Tunneling States in Crystals with Large Random Strains

Abstract

The two-level systems (TLS) that dominate the low temperature properties of amorphous solids have two features, both of which are unexplained---(i) a constant density of states with (ii) nearly the same magnitude in all amorphous solids. We show that feature (i) can also arise in lightly doped crystals subjected to internal strains: A KCl or KBr crystal doped with $\ensuremath{\sim}1$ mole % KCN has a narrow spectrum of tunneling states. When random internal strains are introduced by substituting ${\mathrm{Br}}^{\ensuremath{-}}$ ions for half of the ${\mathrm{Cl}}^{\ensuremath{-}}$, however, these tunneling states are replaced by TLS showing feature (i) of amorphous solids.