Spin dynamics in a quantum point contact showing the 0.7-anomaly

The 0.7-anomaly in the first conductance step of a quantum point contact is believed to arise from an interplay of geometry, spin dynamics and interaction effects. Various scenarios have been proposed to explain it, each evoking a different concept, including spontaneous spin polarization, or a quasi-localized state, or ferromagnetic spin fluctuations, or a van Hove ridge (a geometry-induced maximum in the density-of states). Though these scenarios differ substantially regarding numerous details, they all imply anomalous dynamics for the spins in the vicinity of the QPC. We have performed a detailed study of this spin dynamics in the central region of a parabolic quantum point contact, by using the functional renormalization group to calculate the dynamical spin-spin correlation function $\chi(x,x',\omega) = \int_0^\infty \langle S_z(x,t) S_z(x',0) \rangle e^{i \omega t} $. We will discuss its behavior as function of frequency, interaction strength and gate voltage and comment on the implications of these results for each of the above-mentioned scenarios.