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Naveen Shetty - ​ESR 10,  October 2018 - October 2021

Chalmers University of Technology, Gothenburg, Sweden

Master thesis: "Fabrication and Transport measurement in Semiconductor-Superconductor Hybrid structure"

This thesis has discussions about superconductivity, Josephson junction and application of Josephson junction in a charge qubit system. Josephson Junction is fabricated with Niobium as the superconducting contact and InAs nanowire as the weak link. Here, semiconducting material is used in the place of weak link to control electron transport using a back gate structure. Transport measurements were done in room temperature as well as in 400mK. Low temperature is achieved using He3 cryostat.

Since, low doped nanowire provides a better control over supercurrent, we here discuss about effects of doping on the coherence length of Cooper pair. For a better understanding further measurements are needed. Critical current was controlled within the range of 110nA to 180nA, hence complete control of electron transport over the weak link is achieved. It was possible to achieve zero differential resistance, which indicates the presence of supercurrent in semiconducting nanowire due to proximity effect. Hence, goal of the thesis was attained. Measured current did not indicate the presence of Fraunhofer pattern for varying magnetic field. This supported the theory of narrow junction model and dimensions of the Josephson Junction agreed with the result. For a better control over the transport, a top gate structure was proposed.

Personal Training Committee

Main supervisor: Sergey Kubatkin, CTH

Co-supervisor: Clemens Winkelmann, CNRS

Mentor: Amaia Zurutuza, GRA SEMI

Planned secondments

At CNRS (April 2019) to determine the structure of the nanoribbons edges and achieve scanning tunneling spectroscopy on them and September 2021 for sample measurements,

At GRAPH SEMI (December 2019) to get samples of CVD graphene and locate grain boundaries using TEM,

At RAITH (cancelled due to Covid-19) for training on resistless nanofabrication based on focused ion or EBID.

PhD Project

Quantum transport in graphene edges and grain boundaries

Objectives: Performance of graphene-based electronic devices is limited by defects including edges and grain boundaries. This project aims at clarification and characterisation of the effect of these defects in graphene-based devices, both in epitaxial and CVD graphene. Charge transport will be studied in these devices, particularly in the quantum Hall regime, where the grain boundary may serve as a metallic short between the edge states, preventing complete vanishing of the longitudinal resistance and exact resistance quantisation.

In collaboration with ESR15, I will work on production of Graphene nanoribbons by annealing a silicon carbide substrate in an argon atmosphere at temperatures of 2000 °C. Thus obtained few nanometer high steps serve as a seed for graphene growth. During the growth, the edges of the nanoribbons are self-organised. My objective is to study the effect of this self-organisation on charge transport and explore new possibilities for the charge- and spin-based electronics opened in these self-organised nanostructures. The nanoribbons contacted by standard EBL will be measured in magneto-transport at various temperatures in order to determine the mean free path and phase coherence length of the charge carriers.

The edge disorder often governs electronic properties, leading to CB and masking quantum confinement. By looking at the CB effects in gated nanoribbon devices, I will evaluate and characterise the degree of the edge irregularity achievable and aim at the observation of quantum confinement effects in the ribbons. Local spectroscopy experiments will be pursued using a cryogenic STM at CNRS in collaboration with ESR2.

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