All-temperature superfluidity in two dimensions

In local theories with short-range interactions, continuous symmetries cannot be spontaneously broken in two spatial dimensions at nonzero temperature. $\mathrm{U}(1)$-symmetric models instead admit a Berezinskii–Kosterlitz–Thouless (BKT) phase with quasi-long-range order and nonzero phase stiffness, which is normally destroyed at high temperatures by vortex proliferation. Here, we show that a BKT phase, and with it superfluidity, can persist to arbitrarily high temperatures. Using the large-$N$ expansion, we study a $(2+1)$-dimensional quantum field theory in which a complex scalar is coupled to the critical $\mathrm{O}(N)$ model. At the biconical fixed point, where the inter-sector coupling is negative, the thermal effective potential has a circle of minima with nonzero $\mathrm{U}(1)$ amplitude. Integrating out the gapped fluctuations yields an effective theory for the compact phase mode at distances large compared to $1\mathrm{/}T$. Since temperature is the only scale at the fixed point, the phase stiffness grows linearly with temperature, with a proportionality factor that scales linearly with $N$ and is fixed entirely by the fixed point. This renders vortex operators irrelevant for sufficiently large $N$. We further show that the vortex fugacity is exponentially small in $N$.