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Neutral Quasi-Particles in Mesoscopic Physics

Final Report Summary - NEUTRAL (Neutral Quasi-Particles in Mesoscopic Physics)

Publishable brief summary of the project
The quantum Hall effect (QHE) is a fascinating phenomenon in two-dimensional (2D) electrons bathed in a strong perpendicular magnetic field. Under these conditions, the bulk of the 2D electrons is non-conductive, while current is moving along the edges in a form of chiral skipping orbits, which we call edge modes. The current the edge modes carry is accurately quantized (up to ten digits). However, aside from these easily measured current carrying edge modes, there are energy carrying edge modes (without net charge), which are invisible to simple electrical measurements. Yet, these modes are very important for the understanding and the characterization of the quantum state of the 2D electron system. Moreover, these heat-carrying modes may heat up the electrons, which will render them ‘classical’ particles instead of ‘quantum’ particles –interference of the ‘quantum’ electrons will then vanish.
Being the first group to measure these energy modes, we developed in the past a few measurement techniques for the observation of these modes. Here, we looked for a coincidence between the appearance of the heat modes and the disappearance of interference in the QHE regime. Indeed, such correlation was found, substantiating thus the harmful effect of these modes. Consequently, a way was paved for a search of how to suppress the heat modes to allow measuring electron interference. Such efforts will be pursued in the new ERC grant.
Aside from observing the presence of neutral modes, one needs a quantitative way to determine their ability to carry energy. We carried measurements of the thermal conductance (distinguished from the ubiquitous electrical conductance), and in particular, in exotic QHE states. These states support multiple number of counter-propagating modes, each with a different nature: e.g. modes of electrons, of fractional charges, of neutral (energy carrier) particles, and of especial charges (Majorana - type) that mimic half of a neutral fermionic particle. The total thermal conductance, being the sum of all modes’ thermal conductances, provides a conclusive determination of the true nature of the quantum state.
We carried a series of experiments of measuring the thermal conductance of a variety of quantum states; starting with the more established and relatively simple ones and the more complex and less understood. We were successful in confirming the theorized notion that electron – electron interactions, which is known to strongly affect the electrical conductance of the edge modes, does not affect the thermal conductance. Moreover, we demonstrated that the thermal conductance of the special Majorana - type modes are the only ones that carry a fraction of the thermal conductance. While this was expected, the value of the thermal conductance that we found does not agree with the expected value (via numerical methods). This brought the theoreticians back to action and the experimentalists back to the lab testing this fractional value under different conditions.
The discovery of the QHE in 1980 led to a revolution in condensed matter physics. With time, it became a prototype of new physical realizations of electronic and optical systems with properties governed by their mathematical topological symmetries. After nearly forty years, the QHE and its variety of quantum states, is still not fully understood. We are happy to add a few blocks to the erecting structure of this fascinating phenomenon, which provides a mirror to the most basic phenomena in condensed matter physics.