Magnetic excitation and local magnetic susceptibility of the excitonic insulator Ta2NiSe5 investigated by Se77 NMR

Abstract

$^{77}\mathrm{Se}$ NMR measurements were made on polycrystalline and single-crystalline samples to elucidate local magnetic susceptibility and magnetic excitation of ${\mathrm{Ta}}_{2}{\mathrm{NiSe}}_{5}$, which is proposed to undergo an exciton condensation accompanied by a structural transition at ${T}_{\mathrm{c}}=328\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We determine the $^{77}\mathrm{Se}$ Knight shift tensors for the three Se sites and analyze their anisotropy based on the site symmetry. The temperature dependence of the Knight shift is discussed on the basis of spin and orbital susceptibilities calculated for two-chain and two-dimensional three-band models. The large fraction of the Se $4p$ orbital polarization due to the mixing between Ni $3d$ and Se $4p$ orbitals is estimated from the analysis of the transferred hyperfine coupling constant. Also the nuclear spin-lattice relaxation rate $1/{T}_{1}$ is found not to show a coherent peak just below ${T}_{\mathrm{c}}$ and to obey the thermally activated temperature dependence with a spin gap energy of $1770\ifmmode\pm\else\textpm\fi{}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This behavior of $1/{T}_{1}$ monitors the exciton condensation as proposed by the theoretical study of $1/{T}_{1}$ based on the three-chain Hubbard model for the excitonic insulator.