Investigations of nonlocal transport and current noise in mesoscopic normal metal-superconductor hybrid structures

Spin-based solid-state quantum computing requires the creation of spatially separated, spin-entangled quantum states. The correlated electronic states in a conventional superconductor can serve as an abundant source of entangled spin-singlet electrons provided Cooper pairs can be split without losing wavefunction coherence. In Cooper pair-splitter devices the electrodes (or quantum dots) are coupled to a superconductor with a separation of less than or equal to the superconducting coherence length. At such distances coherent nonlocal subgap transport takes place. It involves Cooper pair splitting among other theoretically predicted transport processes. In this dissertation we investigate signatures of coherence in nonlocal transport. We focus on current correlation measurements in normal metal-superconductor-normal metal (NSN) devices with transparent interfaces for which higher-order processes such as correlated Andreev reflections at both interfaces are predicted to contribute. We verify the distance dependence of nonlocal transport in a superconducting wire to which several normal metal electrodes are attached and obtain a good agreement between the decay length of the nonlocal voltage signal and the theoretical value for the coherence length in the superconductor. To reveal the coherent nature of nonlocal transport we perform a series of experiments involving shot noise and nonlocal current correlation measurements in mesoscopic NSN devices. We extend the model of incoherent shot noise in diffusive normal metal-superconductor (NS) contacts and identify the magnitude of the local self-consistent gap in our devices. We find that the onset of nonequilibrium transport in the superconductor coincides with a steep increase in the nonlocal resistance. We determine the barrier strength at the NS interface by comparing the dynamic resistance to numerical calculations of the contact resistance based on the Keldysh-Usadel equations. The main finding of our work are symmetric, `W-shaped' nonlocal subgap current correlations obtained by sweeping the bias voltage at one contact for fixed current bias at the other contact. This surprising behavior was predicted for NSN devices with similar values of interface transparencies and is caused by higher-order processes. The observation of higher-order processes proves the coherent nature of nonlocal subgap transport.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2013

Author dc:creator

• Stehno, Martin

Contributors dc:contributor

• Van Harlingen, Dale J.
• Mason, Nadya
• Gollin, George D.
• Vishveshwara, Smitha

• Andreev reflection
• Crossed Andreev reflection
• shot noise
• current correlations
• nonequilibrium superconductivity
• nonlocal transport in NSN-hybrid structures

Statement dc:rights

• Copyright 2012 by Martin P. Stehno. All rights reserved.

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/42415

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/42415