Linear Response Theory for Hard and Soft Glassy Materials

Despite qualitative differences in their underlying physics, both hard and soft glassy materials exhibit almost identical linear rheological behaviors. We show that these nearly universal properties emerge naturally in a shear-transformation-zone (STZ) theory of amorphous plasticity, extended to include a broad distribution of internal thermal-activation barriers. The principal features of this barrier distribution are predicted by nonequilibrium, effective-temperature thermodynamics. Our theoretical loss modulus $G"(\omega)$ has a peak at the $\alpha$ relaxation rate, and a power law decay of the form $\omega^{-\zeta}$ for higher frequencies, in quantitative agreement with experimental data.