Probing helicity and the topological origins of helicity via non-local Hanbury-Brown and Twiss correlations

TL;DR

This paper investigates how non-local Hanbury-Brown and Twiss correlations in shot noise can distinguish topological helical edge modes from trivial quasi-helical modes and chiral quantum Hall states.

Contribution

It introduces non-local shot noise as a novel probe to differentiate topological from trivial edge modes and chiral states, highlighting the correlation signatures associated with each.

Findings

Positive non-local HBT correlations indicate topological helical modes.

Negative correlations are characteristic of trivial quasi-helical and chiral states.

Non-local spin correlations and Fano factors provide additional distinguishing features.

Abstract

Quantum Hall edge modes are chiral while quantum spin Hall edge modes are helical. However, unlike chiral edge modes which always occur in topological systems, quasi-helical edge modes may arise in a trivial insulator too. These trivial quasi-helical edge modes are not topologically protected and therefore need to be distinguished from helical edge modes arising due to topological reasons. Earlier conductance measurements were used to identify these helical states, in this work we report on the advantage of using the non local shot noise as a probe for the helical nature of these states as also their topological or otherwise origin and compare them with chiral quantum Hall states. We see that in similar set-ups affected by same degree of disorder and inelastic scattering, non local shot noise "HBT" correlations can be positive for helical edge modes but are always negative for the chiral quantum Hall edge modes. Further, while trivial quasi-helical edge modes exhibit negative non-local "HBT" charge correlations, topological helical edge modes can show positive non-local "HBT" charge correlation. We also study the non-local spin correlations and Fano factor for clues as regards both the distinction between chirality/helicity as well as the topological/trivial dichotomy for helical edge modes.