Using a Boltzmann-like kinetic equation derived in the semiclassical approximation for the partial Wigner distribution function, we determine the ac admittance of a two-dimensional quantum point contact ͑QPC͒ for applied ac fields in the frequency range Ϸ0 -50 GHz. We solve self-consistently an integral equation for the spatial distribution of the potential inside the QPC, taking into account the turning points of the semiclassical trajectories. The admittance of the QPC is a strong function of the gate voltage. This gate voltage can be used to ''tune'' the number of open channels ͑N͒ for electron transport. We show that, for most values of gate voltage, the imaginary part of the total admittance is positive for NϾ1, so that the QPC has an inductive character, because of the predominant role of the open channels. In contrast, for Nϭ0 or 1, for most values of the gate voltage, the imaginary part of the admittance is negative, corresponding to capacitive behavior. For gate voltages near values at which channels open or close, very strong nonlinear effects arise, and the admittance oscillates rapidly ͑with its imaginary part sometimes changing sign͒ both as the function of gate voltage ͑at fixed frequency͒ and as a function of frequency ͑at fixed gate voltage͒. Experimental observation of these oscillations would provide an important test of our semiclassical approach to the ac response of a QPC. We explore the low-frequency regime and investigate the extent to which one can understand the admittance in terms of a static conductance and a ''quantum capacitance'' and a ''quantum inductance.'' We show that it is possible to choose the gate voltage so that there is a large, low-frequency regime in which the admittance is well approximated by a linear function of frequency. In this regime, the admittance can be treated by ''equivalent circuit'' concepts. We study how this approach breaks down at higher frequencies, where strongly nonlinear behavior of the admittance arises. We estimate the value of frequency, c , at which the crossover from the low-frequency linear regime to the high-frequency nonlinear behavior occurs. For chosen parameters of a QPC, c Ϸ10 GHz. ͓S0163-1829͑98͒02339-X͔ PHYSICAL REVIEW B 15 OCTOBER 1998-I VOLUME 58, NUMBER 15 PRB 58 0163-1829/98/58͑15͒/9894͑13͒/$15.00 9894

A new type of collective electromagnetic excitations, namely surface polaritons (SP) -in a 2D electronic layer in a high magnetic field under Quantum Hall Effect (QHE) conditions is predicted. We have found the spectrum, damping, and polarization of the SP in a wide range of frequencies ω and wavevectors k. It is shown that near the Cyclotron Resonance (CR) (ω ∼ Ω = eB/mc) the phase velocity of the SP is drastically slowed down and the group velocity undergoes fundamental steps defined by the Fine Structure Constant α = e 2 /hc. In the vicinity of a CR subharmonic (ω ∼ 2Ω)

The spin filtering effect of the electron current in a double-barrier resonant-tunneling diode ͑RTD͒ consisting of Zn 1−x Mn x Se semimagnetic layers has been studied theoretically. The influence of the distribution of the magnesium ions on the coefficient of the spin polarization of the electron current has been investigated. The dependence of the spin filtering degree of the electron current on the external magnetic field and the bias voltage has been obtained. The effect of the total spin polarization of the electron current has been predicted. This effect is characterized by total suppression of the spin-up component of electron current, which takes place when the Fermi level coincides with the lowest Landau level for spin-up electrons in the RTD semimagnetic emitter.

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