The glassy state — Magnetically viewed from the frozen end

It is argued that the structure of real glasses, especially the multi-component ones, can be investigated at low temperatures by exploiting the existence of tunneling systems (TSs) in the glass which can be described by a suitable extension of the standard tunneling model (STM) as was proposed by one of us. Even the simplest glass - as already argued in 1921 and in 1937 by A.A. Lebedev - is likely to contain elementary crystallites (not yet crystals) embedded in a random network, and our work has shown that this statement is even more true in the case of phase-separating, multi-component glasses, solids containing network-modifying components in their chemical make-up. Using the concept of TSs specifically nesting within the crystallites, we describe how our extended TM can explain puzzling experiments in glasses at low temperatures and in the presence of a magnetic field for a variety of amorphous insulators. The success of the theoretical analysis of a large number of experimental data confirms the crystallite hypothesis, clarifies the nature of the TS and opens the way to a possible new form of spectroscopy for the amorphous solid state in which the TSs play the role the atomic nuclei play in NMR spectroscopy.