Theory of criticality-enabled $U(1)$ symmetry breaking in a class of 1+1D systems
We develop an analytic theory of the recently proposed $U(1)$ symmetry breaking at interacting 1+1D Lifshitz quantum critical points. The low-energy theory contains two compact fields, $θ$ and $ϕ$, coupled by a Berry term that makes $θ$ canonically conjugate to the conserved density $\partial_xϕ$. Within a controlled large-$N$ limit, we establish long-range order (LRO) of $e^{iβθ}$ and stretched-exponential decay of the conjugate vertex, $\log\langle e^{iβϕ(x)}e^{-iβϕ(0)}\rangle\propto-|β|^{4/3}|x|^{2/3}$. Beyond large $N$, we argue that $U(1)$ LRO follows if the dynamical exponent satisfies $z\neq1$, consistent with existing calculations. Using finite-size and infinite-system DMRG, we study an itinerant-electron chain with $U(1)\rtimes\mathbb{Z}_2$ symmetry, whose spin sector realizes this theory at a ferromagnetic quantum critical point. We find spin-nematic LRO in $S_i^+S_{i+1}^+$ and stretched-exponential electron Green's functions, revealing distinctive non-Gaussian signatures of criticality-enabled order.