Unusual spin pseudogap behavior in the spin web lattice Cu$_3$TeO$_6$ probed by $^{125}$Te nuclear magnetic resonance

Abstract

We present a 125 Te nuclear magnetic resonance (NMR) study in the three-dimensional spin web lattice Cu3TeO6 which harbors topological magnons. The 125 Te NMR spectra and the Knight shift K as a function of temperature show a drastic change at TS ∼ 40 K much lower than the Néel ordering temperature TN ∼ 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 * ∼ 75 K, 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.