Spin dynamics in the alternating chain system Li3Cu2SbO6 with defects probed by nuclear magnetic resonance

Abstract

We report the results of the detailed measurements of $^{7}\mathrm{Li}$ nuclear magnetic resonance (NMR) spectra, NMR line shift, spin-lattice relaxation rate, bulk magnetization, magnetic susceptibility, and heat capacity on the spin-$\frac{1}{2}$ Heisenberg honeycomb compound ${\mathrm{Li}}_{3}{\mathrm{Cu}}_{2}{\mathrm{SbO}}_{6}$. This system consists of weakly coupled alternating Cu chains and, owing to the site inversion, some of the magnetic Cu ions are substituted by nonmagnetic Li ions. As a result, the effective spin model for ${\mathrm{Li}}_{3}{\mathrm{Cu}}_{2}{\mathrm{SbO}}_{6}$ is a set of Cu alternating chain fragments of different length (the ``chain-segments spin model''). Due to this unique structure, various kinds of peculiar magnetic features arise and the combination of local (NMR) and global experimental techniques is especially effective in their study. In order to describe the experimental results theoretically, we perform full diagonalization calculations for the chain-segments spin model taking explicit account of all of the possible dimerized chain fragments of different length. From the fitting of measured susceptibility, we estimate the exchange interactions as ${J}_{\mathrm{FM}}=\ensuremath{-}244\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${J}_{\mathrm{AF}}=146\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and find that about 19% of magnetic Cu ions are replaced by nonmagnetic Li ions. Using those values, we can even quantitatively reproduce the observed NMR line shift and the specific heat as well as field- and temperature-dependent magnetization over a wide temperature range. Although small deviations are seen at very low temperature, they can be corrected by considering the effect of small interchain interactions.