EFFECTS OF MACROSCOPIC QUANTUM TUNNELING OF CHARGE IN ULTRASMALL SET DOUBLE JUNCTIONS WITH EXTERNAL ELECTROMAGNETIC ENVIRONMENT

It is known that classical Coulomb blockade effects of SET devices are disturbed by the tunneling effects of higher order denoted as macrosc opic quantum tunneling of charge (q-mqt) or co-tunneling. The influenc e of an external electromagnetic environment modelled by an additional impedance in the circuit on the mean q-mqt current is studied in the high-impedance limit. This is important especially for devices contain ing only two SET junctions, for instance the SET electrometer, where t he influence of the environment on the Coulomb blockade is remarkable. It can be shown that the Coulomb blockade is only partly destroyed pr oducing a new, lower ''quantum'' Coulomb blockade. This holds as well in the case of the incoherent as of the coherent q-mqt. Furthermore, a method is presented on how to regularize the logarithmic singularitie s in the q-mqt current using line width effects.