Physical review. B, Condensed matter
Dynamics of the s=12 linear Heisenberg antiferromagnet in a magnetic field: Experiment and theory
J. P. Groen, Tjeerd O. Klaassen, N.J. Poulis, Gerhard H. Muller, H. Thomas, H. Beck
Abstract
Proton spin-lattice relaxation times have been measured in the quasi-one-dimensional $s=\frac{1}{2}$ Heisenberg antiferromagnetic compounds CuSe${\mathrm{O}}_{4}$\ifmmode\cdot\else\textperiodcentered\fi{}5${\mathrm{H}}_{2}$O and CuS${\mathrm{O}}_{4}$\ifmmode\cdot\else\textperiodcentered\fi{}5${\mathrm{H}}_{2}$O, 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 ${B}_{c}$. Towards lower temperatures a pronounced maximum just below ${B}_{c}$ develops and the decrease above ${B}_{c}$ becomes more abrupt. The longitudinal component behaves more smoothly near and above ${B}_{c}$. 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 ${B}_{c}$. Further, an effective $\mathrm{XY}$ approach is used for low but finite $T$. Here, good agreement is found with the proton relaxation data for very low temperatures.