Observation of neutral modes in the fractional quantum Hall regime

TL;DR

This paper reports the direct detection of neutral modes in fractional quantum Hall states, providing insights into their edge structure and implications for non-abelian wavefunctions at filling 5/2.

Contribution

First direct observation of counter-propagating neutral modes in fractional quantum Hall states at fillings 2/3, 3/5, and 5/2, advancing understanding of edge excitations.

Findings

Neutral modes cause shot noise proportional to applied voltage.

Presence of neutral modes affects Fano factor and temperature of backscattered charge.

Observation at 5/2 may indicate non-abelian wavefunctions.

Abstract

In the quantum Hall effect regime, taking place in a two-dimensional-electron gas under strong magnetic field, currents flow along the edges of the sample. For some particle-hole conjugate states of the fractional regime, e.g., with filling between 1/2 and 1 of the lowest Landau level; early predictions suggested the presence of counter-propagating edge currents in addition to the expected ones. When this did not agree with the measured conductance, it was suggested that disorder and interactions will lead to counterpropagating modes that carry only energy - the so called neutral modes. In addition, a neutral upstream mode (Majorana mode) was also expected for selected wavefunctions proposed for the even denominator filling 5/2. Here we report on the direct observation of counter-propagating neutral modes in fillings 2/3, 3/5 and 5/2. This was done by injecting such modes and allowing them to impinge on a narrow constriction, which partly reflected them, with two main observed effects: (a) A resultant shot noise proportional to the applied voltage on the injecting contact; (b) With simultaneously injecting also a charge mode, the presence of the neutral mode was found to significantly affect the Fano factor and the temperature of the backscattered charge mode. In particular, such observation for filling 5/2, may single out the non-abelian wavefunctions for the state.