Resistively-detected NMR lineshapes in a local filling $\nu < 1$ quantum Hall breakdown

TL;DR

This study investigates the complex lineshapes of resistively-detected NMR signals in a quantum Hall breakdown regime of a quantum point contact, revealing spatial electron density modulation and nuclear spin polarization effects.

Contribution

It provides a systematic analysis of RDNMR lineshapes at various tunneling regimes, uncovering the relation between lineshape features and electron/nuclear spin interactions.

Findings

Observation of a snake-like pattern in transition frequencies indicating spatial electron density modulation.

Identification of dispersive lineshapes linked to dual nuclear spin polarization in weak tunneling regimes.

Evidence of spin-flip scattering occurring near the pinch-off point even at very low conductance.

Abstract

Resistively-detected NMR (RDNMR) is a unique characterization method enabling highly-sensitive NMR detection for a single quantum nanostructure, such as a quantum point contact (QPC). In many studies, we use dynamic nuclear polarization and RDNMR detection in a quantum Hall breakdown regime of a local QPC filling factor of 1 ($\nu_{\rm{qpc}} = 1$). However, the lineshapes of RDNMR signal are proved to be complicated and still not fully understood yet. Here, we systematically polarize the nuclear spins by current-pumping from the close vicinity of $\nu_{\rm{qpc}} = 1$ conductance plateau all the way down to pinch-off point, providing a clear evidence that the spin-flip scattering between two edge channels at the lowest Landau level still occurs in the constriction even when it is close to the pinch off point ($G \approx 10^{-2}$ $2e^{2}/h$). The collected RDNMR spectra reveal two sets of distinguished features. First, in a strong to intermediate tunneling regime, we observe an ordinary resistance dip lineshape but interestingly its transition frequency follows a snake-like pattern, an indicative of spatial modulation of electron density in the QPC. Second, in a weak tunneling regime, the spectrum turns into a dispersive lineshape which we interpret due to the build up of two sets of nuclear spin polarization that are in contact with different electron spin polarization.