Spontaneous Spin Polarization in Quantum Point Contacts

We use spatial spin separation by a magnetic focusing technique to probe the polarization of quantum point contacts. The point contacts are fabricated from $p$-type $\mathrm{GaAs}/\mathrm{AlGaAs}$ heterostructures. A finite polarization is measured in the low-density regime, when the conductance of a point contact is tuned to $<2e^2/h$. Polarization is stronger in samples with a well-defined “0.7 structure.”