Upstream Neutral Modes in the Fractional Quantum Hall Effect Regime: Heat Waves or Coherent Dipoles

Counterpropagating (upstream) chiral neutral edge modes, which were predicted to be present in hole-conjugate states, were observed recently in a variety of fractional quantum Hall states (nu = 2/3, nu = 3/5, nu = 8/3, and nu = 5/2), by measuring the charge noise that resulted after partitioning the neutral mode by a constriction (denoted, as N -> C). Particularly noticeable was the observation of such modes in the nu = 5/2 fractional state-as it sheds light on the non-Abelian nature of the state's wave function. Yet, the nature of these unique, upstream, chargeless modes and the microscopic process in which they generate shot noise, are not understood. Here, we study the ubiquitous nu = 2/3 state and report of two main observations: First, the nature of the neutral modes was tested by "colliding" two modes, emanating from two opposing sources, in a narrow constriction. The resultant charge noise was consistent with local heating of the partitioned quasiparticles. Second, partitioning of a downstream charge mode by a constriction gave birth to a dual process, namely, the appearance of an upstream neutral mode (C -> N). In other words, splitting "hole conjugated" type quasiparticles will lead to an energy loss and decoherence, with energy carried away by neutral modes.