Excess equilibrium noise in topological SNS junction between chiral Majorana liquids

TL;DR

This paper studies the noise behavior in a topological Josephson junction with chiral Majorana modes, revealing enhanced equilibrium noise at low temperatures and phase degeneracy points, contrasting with conventional junctions.

Contribution

It introduces a scattering formalism and Majorana representation to analyze noise in topological SNS junctions, highlighting unique temperature and phase-dependent noise features.

Findings

Equilibrium noise is enhanced to a temperature-independent level near phase degeneracy points.

At high temperatures, noise matches Johnson-Nyquist behavior for single-channel wires.

The impedance's real part remains non-zero, indicating gapless Majorana modes.

Abstract

We consider a Josephson contact mediated by 1D chiral modes on a surface of a 3D topological insulator with superimposed superconducting and magnetic layers. The system represents an interferometer in which 1D chiral Majorana modes on the boundaries of superconducting electrodes are linked by ballistic chiral Dirac channels. We investigate the noise of the Josephson current as a function of the dc phase bias and the Aharonov-Bohm flux. Starting from the scattering formalism, a Majorana representation of the Keldysh generating action for cumulants of the transmitted charge is found. At temperatures higher than the Thouless energy $E_{\rm Th}$, we obtain the usual Johnson-Nyquist noise, $4G_0 k_{\rm B}T$, characteristic for a single-channel wire with $G_0 \equiv e^2/(2\pi \hbar)$. At lower temperatures the behavior is much richer. In particular, the equilibrium noise is strongly enhanced to a temperature-independent value $\sim G_0 E_{\rm Th}$ if the Aharonov-Bohm and superconducting phases are both close to $2\pi n$, which are points of emergent degeneracy in the ground state of the junction. The equilibrium noise is related to the Josephson junction's impedance via the fluctuation-dissipation theorem. In a striking contrast to usual Josephson junctions (tunnel junctions between two $s$-wave superconductors), the real part of the impedance does not vanish, reflecting the gapless character of Majorana modes in the leads.