Tunnelling molecular motion in glassy glycerol at very low temperatures asstudied by H-1 SQUID nuclear magnetic resonance

Citation
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
Citations number
52
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Apllied Physucs/Condensed Matter/Materiales Science
Journal title
JOURNAL OF PHYSICS-CONDENSED MATTER
ISSN journal
0953-8984 → ACNP
Volume
12
Issue
24
Year of publication
2000
Pages
5155 - 5168
Database
ISI
SICI code
0953-8984(20000619)12:24<5155:TMMIGG>2.0.ZU;2-U
Abstract
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.