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High magnetic field studies of 2DEG in graphene on SiC and at the LaAlO³/SrTiO³ interface

Abstract : This thesis is devoted to the study of the magneto-transport properties of two dimensional electron gas (2DEG), and more specifically graphene on silicon carbide (G/SiC) as well as the interface between two complex oxides LaAlO3 / SrTiO3 (LAO/STO). We take advantage of very high magnetic field (up to 80 T) and very low temperature (down to 40 mK) to investigate the quantum transport properties, which are evocative of the underlying electronic band-structure. In G/SiC, close to the quantum Hall breakdown regime, we measure an ultra-broad quantum Hall plateau at R=h/2e² covering a magnetic field range of more than 70 T (from 7 T to 80 T). Accordingly, the longitudinal resistance is close to zero, but displays unexpected weak 1/B-periodic oscillations. Based on microscopic observations, this 2DEG is modeled as a low charge carrier density graphene matrix decorated by micrometers-size puddles with larger doping. Numerical simulations of the transport properties reproduce well both the broad Quantum Hall plateau and the presence of the oscillations. Besides the SiC substrate which acts as a charge reservoir and stabilizes the quantum Hall state at filling factor ν=2, a magnetic field dependent transfer of charges involving the puddles is responsible for the presence of the oscillating features. This original study provides new insights for resistance metrology purposes. The 2DEG arising at the interface between the complex oxides LAO and STO is nowadays envisioned for future multi-functional devices. Their electronic properties are still a matter of debate and require further investigations. The high field magneto-resistance of this 2DEG displays quasi-periodic Shubnikov-de Haas Oscillations (SdHO) and a linear Hall effect up to 55 T at low temperature. We observe a large discrepancy between the carrier density extracted from the period of the SdHO and the slope of the Hall resistance, which constitutes a strong evidence for the presence of many sub-bands crossing the Fermi energy. The quasi-periodic oscillations of the magneto-resistance are well reproduced by numerical simulations taking into account the strong Rashba effect at the interface. In addition, from the back-gate voltage evolution of the SdHO at sub-kelvin temperature, we identify the electronic sub-bands contributing to transport, the orbital symmetry from which they derive, as well as their spatial localization along the interface.
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Submitted on : Friday, November 23, 2018 - 1:13:07 PM
Last modification on : Friday, July 10, 2020 - 7:58:47 AM


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  • HAL Id : tel-01932808, version 1


Ming Yang. High magnetic field studies of 2DEG in graphene on SiC and at the LaAlO³/SrTiO³ interface. Physics [physics]. INSA de Toulouse, 2018. English. ⟨NNT : 2018ISAT0015⟩. ⟨tel-01932808⟩



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