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Drayne, Johann Peter

University of British Columbia

The Kondo effect, first discovered in impure bulk metals during the 1930s and explained in the 1960s, has gained significant interest within the field of quantum devices. These devices offer a high degree of tunability and control, enabling rigorous testing of theoretical predictions. Previous studies on the Kondo effect have measured conductance through a quantum dot and observed a zero-bias peak between Coulomb peaks. This effect requires strong coupling between the quantum dot and leads. This work studies a relatively weak coupling, where the characteristic zero-bias peak between Coulomb peaks is not observed. However, as charge degeneracy of the quantum dot is approached, the Kondo temperature increases. This results in a small conductance enhancement at the shoulder of the Coulomb peak. We show that a simultaneous measurement of the quantum dot’s occupation can unveil the small enhancement of conductance due to the Kondo effect. To compare with Numerical Renormalisation Group (NRG) theory, conductance and occupation are measured across a range of temperatures to determine fitting parameters. Good agreement is found at a range of coupling strengths and charge sensor current setpoints. However, a discrepancy is found when the tunnel barriers to the quantum dot are asymmetrically tuned. Strong asymmetric coupling approaches a regime where the quantum dot is coupled to a single lead. A recent measurement of the entropy of a quantum dot coupled to a single lead where Kondo correlations were expected also observed a discrepancy between data and NRG. Interestingly, this work shows that conductance data displays greater Kondo enhancement than NRG, whereas previously measured entropy showed less Kondo enhancement than NRG. A direct comparison of conductance and entropy measured in the same device under similar settings holds promise for illuminating this discrepancy.

Text

2024-04-29

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/88046

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/