Bias and temperature dependence of the 0.7 conductance anomaly in quantum point contacts

The 0.7 (2e²/h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e²/h) at finite bias. On varying the gate voltage the transition between the 1.0 and 0.85 (2e²/h) plateaus occurs for definite bias voltages, which define a gate-voltage-dependent energy difference Δ. This energy difference is compared with the activation temperature T_a extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find Δ = k_BT_a, which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge Δ below the chemical potential becomes thermally depopulated as the temperature is increased.