SEARCH FOR LONG-RANGE COULOMB INTERACTION IN DISORDERED METALS

The theory of disordered electron systems predicts that the dynamical Coulomb interaction (omega > 0) is strongly enhanced and in particular long ranged. This modification of the Coulomb interaction has many important consequences on the behavior of disordered electron systems. However, its effect has only been observed indirectly. Therefore we have looked for a direct observation of this long-ranged character. We used a sandwich of two disordered metal films separated by an insulating layer. For a long-range Coulomb interaction the two films should assist each other in screening. The ln(T) dependence of the resistance, i.e., the Coulomb anomaly, is used to determine the strength of the Coulomb interaction. The coupled two films represent a tunneling junction with a tunneling resistance of a few mu-OMEGA. The electronic properties of the junction, in particular, the tunneling time, are measured by means of weak localization. The thickness of the tunneling barrier is chosen so that the tunneling time of the conduction electrons is larger than the thermal coherence time h/k(B) T. Our measurements do not support the current theory of the long-range character of the dynamical Coulomb interaction and represent a challenge to our present understanding of the Coulomb effect in disordered electron systems.