Unusual spin pseudogap behavior in the spin web lattice Cu3TeO6 probed by Te125 nuclear magnetic resonance

We present a $^{125}$Te nuclear magnetic resonance (NMR) study in the three-dimensional spin web lattice Cu$_3$TeO$_6$, which harbors topological magnons. The NMR spectra and Knight shift $\mathcal{K}$ as function of temperature show drastic change at $T_\text{S}\sim 40$ K much lower than N\'eel ordering $T_\text{N}\sim 61$ K, providing evidence for first-order structural phase transition within magnetically ordered state. Most remarkably, dependence spin-lattice relaxation rate $T_1^{-1}$ unravels spin-gap-like excitations, sharply sets $T^*\sim 75$ well above $T_\text{N}$. gap behavior may be understood by weakly dispersive optical magnon branches high-energy excitations originating from unique corner-sharing Cu hexagon spin-1/2 network with low coordination number.