Spin dynamics and transport in gapped one-dimensional Heisenberg antiferromagnets at nonzero temperatures

We present the theory of nonzero temperature ($T$) spin dynamics and transport in one-dimensional Heisenberg antiferromagnets with an energy gap $\Delta$. For $T << \Delta$, we develop a semiclassical picture of thermally excited particles. Multiple inelastic collisions between the particles are crucial, and are described by a two-particle S-matrix which has a super-universal form at low momenta. This is established by computations on the O(3) $\sigma$-model, and strong and weak coupling expansions (the latter using a Majorana fermion representation) for the two-leg S=1/2 Heisenberg antiferromagnetic ladder. As an aside, we note that the strong-coupling calculation reveals a S=1, two particle bound state which leads to the presence of a second peak in the T=0 inelastic neutron scattering (INS) cross-section for a range of values of momentum transfer. We obtain exact, or numerically exact, universal expressions for the thermal broadening of the quasi-particle peak in the INS cross-section, for the magnetization transport, and for the field dependence of the NMR relaxation rate $1/T_1$ of the effective semiclassical model: these are expected to be asymptotically exact for the quantum antiferromagnets. The results for $1/T_1$ are compared with the experimental findings of Takigawa et al and the agreement is quite good. In the regime $\Delta < T < (a typical microscopic exchange)$ we argue that a complementary description in terms of semiclassical waves applies, and give some exact results for the thermodynamics and dynamics.