Free energy and spectral edge of the SYK model

Abstract

We introduce a microscopic framework that gives the first rigorous derivation of the Schwinger--Dyson thermodynamics of the Sachdev--Ye--Kitaev model. For fixed, even interaction order $q\ge 4$, we prove that the annealed and quenched normalized pressures converge to the Schwinger--Dyson pressure $p_{\mathrm{SD},q}(\beta )$, at any fixed inverse temperature $\beta >0$. The proof is derived from the finite-$N$ Gibbs state, and it contains three new ingredients: a single-site cavity expansion that keeps the bulk Gibbs state intact, a finite-dimensional locality estimate yielding label-uniform conditional factorization of the Euclidean cavity fields, and an exact Majorana-bath representation of the leading diagrams. We further determine the asymptotic location of the largest eigenvalue, which answers a question posed by Feng--Tian--Wei. Consequently, the sample free-energy density converges almost surely to the zero-temperature value along every sequence $\beta \equiv {\beta }_N\to \infty$.

Streamlined the argument and modified the title. 49 pages