Evidence for the crucial role of nuclear spins for the magnetic field effect in polarization echoes in glasses

Abstract

PACS 61.43.Fs, A few years ago, pronounced magnetic field effects have been observed in the low-temperature dielectric properties of glasses and disordered crystals. In particular, dielectric susceptibility and coherent polarization echoes were reported to be strongly influenced by external magnetic fields. It is well-known that the lowtemperature dielectric properties of disordered solids are dominated by atomic tunneling. Recently, it has been suggested that the surprising magnetic field dependence is caused by the nuclear quadrupole moment associated with the tunneling particles. To investigate the role of the nuclear quadrupole we have studied the magnetic field dependence of the amplitude of spontaneous polarization echoes generated in ordinary and deuterated glycerol. The observed isotope effect gives striking evidence for the explanation involving nuclear quadrupole moments. In addition, the quantum beating of the nuclear spin states has been observed in our echo experiments.

Realistic tunnelling states for the magnetic effects in non-metallic real glasses

“…Yet, deviations from the behaviour predicted by the standard tunneling model (STM) have challenged the validity of the model in the case of multi-component glasses with variable content of the good crystal- 18,19]) and especially in glasses of compositions like BaO-Al 2 O 3 -SiO 2 in the presence of a weak magnetic field [20]. In such glasses (the mixed alkali-silicates have not yet been investigated in a field, but we predict important magnetic effects there, and x-dependent, too) a puzzling non-monotonous magnetic-field dependence has been revealed in most physical properties [6,7,8,9]. The magnetic effect is normally weak, but orders of magnitude larger than expected from basic thermodynamic considerations.…”

“…Glycerol-d 0 has none, other than the natural abundance of deuterium, some 125 ppm, and of 17 O, about 500 ppm, concentrations which Figure 14: (colour online) The integrated echo amplitude as a function of the magnetic field at T =13 mK, generated in partially deuterated glycerol (glycerol-d 3 ) and in ordinary glycerol (glycerol-d 0 ). On the lefthand side the figure shows that the echo amplitude for deuterated glycerol-d 3 is much more sensitive to the magnetic field in comparison with non-deuterated glycerol-d 0 , shown at the right-hand side (from [9]).…”

“…With this characterization the 2LSs have been employed in the 1970s and 80s to explain with some success the anomalies in the properties of glasses at low temperatures. Recently, the field has been witnessing a renaissance since the discovery that there are unexpected magnetic effects in insulating glasses [6,7,8,9] and that the TSs are responsible for the decoherence of phase qubits made of superconducting Josephson-junctions [10,11,12,13,14]. The 2LSs are thought to be ubiquitous in the junction's tunneling barrier which, though very thin, is considered to be amorphous and described by Zachariasen-Warren's model [4,5].…”