We consider the isotropic
S=1 Heisenberg chain with a finite Haldane gap $\mathrm{\ensuremath{\Delta}}$ and use state-of-the-art numerical techniques to investigate its dynamical properties at finite temperature, focusing on the nuclear spin-lattice relaxation rate
1/T1 measured in nuclear magnetic resonance (NMR) experiments, for instance. In particular, we analyze the contributions from modes with momenta close to $q\ensuremath{\approx}0$ and $q\ensuremath{\approx}\ensuremath{\pi}$ as a function of temperature. At high-temperature we observe spin diffusion, while at low-temperature we argue that a simple activated behavior $1/{T}_{1}\ensuremath{\propto}exp(\ensuremath{-}\mathrm{\ensuremath{\Delta}}/T)$ can be observed only at temperatures much smaller than the gap $\mathrm{\ensuremath{\Delta}}$.
Proton spin-lattice relaxation times have been measured in the quasi-one-dimensional
s=21 Heisenberg antiferromagnetic compounds CuSe
O4\ifmmode\cdot\else\textperiodcentered\fi{}5
H2O and CuS
O4\ifmmode\cdot\else\textperiodcentered\fi{}5
H2O, in order to study the magnetic field dependence of the 1D quantum spin dynamics at various temperatures. Information is obtained on the low-frequency behavior of the spin autocorrelation functions for the linear chains. The transverse component shows at high
T a marked decrease with magnetic field above the saturation field
Bc. Towards lower temperatures a pronounced maximum just below
Bc develops and the decrease above
Bc becomes more abrupt. The longitudinal component behaves more smoothly near and above
Bc. The field dependence of the relaxation rates is well explained at all nonzero temperatures by finite-chain calculations. For
T=0 we present analytical calculations of autocorrelation functions based on the predominance of a two-parameter spin-wave continuum for the quantum spin dynamics yielding a nondivergent behavior at $\ensuremath{\omega}=0$ and
B=0. Exact results are obtained for fields above
Bc. Further, an effective
XY approach is used for low but finite
T. Here, good agreement is found with the proton relaxation data for very low temperatures.
The NMR relaxation data on
Sr2CuO3 [Phys. Rev. Lett. 76, 4612 (1996)] are reexamined and compared with the analytic theory of the dynamic susceptibility in the
S=1/2 antiferromagnetic Heisenberg chain including multiplicative logarithmic corrections [Phys. Rev. Lett. 78, 539 (1997)]. Comparisons of the spin-lattice and the Gaussian spin-echo decay rates (
1/T1 and
1/T2G) and their ratio all show good quantitative agreement. Our results demonstrate the importance of the logarithmic corrections in the analysis of experimental data for quasi-one-dimensional systems and indicate that the dynamics of
Sr2CuO3 is well described by an
S=1/2 one-dimensional Heisenberg model with a nearest-neighbor exchange.
We used
17O NMR to probe the uniform (wave vector
q=0) electron spin excitations up to 800 K in
Sr2CuO3 and separate the
q=0 from the $q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ifmmode\pm\else\textpm\fi{}\frac{\ensuremath{\pi}}{a}$ staggered components. Our results support the logarithmic decrease of the uniform spin susceptibility below $T\ensuremath{\sim}0.015J$, where
J=2200K. From measurement of the dynamical spin susceptibility for
q=0 by the spin-lattice relaxation rate
1/T1, we demonstrate that the
q=0 mode of spin transport is ballistic at the
T=0 limit, but has a diffusion-like contribution at finite temperatures even for $T\ensuremath{\ll}J$.
The spin-lattice relaxation rate 1/
T1 and the spin-echo decay rate 1/
T2G for the spin-1/2 antiferromagnetic Heisenberg chain are calculated using quantum Monte Carlo and maximum-entropy analytic continuation. The results are compared with recent analytical calculations by Sachdev. If the nuclear hyperfine form factor
Aq is strongly peaked around q=\ensuremath{\pi}, the predicted low-temperature behavior [1/
T1\ensuremath{\sim}
ln1/2(1/T), 1/
T2G\ensuremath{\sim}
ln1/2(1/T)/\ensuremath{\surd}T] extends up to temperatures as high as T/J\ensuremath{\approxeq}0.5. If
Aq has significant weight for q\ensuremath{\approxeq}0 there are large contributions from diffusive long-wavelength processes not taken into account in the theory, and very low temperatures are needed in order to observe the asymptotic T\ensuremath{\rightarrow}0 forms.
We study dynamical properties at finite temperature
(T) of Heisenberg spin chains with random antiferromagnetic exchange couplings, which realize the random singlet phase in the low-energy limit, using three complementary numerical methods: exact diagonalization, matrix-product-state algorithms, and stochastic analytic continuation of quantum Monte Carlo results in imaginary time. Specifically, we investigate the dynamic spin structure factor $S(q,\ensuremath{\omega})$ and its $\ensuremath{\omega}\ensuremath{\rightarrow}0$ limit, which are closely related to inelastic neutron scattering and nuclear magnetic resonance (NMR) experiments (through the spin-lattice relaxation rate
1/T1). Our study reveals a continuous narrow band of low-energy excitations in $S(q,\ensuremath{\omega})$, extending throughout the
q space, instead of being restricted to $q\ensuremath{\approx}0$ and $q\ensuremath{\approx}\ensuremath{\pi}$ as found in the uniform system. Close to $q=\ensuremath{\pi}$, the scaling properties of these excitations are well captured by the random-singlet theory, but disagreements also exist with some aspects of the predicted
q dependence further away from $q=\ensuremath{\pi}$. Furthermore we also find spin diffusion effects close to
q=0 that are not contained within the random-singlet theory but give non-negligible contributions to the mean
1/T1. To compare with NMR experiments, we consider the distribution of the local relaxation rates
1/T1. We show that the local
1/T1 values are broadly distributed, approximately according to a stretched exponential. The mean
1/T1 first decreases with
T, but below a crossover temperature it starts to increase and likely diverges in the limit of a small nuclear resonance frequency ${\ensuremath{\omega}}_{0}$. Although a similar divergent behavior has been predicted and experimentally observed for the static uniform susceptibility, this divergent behavior of the mean
1/T1 has never been experimentally observed. Indeed, we show that the divergence of the mean
1/T1 is due to rare events in the disordered chains and is concealed in experiments, where the typical
1/T1 value is accessed.