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arXiv:2609.16438

Analytical solutions to the non-equilibrium Green's functions in large-$q$ SYK models

Jan C. Louw

Published Sep 14, 2026• 0 citations
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Abstract

It is known that the Sachdev-Ye-Kitaev (SYK) model is exactly solvable at leading order in $1/q$; specifically, the instantaneously thermal time block after the quench is known in closed form. Thus far the remaining (inherently out of equilibrium) blocks have only been numerically accessible. Here we provide their analytical solutions for a quench between non-commuting $q/2$-body SYK Hamiltonians. We obtain the Green's functions in closed form for every time block. We extract a simple relation between the pre- and post-quench energies $ε_1 \propto ε_0$ leading to an exact temperature update rule. Via a Cauchy-Schwarz inequality, we show that heating is inevitable for the closed system; hence, the second law emerges geometrically. The solution is used in a companion paper to address what ``instantaneous thermalization'' means at finite $q$.

DOI
10.48550/arxiv.2609.16438

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Analytical solutions to the non-equilibrium Green's functions in large-qqq SYK models
It is known that the Sachdev-Ye-Kitaev (SYK) model is exactly solvable at leading order in 1/q1/q1/q; specifically, the instantaneously thermal time block after the quench is known in closed form. Thus far the remaining (inherently out of equilibrium) blocks have only been numerically accessible. Here we provide their analytical solutions for a quench between non-commuting q/2q/2q/2-body SYK Hamiltonians. We obtain the Green's functions in closed form for every time block. We extract a simple relation between the pre- and post-quench energies ε1∝ε0ε_1 \propto ε_0ε1​∝ε0​ leading to an exact temperature update rule. Via a Cauchy-Schwarz inequality, we show that heating is inevitable for the closed system; hence, the second law emerges geometrically. The solution is used in a companion paper to address what ``instantaneous thermalization'' means at finite qqq.
We investigate the non-equilibrium dynamics of complex Sachdev-Ye-Kitaev (SYK) models in the q→∞q\rightarrow\inftyq→∞ limit, where q/2q/2q/2 denotes the order of the random Dirac fermion interaction. We extend previous results by Eberlein et al. [Phys. Rev. B 96, 205123 (2017)] to show that a single SYK q→∞q\rightarrow\inftyq→∞ Hamiltonian for t≥0t\geq 0t≥0 is a perfect thermalizer in the sense that the local Green's function is instantaneously thermal. The only memories of the quantum state for t<0t<0t<0 are its charge density and its energy density at t=0t=0t=0. Our result is valid for all quantum states amenable to a~1/q1/q1/q-expansion, which are generated from an equilibrium SYK state in the asymptotic past and acted upon by an arbitrary combination of time-dependent SYK Hamiltonians for t<0t<0t<0. Importantly, this implies that a single SYK q→∞q\rightarrow\inftyq→∞ Hamiltonian is a perfect thermalizer even for non-equilibrium states generated in this manner.
What does "instant thermalization" in large-qqq SYK models mean?
Motivated by the Planckian thermalization rate Γ∼TΓ\sim TΓ∼T observed in the two-body interacting SYK model, we study thermalization in the q/2q/2q/2-body case. Previous studies of such systems have established the notion of instantaneous thermalization to leading order in 1/q1/q1/q. It was conjectured that the thermalization may still be Planckian but with a divergent rate Γ∼TqΓ\sim T qΓ∼Tq, explaining the ``instantaneous'' part. For an analytically and numerically tractable system, we calculate the effective temperatures after a quench at time t=0t = 0t=0 and indeed find a Planckian rate, albeit with the unexpected decaying behavior $Γ\sim T q^{-1} $. This is contrasted with the behavior of the causal Green's function G(t1,t2)\mathcal{G}(t_1, t_2)G(t1​,t2​), which instantly acquires a thermal form in the two-time plane block t1,t2>0t_1, t_2 > 0t1​,t2​>0. The resulting picture is that instant thermalization is a meaningful concept only for this block, while the off-diagonal blocks t1⋅t2<0t_1 \cdot t_2 < 0t1​⋅t2​<0 are inherently non-thermal at any qqq. Since the effective temperature is obtained from correlations spanning all blocks, it inherits a finite rate set by the off-diagonal. We illustrate this directly by studying the non-thermal correlations' time dependence.
We present an exact solution in the large-NNN limit of the Lévy Sachdev-Ye-Kitaev (LSYK) model introduced in Ref. [1], wherein the couplings are drawn from a Lévy Stable distribution parameterized by a tail exponent μ∈[0,2]μ\in [0, 2]μ∈[0,2]. Starting from the Hamiltonian and its associated partition function, we highlight the key differences from the standard Gaussian SYK model and derive the large-NNN Schwinger-Dyson equations via a bosonic oscillator representation of the action. These equations are solved both numerically and analytically in the large-qqq and infrared limits. We subsequently analyze the chaotic properties of the model by computing the Krylov exponent from the large-qqq Green's function and extracting the Lyapunov exponent from the 444-point function. The parameter μμμ continuously interpolates between a free theory at μ=0μ= 0μ=0 and the conventional, maximally chaotic Gaussian SYK model at μ=2μ= 2μ=2, with non-maximal chaos persisting throughout the intermediate regime 0<μ<20 < μ< 20<μ<2. Thermodynamic quantities, including the entropy, free energy, average energy, and specific heat capacity, are computed and compared with their Gaussian SYK counterparts. The interpretations of the thermodynamics are discussed with respect to the holographic dual and non-Fermi liquid theory. Finally, we discuss an alternative representation of the LSYK model based on a distinct decomposition of the Lévy Stable distribution, which establishes a non-trivial connection to Gaussian SYK, and provide supporting analytical and numerical results in the appendices.
The question of thermalization of a closed quantum system is of central interest in nonequilibrium quantum many-body physics. Here we present one such study analyzing the dynamics of a closed coupled Majorana SYK system. We have a large- q SYK model prepared initially at equilibrium quenched by introducing a random hopping term, thus leading to nonequilibrium dynamics. We find that the final stationary state reaches thermal equilibrium with respect to the Green's functions and energy. Accordingly, the final state is characterized by calculating its final temperature and the thermalization rate. We provide a detailed review of analytical methods and derive the required Kadanoff-Baym equations, which are then solved using the algorithm developed in this work. Our results display rich thermalization dynamics in a closed quantum system in the thermodynamic limit.
We describe the nonequilibrium quench dynamics of the Sachdev-Ye-Kitaev models of fermions with random all-to-all interactions. These provide tractable models of the dynamics of quantum systems without quasiparticle excitations. The Kadanoff-Baym equations show that, at long times, the fermion two-point function has a thermal form at a final temperature determined by energy conservation, and the numerical analysis is consistent with a thermalization rate proportional to this temperature. We also obtain an exact analytic solution of the quench dynamics in the large qqq limit of a model with qqq fermion interactions: in this limit, the thermalization of the two-point function is instantaneous.
We investigate the nonequilibrium dynamics of complex Sachdev-Ye-Kitaev (SYK) models in the $q\ensuremath{\rightarrow}\ensuremath{\infty}$ limit, where q/2q/2q/2 denotes the order of the random Dirac fermion interaction. We extend previous results by Eberlein et al. [Phys. Rev. B 96, 205123 (2017)] to show that a single SYK $q\ensuremath{\rightarrow}\ensuremath{\infty}$ Hamiltonian for $t\ensuremath{\ge}0$ is a perfect thermalizer in the sense that the local Green's function is instantaneously thermal. The only memories of the quantum state for t<0t<0t<0 are its charge density and its energy density at t=0t=0t=0. Our result is valid for all quantum states amenable to a 1/q1/q1/q expansion, which are generated from an equilibrium SYK state in the asymptotic past and acted upon by an arbitrary combination of time-dependent SYK Hamiltonians for t<0t<0t<0. Importantly, this implies that a single SYK $q\ensuremath{\rightarrow}\ensuremath{\infty}$ Hamiltonian is a perfect thermalizer even for nonequilibrium states generated in this manner.