||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, many transport properties of QPCs remain mysterious, including the extra plateau commonly observed around 0.7x2e2/h (0.7structure) and a zero-bias peak in conductance on the steps between plateaus. Differential conductance is often used to study these anomalies, but this type of measurement is difficult to interpret and the underlying mechanisms are still a matter of debate. Here, we present measurements of differential thermopower (as opposed to conductance) of QPCs, both at and away from the Fermi energy. These measurements provide more direct access to the QPC transmission spectrum, and can be used to detect high-entropy states such as those characterized by spin fluctuations. We clearly observe evidence of a Kondo-screened localized state in the QPC near pinchoff, as has been predicted theoretically. For higher conductance--around 0.7 structure - the localized state gives way to a diffuse band suggestive of a high degree of spin entropy. We show comparisons to (and contrasts) with numerical simulations that as an aid to understanding the microscopic origins of QPC phenomenology.
Presented by: Joshua Folk, University of British Columbia Department of Physics and Astronomy