CPM Seminar

The Glassy State, Magnetically Viewed from the Frozen End

Giancarlo Jug

Università degli Studi dell'Insubria

It is by now an established fact that the glass transition is accompanied by the formation of so-called dynamical heterogeneities (DHs) or coherently re-arranging regions (CRRs) in the supercooled liquid below the melting temperature T_m. These DHs continue to exist well below T_m to the Kauzmann temperature T_K and their mutual hindering is responsible for the rigidity of the amorphous solid[1]. However, below T_K the DHs become static inhomogeneities which can take the form of crystal embryos (CEs), regions of enhanced regularity (RERs) (in the purest glass) or even nano- and micro-crystals (in the multi-component glasses). These entities are the frozen remnants of the DHs or CRRs and are ubiquitous in the solid amorphous state. Their manifestation has been discovered[2] at the lowest temperatures (T < 1 K) when the remaining degrees of freedom are acoustic phonons and the so-called tunneling systems (TSs). The TSs thus become probes of the existence of the CEs and specialize into two types: ordinary two-level systems (2LSs) and anomalous TSs (ATSs) which become extremely sensitive to weak magnetic fields (a few Oe). A simple model made up of a dilute gas of 2LSs+ATSs will be shown to be able to explain all of the magnetic effects reported for the cold multi-silicate and organic glasses in the last 15 years or so: thermal, dielectric constant, dielectric loss, and polarization echo response to weak magnetic fields. In particular, the model stemming from the existence of the CEs probed by the ATSs explains the dependence of the polarization echo amplitude on the magnetic field, on the electric field, on the isotopic composition (novel isotope effect) and on the pulses waiting time (oscillations in the amplitude disappearing at the higher fields). The agreement between theory and experiment is excellent and there is no need to resort to nuclear electric quadrupoles in the glasses to explain the experimental data. At the lowest temperatures (ca. 5 mK) the interactions between ATSs lead to the freezing of the dynamics of the ATS electric dipoles which manifests itself in the form of a spin-glass like transition and a cusp in the dielectric constant (as observed2).

[1] M.M. Hurley and P. Harrowell, Phys. Rev. E, 1995, 52, 1694
[2] P. Strehlow, C. Enss and S. Hunklinger, Phys. Rev. Lett. 1998, 80, 5361; P. Strehlow, M. Wohlfahrt, A.G.M. Jansen, R. Haueisen, G. Weiss, C. Enss and S. Hunklinger, Phys. Rev. Lett. 2000, 84, 1938; P. Nagel, A. Fleischmann, S. Hunklinger, and C. Enss, Phys. Rev. Lett. 2004, 92, 245511