The spectra of plasma and magnetoplasma excitations in a two-dimensional system of anisotropic heavy fermions were investigated for the first time. The spectrum of microwave absorption by disklike samples of stressed AlAs quantum wells at low electron densities showed two plasma resonances separated by a frequency gap. These two plasma resonances correspond to electron mass principle values of (1.10 ± 0.05)m0 and (0.20 ± 0.01)m0. The observed results correspond to the case of a single valley strongly anisotropic Fermi surface. It was established that electron density increase results in population of the second valley, manifesting itself as a drastic modification of the plasma spectrum. We directly determined the electron densities in each valley and the inter-valley splitting energy from the ratio of the two plasma frequencies.

Dependence of cyclotron magneto-plasma mode relaxation time on electron concentration and temperature in the two-dimensional electron system in GaAs/AlGaAs quantum wells has been studied. Comparative analysis of cyclotron and transport relaxation time has been carried out. It was demonstrated that with the temperature increase transport relaxation time tends to cyclotron relaxation time. It was also shown that cyclotron relaxation time, as opposed to transport relaxation time, has a weak electron density dependence. The cyclotron time can exceed transport relaxation time by an order of magnitude in a low-density range.

It is established that cyclotron resonance (CR) in a high-quality GaAs/AlGaAs two-dimensional electron system (2DES) originates as a pure resonance, that does not hybridize with dimensional magnetoplasma excitations. The magnetoplasma resonances form a fine structure of the CR. The observed fine structure of the CR results from the interplay between coherent radiative and incoherent collisional mechanisms of 2D plasma relaxation. We show that the range of 2DES filling factors from which the phenomenon arises is intimately connected to the fundamental fine-structure constant.Cyclotron resonance (CR) spectroscopy is the most direct and convenient method of characterizing the Fermi surface and determining the effective mass of semiconductors [1][2][3]. In a two-dimensional electron system (2DES), a perpendicular magnetic field quenches the inplane motion of electrons into cyclotron orbits. In turn, if the phase and polarization of the incident electromagnetic radiation are synchronized with the electron orbital motion, CR is triggered. The first observation of CR in 2DESs was reported for charged carriers in an inversion layer on Si [4, 5]. Subsequently, CR spectroscopy has been successfully applied to research on many two-dimensional systems, for example, isotropic 2D carriers in GaAs heterostructures [6][7][8], composite fermions [9], heavy fermions in MgZnO/ZnO heterojunctions [10,11], and anisotropic heavy fermions in AlAs quantum wells [12,13].It is widely believed that the cyclotron resonance originates from the dimensional magnetoplasma resonance [15,16,20]. The frequency of the hybrid cyclotron magnetoplasma mode is described by the equationwhere ω c = eB/m * c is the cyclotron frequency, and ω p is the dimensional plasmon frequency [17] (Gaussian units are used in this Letter unless otherwise stated):Here, n s and m * are the density and the effective mass of the 2D electrons, and ε is the effective permittivity of the surrounding medium. For a narrow 2DES stripe of width W , the plasmon wavevector can be approximately described by q = πN/W (N = 1, 2, . . . is the number of the plasmon harmonic).Contrary to general believe, we discovered that the CR originates as a pure resonance, that does not hybridize with dimensional magnetoplasma excitations. The CR rises as a single peak with superimposed contribution from different dimensional magnetoplasma modes. Therefore, initially the CR line shape exhibits a multipeak structure. We show that the CR fine structure is resolved when the coherent radiative 2D plasma relaxation dominates the incoherent collisional damping. In large samples, this regime appears when the 2D conductivity 2πσ xx is much larger than c. Moreover, the range of 2DES filling factors ν in which the CR fine structure necessarily arises is dictated by the relation 2πσ xy > c. The latter can be rewritten very elegantly as ν > 1/α, where α = e 2 / c ≈ 1/137 is the fundamental fine-structure constant.Experiments were performed on a set of high-quality GaAs/AlGaAs single quantum wells with a well w...