Does shot noise always provide the quasiparticle charge?

TL;DR

This paper investigates the universality of shot noise and quasiparticle charge measurements in quantum Hall states, revealing the influence of neutral modes and proposing a new theoretical framework for understanding Fano factors.

Contribution

It introduces a new paradigm explaining the universal Fano factor through the interplay of charge and neutral modes, challenging previous assumptions about shot noise.

Findings

Fano factor equals the bulk filling factor at very low temperatures.

Unexpected Fano factors observed on intermediate conductance plateaus.

Neutral modes influence shot noise measurements and Fano factor universality.

Abstract

The fractional charge of quasiparticles is a fundamental feature of quantum Hall effect (QHE) States. The charge has long been measured via shot-noise at moderate temperatures ($T$>30mK), with the Fano factor $F=e^*/e$ revealing the charge $e^*$ of the quasiparticles. However, at sufficiently low temperatures ($T\approx$10mK), we consistently find $F$ being equal to the bulk filling factor, $\nu_b$. Surprisingly, noise with $F=\nu_b$ is also observed on intermediate conductance plateaus in the transmission of the quantum point contact (QPC), where shot noise is not expected. We attribute the unexpected Fano factor to upstream neutral modes, which proliferate at the lowest spinless Landau level. The universality of the Fano factor is also confirmed when the edge modes do not conform to the bulk. For this, the ubiquitous edge modes at the periphery of the sample are replaced by artificially constructed 'interface modes', propagating at the interface between two adjoined QHE states: the tested state and a different state. We present a new theoretical paradigm based on an interplay between charge and neutral modes, explaining the origin of the universal Fano factor.