High temperature modulations, meso-scale interactions and hyperscaling breakdown in Ising models with frustration: some insights from thermodynamic geometry

In this work we revisit the Axial Third Nearest Neighbour Ising (A3NNI) chain and examine in detail some aspects of its phase behaviour ensuing from competing interactions and resulting frustration. We probe the phase behaviour with two complimentary tools: a microscopic two-point correlation function $\mathcal{C}(n)$ which we carefully construct after appropriate spin transformations, and a macroscopic, thermodynamic curvature $R$ which we obtain from the free entropy. We report novel observations of phenomena such as hyperscaling breakdown and intermediate-ranged modulations among others. The zero field thermodynamic curvature $R_0$ is shown to systematically sub-divide the ground state phases into regions of attractive or repulsive effective interactions of varying strength. Furthermore, $R_0$ brings forth the significance of some third order moments in describing the effects of frustration, including the multiphase lines. Combined use of both the probes in the short-ranged modulated order regime confirms and further clarifies the discussion in [1] regarding the appropriate measure of high temperature correlation length in this regime.