Moment method and continued fraction expansion in Floquet Operator Krylov Space

TL;DR

This paper develops a moment method using continued fraction expansion to analyze Floquet operators via Krylov space, enabling the construction of Floquet models and identifying conditions for non-unitary dynamics and stable edge modes.

Contribution

It introduces a novel moment method linking autocorrelation functions to Floquet models through Krylov angles, revealing insights into edge modes and non-unitary dynamics in Floquet systems.

Findings

Constructed Floquet-ITFIM from autocorrelation functions.

Identified conditions indicating non-unitary dynamics.

Demonstrated stable m-periodic edge modes in Floquet systems.

Abstract

Recursion methods such as Krylov techniques map complex dynamics to an effective non-interacting problem in one dimension. For example, the operator Krylov space for Floquet dynamics can be mapped to the dynamics of an edge operator of the one-dimensional Floquet inhomogeneous transverse field Ising model (ITFIM), where the latter, after a Jordan-Wigner transformation, is a Floquet model of non-interacting Majorana fermions, and the couplings correspond to Krylov angles. We present an application of this showing that a moment method exists where given an autocorrelation function, one can construct the corresponding Krylov angles, and from that the corresponding Floquet-ITFIM. Consequently, when no solutions for the Krylov angles are obtained, it indicates that the autocorrelation is not generated by unitary dynamics. We highlight this by studying certain special cases: stable $m$-periodic dynamics derived using the method of continued fractions, exponentially decaying and power-law decaying stroboscopic dynamics. Remarkably, our examples of stable $m$-periodic dynamics correspond to $m$-period edge modes for the Floquet-ITFIM where deep in the chain, the couplings correspond to a critical phase. Our results pave the way to engineer Floquet systems with desired properties of edge modes and also provide examples of persistent edge modes in gapless Floquet systems.