Development of graphene-based sensors for scanning Hall microscopy and imaging of vortex matter in unconventional superconductors

Student thesis: Doctoral ThesisPhD

Abstract

The isolation of graphene in 2004 started a new development in the field of condensed matter physics and materials science to use graphene’s impressive electronic properties and mechanical versatility to improve upon existing electrical devices. One of several types of key devices that graphene could become incorporated into is the Hall-effect magnetic field sensor. Within this project CVD graphene Hall probes with wire widths from 1.5mu m down to 50nm have been fabricated and characterised to explore the possibility of replacing III-V semiconductor-based 2D electron gas Hall probes that are the current standard in scanning Hall probe microscopy. The minimum detectable field of the graphene Hall probes was estimated by measuring their Hall coefficients, and characterising the low frequency electronic noise between 1Hz and 1kHz using a dynamic spectrum analyser. The minimum detectable field was mapped as a function of wire width, carrier density and current density. This revealed that it can be optimised by increasing the current density, with performance improvements being particularly pronounced in devices with micrometer-scale dimensions. The minimum detectable field can also be tuned by back gating the devices on a Si/SiO2 substrate, allowing the carrier density and type to be controlled. In this way, the minimum detectable field can be substantially improved by reducing the carrier density close to the charge neutrality point. In practice its dependence on the wire width is surprisingly weak. However, a large increase in the minimum detectable field is seen when the width drops below 85nm, associated with a drop in mobility as edge roughness-induced charge scattering begins to dominate. To improve the minimum detectable field, graphene devices passivated by HSQ and encapsulated in exfoliated hBN flakes have been fabricated and characterised. Passivation by HSQ yields a small improvement in the electrical mobility as estimated from Hall measurements, while the biggest improvements are found by encapsulation with hBN. This improvement in mobility substantially improves the minimum detectable fields, with encapsulated devices exhibiting an order of magnitude better detection limit than unencapsulated CVD graphene. Resistance versus back gate voltage measurements on devices encapsulated with hBN also show significantly lower extrinsic doping levels than unencapsulated devices, thus removing the need to gate such devices to achieve optimal carrier densities near the CNP. These encouraging results reveal that encapsulated graphene is a promising material for the sensors of the next generation of high resolution scanning Hall probe microscopes.

Scanning Hall probe microscopy with a GaAs/AlGaAs 2D electron gas-based Hall probe has been used to explore the unique interplay between superconductivity and ferromagnetism in the Fe-based material, RbEuFe4As4. Quantitative magnetic images of superconducting vortices allows the direct extraction of the superconducting penetration depth from the vortex profiles as a function of temperature. This reveals a pronounced increase in the penetration depth near the magnetic transition temperature, in good agreement with a recently developed model describing the change in superfluid density as a result of the ordering of the magnetic moments. This shows that there is a significant interplay between the two types of order within the material, yet the exchange interaction between superconducting and magnetic sublattices is still weak enough that both are able to coexist. Scanning Hall probe microscopy has also been used to characterise the vortex pinning landscape in second generation GdBaCuO high temperature superconducting tapes. Local Hall probe magnetometry was employed to estimate the critical current, while topographic and magnetic imaging reveals that the films contain strong distinct vortex pinning sites. Comparing magnetic and topographic images reveals that these are most likely linked to the GdO second phases formed during thin film growth, as opposed to grain boundaries and normal CuO phases that protruding from the surface of the superconducting layer. This suggests that the engineering of high critical current tapes should focus on optimising the film microstructure and pinning sites during growth, rather than the addition of extra artificial pinning post-growth sites.
Date of Award26 May 2021
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorSimon Bending (Supervisor) & Daniel Wolverson (Supervisor)

Keywords

  • Graphene
  • Unconventional
  • Superconductors
  • Hall
  • Sensors
  • Boron nitride

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