Quantum Coherent Phenomena in Superconducting Heterostructures

Northwestern University, Evanston IL

Investigators

Abstract

****NON-TECHNICAL ABSTRACT**** According to quantum mechanics, small objects such as electrons behave like waves in addition to behaving like discrete particles. As waves, electrons show many interesting effects. An example is the interference of waves: think of the patterns of waves on the surface of a small pond when a stone is thrown in it. Quantum waves also encode the results of various interactions and correlations between particles, and devices that take advantage of these effects are the fundamental building blocks of many modern electronic devices. The effects of interference and correlations between electrons due to their wave nature can be seen by measuring the electric currents that are generated by them. However, these fragile interference and correlation effects are easily destroyed, and extremely low temperatures (close to absolute zero) and very sensitive measurement techniques are required to observe them. This project will explore correlations between electrons in metals that are induced by their interactions with a superconductor, which is a material that can carry an electrical current without resistance at very low temperatures. The experimental techniques that will be used include fabrication of very small devices that include metallic and superconducting parts, and measurements of the currents and voltages through these devices, as well as the electrical noise generated by these currents and voltages at very low temperatures. A major component of this project is the involvement of undergraduate and graduate students in research, who will learn sophisticated techniques of nanofabrication and device measurements. This training will enable the students to have careers in academia or high-technology industries. ****TECHNICAL ABSTRACT**** This project will explore quantum correlations induced in normal metals and ferromagnets due to their interactions with superconductors. In particular, the primary focus of this project will be on non-local correlations introduced in spatially separated normal metals and ferromagnets induced by crossed Andreev reflection (CAR) and elastic co-tunneling (EC). CAR and EC will be probed using nonlocal electrical transport measurements as well as noise measurements. The noise measurements will also be used to probe for spin entanglement of quasiparticles using samples with ferromagnetic elements. A second set of measurements will focus on the conditions under which one can observe superconducting correlations over long length scales in systems with finite spin polarization. While superconducting correlations have been observed in ferromagnets, the length scale over which these correlations extend has been extremely short. The new material systems that will be investigated may extend this length scale, and lead to the possibility of new superconducting devices in addition to furthering our understanding of exotic forms of superconductivity in ferromagnets. Integral to the project is the training of graduate and undergraduate students in nanofabrication and sophisticated measurement techniques.