Two-Terminal Transport of Quantum Spin Superfluidity

We theoretically study the spin current between two quantum spin chains with easy plane anisotropy separated by an intermediate quantum spin chain with easy axis anisotropy in the same plane. Upon increasing the magnetic field, the middle section undergoes a quantum phase transition from a doubly degenerate ordered phase to a nondenerate quantum paramagnet. In the nongenerate phase, we find that zero energy magnons are exponentially suppressed by the middle section and perfectly reflected when the middle section is sufficiently long. In the degenerate phase, the tunneling is exponentially suppressed until a resonant length is reached in which there is a peak in the transmission of magnons which facilitates transfer of a spin superfluid. We identify a twist in the magnetic order connecting the two degenerate ground states in the middle region as the mechanism responsible for the enhanced tunneling at this resonant length beyond which incident magnons are perfectly reflected with opposite spin.