From the degree of circular polarization of the time-resolved radiative recombination of 2D electrons with photoexcited holes bound to acceptors we have measured the magnetic field dependencies of the electron spin polarization for various fractional (n 2͞3, 3͞5, 4͞7, 2͞5, 3͞7, 4͞9, 8͞5, 4͞3, and 7͞5) and composite fermions (n 1͞2, 1͞4, and 3͞2) states. The Fermi energies of these composite fermion states are measured for the first time and the corresponding value of the composite fermion density of states mass at n 1͞2 is found to be about 4 times heavier than the previously reported values of the "activation" mass. [S0031-9007 (99)09059-6] PACS numbers: 71.10.Pm, 73.40.HmThe existence of unusual new quasiparticles, composite fermions (CFs), assembled from several magnetic flux quanta and an electron, has been demonstrated in various experiments on two-dimensional (2D) electron systems in high magnetic fields [1-3]. These CFs were introduced theoretically [4,5] to explain the fractional quantum Hall effect (FQHE). According to this theory, at half filling of the Landau level (n 1͞2), two flux quanta are attached to an electron to form a CF, which moves in zero effective magnetic field, since external field is compensated by attached fluxes. The CF concentration is equal to that of 2D electrons and the system of these quasiparticles can be characterized by a Fermi wave vector and a Fermi energy. Any deviation of the magnetic field from exactly half filling of the Landau level results in the appearance of an effective magnetic field, which quantizes the CF motion and splits their energy into Landau levels. Every integer filling of the CF Landau levels corresponds to a specific FQHE state. The recent experiments [6-8] not only supported the validity of this theoretical concept but demonstrated also the semiclassical behavior of these strange quasiparticles.

The rich correlation physics in two-dimensional (2D) electron systems is governed by the dispersion of its excitations. In the fractional quantum Hall regime, excitations involve fractionally charged quasi particles, which exhibit dispersion minima at large momenta referred to as rotons. These rotons are difficult to access with conventional techniques because of the lack of penetration depth or sample volume. Our method overcomes the limitations of conventional methods and traces the dispersion of excitations across momentum space for buried systems involving small material volume. We used surface acoustic waves, launched across the 2D system, to allow incident radiation to trigger these excitations at large momenta. Optics probed their resonant absorption. Our technique unveils the full dispersion of such excitations of several prominent correlated ground states of the 2D electron system, which has so far been inaccessible for experimentation.

Retardation effects, theoretically predicted more than 35 years ago, are observed in the spectrum of two-dimensional plasmons in high-electron-mobility GaAs/AlGaAs quantum wells. In zero magnetic field, a strong reduction of the resonant plasma frequency is observed due to the hybridization of the plasma and light modes. In a perpendicular magnetic field B, hybrid cyclotron-plasmon modes appear with a very unusual dependence of the frequency on B field. Experimental results are in excellent agreement with the theory.