Dissertations, Theses, and Capstone Projects

Date of Degree

9-2017

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

David Schmeltzer

Committee Members

David Schmeltzer

Joel Gersten

Anatoly Kuklov

Avadh Saxena

Jianxin Zhu

Subject Categories

Condensed Matter Physics

Keywords

Majorana fermions, Aharonov-Bohm effect, Andreev reflection, ballistic transport, semiconductors, superconductors, topological materials

Abstract

This dissertation is devoted to a study of detecting the Majorana fermion induced crossed Andreev reflection.

Majorana fermions are particles that constitute their own antiparticles. In condensed matter physics, Majorana fermions are zero energy modes that reside at edges or around vortices of topological superconductors. The special properties of Majorana fermions result in their potential to conduct topological quantum computation, which has been attracting a lot of current research. One of the most important issues in the field of the Majorana fermion physics now is to detect their existence in realistic systems. Among many classes of detecting methods, a transport experiment is the most direct one. The existence of Majorana fermions induces uncommon Andreev reflections that can happen at interfaces between normal regions and superconducting regions. There are two types of Andreev reflections: local Andreev reflection and crossed Andreev reflection. The Majorana fermion induced local Andreev reflection yields zero-bias conductance peaks that have been observed by many experimentalists. Nonetheless, the zero-bias peak alone cannot be taken as a conclusive evidence for the existence of Majorana fermions, because several other mechanisms can lead to analogous results. On the other hand, the Majorana fermion induced crossed Andreev reflection yields equal probabilities of electron tunneling and hole tunneling. It is thus usually believed that a direct measurement of the tunneling current is impossible and a measurement of the shot noise is necessary for the detection of the Majorana fermion induced crossed Andreev reflection.

My main contribution is a new experimental proposal that is aimed at changing this opinion. In my proposal, a metallic ring structure is employed to separate the electron tunneling signals and the hole tunneling signals, and as a result the tunneling current is measurable. The key idea behind this proposal is that the constructive interference condition for the tunneling electrons in the metallic ring is different from that for the tunneling holes, utilizing the fact that their wave-vectors are different. The signature of the tunneling current in my intended set-up is that it changes sign from the electron tunneling dominated regime to the hole tunneling dominated regime, where the control parameter is the magnetic flux threading the ring.

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