Microwave Photoconductivity of Gapless Dirac Fermions in HgTe Quantum Wells

The microwave photoconductivity of a system of gapless Dirac fermions in HgTe quantum wells with a critical thickness has been experimentally and theoretically investigated. It has been found that the photoconductivity fluctuates depending on the gate voltage near the Dirac point, and the fluctuation amplitude increases with an increase in the conductor size and a decrease in temperature. A theoretical explanation of the microwave response based on the assumption of the existence of a percolation two-dimensional fractal network of helical edge current states induced by fluctuations in the well thickness near the critical value has been proposed. It has been shown that the microwave photoconductivity of this network fluctuates with a change in the Fermi energy and the behavior of the fluctuation amplitude is in qualitative agreement with the corresponding experimental data.