Quantized conductance with non-zero shot noise as a signature of Andreev edge state

TL;DR

This study demonstrates that non-zero shot noise with quantized conductance in a graphene quantum Hall-superconductor junction reveals the presence of Andreev edge states, providing a new detection method for topological excitations.

Contribution

We show that shot noise measurements can detect Andreev edge states in QH-SC junctions, revealing a half-Fano factor signature not observable through conductance alone.

Findings

Fano factor approaches 0.5 below the superconducting gap

Electrical conductance remains quantized at 2e^2/h across the gap

Shot noise provides a signature of Andreev edge states

Abstract

Electrical conductance measurements have limited scope in identifying Andreev edge states (AESs), which form the basis for realizing various topological excitations in quantum Hall (QH) - superconductor (SC) junctions. To unambiguously detect AESs, we measure shot noise along with electrical conductance in a graphene based QH-SC junction at integer filling nu=2. Remarkably, we find that the Fano factor of shot noise approaches half when the bias energy is less than the superconducting gap, whereas it is close to zero above the superconducting gap. This is striking, given that, at the same time, the electrical conductance remains quantized at 2e^2/h within and above the superconducting gap. A quantized conductance is expected to produce zero shot noise due to its dissipationless flow. However, at a QH-SC interface, AESs carry the current in the zero-bias limit and an equal mixing of electron and hole like states produces half of the Poissonian shot noise with quantized conductance. The observed results are in accord with our detailed theoretical calculations of electrical conductance and shot noise based on non-equilibrium Green's function method in the presence of disorder. Our results pave the way in using shot noise as a detection tool in the search of exotic topological excitations in QH-SC hybrids.