Finite Temperature Dynamics of the Spin 1/2 Bond Alternating Heisenberg Antiferromagnetic Chain

TL;DR

This paper investigates the finite temperature dynamic structure factor of the S=1/2 bond alternating Heisenberg chain using numerical methods, revealing temperature-dependent features like a central peak, asymmetric one-magnon peaks, and a persistent two-magnon bound state.

Contribution

It provides a comprehensive numerical analysis of the dynamic structure factor across various temperatures and frequencies, extending understanding of spin dynamics in bond alternating chains.

Findings

Identification of a temperature-induced central peak around zero frequency.

Observation of asymmetric one-magnon peaks with increasing temperature.

Persistence of the two-magnon bound state up to high temperatures.

Abstract

We present results for the dynamic structure factor of the S=1/2 bond alternating Heisenberg chain over a large range of frequencies and temperatures. Data are obtained from a numerical evaluation of thermal averages based on the calculation of all eigenvalues and eigenfunctions for chains of up to 20 spins. Interpretation is guided by the exact temperature dependence in the noninteracting dimer limit which remains qualitatively valid up to an interdimer exchange $\lambda \approx 0.5$. The temperature induced central peak around zero frequency is clearly identified and aspects of the crossover to spin diffusion in its variation from low to high temperatures are discussed. The one-magnon peak acquires an asymmetric shape with increasing temperature. The two-magnon peak is dominated by the S=1 bound state which remains well defined up to temperatures of the order of J. The variation with temperature and wavevector of the integrated intensity for one and two magnon scattering and of the central peak are discussed.