Hybrid superconductor-semiconductor nanowire junctions as useful platforms to study majorana bound states

• Autores: Jorge Luis Cayao Diaz
• Directores de la Tesis: Ramón Aguado Sola (dir. tes.), Elsa Prada (tut. tes.)
• Lectura: En la Universidad Autónoma de Madrid ( España ) en 2016
• Idioma: español
• Tribunal Calificador de la Tesis: Rosa López Gonzalo (presid.), Alfredo Levy-Yeyati (secret.), Fernando Sebastián Bergeret Sbarbaro (voc.), Karsten Flensberg (voc.), Pascal Simon (voc.)
• Materias:
  • Física
    • Electromagnetismo
      • Superconductividad
    • Física del estado sólido
      • Propiedades de transporte de electrones
      • Semiconductores
      • Dispositivos de estado sólido
• Enlaces
  • Tesis en acceso abierto en: Biblos-e Archivo
• Resumen

  • One of the most promising platforms for one-dimensional topological superconductivity is based on semiconducting nanowires with strong spin-orbit coupling (SOC), where $s$-wave superconductivity is induced by proximity effect and an external Zeeman field drives the system into the topological superconducting phase with Majorana bound states (MBSs) at the end of the wire. During last years this idea has led to a great number of important experiments in hybrid superconductor-semiconductor systems, where the main signature is an emergent zero-bias peak (ZBP) in the differential conductance as a magnetic field is increased.

    This thesis focuses on the study of hybrid superconductor-semiconductor junctions made of semiconducting nanowires with Rashba SOC. In the first part, we introduce the emergence of one-dimensional topological superconductivity, and then we show details on modeling one-dimensional hybrid junctions made of semiconducting nanowires with SOC.

    Afterwards, we fully analyse the Andreev spectrum and study phase-biased transport, which exhibit non-trivial signatures in the topological phase with MBSs.

    In the second part, we focus on transport in a voltage-biased short superconductor-normal-superconductor (SNS) junction made of semiconducting nanowires with SOC as the applied Zeeman field drives the system into the topological superconducting phase.

    We show that the dissipative multiple Andreev reflection (MAR) current at different junction transparencies exhibits unique features related to topology such as gap inversion, the formation of MBSs and fermion-parity conservation.

    In the third part, we carried out a detailed study of helicity and confinement in long SNS junctions based on semiconducting nanowires. The main conclusion in this part is that a long junction with a helical normal section but still in the topologically trivial regime, supports a low-energy sub-gap spectrum consisting of multiple zero-energy crossings that smoothly evolve towards MBSs as the junction becomes topological.

    In the fourth part, we investigate a novel approach to engineer MBSs in non-topological superconducting wires, where we propose to create a sufficiently transparent normal-superconductor (NS) junction on a Rashba wire with a topologically trivial superconducting side and a helical normal side.

    We finish with an analysis of screening properties of proximitized nanowires with SOC and Zeeman fields within linear response theory, relevant for experiments trying to measure Majorana bound states and their non-trivial overlap.

    Along this thesis, we emphasise the importance of employing hybrids superconductor-semiconductor nanowire junctions towards the unambiguously detection of MBSs beyond zero-bias anomalies.