Evidence for many-electron composite charge excitations in a Coulomb glass

We present experimental evidence that the elementary charge excitations in a Coulomb glass are composite entities comprised of a correlated motion involving many electrons, rather than the ordinary single-point particles previously believed. The Coulomb interaction energy in localized boron-doped silicon is measured by dc transport and electron tunneling. The transport excitations are found to have a significantly lower Coulomb energy than single-particle point charges introduced by tunneling. This reduction in the transport Coulomb energy is clearly inconsistent with the conventional model of single-particle excitation but agrees semiquantitatively with new theories of many-electron composite excitations.