On Bunching Of Fractionally-Charged Quasiparticles

Shot noise measurements are an important tool for characterizing the charge of current carriers and the correlation among them. Most notable recently was the verification that quasiparticles in the Fractional Quantum Hall (FQH) regime have fractional charge q=e/n, with e the charge of the electron, n an odd integer,:and v=p/n the fractional filling of the first Landau level (such as 1/3, 2/5, 3/7, etc.). We investigate here the interaction of quasiparticles with a strong back scatterer and find unexpected results. When a weak back scatterer (with transmission t1) is introduced in the path of an otherwise noiseless current of quasiparticles (a fully occupied beam), stochastic partitioning and noise proportional to the quasiparticles charge appear. Specifically, at v=1/3 noise corresponding to a partitioned charge q=e*=e/3 is measured. However, this charge increases monotonically as backscattering becomes stronger - approaching q=e for t- > 0. In other words, only electrons (or, alternatively, three bunched quasiparticles) are found to tunnel through high potential barriers when impinged by a noiseless current of quasiparticles. Here we show that the partitioned charge, or the bunching of quasiparticles, after a strong back scatterer, depends on the average state occupation of the impinging current (the dilution of the quasiparticles beam). In the limit of a very dilute impinging current, when quasiparticles arrive one by one, bunching ceases altogether and the transferred charge approaches q=e*=e/3. These surprising results, not yet understood, prove that sparse quasiparticles, with charge e*=e/3, can tunnel through high potential barriers, originally thought to be opaque.