NMR investigation of a structural phase transition and temperature-induced magnetism in copper thiospinelCuV2S4s

Metallic thiospinel ${\mathrm{CuV}}_2$ ${\mathrm{S}}_4$ transforms from the high-T cubic (HTC) crystal phase to a low-T tetragonal (LTT) phase below a transition temperature ${\mathrm{T}}_{\mathrm{f}}$ =90 K. The electronic and magnetic properties of ${\mathrm{CuV}}_2$ ${\mathrm{S}}_4$ have been investigated with magnetic susceptibility χ, Knight shift K, and nuclear spin-lattice relaxation time ${\mathrm{T}}_1$ measurements between T=4.2 and 300 K at 78 MHz. With the phase transition from HTC to LTT, the d-spin hyperfine field of ${}_{}{}^{51}{}_{}{}^{}$ V shows a large change from -29.4 to 2.5 kOe/${\mathrm{\mu }}_{\mathrm{B}}$ and of ${}_{}{}^{63}{}_{}{}^{}$ Cu from 19.5 to 11.9 kOe/${\mathrm{\mu }}_{\mathrm{B}}$ . The small increase in χ and large increase in ${\mathrm{K}}^2$ ${\mathrm{T}}_1$ T above ${\mathrm{T}}_{\mathrm{f}}$ are described by an energy band scheme of ${\mathrm{V}}^{3+}$ (${\mathrm{d}}^2$ ) with a singlet ground state (S=0) and triplet first-excited state (S=1). A large Jahn-Teller local distortion in the LTT phase results in the singlet ground state. In the HTC phase, on the other hand, the distortion is so small as to allow a thermal excitation to the triplet state with an activation energy of ΔE≃13 meV.