He, solids, spinor BEC

Spin superfluidity has already been discussed from 70s of the last century. The interest to this phenomenon was revived after emergence of spintronics. Manifestation of spin superfluidity is a metastable spin supercurrent proportional to the gradient of the phase (spin rotation angle in a plane) and is not accompanied by dissipation, in contrast to a dissipative spin diffusion current proportional to the gradient of spin density. The supercurrent state is a helical spin structure, but in contrast to equilibrium helical structures is metastable. An elementary process of relaxation of the supercurrent is phase slip. In this process, a vortex with 2π phase variation around it crosses streamlines of the supercurrent decreasing the total phase variation across streamlines by 2π. Phase slips are suppressed by energetic barriers for vortex creation, which disappear when phase gradients reach critical values determined by the criterion similar to the famous Landau criterion for mass (charge) superfluidity.

Spin superfluidity is realized at special topology of the magnetic-order-parameter space, such as, e.g., that in easy-plane ferro- and antiferromagnets, or in coherent spin precession states supported by pumping of energy and magnons. It was discussed by theorists for superfluid ³He, magnetically ordered solids, and spinor BEC of cold atoms. Efforts in experimental detection of spin superfluidity in superfluid ³He and solids were also undertaken, which are assessed in the talk. Finally, connection between concepts of spin superfluidity and of magnon BEC is discussed.