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Title: Nuclear spin phenomena in III-V and II-VI semiconductor quantum dots
Author: Ragunathan, Gautham
ISNI:       0000 0004 8504 8846
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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This thesis contains experimental observations of fundamental nuclear spin properties in single self-assembled quantum dots (QDs). Using optically detected nuclear magnetic resonance (NMR) techniques, we investigate both the dense quadrupolar nuclear spin system of III-V InGaAs/GaAs dots and for the first time the dilute nuclear spin bath of II-VI CdTe/ZnTe QDs at large external magnetic fields. Non-resonant optical pumping can be used to demonstrate dynamic nuclear polarisation (DNP) of the spin bath (> 60 %) and changes in the nuclear magnetisation can be probed through changes in the hyperfine shifts of the spectral exciton emission lines. We present the first direct measurement of DNP in the CdTe/ZnTe QD system containing only a few hundred nuclear spins. We observe a robust nuclear polarisation formed via quantum well states and a system governed by a strong Knight field (from the confined electron) as well as spectral wandering effects due to charge fluctuations in the environment. Using a newly developed continuous wave (cw) NMR technique as a tool for QD structural analysis, the structure of single, self-assembled QDs (with only 105 nuclei) in different InGaAs/GaAs samples were determined. The first measurement of NMR signal from low strain nuclei revealed shallow, disc-like shapes for the QDs with the electron wavefunction penetrating significantly into the weakly strained surrounding GaAs barrier layers. Finally, we compare the nuclear spin bath properties for an InGaAs/GaAs QD with and without a resident electron. Using cw radiofrequency excitation with a spectral shape of a frequency comb, the underlying homogenous nuclear linewidth could be extracted from the large strain induced inhomogeneous broadening. By all indications, the presence of a single electron spin trapped in a QD has a detrimental effect on the nuclear spin bath due to cotunnelling with electrons in the Fermi sea and the Knight field.
Supervisor: Chekhovich, Evgeny ; Skolnick, Maurice Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available