Bernstein modes and giant microwave response of a two-dimensional electron system

Abstract

We report on a contribution to the microwave response of a two-dimensional electron system in a magnetic field which originates from excitation of virtual Bernstein modes. These collective modes emerge as a result of interaction between the usual magnetoplasmon mode and cyclotron resonance harmonics. The electrons are found to experience a strongly enhanced radiation field when its frequency falls in a gap of the Bernstein modes spectrum. This field can give rise to nonlinear effects, one of which, the parametric cyclotron resonance, is discussed. We argue that this resonance leads to a plasma instability in the ultraclean system. The instability-induced heating is responsible for the giant photoresistivity spike recently observed in the vicinity of the second cyclotron resonance harmonic.PACS numbers: 73.21. Fg, 73.20.Mf, 73.43.Qt, 72.30.+q Plasma oscillations are well known; their study began about a century ago by Langmuir and Tonks. These oscillations are mostly investigated in two different types of systems: low-density nondegenerate gas plasma [1] and the degenerate plasma of solids [2]. In the latter case plasma excitations are often referred to as plasmons. A characteristic feature of the solid state systems is that the motion of particles can be easily restricted in one or more directions and thus low-dimensional systems can be created. Properties of plasmons in these systems differ dramatically from those in the three-dimensional (3D) systems. For instance, the plasmon dispersion law in a twodimensional (2D) electron system (ES) can be written as ω (2) p (q) = 2πn s e 2 q/κm, where n s is 2D electron concentration, κ is the surrounding dielectric constant, m is electron effective mass, and q is the 2D wave vector of the plasmon. The spectrum of 2D plasmons is gapless and strongly depends on q in contrast to the dispersionless spectrum of the 3D plasmon ω (3) p (q) = 4πn 3D e 2 /κm, where n 3D is the electron concentration of 3DES.Recently an ultrastrong radiation-plasmon coupling in 2DES in magnetic field has been observed [3,4]. Plasma oscillation in the perpendicular magnetic field B is called the upper hybrid mode in gas plasma or the magnetoplasmon mode in solid state plasma. The dispersion relation of the excitation is as follows:where ω c = eB/mc is the electron cyclotron frequency, and ω (i)p is the plasmon frequency in a 3D (i = 3) or 2D (i = 2) system in the absence of a magnetic field.Plasma oscillations determined by Eq. (1) can interact with cyclotron resonance harmonics due to finite value of qR c , where R c = v F /ω c is the electron cyclotron radius, and v F is the Fermi velocity (in the case of degenerate plasma). This interaction splits mode (1) into the so-called Bernstein modes. In 2DES these modes, see Consider the influence of the Bernstein modes on the screening of incident radiation by magnetoplasmons in 2DES. If a wave vector of radiation q is nonzero and the radiation frequency Ω lies in one of the gaps, see Fig. 1(b), then real magnetoplasma modes are not excited but elect...

Analog of microwave-induced resistance oscillations induced in GaAs heterostructures by terahertz radiation

“…Consequently, new materials have been studied [22][23][24][25] and new theoretical models have been put forward [26][27][28][29][30]. To the most pressing issues which need to be clarified experimentally and which might help to differentiate between the different models belong the MIRO polarization dependence [7,8,31] and the "bulk" or "edge" nature of the effect.…”

Cyclotron resonance overtones and near-field magnetoabsorption via terahertz Bernstein modes in graphene

“…The emergent collective excitations are dubbed Bernstein modes 23 (BMs) and were first theorized to exist in the field of plasma physics many decades ago 23,24 and later explored in 2DES. 22,[25][26][27][28][29] We show that the vanishing group velocity of the BMs and large plasmonic density of states 30 facilitate strong magnetoabsorption in graphene devices in the vicinity of the CR overtones. We detect it as a giant photoresistance peak, exceeding the signal due to the ordinary CR by few orders of magnitude.…”

“…Next, we note that although BMs were discussed in relation with the resonant photoresistance close to the 2nd CR harmonics observed in GaAs-based heterostructures, 31 the described scenario relied on the emergence of electron plasma instability mediated by the nonlinearity of the Euler equation and parametric resonance. 30 The effect described here is linear in P which excludes any instabilities-related mechanisms. The same observation also excludes multiphoton mechanisms which would inevitably lead to non-linear scaling with P .…”

Microwave-induced zero-resistance states and second-harmonic generation in an ultraclean two-dimensional electron gas

“…The origin of the recently observed [22][23][24][25] , in the vicinity of the 2nd cyclotron harmonic ω = 2ω c , giant photoresistivity spike is not yet understood. It is noticeable, however, that the first theoretical attempt to explain this effect 45 employs the ideas of the ponderomotive-force theory 26,34 . In this paper it is assumed that a strongly inhomogeneous near-contact electric field causes a parametric resonance in the system, which then leads to the observed giant U xx -spike.…”