|project title:||Noise in ballistic superconducting quantum nanostructures|
|project leader:||Prof. Yuri Galperin and Mr. Dmitri Denisov|
|access given (in days):||30|
|access used (in days):||23|
|local host:||N. Kopnin|
|home institution:||Department of Physics, University of Oslo|
|country of institution:||Norway|
|starting date (yyyy-mm-dd):||2007-01-25|
We will study noise in ballistic superconducting nanostructures such as quantum superconductor-insulator-normal-insulator-superconductor (SINIS) contacts using the microscopic nonequilibrium theory. Up to now, the noise in superconducting weak links has been extensively studied either for diffusive conductors or for short ballistic junctions (point contacts). The progress in nanotechnology opens a possibility to fabricate a SINIS heterostructure containing an extended ballistic normal quantum conductor connected to the superconducting leads through low resistive contacts. The quantum interference of Andreev and normal reflection processes at the contacts becomes of a primary importance for transport and noise characteristics of such structures. Studies of noise offers a direct access to the microscopic characterization of quantum states in superconducting nanostructures where we expect profound effects of quantum interference on the noise characteristics.
The charge transport and noise in ballistic normal and superconducting nanostructures such as quantum superconductor-insulator-normal-insulator-superconductor (SINIS) contacts have been studied. The progress in nanotechnology opens a possibility to fabricate a SINIS heterostructure containing an extended ballistic normal single-channel quantum conductor connected to the leads through low resistive contacts. The quantum interference of Andreev and normal reflection processes at the contacts is of a primary importance for transport and noise characteristics of such structures which offer a direct access to the microscopic characterization of quantum states in superconducting nanostructures. Within the framework of the project, the noise spectrum has been calculated in a single-mode long SINIS junction with the length of the normal conductor much larger than the superconducting coherence length but shorter than the inelastic mean-free path. The current noise in long SINIS junctions at low temperatures is shown to be sensitive to the population of the sub-gap states which is far from equilibrium even at low bias voltages. Nonequilibrium distribution establishes due to an interplay between voltage-driven inter-level Landau-Zener transitions and intra-level inelastic relaxation. The Fano factor is enhanced drastically, being proportional to the number of times which particle can fly along the Andreev trajectory before it escapes from the level due to inelastic scattering. Combining the dc current and noise measurements one can fully characterize the non-equilibrium kinetics in SINIS junctions. The resonant transmission mechanism of charge transport through a finite-length quantum wire connected to leads via finite transparency junctions has been considered. The coherent electron transport was found to be strongly modified by the Coulomb interaction. The low-temperature current-voltage (IV) curves show step-like dependence on the bias voltage determined by the distance between the quantum levels inside the conductor, the pattern being dependent on the ratio between the charging energy and level spacing. If the system is tuned close to the resonance condition by the gate voltage, the low-voltage IV curve is Ohmic. At large Coulomb energy and low temperatures, the conductance is temperature-independent for any relationship between temperature, level spacing, and coupling between the wire and the leads. Publications:  Kopnin, Y.M. Galperin, and V. Vinokur, ``Low-voltage current noise in long quantum superconductor/insulator/normal-metal/insulator/superconductor junctions'' Phys. Rev. B 76, 100504® (2007).  N. B. Kopnin, Y. M. Galperin, and V. M. Vinokur ``Charge transport through weakly open one dimensional quantum wires'', Submitted to PRL; preprint arXiv:0804.3979v1.