The Joint Condensed Matter Seminar (JCMS) series is organised by KTH Royal Insitute of Technology, Nordita, and Stockholm University.
On Monday, May 29th, 2023 from 11 am to 12 am we will host a seminar by Christian Spånslätt from Chalmers University of Technology.
Detecting neutral modes in fractional quantum Hall edge transport
The composition of edge modes on boundaries of quantum Hall (QH) states is crucial for understanding their low energy transport properties. In turn, this understanding is pivotal in many situations, e.g., for using QH edges to detect and manipulate anyons.
The integer and simple fractional QH states, e.g. at fillings 1 and 1/3, have edge modes propagating only in one direction, called “downstream”. In contrast, many other states, e.g., at fillings 2/3 and 3/5, are predicted to host edge modes propagating also in the opposite, “upstream”, direction. However, non-universal edge phenomena, such as disorder and interactions, tend to cause all charge to flow downstream, causing the upstream modes to become charge-neutral. This effect makes the detection of such modes a challenging problem in edge transport experiments.
In this talk, I describe why and how the detection of excess charge noise on a current biased QH edge segment solves this problem and thus acts as a smoking gun for the presence of upstream neutral modes [1]. I also present the results of a recent experiment [2] in graphene detecting such noise at fillings 2/3 and 3/5, but not at fillings 1 and 1/3, in full accordance with theoretical predictions. These findings provide the means to investigate generic QH edges, in particular the edge of the exotic non-Abelian 5/2 state [3].
[1] Topological classification of shot noise on fractional quantum Hall edges, C. Spånslätt, J. Park, Y. Gefen, and A.D. Mirlin, Phys. Rev. Lett. 123, 137701 (2019).
[2] Observation of ballistic upstream modes at fractional quantum Hall edges of graphene, R. Kumar, S.K. Srivastav, C. Spånslätt, K. Watanabe, T. Taniguchi, Y. Gefen, A.D. Mirlin, and A. Das Nat. Commun. 13, 213 (2022).
[3] Thermal conductance and noise of Majorana modes along interfaced ν=5/2 fractional quantum Hall states, M. Hein and C. Spånslätt arXiv:2211.08000.