Recent studies report on anomalous spin transport for the integrable Heisenberg spin chain at its isotropic point. Anomalous scaling is also observed in the time-evolution of non-equilibrium initial conditions, the decay of current-current correlations, and non-equilibrium steady state averages. These studies indicate a space-time scaling with x∼t2/3 behaviour at the isotropic point, in sharp contrast to the ballistic form x∼t expected for integrable systems. In our contribution we study the scaling behaviour for the Faddeev-Takhtajan model, which is an integrable classical spin chain with the same symmetries as the quantum Heisenberg model. We report on equilibrium time correlations and the evolution with step initial conditions. Remarkably, the scaling function is identical to the one obtained from Kardar-Parisi-Zhang equation. In addition, we present results for the easy-plane and easy-axis regimes for which, repectively, ballistic and diffusive spin transport is observed, whereas the energy remains ballistic over the entire parameter regime.
Digamma-Function Representation of the Ground-State Energy in Antiferromagnetic Heisenberg XXXs Spin Chains
The antiferromagnetic Heisenberg spin chain remains a central framework for exploring exactly solvable models within quantum integrable systems. For the isotropic XXX chain, the ground-state energy per site of the spin-1/2 system is famously given by ln2. Extending the classic formulations to arbitrary spin-s, Takhtajan and Babujian derived two separate finite-series expressions for integer and half-integer spin representations. Current work introduces a unified analytical expression for the ground-state energy density in terms of the digamma function. This compact formulation reproduces both Takhtajan Babujian series.