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Exact form of the exponential correlation function in the glassy super-rough phase

We consider the random-phase sine-Gordon model in two dimensions. It describes two-dimensional elastic systems with random periodic disorder, such as pinned flux-line arrays, random field XY models, and surfaces of disordered crystals. The model exhibits a super-rough glass phase at low temperature $T<T_{c}$ with relative displacements growing with distance $r$ as $\bar{\langle [θ(r)-θ(0)]^2\rangle} \simeq A(τ) \ln^2 (r/a)$, where $A(τ) = 2 τ^2- 2 τ^3 +\mathcal{O}(τ^4)$ near the transition and $τ=1-T/T_{c}$. We calculate all higher cumulants and show that they grow as $\bar{\langle[θ(r)-θ(0)]^{2n}\rangle}_c \simeq [2 (-1)^{n+1} (2n)! ζ(2n-1) τ^2 + \mathcal{O}(τ^3) ] \ln(r/a)$, $n \geq 2$, where $ζ$ is the Riemann zeta function. By summation, we obtain the decay of the exponential correlation function as $\bar{\langle e^{iq\left[θ(r)-θ(0)\right]}\rangle} \simeq (a/r)^{η(q)} \exp\boldsymbol(-\frac{1}{2}\mathcal{A}(q)\ln^2(r/a)\boldsymbol)$ where $η(q)$ and ${\cal A}(q)$ are obtained for arbitrary $q \leq 1$ to leading order in $τ$. The anomalous exponent is $η(q) = c q^2 - τ^2 q^2 [2γ_E+ψ(q)+ψ(-q)]$ in terms of the digamma function $ψ$, where $c$ is non-universal and $γ_E$ is the Euler constant. The correlation function shows a faster decay at $q=1$, corresponding to fermion operators in the dual picture, which should be visible in Bragg scattering experiments.