NMR evidence of hidden order in the high-temperature phase of (TaSe4)2I

Abstract

(TaSe 4 ) 2 I has a quasi-one-dimensional electronic structure with charge-density wave (CDW) ground state forming in a phase transition at Tp = 265 K. Here we report on 77 Se nuclear magnetic resonance (NMR) spectrum and spin-spin relaxation time (T 2 ) measurements in the temperature range of 100 to 380 K. The line positions are shown to couple to the CDW order parameter. The spin-spin relaxation rate displays a sharp minimum at Tp; signs of critical slowing down are absent. Together with the fast increase of the order parameter, these findings are consistent with a true gap due to hidden order in the high-temperature phase.