaces rapidly form. At the interfaces, we observe the formation of short-wavelength 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 far-from-equilibrium spin dynamics in regimes that are not easily accessible in solid-state systems.
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Data availability
Source data are provided with this paper. The data that support the findings of this study are available from the corresponding authors upon reasonable request.
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Acknowledgements
We thank F. Dalfovo for his critical reading of the manuscript and M. Oberthaler, G. Consolo, D. Go, E. Mendive-Tapia and N. Pavloff for fruitful discussions. We acknowledge funding from INFN through the FISH project, from the European Union’s Horizon 2020 Programme through the NAQUAS project of QuantERA ERA-NET Cofund in Quantum Technologies (grant agreement no. 731473) and from Provincia Autonoma di Trento. We thank the BEC Center, the Q@TN initiative and QuTiP.
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A. Recati and G.F. conceived the project. A.F. performed the experiment. A.F. and A.Z. analysed the experimental data. D.T. set up the experiment control. A.G., A. Recati and A. Roy developed the theory and performed the corresponding numerical simulations. A. Recati, A.Z. and G.L. wrote the manuscript. All the authors contributed to the discussion and interpretation of results.
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Extended data
Extended Data Fig. 1 Comparison between spin hydrodynamic (SH) and LLE simulation.
Total density relative modulation (a), modulus of the total current (b), magnetization (c) and modulus of the spin current (d) simulated according to the SH around the interface before and after its breaking. The magnetization resulting from LLE (e) shows a very good agreement, accordingly to the negligible role of the density modulation in the dynamics. f, Contribution of the spin current term related to the quantum torque. Also in this case, panel f shows that the second term in Eq. (2) dominates over the first one, hence the quantum torque drives the dynamics. The colorscale units of plots b,d,f is atoms/ms.
Source data
Source Data Fig. 2
Source data: {x,y,z} for Fig. 2c,d and {x,y} for Fig.2a,b,e,g and the insets.
Source Data Fig. 3
Source data: {x,y,z} for Fig. 3a,b and {x,y} for Fig. 3c–h.
Source Data Fig. 4
Source data: {x,y,z} for the insets and {x,y,dy} for Fig. 4a,b.
Source Data Extended Data Fig. 1
Source data: {x,y,z} for Extended Data Fig. 1.
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Farolfi, A., Zenesini, A., Trypogeorgos, D. et al. Quantum-torque-induced breaking of magnetic interfaces in ultracold gases. Nat. Phys. 17, 1359–1363 (2021). https://doi.org/10.1038/s41567-021-01369-y
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DOI: https://doi.org/10.1038/s41567-021-01369-y
