Electrical read-out of the local nuclear polarization in the quantum Hall effect

TL;DR

This paper demonstrates a method to electrically read out local nuclear polarization in the quantum Hall effect by utilizing edge states, revealing that nuclear spin polarization can act as an energy source exceeding Zeeman splitting.

Contribution

It introduces a novel electrical measurement technique for local nuclear polarization in the quantum Hall regime using edge state configurations.

Findings

Achieved near-maximum local nuclear polarization via current-driven spin exchange.

Observed voltage signals up to a few tenths of meV decaying with nuclear relaxation.

Explained the energy source as a local Overhauser shift affecting edge state reconstruction.

Abstract

It is demonstrated that the now well-established `flip-flop' mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization --close to the maximum value achievable-- by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a meV, which decays with a time constant typical for the nuclear spin relaxation. The surprizing fact that a sample with a local nuclear spin polarization can act as a source of energy and that this energy is well above the nuclear Zeeman splitting is explained by a simple model which takes into account the effect of a local Overhauser shift on the edge state reconstruction.