Dynamics of impurity in the environment of Dirac fermions

TL;DR

This paper investigates the impurity dynamics on topological insulator surfaces, revealing power-law decay of Greens functions and mobility divergence at low temperatures, indicating breakdown of quasiparticle descriptions in certain regimes.

Contribution

It develops a formalism for Greens functions of impurities interacting with Dirac fermions and analyzes their dynamics, including effects of recoil, momentum, and magnetic fields.

Findings

Greens function exhibits power-law decay in long-time limit for non-recoil cases.

Impurity mobility diverges at low temperatures, especially under magnetic fields.

Quasiparticle picture breaks down in specific impurity regimes, but is restored at high momentum.

Abstract

We study the dynamics of a non-magnetic impurity interacting with the surface states of a 3D and 2D topological insulator. Employing the linked cluster technique we develop a formalism for obtaining the Greens function of the mobile impurity interacting with the low-energy Dirac fermions. We show that for the non-recoil case in 2D, similar to the case involving the parabolic spectrum, the Greens function in the long-time limit has a power-law decay in time implying the breakdown of the quasiparticle description of the impurity. The spectral function, in turn, exhibits a weak power-law singularity. In the recoil case, however, the reduced phase-space for scattering processes implies a non-zero quasiparticle weight and the presence of a coherent part in the spectral function. Performing a weak coupling analysis we find that the mobility of the impurity reveals a divergence at low temperatures. In addition, we show that the Greens function of an impurity interacting with the helical edge modes (surface states of 2D TI), exhibit power-law decay in the long-time limit for both the non-recoil and recoil case (with low impurity momentum), indicating the break down of the quasiparticle picture. However, for impurity with high momentum, the quasiparticle picture is restored. Using the Boltzmann approach we show that the presence of the magnetic field results in a power-law divergence of the impurity mobility at low-temperatures.