Photoconductivity in AC-driven modulated two dimensional electron gas in a perpendicular magnetic field

TL;DR

This paper investigates microwave-induced photoconductivity oscillations and negative resistance states in a modulated two-dimensional electron gas under magnetic fields, using a non-perturbative Landau-Floquet formalism.

Contribution

The study introduces a formalism combining Landau-Floquet states with a perturbative treatment of the lattice potential to analyze photoconductivity in 2DES under microwave radiation.

Findings

Strong oscillations in conductivity and resistivity depending on microwave frequency.

Identification of specific oscillation patterns with minima and maxima related to cyclotron resonance.

Observation of negative resistance states in certain lattice parameter regimes.

Abstract

In this work we study the microwave photoconductivity of a two-dimensional electron system (2DES) in the presence of a magnetic field and a two-dimensional modulation (2D). The model includes the microwave and Landau contributions in a non-perturbative exact way, the periodic potential is treated perturbatively. The Landau-Floquet states provide a convenient base with respect to which the lattice potential becomes time-dependent, inducing transitions between the Landau-Floquet levels. Based on this formalism, we provide a Kubo-like formula that takes into account the oscillatory Floquet structure of the problem. The total longitudinal conductivity and resistivity exhibit strong oscillations, determined by $\epsilon = \omega / \omega_c$ with $\omega$ the radiation frequency and $\omega_c$ the cyclotron frequency. The oscillations follow a pattern with minima centered at $\omega/\omega_c =j + {1/2} (l-1) + \delta $, and maxima centered at $\omega/\omega_c =j + {1/2} (l-1) - \delta $, where $j=1,2,3.......$, $\delta \sim 1/5$ is a constant shift and $l$ is the dominant multipole contribution. Negative resistance states (NRS) develop as the electron mobility and the intensity of the microwave power are increased. These NRS appear in a narrow window region of values of the lattice parameter ($a$), around $a \sim l_B$, where $l_B$ is the magnetic length. It is proposed that these phenomena may be observed in artificially fabricated arrays of periodic scatterers at the interface of ultraclean $GaAs/Al_xGa_{1-x} As$ heterostructures.