Low
temperature
universality in disordered solids A
large variety of otherwise very different amorphous and disordered
solids show, at temperatures lower than 3 Kelvin, remarkable
universality in their properties.
Most astounding, the ratio λ/l of the phonon wavelength divided by its mean free path is roughly 1/150, this value being constant in wavelength, temperature, and very similar between otherwise very different materials. For 4 decades this phenomenon is discussed within the framework of the phenomenological model of tunneling two level systems (TLSs). However, crucial questions such as the nature of the tunneling states, the universality and smallness of the ratio λ/l, and the energy scale dictating the temperature of 3K below which the phenomenon is observed, remain unanswered. We have recently suggested
a novel model to explain the above
questions, where tunneling states are classified to symmetric and
asymmetric with respect to local inversion (examples are 180°
flips and 90° rotations of the CN impurity in the picture at
the bottom left). The “symmetric” TLSs couple
weakly to the phonons, yet gap the asymmetric TLSs at low energies
– the DOS of the latter is shown at the bottom right. We are
now interested in using the theory to calculate further relevant
physical quantities, and in the rigorous generalization of the theory
to amorphous solids. For more details see arXiv:0910.1283.
