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Noise in the helical edge channel anisotropically coupled to a local spin
We calculate the frequency-dependent shot noise in the edge states of a two-dimensional topological insulator coupled to a magnetic impurity with spin S = 1/2 of arbitrary anisotropy. If the anisotropy is absent, the noise is purely thermal at low frequencies, but tends to the Poissonian noise of the full current I at high frequencies. If the interaction only flips the impurity spin but conserves those of electrons, the noise at high voltages eV ≫ T is frequency-independent. Both the noise and the backscattering current Ibs saturate at voltage-independent values. Finally, if the Hamiltonian contains all types of non-spin-conserving scattering, the noise at high voltages becomes frequency-dependent again. At low frequencies, its ratio to 2eIbsis larger than 1 and may reach 2 in the limit Ibs → 0. At high frequencies it tends to 1.