Spontaneous PT symmetry breaking of a ferromagnetic superfluid in a gradient field

We consider the interaction of a ferromagnetic spinor Bose-Einstein condensate with a magnetic-field gradient. The magnetic-field gradient realizes a spin-position coupling that explicitly breaks time-reversal symmetry T and space parity P, but preserves the combined PT symmetry. We observe, using numerical simulations, a phase transition spontaneously breaking this remaining symmetry. The transition to a low-gradient phase, in which gradient effects are frozen out by the ferromagnetic interaction, suggests the possibility of high-coherence magnetic sensors unaffected by gradient dephasing. Copyright (C) EPLA, 2015