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Journal of the Physical Society of Japan

Spin Current of Fermions Induced in Spin–Orbit Coupled Bose–Fermi Mixture

Ryohei Sakamoto, Emiko Arahata, 弘之 森

Published Jun 26, 2018• 0 citations
Publisher

Abstract

The novel controllability of a spin current in cold atoms is studied. We show that a mixture of spin-1/2 fermions and spin–orbit (SO) coupled pseudospin-1/2 bosons at zero temperature can form a fermionic spin current, which is experimentally controllable with an external laser applied to the bosons to create the SO coupling. We derive the ground-state phase diagram of the bosons using a variational wave function and identify the magnetic structure of each phase. The spin current of the fermions with the background of the spiral/precessed magnetization of the bosons is calculated as a function of the wave vector and intensity of the external laser. We show that the spin current increases and then decreases, forming a peak, as the wave vector of the laser increases, which indicates the controllability of the spin current with the laser parameters.

DOI
10.7566/jpsj.87.074004

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The novel controllability of a spin current in cold atoms is studied. We show that a mixture of spin-1/2 fermions and spin–orbit (SO) coupled pseudospin-1/2 bosons at zero temperature can form a fermionic spin current, which is experimentally controllable with an external laser applied to the bosons to create the SO coupling. We derive the ground-state phase diagram of the bosons using a variational wave function and identify the magnetic structure of each phase. The spin current of the fermions with the background of the spiral/precessed magnetization of the bosons is calculated as a function of the wave vector and intensity of the external laser. We show that the spin current increases and then decreases, forming a peak, as the wave vector of the laser increases, which indicates the controllability of the spin current with the laser parameters.
Atomtronics, a technique to control atoms for an application of devices, has been studied in recent years.The purpose of this thesis is to propose a novel method to control atoms for the application to the atomtronics.To achieve this purpose, we use a spin current in a spin-orbit (SO) coupled cold atoms.The spin current, that is a counterflow of particles with different spins, has attracted attention as a method to control electrons.On the other hand, the artificial SO coupled cold atoms were experimentally realized in 2011 and have been studied theoretically and experimentally.We can expect that a spin current can be formed in the SO coupled cold atoms because the SO coupling is known to induce a spin current and controlled because the cold atoms have various experimentally controllable parameters.However, little has been studied so far on the spin current in the cold atom.Therefore, we studied novel controllability of the spin current in the SO coupled cold atom.An SO coupled Bose-Einstein condensate (BEC) was experimentally realized in 2011 and then an SO coupled degenerate fermions became available in 2012.In these systems, since the SO coupling was engineered in a neutral atom by dressing two atomic spin states with a pair of Raman lasers and an external magnetic field, it is called artificial SO coupling.In addition, some parameters such as intensity of the SO coupling can be experimentally tuned by the lasers and the external magnetic field in the artificial SO coupled cold atoms.Therefore, by investigating parameter dependence of the spin currents, we reveal controllability of the atoms in the artificial SO coupled Bose and Fermi systems.As a result, in the SO coupled Bose system, the spin current changes discontinuously and non-smoothly across the phase boundaries, while in the SO coupled Fermi system, the spin current changes continuously and smoothly at zero temperature.On the other hand, Bose-Fermi mixture was experimentally realized and have been studied from different viewpoints.Therefore, we can expect to realize an artificial SO coupled Bose-Fermi mixture in near future.In this system, since atoms of different quantum statistics coexist under an influence of the SO coupling, novel phenomena can be expected.Based on these ideas, we investigate a ground state and a spin current in a mixture of fermions and SO coupled bosons.The ground states are calculated in the SO coupled Bose-Fermi mixture with high density or low density of the atoms by using a variational method and some approximations such as tight-binding approximation.For the high density, the fermions have a large influence on the Bose system, while, for the low density, the fermions have little influence although the relative amounts of the bosons and the fermions are comparable in the two cases.In addition, we show that the spin current of the i
We study a mixture of Bose and Fermi gases with equal Rashba and Dresselhaus spin–orbit coupling. The phase diagram of bosons with the same coupling was already studied in a mean-field approximation and shown to have three kinds of ground states. In the present study, the ground state of the spin–orbit coupled Bose–Fermi mixture is investigated in the same approach to analyze effects of introducing fermions to the bosonic system. The result shows that even a small amount of fermions could drastically change the phase diagram of the bosons.
We investigate the ground-state properties of a mixture of spin-1/2 Bose-Einstein condensate and spin-1/2 Fermi superfluid. In our system, bosons are subjected to a pair of Raman laser beams that induces spin-orbit coupling, while fermions are not coupled to the Raman laser. We show that the fermions can acquire an effective spin-orbit coupling from the spin-exchange interaction between bosons and fermions. In the regime where the boson number is more than the fermion number, we study the topological phase transition of the Fermi superfluid by monitoring the close and re-open of the energy gap, the Berry phase, and the superfluid order parameter, where the back-action to the Bose condensate is also taken into consideration. Our work provides a new way of achieve topological Fermi superfluid … The American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics.
We consider a mixture of spin-1/2 bosons and fermions, where only the bosons are subjected to the spin-orbit coupling induced by Raman beams. The fermions, although not directly coupled to the Raman lasers, acquire an effective spin-orbit coupling through the spin-exchange interaction between the two species. Our calculation shows that this is a promising way of obtaining spin-orbit coupled Fermi gas without Raman-induced heating, where the long-sought topological Fermi superfluids and topological bands can be realized. Conversely, we find that the presence of fermions not only provides a new way to create the supersolid stripe phase of the bosons, but more strikingly it can also greatly increase the spatial period of the bosonic density stripes, and hence makes this phase directly observable in the experiment. This system provides a new and practical platform to explore the physics of spin-orbit coupling, which possesses a dynamic nature through the interaction between the two species.