PhD defense - Kristján Óttar Klausen

Majorana Zero Modes in Tubular Nanowires

  • 30.9.2022, 14:00 - 15:00

Kristján Óttar Klausen will be defend his PhD thesis on Friday September 30th at 14:00 in M108.

Title: Majorana Zero Modes in Tubular Nanowires
Candidate: Kristján Óttar Klausen
Date and time: September 30th 2022 at 14:00

Thesis Committee:
Andrei Manolescu, Supervisor, Professor, Reykjavik University
Sigurður I. Erlingsson, Co-supervisor, Professor, Reykjavik University
Viðar Guðmundsson, Advisor, Professor, University of Iceland

Habib Rostami, Nordita Assistant Professor, Stockholm University, Sweden


Majorana zero modes have recently been proposed as a platform for fault-tolerant quantum computation. They are a specific kind of particle formation in quantum systems at the nanoscale with the potential of functioning as a qubit, the basic building block of quantum computers. Their topological property holds promise to counter the decoherence problem, which is one of the main challenges for upscaling the number of qubits in a quantum computer. In this work, Majorana zero modes in core-shell nanowire systems are investigated. Core-shell nanowires are tubular conductors that commonly have polygonal cross-section geometry due to crystallographic qualities. Semiconductors with proximity-induced superconductivity, large g-factors and Rashba spin-orbit coupling, can be tuned by an external magnetic field to host Majorana zero modes. Corner-localization of the lowest energy states in polygonal core-shell nanowires allows for hosting multiple Majorana zero modes within a single nanowire system. Finite and infinite three-dimensional nanowires are modeled using the Bogoliubov-de Gennes Hamiltonian, which is solved numerically by diagonalization. Effects of variable core and shell geometry are analyzed and triangular wires with hexagonal cores are found to have particularly large energy separations between the first and second groups of energy states, which is favorable for experimental hosting of Majorana zero modes. Braiding in such a system is discussed and for the realization of real space braiding, a dual core wire is suggested to include the necessary degrees of freedom for the simplest non-commutative braiding operation. The relation between Andreev reflection and propagation of the superconductivity property is studied by modeling partially proximitized wire shells, with radial, angular and longitudinal interfaces, corresponding to common experimental platforms. Varying compatibility is found with Andreev reflection. Flux-periodic oscillations in the energy spectra of proximitized shells are explored along with the effects of geometry, Zeeman and spin-orbit interaction. Instances are shown where the periodicity of the lowest energy state is found to separately display normal, superconducting and fractional flux quanta. To gain insight into the structure of topological invariants, the first Chern number in particular, Stokes' theorem for bivector valued fields is analyzed and visualized using geometric calculus. The findings show how the multiple components necessary for the emergence of Majorana zero modes each have complex subtleties and interdependencies, and in what way they should be investigated to come closer to the technological realization of Majorana zero modes.

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