Y. Akagi et N. Nakamura, Tunnelling molecular motion in glassy glycerol at very low temperatures asstudied by H-1 SQUID nuclear magnetic resonance, J PHYS-COND, 12(24), 2000, pp. 5155-5168
The H-1 nuclear spin-lattice relaxation process in glycerol has been studie
d at temperatures from 3.5 K to 300 K over a very wide range of Larmor freq
uency between 236 kHz (0.00554 T) and 21.0 MHz (0.4932 T). A superconductin
g quantum interference device (SQUID) was used to detect the longitudinal c
omponent of magnetization of the proton at very low frequencies below 1.62
MHz. At sufficiently low temperatures the nuclear spin-lattice relaxation r
ate obeys a relation 1/T-1 proportional to (T-2/omega(beta))integral(0)(6/T
) [(x dx)/sinh x], (with beta around 0.9 below 25 K), implying that the rel
axation rare is governed by an excitation of low-frequency disordered modes
inherent to the glassy state of glycerol and becomes asymptotically 1/T-1
proportional to T-2 below T = 3 K and 1/T-1 proportional to T above T = 3 K
. The relaxation phenomena can be interpreted as the nuclear spin dipping a
ssociated with a Raman process which is induced by a coupling of thermally
activated low-frequency disordered modes or low-frequency excitation (LFE)
with a phonon bath. The LFE originates from a quantum-mechanical two-level
system (TLS) reflecting an asymmetric-double-well (ASDW) potential which is
formed by the hydrogen bonding configuration in the glassy stare of glycer
ol. The maximum characteristic asymmetry of the double-well potential was f
ound to be (3 +/- 1) K. This quantum-mechanical molecular motion dominates
the other relaxation mechanisms at low temperatures, such as the dipolar re
laxation due to molecular classical reorientation with distributed correlat
ion times.