Unusual spin pseudogap behavior in the spin web lattice Cu3TeO6 probed by Te-125 nuclear magnetic resonance

Abstract

We present a Te-125 nuclear magnetic resonance (NMR) study in the three-dimensional spin web lattice Cu3TeO6 which harbors topological magnons. The Te-125 NMR spectra and the Knight-shift K as a function of temperature show a drastic change at T-S similar to 40 K much lower than the Neel ordering temperature T-N similar to 61 K, 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 T-1(-1) unravels spin-gap-like magnetic excitations, which sharply sets in at T* similar to 75 K, the temperature well above T-N. 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.