Giancarlo	Jug -	Università dell'Insubria	#	
Magnetic effect(s) in the dipole echo of non-magnetic cold 
glasses: the solution of a riddle #
	Startling magnetic 
effects have been reported, in the last decade or so, when 
structural glasses (multi-silicates, but also amorphous 
glycerol) are studied at low and ultra-low (mK) temperatures. 
The heat capacity and dielectric constant of glasses, 
dominated by tunneling systems at these temperatures, ought to 
display universal features and to be oblivious to the magnetic 
field. Instead, small non-monotonic deviations have been 
observed in the dielectric constant (real part and loss) when 
the glasses are immersed in weak magnetic fields (10 mT up to 
1 T). Moreover significant deviations have been reported for 
the heat capacity and also for the amplitude of the dipole or 
polarization echo. We have developed a theory able to explain 
quantitatively the magnetic effects in the heat capacity, and 
present our best results for the dielectric constant and loss 
in a magnetic field. Also, we have solved the problem of the 
astonishing magnetic effects reported on the echo amplitude, 
and using the very same model. We present our explanation, for 
ALL of the magnetic effects, in terms of special tunneling 
systems coupled orbitally to the magnetic field and residing 
in ``crystal embrios'' (nano-crystals or smaller) within the 
otherwise homogeneously disordered solid. This theory shows 
that the glass transition is more likely to be associated to 
the formation of crystal droplets around Tg than to 
frustration as is the case in the spin-glasses. The 
``magnetic'' tunneling systems become therefore a viable probe 
to study these crystal embryos when other spectroscopies would 
fail.