Michael Straß
Shot noise control in coherent nanoscale conductors
Supervisor: Priv. Doz. Dr. Sigmund Kohler [Theoretical physics I]
Date of oral examination: 03/15/2006
95 pages, english , OPUS-Online Veröffentlichung
Subject of this thesis is the theoretical investigation of the noise characteristics of nanoscale conductors under the influence of a time-dependent external field. The electron transport through the nanoconductor is enabled via the coupling to macroscopic metal electrodes. In a Floquet-Green formalism, general expressions for the time-averaged electrical current and the associated current noise in terms of the retarded Green function are derived. Thereby, the Green function is expanded in terms of a Floquet ansatz and the metallic leads are eliminated in favor of a self-energy term. This approach can be interpreted as an extension of the Landauer formalism to time-dependent scattering. In addition, a rotating-wave approximation applicable for high-frequency driving is developed which allows the analytical solution of the transport problem by an approximate reduction to a time-independent system with renormalized parameters. With the scattering formalism at hand, an unbiased two-level system is described modelling the transport through two orbitals of a molecule excited by infrared laser light, as well as the transport through a double-well heterostructure driven by a gate voltage. Depending on the ratio of the driving amplitude and frequency, the time-averaged current and the noise power can be suppressed coherently. Moreover, regarding the relative noise strength by means of the Fano factor, remarkably low minimal values, i.e., strong shot noise reductions, are observed in the vicinity of the current suppressions. As a further example, the shot noise behavior in an asymmetric double quantum dot is investigated. Driven by microwave radiation, this setup serves as a nonadiabatic electron pump if no external bias voltage is applied. The pump current assumes a maximum for resonant driving while the noise power exhibits a at the same time a minimum. Therefore, coupled quantum dots are ideal for pumping electrons effectively and reliably at a low noise level.