paper

Velocity-dependent Lyapunov exponents in many-body quantum, semi-classical, and classical chaos

arXiv:1803.05902 · doi:10.1103/PhysRevB.98.144304

Abstract

The exponential growth or decay with time of the out-of-time-order commutator (OTOC) is one widely used diagnostic of many-body chaos in spatially-extended systems. In studies of many-body classical chaos, it has been noted that one can define a velocity-dependent Lyapunov exponent, , which is the growth or decay rate along "rays" at that velocity. We examine the behavior of for a variety of many-body systems, both chaotic and integrable. The so-called light cone for the spreading of operators is defined by , with a generally direction-dependent "butterfly speed" . In spatially local systems, is negative outside the light cone where it takes the form near , with the exponent taking on various values over the range of systems we examine. The regime inside the light cone with positive Lyapunov exponents may only exist for classical, semi-classical or large- systems, but not for "fully quantum" chaotic systems with strong short-range interactions and local Hilbert space dimensions of order one.

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