Upper deviations of the Sherrington-Kirkpatrick free energy: the fifth-order law and the onset of constrained Parisi measures

Abstract

Let $Z_N$ be the partition function of the Sherrington-Kirkpatrick model at inverse temperature $β>1$. Replica theory (Kondor near the critical temperature, Parisi and Rizzo at all temperatures) predicts that the probability that $N^{-1}\log Z_N$ exceeds its typical value by $δ$ decays like $\exp(-Naδ^{6/5})$ with an explicit constant $a$; Aronow and Lopatto recently proved bounds of this order. The exponent is the Legendre transform of the Parisi problem constrained to carry an atom $θ$ at zero. We show that the constrained minimum exceeds the equilibrium free energy by $c_βθ^5(1+o(1))$, where $c_β=\frac{9}{640}β^3ρ_β(0)^{-3}$ and $ρ_β(0)>0$ is the density at zero of the Parisi measure. With the fractional-moment formula, valid for this model, this gives the exponent $\frac56(6c_β)^{-1/5}δ^{6/5}(1+o(1))$ as $N\to\infty$ and then $δ\to0$. The constrained minimizer has a marginal first positive contact, to leading order at $3θ/(2ρ_β(0))$ with an atom $θ/2$, below which a function equal to the identity at the contacts differs from it by a universal cubic to leading order. This plateau calculus applies to every mixture with $ξ'''(0)=0$ whose Parisi measure accumulates at zero; it also shows that a small field $h$ opens a gap of order $|h|^{2/3}$ and adds a term of order $|h|^{10/3}$ to the free energy, and that an atom of the Parisi measure at zero, present in every pure $p$-spin model with $p\ge3$, makes the rate leave zero linearly, with slope equal to the atom. Near criticality the constant tends to Kondor's value $9/5120$, the marginal regime ends at a de Almeida-Thouless threshold in the constraint, and above its first contact the constrained measure agrees with the Parisi measure up to $O(θ^5)$. The results at every $β>1$ use Lopatto's theorem that the Parisi measure accumulates at zero.