Dynamic nuclear spin polarization in the fractional quantum Hall effect spin transitions

TL;DR

This paper theoretically investigates how dynamic nuclear polarization influences spin transitions in the fractional quantum Hall effect, revealing mechanisms for controlling phase boundaries and potential parafermion modes via hyperfine interactions.

Contribution

It introduces a theoretical model of DNP effects on spin transitions and phase boundary reconstructions in fractional QHE systems near quantum point contacts.

Findings

Nuclear spins can be nearly fully polarized by electric current.

Overhauser fields can induce or modify phase transitions between polarized and unpolarized states.

Asymmetry in nuclear polarization and domain wall displacement depends on current direction.

Abstract

Experiments suggest that nuclear spins play a significant role in the quantum Hall effect (QHE) near integer and fractional QHE spin transitions, but many of these phenomena still remain to be understood. Here we study theoretically the dynamic nuclear polarization (DNP) in the two-dimensional electron liquid near a quantum point contact (QPC) or a domain wall between spin polarized and unpolarized phases induced by electrostatic gating in the fractional QHE at a filling factor 2/3 and analyze the dependence of the spin transition on temperature and the magnitude of the flowing current. We demonstrate that nearly all nuclear spins in the QPC or in the domain wall can be polarized by the electric current. The Overhauser effective magnetic field from the DNP can be strong enough to induce (or modify) a phase transition between polarized and unpolarized phases. This changes the gate voltages and magnetic fields required for the spin transitions, and leads to the reconstruction of the boundary between two phases and a domain wall and a current path displacement. The spread of nuclear spin polarization and the domain wall displacement are strongly asymmetric with respect to the direction of the current flow. Equilibration due to hyperfine interactions and its role on the nuclear spin polarization, domain wall displacements and spin transitions is studied. Back and forth oscillatory transitions between polarized and unpolarized phases near a source contact are discussed. Hyperfine interactions of nuclear spins provide a route for observation and control of the parafermion zero modes that can be induced when the domain wall between the polarized and unpolarized regions is placed in the proximity of a superconductor