Tunnelling molecular motion in glassy glycerol at very low temperaturesas studied by 1H SQUID nuclear magnetic resonance

Abstract

The 1H nuclear spin-lattice relaxation process in glycerol has beenstudied at temperatures from 3.5 K to 300 K over a very wide range of Larmorfrequency between 236 kHz (0.00554 T) and 21.0 MHz (0.4932 T). Asuperconducting quantum interference device (SQUID) was used to detect thelongitudinal component of magnetization of the proton at very low frequenciesbelow 1.62 MHz. At sufficiently low temperatures the nuclear spin-latticerelaxation rate obeys a relation1/T1∝(T2/ωβ)∫6/T0[(x dx)/sinh x], (withβ around 0.9 below 25 K), implying that the relaxation rate is governedby an excitation of low-frequency disordered modes inherent to the glassystate of glycerol and becomes asymptotically 1/T1∝T2 belowT = 3 K and 1/T1∝T above T = 3 K. The relaxationphenomena can be interpreted as the nuclear spin flipping associated with aRaman process which is induced by a coupling of thermally activatedlow-frequency disordered modes or low-frequency excitation (LFE) with a phononbath. The LFE originates from a quantum-mechanical two-level system (TLS)reflecting an asymmetric-double-well (ASDW) potential which is formed by thehydrogen bonding configuration in the glassy state of glycerol. The maximumcharacteristic asymmetry of the double-well potential was found to be(3±1) K. This quantum-mechanical molecular motion dominates theother relaxation mechanisms at low temperatures, such as the dipolarrelaxation due to molecular classical reorientation with distributedcorrelation times.