Unusual spin pseudogap behavior in the spin web lattice Cu3Te O6 probed by Te 125 nuclear magnetic resonance

We present a Te125 nuclear magnetic resonance (NMR) study in the three-dimensional spin web lattice Cu3TeO6 which harbors topological magnons. The Te125 NMR spectra and the Knight-shift K as a function of temperature show a drastic change at TS∼40K much lower than the Néel ordering temperature TN∼61K, providing evidence for the first-order structural phase transition within the magnetically ordered state. Most remarkably, the temperature dependence of the spin-lattice relaxation rate T1-1 unravels spin-gap-like magnetic excitations, which sharply sets in at T∗∼75K, the temperature well above TN. The spin-gap behavior may be understood by weakly dispersive optical magnon branches of high-energy spin excitations originating from the unique corner-sharing Cu hexagon spin-1/2 network with low coordination number.