Universality and Thouless energy in the supersymmetric Sachdev-Ye-Kitaev Model

TL;DR

This paper analyzes the spectral and dynamical properties of the supersymmetric Sachdev-Ye-Kitaev model, revealing universality, symmetry effects, and ergodic behavior consistent with random matrix theory, especially near zero energy and up to the Thouless energy.

Contribution

It provides a detailed spectral analysis of the supersymmetric SYK model, highlighting universality, symmetry effects, and the scaling of the Thouless energy, extending understanding of quantum chaos and black hole analogs.

Findings

Spectral density grows exponentially with N near zero energy.

Spectral properties match chiral and superconducting random matrix ensembles.

Spectral form factor decays as 1/t^2 for times shorter than the Thouless time.

Abstract

We investigate the supersymmetric Sachdev-Ye-Kitaev (SYK) model, $N$ Majorana fermions with infinite range interactions in $0+1$ dimensions. We have found that, close to the ground state $E \approx 0$, discrete symmetries alter qualitatively the spectral properties with respect to the non-supersymmetric SYK model. The average spectral density at finite $N$, which we compute analytically and numerically, grows exponentially with $N$ for $E \approx 0$. However the chiral condensate, which is normalized with respect the total number of eigenvalues, vanishes in the thermodynamic limit. Slightly above $E \approx 0$, the spectral density grows exponential with the energy. Deep in the quantum regime, corresponding to the first $O(N)$ eigenvalues, the average spectral density is universal and well described by random matrix ensembles with chiral and superconducting discrete symmetries. The dynamics for $E \approx 0$ is investigated by level fluctuations. Also in this case we find excellent agreement with the prediction of chiral and superconducting random matrix ensembles for eigenvalues separations smaller than the Thouless energy, which seems to scale linearly with $N$. Deviations beyond the Thouless energy, which describes how ergodicity is approached, are universality characterized by a quadratic growth of the number variance. In the time domain, we have found analytically that the spectral form factor $g(t)$, obtained from the connected two-level correlation function of the unfolded spectrum, decays as $1/t^2$ for times shorter but comparable to the Thouless time with $g(0)$ related to the coefficient of the quadratic growth of the number variance. Our results provide further support that quantum black holes are ergodic and therefore can be classified by random matrix theory.