Bound on Lyapunov exponent in Einstein-Maxwell-Dilaton-Axion black holes*

In this study, we investigate the influence of the angular momentum of a charged particle around non-extremal and extremal Einstein-Maxwell-Dilaton-Axion black holes on the Lyapunov exponent. The angular momentum's ranges and spatial regions where the bound of the exponent is violated are found for certain values of the rotation parameter and dilatonic constant of the black holes. This violation always exists when the rotation parameter is large enough and the rotation direction of the particle is opposite to that of the black holes. The spatial region outside the extremal black hole of the violation is relatively large. In the near-horizon regions of the extremal black holes, the violation depends on the rotation directions of the black holes and particles and not depend on the value of the angular momentum.