NONLINEAR DIELECTRIC RESPONSE OF GLASSES AT LOW-TEMPERATURE

We have measured the dielectric response of amorphous insulators in th e audio frequency range at temperatures between 500 mu K and 400 mK. W e compare the measured superlinear behavior with a model incorporating higher order terms at low frequencies. Temperature independent dielec tric response at low fields and low temperatures has also been observe d which may indicate a low energy cutoff in the two-level system distr ibution of order 1 mK in some materials. We also find anomalously high sensitivity of the dielectric response to rf noise.