Coherence, charging, and spin effects in quantum dots and point contacts.

摘要

A series of experiments is presented on electron transport through quantum dots and quantum point contacts in the strong-tunneling regime where electron coherence, charging, and spin play significant roles. In the first experiment, transport measurements are presented for quantum dots in the strong-tunneling Coulomb blockade (CB) regime. Fluctuations of the conductance, evidence of quantum interference, show sensitivity to charging effects in the CB valleys where elastic cotunneling is the dominant transport mechanism. The effects of interactions are accounted for using a single parameter—the charging energy (the energy required to add an additional electron to the dot)—and are measured using the magnetic field correlation length of conductance fluctuations. The second experiment shows CB in a quantum dot with one fully-transmitting lead and one weak-tunneling lead. In this system, the CB appears only due to constructive interference of backscattered trajectories. The predominant effect of interactions, namely the Coulomb blockade, is turned on and off by quantum interference. The third and fourth experiments investigate the Kondo effect, a classic many-body problem where the effects of coherence and interactions cannot be trivially separated. Quantum dots are well suited to study the Kondo effect as many parameters can be varied in dots that cannot be changed in traditional bulk metal systems. The second observation of the Kondo effect in dots is presented here, including the first demonstration of a gate voltage controlled Kondo temperature. The last part of this dissertation describes quantum point contacts (QPCs) which show a number of remarkable similarities to the Kondo effect in dots. Transport measurements of the “0.7 structure”, a robust extra plateau or shoulder-like feature in the lowest mode of a QPC, show evidence for a lifted spin-degeneracy. The formation of a zero-bias peak at low temperature, the collapse of the temperature-dependent conductance to a single curve when scaled by one parameter (the Kondo temperature), and the correspondence of this temperature scaling parameter to the source-drain bias voltage width of the zero-bias peak strongly suggest that a Kondo-like correlated many-electron state forms in the QPC at low temperatures. This suggestion is currently under discussion theoretically.