Emulation of quantum correlations by classical dynamics
in quantum magnets

Chaebin Kim

School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

In the last decade, continua of magnetic excitations have been evidenced in various magnetic materials by neutron scattering experiments. This has led to rapid developments in numerical techniques to reproduce and explain the physical origin of these continua, which include spin fractionalization, spontaneous magnon decay, cooperative paramagnetism, and chemical disorder. One such approach, classical dynamics, does not rely on an accurate quantum representation of the ground state and has been surprisingly effective due to its speed and versatility. However, the limits of applicability of classical dynamics are not yet fully understood. In this talk, I’ll present how classical spin dynamics can emulating the quantum effects depending on the temperature in the well-known quantum system; quantum S = ½ chain. By employing Landau-Lifshitz Dynamics, we emulate quantum correlations through temperature-dependent corrections. Our results demonstrate that the quantum-equivalent dynamical spin structure factor (DSSF) closely matches Quantum Monte-Carlo calculations for k_BT/J > 1, capturing the continuum of magnetic excitations observed in neutron scattering measurements. At higher temperatures, our simulations comply with general predictions by De Gennes, where the pinched-oval spectral continuum reflects the uncorrelated paramagnetic fluctuations in the infinite temperature limit. Surprisingly, entanglement witnesses extracted from the quantum-equivalent DSSF mimic quantum calculations across all temperatures, despite relying on classical dynamics. Our approach highlights the potential of classical dynamics to emulate quantum effects even in the most extreme spin systems, with implications for future experimental and theoretical investigations.