The introduction of a steady-state spatially periodic Raman coupling between two components of an ultracold atomic gas produces a dressed-state gas with an anisotropic and tunable dispersion relation. A Bose-Einstein condensate (BEC) formed in such a gas is consequently characterized by an anisotropic superfluid critical velocity. The anisotropic dissipation of superfluid flow is quantified by considering the scattering of impurities flowing through this superfluid. A gradual transition from the isotropic nature of an uncoupled BEC to the anisotropic periodically-dressed condensate is obtained as the strength of the Raman coupling is varied. These results present a clear signature for future experimental realizations of this novel superfluid.
The introduction of a steady-state spatially periodic Raman coupling between two components of an ultracold atomic gas produces a dressed-state gas with an anisotropic and tunable dispersion relation. A Bose-Einstein condensate (BEC) formed in such a gas is consequently characterized by an anisotropic superfluid critical velocity. The anisotropic dissipation of superfluid flow is quantified by considering the scattering of impurities flowing through this superfluid. A gradual transition from the isotropic nature of an uncoupled BEC to the anisotropic periodically-dressed condensate is obtained as the strength of the Raman coupling is varied. These results present a clear signature for future experimental realizations of this novel superfluid.
We consider a two-component atomic gas illumined by two intersecting laser beams which induce Raman coupling between the components. This spatially periodic coupling modifies the dispersion relation of the gas. Properties of a Bose-Einstein condensate of such a gas are strongly affected by this modification. Using the quasiparticle excitation spectrum derived from a Bogoliubov transformation, the Landau critical velocity is found to be anisotropic and can be widely tuned by varying properties of the dressing laser beams.
In this work we discuss the Landau criterion for anisotropic superfluidity. To this end we consider a pointlike impurity moving in a uniform Bose-Einstein condensate with either interparticle dipole-dipole interaction or Raman-induced spin-orbit coupling. In both cases we find that the Landau critical velocity vc is generally smaller than the sound velocity in the moving direction. Beyond vc, the energy dissipation rate is explicitly calculated via a perturbation approach. In the plane-wave phase of a spin-orbit-coupled Bose gas, the dissipationless motion is suppressed by the Raman coupling even in the direction orthogonal to the recoil momentum. Our predictions can be tested in the experiments with ultracold atoms.
The ability to support frictionless motion is one of the manifestations of superfluidity. An impurity immersed in a superfluid can move without dissipation below the critical velocity, which, according to the Landau criterion, is determined by the elementary excitation spectrum of the system. In a quantum gas of the ultracold atoms, the critical velocity can be measured by stirring a laser beam through the atomic cloud, and the emergence of dissipation can be observed via the heating effect above the threshold stirring speed. Recently, such a technique is exploited to study the superfluidity of the Bose-Einstein condensate (BEC) of 162Dy atoms with dipole-dipole interactions. It is shown that both the critical velocity and the heating rate reflect the anisotropy of the underlying dipolar excitation spectrum. In this work, we theoretically investigate the anisotropic dissipation of a point-like impurity moving through a dipolar BEC. For the motion along the principal axis, the dissipation rate above the critical velocity is analytically derived according to the linear response theory. At a given reduced velocity, we find the dissipation rate being of a higher value in the direction parallel to the dipole moment, which qualitatively explains the recent experimental observation in dysprosium atoms. Moreover, in the moving direction away from the principal axis, the asymptotic expressions for the dissipation rate are obtained in the high-speed limit, as well as in the regime close to the dissipation threshold. By combining these analytical results with the numerical calculations, we conclude that, in a dipolar BEC, the angular dependence of the dissipation rate always shows the same anisotropy as the critical velocity. Our predictions can be examined in the current experiments with cold atoms, and the results presented here may be also helpful in understanding the anisotropic superfluidity in other systems.
An impurity immersed in a superfluid can move without friction when its velocity is below a critical value. This phenomenon can be explained by the famous Landau criterion, according to which, the critical velocity is determined by the elementary excitation spectrum of the superfluid. Landau critical velocity has been measured in the isotropic superfluid, such as the liquid He-Ⅱ and the Bose-Einstein condensates of dilute atomic gases, where the onset of dissipation is due to the creation of roton and phonon, respectively. The recent realization of synthetic spin-orbit coupling in quantum gas opens up possibilities for the study of novel superfluidity with ultracold atoms. To date, a specific type of spin-orbit coupling, which is generated by a pair of Raman laser beams, has been achieved in a Bose-Einstein condensate of 87Rb experimentally. Remarkably, the excitation spectrum of this system is anisotropic and can be feasibly tuned by the external laser field. While the anisotropic dynamics has been observed experimentally, the critical velocity has not been measured so far. It is a conventional wisdom that in an anisotropic superfluid, the critical velocity is determined by the excitation spectrum in the moving direction of the impurity. However, this is not always the case. In this work, we investigate the motion of a point-like impurity in a spin-orbit-coupled condensate with the spin-dependent interatomic interaction. In the vicinity of the quantum phase transition between the plane-wave (PW) phase and the zero-momentum (ZM) phase, the onset of the dissipation is due to the emission of a phonon, and the Landau critical velocity vc depends on the anisotropic sound velocity. While the sound velocity varies smoothly across the PW-ZM phase transition, the critical velocity in the direction perpendicular to the axis of spin-orbit coupling exhibits a sudden jump at the phase boundary. The value of vc on the PW phase side of the transition is generally smaller than the one on the ZM phase side, and the jump amplitude of vc is an increasing function of the spin-dependent interaction strength. Beyond the critical velocity, the energy dissipation rate of the impurity is explicitly calculated via a perturbation approach. The discontinuity of vc at the phase boundary can be clearly seen from the dissipation curves, which can be measured through the heating of the condensate. Our prediction can be tested in the current experiments with ultracold atoms.
We theoretically study the ground-state phases and superfluidity of tunable spin-orbit-coupled Bose-Einstein condensates (BECs) under the periodic driving of Raman coupling. An effective time-independent Floquet Hamiltonian is proposed by using a high-frequency approximation, and we find single-particle dispersion, spin-orbit-coupling, and asymmetrical nonlinear two-body interaction can be modulated effectively by the periodic driving. The critical Raman coupling characterizing the phase transition and relevant physical quantities in three different phases (the stripe phase, plane-wave phase, and zero momentum phase) are obtained analytically. Our results indicate that the boundary of ground-state phases can be controlled and the system will undergo three different phase transitions by adjusting the external driving. Interestingly, we find the contrast of the stripe density can be enhanced by the periodic driving in the stripe phase. We also study the superfluidity of tunable spin-orbit-coupled BECs and find the dynamical instability can be tuned by the periodic driving of Raman coupling. Furthermore, the sound velocity of the ground-state and superfluidity state can be controlled effectively by tuning the periodic driving strength. Our results indicate that the periodic driving of Raman coupling provides a powerful tool to manipulate the ground-state phase transition and dynamical instability of spin-orbit-coupled BECs.
Dilute gas Bose-Einstein condensates (BECs) are an example of a superfluid, and exhibit many of the same phenomena as the strongly interacting superfluid helium. However, their isolation from the environment combined with their inhomogeneous density profiles make it difficult to observe some phenomena, such as the thermo-mechanical effect, in experiments. Here we perform dynamical simulations of a finite temperature Bose-Einstein condensate coupled to spatially separated heat and particle reservoirs to observe such non-equilibrium superfluid phenomena. We analyse the resulting steady-state non-equilibrium flows in the BEC, and consider the prospect of observing sustained superfluid turbulence.