Has long-distance spin-superfluid transport been demonstrated?
In plain words
In magnets whose spins lie in a plane, a steady twist of the spin direction should carry spin current with little loss, decaying only slowly with distance, like a superfluid. Some experiments claim this, and magnon condensation in yttrium iron garnet is established, but other explanations of the signals remain.
Precise statement
Spin superfluidity predicts a nonlocal spin signal decaying algebraically, $\sim 1/(L + L_0)$, instead of exponentially, $\sim \exp(-L/\lambda)$, and a critical spin current. Determine whether claimed observations in spin-flop $\mathrm{Cr_2O_3}$ (Yuan et al. 2018), the graphene quantum Hall antiferromagnet (Stepanov et al. 2018) and magnon condensate supercurrents in YIG (Bozhko et al. 2016) show superfluid spin transport, by measuring the distance dependence and a critical current in a single device free of thermal artifacts. An answer is yes or no.
What would settle it
A nonlocal measurement over a range of injector-detector distances in one device showing algebraic decay and a critical current, with thermal spin Seebeck backgrounds excluded.
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