Quasibound states and evidence for a spin-1 Kondo effect in asymmetric quantum point contacts

Linear conductance below 2e super(2)/h shows resonance peaks in highly asymmetric quantum point contacts (QPCs). As the channel length increases, the number of peaks also increases. At the same time, differential conductance exhibits zero-bias anomalies (ZBAs) in correspondence with every other peak in the linear conductance. This even-odd effect, observable in the longer channels, is consistent with the formation of correlation-induced quasilocalized states within the QPC. In rare cases, triple peaks are observed, indicating the formation of a spin-1 Kondo effect when the electron filling number is even. Changing the gate voltage tunes this spin triplet to a singlet which exhibits no ZBA. The triple peak provides evidence suggestive of a spin singlet-triplet transition in a QPC, and the presence of a ferromagnetic spin interaction between electrons.