Bias Dependent Subband Edges and the 0.7 Conductance Anomaly

The 0.7 (2e2/h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts by measuring the differential conductance G as a function of source-drain bias Vsd and gate-source bias Vgs as well as a function of temperature. In the Vgs- Vsd plane we use a grayscale plot of the transconductance dG/dVgs to map out the bias dependent transitions between the normal and anomalous conductance plateaus. Any given transition is interpreted as arising when the bias controlled chemical potential μd (μs) of the drain (source) reservoir crosses a subband edge εα in the point contact. From the grayscale plot we extract the constant normal subband edges ε0, ε1,... and most notably the bias dependent anomalous subband edge ε'0 (μd) split off from ε0. We show by applying a finite-bias version of the recently proposed BCF model, how the bias dependence of the anomalous subband edge is the key to analyze various experimental observations related to the 0.7 anomaly.