ELECTRON-TRANSPORT IN DOUBLE-QUANTUM WELLS UNDER THE LONGITUDINAL SIZE-EFFECT REGIME

We study electron transport in contacted double quantum wells (DQW's) under conditions when a competition between the longitudinal (Ohmic) c urrents and transverse (tunneling) current leads to inhomogeneous dist ributions of the currents in the wells. if the characteristic length k appa(-1) (estimated as a square root from the ratio of the averaged in -plane conductivity to tunneling conductance) of such inhomogeneities is comparable with the length L of the DQW's, the conductivity of the system depends on the size factor kappa L, i.e., the longitudinal size effect takes place. Basic equations describing in-plane distribution of the potentials in the wells are derived for a linear response regim e-in a phenomenological approach, while transport coefficients appeari ng in these equations are calculated from a microscopic theory. Genera l form of boundary conditions (relations between the currents and pote ntials at the contacts) for the basic equations is presented. Differen t contacting schemes containing independent and common contacts are un der consideration. When the frequency of the applied voltage becomes c omparable with the ratio of the tunneling conductance to the transvers e polarizability of DQW's, the appearance of retardation effects gives rise to a nontrivial size dependence of the ac response and transient characteristics of the DQW's under the longitudinal size-effect regim e.