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)

We have calculated the admittance of a two-dimensional quantum point contact (QPC) using a novel variant of the Wigner distribution function (WDF) formalism. In the semiclassical approximation, a Boltzman-like equation is derived for the partial WDF describing both propagating and nonpropagating electron modes in an effective potential generated by the adiabatic QPC. We show that this quantum kinetic approach leads to the well-known stepwise behavior of the real part of the admittance (the conductance) [1], and of the imaginary part of the admittance (the emittance), in agreement with the latest results derived in [18], which is determined by the number of propagating electron modes.It is shown, that the emittance is sensitive to the geometry of the QPC, and can be controlled by the gate voltage. We established that the emittance has contributions corresponding to both quantum inductance and quantum 1 capacitance. Stepwise oscillations in the quantum inductance are determined by the harmonic mean of the velocities for the propagating modes, whereas the quantum capacitance is a significant mesoscopic manifestation of the nonpropagating (reflecting) modes.

We investigate resonant interaction of conduction electrons with an electromagnetic field that irradiates a point contact between a ferromagnetic and a normal metal in the presence of a strong magnetic field of order 1 T. We show that electron spin-flips caused by resonant absorption and stimulated emission of photons result in a sharp peak in the magnetic-field dependence of the point-contact resistance. The height of the peak is shown to be directly proportional to the net rate of energy transfer to the electromagnetic field in the point contact due to absorption and stimulated emission of photons.