Observation of neutral modes in the fractional quantum Hall regime (1005.5724v1)

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.

Observation of Neutral Modes in the Fractional Quantum Hall Regime

The paper under discussion reports on the direct observation of counter-propagating neutral modes in the fractional quantum Hall effect (FQHE) regime, focusing on particular filling fractions, specifically in the 2/3, 3/5, and 5/2 states. These observations mark a significant advancement in understanding the dynamics of neutral modes, which were theorized but had eluded experimental confirmation until now. The paper was conducted in a two-dimensional electron gas (2DEG) under strong magnetic fields, creating conditions conducive to observing the quantum Hall effects along the edges of the samples.

The researchers employed a quantum point contact (QPC) constriction to paper these neutral modes. By injecting and reflecting these modes at a QPC, the researchers observed shot noise that varied with the applied voltage. This variation indicated the presence of neutral modes affecting both the Fano factor and the temperature of the backscattered charge mode. Of particular interest is the filling 5/2, where such observations may indicate the presence of non-abelian wavefunctions.

Key Findings

1. Observation of Neutral Modes:
   • The paper provides the first empirical evidence for the existence of neutral modes in specific fractional quantum Hall states. At fillings 2/3, 3/5, and 5/2, the researchers observed upstream neutral modes via the shot noise generated at the QPC constriction.
2. Impact on Charge Modes:
   • It was demonstrated that neutral modes, while carrying no net charge themselves, influence the behavior of charge modes. This interaction was evident through the modifications in shot noise when both charge and neutral modes co-occurred at the QPC.
3. Dependence on Filling Factors:
   • For the 2/3 filling, a neutral mode propagates upstream, corroborated by the shot noise measurements. The presence of this mode suggests that neutral quasiparticles are fragmented into charge carriers upon interaction with the QPC.
   • The 5/2 state provided evidence supporting the presence of a neutral Majorana mode, challenging the assumptions of Abelian states at this filling and implying a more complex underlying physics potentially involving non-abelian states.
4. Fractional States Examination:
   • Fractional states 1/3, 2/5, and 1 were analyzed, showing an absence of similar effects, confirming that neutral modes are specific to certain fillings, primarily the ones with complex edge reconstruction.

Implications for Theory and Experimentation

The results of this paper have notable implications for theoretical models of quantum Hall systems, particularly those involving edge states and quasiparticles. The confirmation of neutral modes in the 5/2 state might be pivotal in the pursuit of topological quantum computing, given the hypothesized non-abelian statistics of these states.

Furthermore, identifying and characterizing these neutral modes broaden experimental methodologies for quantum Hall systems. The QPC-based approach to measuring shot noise and its correlation to neutral mode presence could serve as a standard technique for future studies.

Future Directions

This research opens several avenues for future exploration. One immediate pathway would involve a deeper theoretical investigation into the non-linearity of interactions between charge and neutral modes, potentially bridging gaps between theoretical predictions and experimental realities. Further studies could focus on isolating and characterizing the specific properties of Majorana modes anticipated in the 5/2 state. Additionally, understanding temperature dependencies and the microscale dynamics of these neutral modes remains a fertile ground for research.

In conclusion, this paper provides a significant stride in experimental condensed matter physics, offering valuable insights and methodologies that enhance our understanding of edge state dynamics in the FQHE regime. The implications for theoretical physics and potential applications, particularly in quantum computing, make this a critical area for continued investigation.