Skyrmion Dynamics and NMR Line Shapes in QHE Ferromagnets

TL;DR

This paper models skyrmion diffusion in quantum Hall ferromagnets to explain NMR line shape transitions observed experimentally, revealing a relaxation time of about 50 microseconds and the influence of nuclear polarization profiles.

Contribution

It introduces a skyrmion diffusion model that qualitatively matches experimental NMR line shape transitions and explores the sensitivity to nuclear polarization and excitation dynamics.

Findings

Skyrmion transport relaxation time is approximately 50 microseconds.

Transition from motional narrowing to frozen line shape occurs with temperature decrease.

Spectral line shape is highly sensitive to nuclear polarization profiles.

Abstract

The low energy charged excitations in quantum Hall ferromagnets are topological defects in the spin orientation known as skyrmions. Recent experimental studies on nuclear magnetic resonance spectral line shapes in quantum well heterostructures show a transition from a motionally narrowed to a broader `frozen' line shape as the temperature is lowered at fixed filling factor. We present a skyrmion diffusion model that describes the experimental observations qualitatively and shows a time scale of $\sim 50 \mu{\rm sec}$ for the transport relaxation time of the skyrmions. The transition is characterized by an intermediate time regime that we demonstrate is weakly sensitive to the dynamics of the charged spin texture excitations and the sub-band electronic wave functions within our model. We also show that the spectral line shape is very sensitive to the nuclear polarization profile along the z-axis obtained through the optical pumping technique.