Abstract
The quantum Hall effect (QHE) is used realise the standard for the unit of electrical resistance, the ohm, in terms of fundamental constants h=e^2. Epitaxial graphene on silicon carbide has become the system of choice for this because of the large inter-Landau level spacing, fast energy relaxation and very robust quantum Hall state, which is owed to the charge transfer between graphene and the substrate. There are however non-equilibrium dynamics in the quantum Hall regime in graphene, which need to be understood in order to better implement the resistance standard in the future.
This thesis concerns the study of this non-equilibrium dynamics in graphene, and in particular the breakdown of the QHE. In conventional semiconductor 2DEGs it is achieved by the study of the spectrum of the terahertz radiation emitted from the hot spots, where
the electrons enter and leave the 2D conductor. In graphene the non-equilibrium dynamics can be investigated using the weak localisation
corrections to the conductivity, which are sensitive to any time-reversal symmetry breaking perturbation, including temperature, magnetic field or ac electric field. High-frequency microwave radiation was employed to generate hot electrons and showed that the response
of graphene is entirely bolometric.
The response to MIR radiation (5-10 um) was studied by measuring magnetoresistance of a graphene Hall bar in a wide range of magnetic fields. At intermediate magnetic fields, the response is a combination of the direct bolometric effect and a change in the carrier density due to radiation. The cyclotron resonance did not appear as expected at the appropriate magnetic fields, which is discussed.
Finally, the breakdown of the QHE in graphene was investigated by measuring cross-spectral noise power density over a 500 kHz window centred at 3.22 MHz. Comparing the spectra to those of a GaAs sample it seems that the breakdown in graphene may occur in the bulk of the graphene sample rather than in well-defined hot spots.
This thesis concerns the study of this non-equilibrium dynamics in graphene, and in particular the breakdown of the QHE. In conventional semiconductor 2DEGs it is achieved by the study of the spectrum of the terahertz radiation emitted from the hot spots, where
the electrons enter and leave the 2D conductor. In graphene the non-equilibrium dynamics can be investigated using the weak localisation
corrections to the conductivity, which are sensitive to any time-reversal symmetry breaking perturbation, including temperature, magnetic field or ac electric field. High-frequency microwave radiation was employed to generate hot electrons and showed that the response
of graphene is entirely bolometric.
The response to MIR radiation (5-10 um) was studied by measuring magnetoresistance of a graphene Hall bar in a wide range of magnetic fields. At intermediate magnetic fields, the response is a combination of the direct bolometric effect and a change in the carrier density due to radiation. The cyclotron resonance did not appear as expected at the appropriate magnetic fields, which is discussed.
Finally, the breakdown of the QHE in graphene was investigated by measuring cross-spectral noise power density over a 500 kHz window centred at 3.22 MHz. Comparing the spectra to those of a GaAs sample it seems that the breakdown in graphene may occur in the bulk of the graphene sample rather than in well-defined hot spots.
| Original language | English |
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| Qualification | Ph.D. |
| Awarding Institution |
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| Supervisors/Advisors |
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| Thesis sponsors | |
| Award date | 1 Jun 2017 |
| Publication status | Unpublished - 2017 |
Keywords
- epitxial graphene
- weak localization
- quantum Hall effect
- noise measurement
- photoconductivity