Scattering Theory of Current-Induced Spin Polarization

TL;DR

This paper develops a scattering theory to analyze current-induced spin polarization in mesoscopic systems with spin-orbit coupling, revealing large fluctuations and weak correlations with spin currents, supported by numerical simulations.

Contribution

It introduces a novel scattering approach linking local spin polarization to spin-dependent potentials, accounting for mesoscopic fluctuations and correlations in spin transport.

Findings

Local spin polarizations can exceed known values by large factors.

Spin polarizations show large sample-to-sample fluctuations.

Weak correlation between spin polarization and spin currents in leads.

Abstract

We construct a novel scattering theory to investigate magnetoelectrically induced spin polarizations. Local spin polarizations generated by electric currents passing through a spin-orbit coupled mesoscopic system are measured by an external probe. The electrochemical and spin-dependent chemical potentials on the probe are controllable and tuned to values ensuring that neither charge nor spin current flow between the system and the probe, on time-average. For the relevant case of a single-channel probe, we find that the resulting potentials are exactly independent of the transparency of the contact between the probe and the system. Assuming that spin relaxation processes are absent in the probe, we therefore identify the local spin-dependent potentials in the sample at the probe position, and hence the local current-induced spin polarization, with the spin-dependent potentials in the probe itself. The statistics of these local chemical potentials is calculated within random matrix theory. While they vanish on spatial and mesoscopic average, they exhibit large fluctuations, and we show that single systems typically have spin polarizations exceeding all known current-induced spin polarizations by a parametrically large factor. Our theory allows to calculate quantum correlations between spin polarizations inside the sample and spin currents flowing out of it. We show that these large polarizations correlate only weakly with spin currents in external leads, and that only a fraction of them can be converted into a spin current in the linear regime of transport, which is consistent with the mesoscopic universality of spin conductance fluctuations. We numerically confirm the theory.