Quantum Spin Systems and Classical Dynamics

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For the temperature regime where time evolution of a quantum spin system is described by a classical field theory, spectral shape of the Electron Spin Resonance is determined using classical equation of motion. It is difficult to obtain the spectral shape with other methods. On Heisenberg antiferromagnetic chain under a staggered field, a crossover between paramagnetic and antiferromagnetic resonances, and accompanying change in the spectral shape are demonstrated. Our approach based on the classical equation motion provides insights complimentary to those obtained in other approaches.