Experimental study of high-temperature spin dynamics in one-dimensional Heisenberg paramagnets

Abstract

From room-temperature measurements of proton spin-lattice relaxation rate in (${\mathrm{C}{\mathrm{H}}_{3})}_{4}$NMn${\mathrm{Cl}}_{3}$ (TMMC) and CsMn${\mathrm{Cl}}_{3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 2${\mathrm{H}}_{2}$O (CMC) as a function of magnetic field $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$ the shape of the low-frequency spectral density of the spin fluctuations is directly obtained. For $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$ parallel to the chain axis the spectral density contains a diffusive term which diverges as ${\ensuremath{\omega}}^{\ensuremath{-}\frac{1}{2}}$. The experimental results for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}\ensuremath{\parallel}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ are in good agreement with a theoretical calculation in TMMC in which the two-spin correlation function is assumed to decay at long times as ${t}^{\ensuremath{-}\frac{1}{2}}$. The lack of frequency dependence of $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$ perpendicular to the chain axis indicates a difference in the "cutoff" time of the two-spin correlation function for the spin components parallel and perpendicular with respect to the external magnetic field.