Effects of stickiness in the classical and quantum ergodic lemon billiard

TL;DR

This paper investigates classical and quantum properties of the ergodic lemon billiard with stickiness regions, revealing nonuniversal eigenstate localization, complex spectral statistics, and classical-quantum phase space division effects.

Contribution

It provides a detailed analysis of classical stickiness and quantum localization in the ergodic lemon billiard, highlighting deviations from universal chaotic behavior and introducing new spectral statistical measures.

Findings

Eigenstates are chaotic but localized, as shown by Poincaré-Husimi functions.

Energy level spacing follows Berry-Robnik-Brody distribution, indicating phase space division.

Localization measures exhibit nonuniversal, bimodal distributions, deviating from typical chaotic systems.

Abstract

We study the classical and quantum ergodic lemon billiard introduced by Heller and Tomsovic in Phys. Today 46(7), 38 (1993), for the case $B=1/2$, which is a classically ergodic system (without a rigorous proof) exhibiting strong stickiness regions around a zero-measure bouncing ball modes. The structure of the classical stickiness regions is uncovered in the S-plots introduced by Lozej [Phys. Rev. E 101, 052204 (2020)]. A unique classical transport or diffusion time cannot be defined. As a consequence the quantum states are characterized by the following nonuniversal properties: (i) All eigenstates are chaotic but localized as exhibited in the Poincar\'e-Husimi (PH) functions. (ii) The entropy localization measure A (also the normalized inverse participation ratio) has a nonuniversal distribution, typically bimodal, thus deviating from the beta distribution, the latter one being characteristic of uniformly chaotic systems with no stickiness regions. (iii) The energy-level spacing distribution is Berry-Robnik-Brody (BRB), capturing two effects: the quantally divided phase space (because most of the PH functions are either the inner-ones or the outer-ones, dictated by the classical stickiness, with an effective parameter $\mu_1$ measuring the size of the inner region bordered by the sticky invariant object, namely, a cantorus), and the localization of PH functions characterized by the level repulsion (Brody) parameter $\beta$. (iv) In the energy range considered (between 20 000 states to 400 000 states above the ground state) the picture (the structure of the eigenstates and the statistics of the energy spectra) is not changing qualitatively, as $\beta$ fluctuates around 0.8, while $\mu_1$ decreases almost monotonically, with increasing energy.