Quantum phase transition in a parallel -coupled double quantum dot
Abstract
The interplay of strongly correlated electrons is a fundamental issue in condensed-matter physics. A well-known example manifesting this type of complicated interaction is the two-magnetic-impurities problem. The competition between Kondo binding at energy TK and antiferromagnetic-correlations due to a coupling between impurity spins J gives rise to a striking phase transition extensively studied by theorists. Moreover, the correlated behavior of a pair of impurity spins is central to our understanding of heavy-fermion systems as well. To date experiments have clearly demonstrated the quantum dot system to be an excellent model system for studying the Kondo problem. Because of these experimental breakthroughs, the coupled double quantum dots have emerged as a prototypical system for investigating the interplay between Kondo screening and interdot spin correlation. In particular, theoretical investigations have uncovered an even richer phase diagram accessible by tuning key quantum dots parameters. Intriguingly, a recent experiment has provided the first glimpse of a novel Kondo phenomenon. Nevertheless, conclusive evidence for the existence of a phase transition has yet to be observed. This thesis describes the transport properties in an artificial molecule formed by two-path, parallel-coupled double-quantum-dots. Coulomb blockade spectroscopy is systematically measured to determine how the binding energy of the artificial molecule evolves with the interdot coupling strength. In the Kondo regime, by tuning the gating voltages and interdot coupling strength to the appropriate regime, the differential conductance dI/dV exhibits a splitting in the zero-bias anomaly. As the inter-dot coupling is reduced, the splitting is observed to merge while the conductance shows a maximum at the crossover region. This merging of the splitting in conjunction with other evidence demonstrates a direct observation of a quantum phase transition associated with interdot exchange coupling in the two-impurity Kondo effect.
Degree
Ph.D.
Advisors
Chang, Purdue University.
Subject Area
Condensation
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