Lyapunov Exponent Approach to Phase Structure of Schwarzschild AdS Black Holes Surrounded by a Cloud of Strings

Abstract: We investigate Schwarzschild black holes in anti-de Sitter (AdS) spacetimes surrounded by a cloud of strings (BH-AdS-CoS), incorporating both electric- and magnetic-like components of the string bi-vector. Thermodynamically, these systems exhibit small/intermediate/large black hole phases with first- and second-order transitions governed by the string parameter $c_0$. Dynamically, we probe the phase structure using Lyapunov exponents $\lambda$ from unstable circular geodesics. For massless particles ($\delta = 0$), analytical expressions $\lambda$ reveal multivalued behavior in first-order transition regimes ($c_0 < c_{\text{cri}}$), with branches mapping to thermodynamic phases ($\lambda_{\text{SBH}}, \lambda_{\text{IBH}}, \lambda_{\text{LBH}}$). The discontinuity $\Delta\lambda = \lambda_{\text{SBH}} - \lambda_{\text{LBH}}$ at $T_p$ follows mean-field scaling: $\Delta\lambda / \lambda_{\text{cri}} \propto (T_\text{cri} - T)^{1/2} \quad (\beta = 1/2)$. For massive particles ($\delta = 1$), numerical computation of timelike geodesics confirms $\lambda$ as an order parameter, with critical exponent $\beta = 1/2$ universally. Key distinctions emerge: $\lambda\to 1$ asymptotically for photons, while $\lambda\to 0$ in the significant black hole phase for massive particles due to vanishing unstable orbits. The transition of $\lambda$ from multivalued to single-valued at $c_0 = c_{\text{cri}}$ establishes it as a universal dynamical probe of black hole criticality. The universal critical exponent of 1/2 for (\Delta\lambda) further reinforces the analogy with conventional thermodynamic systems. Our results confirm a direct connection between the thermodynamic phase structure of BH-AdS-CoS and the dynamics of test particles, with the Lyapunov exponent emerging as a sensitive diagnostic of black hole criticality.