Observation of neutral modes in the fractional quantum Hall regime

• Published in: Nature (London) Vol. 466; no. 7306; pp. 585 - 590
• Main Authors: Bid, Aveek, Ofek, N., Inoue, H., Heiblum, M., Kane, C. L., Umansky, V., Mahalu, D.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 29.07.2010; Nature Publishing Group
• Subjects: 639/766/119; 639/766/25; 639/766/483; Backscattering; Charge; Classical and quantum physics: mechanics and fields; Conjugates; Exact sciences and technology; Humanities and Social Sciences; Magnetic fields; multidisciplinary; Noise; Observations; Physics; Quantum Hall effect; Science; Science (multidisciplinary); Shot noise; Upstream; Wavefunctions; United States; Italy, Marche, Pesaro, Fano; Quasiparticles; High field; Quantum Hall effect; Wave functions; Shot noise; Landau levels; Magnetic fields; Electron gas
• DOI: 10.1038/nature09277
• ISSN: 0028-0836; 1476-4687

The quantum Hall effect takes place in a two-dimensional electron gas under a strong magnetic field and involves current flow along the edges of the sample. For some particle–hole conjugate states of the fractional regime (for example, with fillings 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 counter-propagating modes that carry only energy—the so called neutral modes. In addition, a neutral upstream mode (the Majorana mode) was expected for selected wavefunctions proposed for the even-denominator filling 5/2. Here we report the direct observation of counter-propagating neutral modes for fillings of 2/3, 3/5 and 5/2. The basis of our approach is that, if such modes impinge on a narrow constriction, the neutral quasiparticles will be partly reflected and fragmented into charge carriers, which can be detected through shot noise measurements. We find that the resultant shot noise is proportional to the injected current. Moreover, when we simultaneously inject 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 observations for filling 5/2 may single out the non-Abelian wavefunctions for the state. Neutral modes unmasked The quantum Hall effect involves current propagation along the edges of a two-dimensional electron gas in a strong magnetic field. It was predicted more than a decade ago that in the fractional regime (characterized by quasiparticles carrying fractional charges), counter-propagating modes would develop, carrying energy but not charge. These 'neutral modes' have proved elusive in experimental situations, but they have now been observed through measurements of electrical noise. This discovery opens up a new area of study, adding previously unknown energy modes to the well-studied charge modes in this field. The quantum Hall effect takes place in a two-dimensional electron gas under a strong magnetic field and involves current flow along the edges of the sample. In the fractional regime, counter-propagating modes that carry energy but not charge — the so-called neutral modes — have been predicted but never observed. These authors report the first direct observation of these elusive modes.