Restricted and unrestricted Hartree-Fock calculations of conductance for a quantum point contact

TL;DR

This paper uses restricted and unrestricted Hartree-Fock calculations to analyze the 0.7 conductance anomaly in quantum point contacts, examining electron interactions and spin effects, and clarifies the underlying mechanism.

Contribution

It compares restricted and unrestricted Hartree-Fock methods to validate the electron interaction effects and spin dependence in quantum point contact conductance.

Findings

Unrestricted Hartree-Fock reveals spin-dependent bound states.

Results do not support the Kondo model for the 0.7 structure.

The effect depends on electron density in leads.

Abstract

Very short quantum wires (quantum contacts) exhibit a conductance structure at a value of conductance close to $0.7 \times 2e^2/h$. It is believed that the structure arises due to the electron-electron interaction, and it is also related to electron spin. However details of the mechanism of the structure are not quite clear. Previously we approached the problem within the restricted Hartree-Fock approximation. This calculation demonstrated a structure similar to that observed experimentally. In the present work we perform restricted and unrestricted Hartree-Fock calculations to analyze the validity of the approximations. We also consider dependence of the effect on the electron density in leads. The unrestricted Hartree-Fock method allows us to analyze trapping of the single electron within the contact. Such trapping would result in the Kondo model for the ``0.7 structure''. The present calculation confirms the spin-dependent bound state picture and does not confirm the Kondo model scenario.