UBC Theses and Dissertations
Spin effects in quantum point contacts Ren, Yuan
Quantum point contacts (QPCs) are narrow constrictions between large reservoirs of two-dimensional electron gas, with conductance quantized in units of G=2e²/h at zero magnetic field. Despite decades of investigation, some conductance features of QPCs remain mysterious, such as an extra conductance plateau at 0.7(2e²/h) (0.7 structure) and a zero-bias peak (ZBP) in nonlinear conductance. In this thesis, we present experimental studies of transport anomalies in QPCs, aiming at shedding more light on these features. Conductance measurements are performed for ZBPs in a much wider range than in most previous work, focused especially on the low- and high-conductance regimes. The Kondo model and a model of subband motion are compared with experimental results, but both of them fall short of explaining the data. The subband-motion model is not spin-dependent, so it conflicts with the spin-related nature of ZBPs as confirmed by measurements of nuclear spin polarization in QPCs in an in-plane magnetic field. However, the motion of subbands and the spin dependence of these motions are clearly shown by thermopower spectroscopy. These results may help understand the origin of ZBPs and 0.7 structure.
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