Very Fast Relaxation at Low Temperature of Tunneling States in Rare-Earth Doped Glasses

Abstract

To explain the low temperature properties of amorphous materials, the existence of Tunneling States (T.S.) has been assumed [1,2]. We report on the existence of a much faster relaxation than usual in a new type of insulating glass: an aluminosilicate glass doped with magnetic rare-earth ions (Dysprosium and Gadolinium). We present here the results of a saturation recovery experiment: we send in our sample a first strong acoustic pulse which saturates the resonant T.S. and then, we look at the saturation rate as a function of the elapsed time (fig.1). We have performed this experiment at about 15mK, for several magnetic ions concentrations from 0% to 10% and for several values of the magnetic field from 0 to 5 Tesla. Our results are shown in Fig.1 for different magnetic fields. The decay of the saturation rate is non exponential. This usual property is due to a distribution of relaxation time T1. This distribution has a lower boundary, which can be usually evaluated from the initial decay. We can see on Fig.1, that for the Dysprosium-doped glass the initial decay is very strong. The lower limit T1, min of the T1-distribution is less than 5μs. This value is to be compared with T1, min=200μs we have found in the non magnetic glass at same frequency and temperature. The magnetic field seems to have a little effect; it seems only to induce a little vertical translation of the curve. The increase of the Dysprosium concentration speeds up the relaxation: for an atomic concentration of 10%, the recovery of the thermal equilibrium is already completed after less than lμs.