Sub- to super-Poissonian crossover of current noise in helical edge states coupled to a spin impurity in a magnetic field

Edge states of two-dimensional topological insulators are helical and single-particle backscattering is prohibited by time-reversal symmetry. In this paper, we show that an isotropic exchange coupling edge (HES) to a spin 1/2 impurity subjected magnetic field results in characteristic current noise (BCN) as function bias voltage tilt angle between the direction quantization axis HES. particular, find transitions from sub-Poissonian (antibunching) super-Poissonian (bunching) behavior direct consequence helicity state electrons. We use method full counting statistics within master equation approach treating spin-1/2 HES perturbatively. express BCN via coincidence correlation functions scattering processes HES, which gives precise interpretation Fano factor terms bunching antibunching electron jump events. also investigate effect electron-electron interactions Tomonaga-Luttinger liquid theory.