Fractional Quantum Hall effect
Electrical conduction in the quantum Hall regime is through co-propagating one dimensional edge excitations moving in a direction dictated by the magnetic field the so called ‘downstream’ direction. It was theoretically proposed that in certain fractional states (like the ‘hole conjugate’ states at filling factors 2/3, 3/5, 5/2), however there can exist counter-propagating channels on edges of the device – the ‘upstream’ modes. These modes were predicted to carry only energy and no net electrical current and hence were dubbed neutral modes. A neutral upstream mode (Majorana mode) was also expected for certain wave-functions for the even denominator filling 5/2. Although predicted more than two decades ago as a crucial ingredient to our understanding of the FQH effect, these neutral modes had not been experimentally detected until very recently.
Recently a novel technique was developed by us using which these neutral modes could be detected by the effect they have on the shot noise produced at a quantum point contact (QPC). This was done by injecting such modes and allowing them to impinge on a narrow constriction, which partly reflected them. It was observed that this process resulted in the generation of shot noise proportional to the voltage applied to the neutral mode contact. When in addition to the neutral mode, a charge mode also was simultaneously introduced, the presence of the neutral mode was found to significantly affect the Fano factor and the temperature of the backscattered charge mode.
We could further show using this technique that the ground state of the quantum Hall state 5/2 is most likely a non-abelian state making it a very strong candidate for application in Topological Quantum computing. Although it was shown that neutral modes exist for certain fractional quantum Hall states, very little is actually known about these modes, for example, unknowns are the energy they carry; their decay length scales; their temperature dependence; their velocity; and how they interact with charge modes. Thus a complete understanding of the nature of these new ‘quasiparticles’ will require extensive further studies.
