The Miles theory of wave amplification by wind is extended to the case of finite depth h and a shear flow with (constant) vorticity {\Omega}. Vorticity is characterised through the non-dimensional parameter {\nu} = {\Omega} U_1 /g, where g the gravitational acceleration, U_1 a characteristic wind velocity and k the wavenumber. The notion of 'wave age' is generalised to account for the effect of vorticity. Several widely used growth rates are derived analytically from the dispersion relation of the wind/water interface, and their dependence on both water depth and vorticity is derived and discussed. Vorticity is seen to shift the maximum wave age, similar to what was previously known to be the effect of water depth. At the same time, a novel effect arises and the growth coefficients, at identical wave age and depth, are shown to experience a net increase or decrease according to the shear gradient in the water flow.

We study the low frequency admittance of a quantum Hall bar of size much larger than the electronic coherence length. We find that this macroscopic conductor behaves as an ideal quantum conductor with vanishing longitudinal resistance and purely inductive behavior up to f 1 MHz. Using several measurement configurations, we study the dependence of this inductance on the length of the edge channel and on the integer quantum Hall filling fraction. The experimental data are well described by a scattering model for edge magnetoplasmons taking into account effective long range Coulomb interactions within the sample. This demonstrates that the inductance's dependence on the filling fraction arises from the effective quantum inertia of charge carriers induced by Coulomb interactions within an ungated macroscopic quantum Hall bar.

In a two dimensional electron gas, low energy transport in presence of a magnetic field occurs in chiral 1D channels located on the edge of the sample. In the AC description of quantum transport, the emittance determines the amplitude of the imaginary part of the admittance, whose sign and physical meaning are determined by the sample topology: a Hall bar is inductive while a Corbino ring is capacitive. In this article, the perfect capacitive character of Corbino samples in the quantum Hall effect regime is shown. A vanishing conductance and an electrochemical capacitance which depends on the density of states of 1D channels are measured. Our samples have no gate, neither on the side nor on the top, and the inner capacitances are measured. The transit time of electrons across the device is obtained and the drift velocity of carriers is deduced.

Low‐temperature current–voltage characteristics of n‐type GaAs/GaAlAs quantum wells delta‐doped in GaAs channel with Be acceptors are studied in the presence of a magnetic field. Negatively charged acceptor ions localize 2D conduction electrons by a combined effect of a quantum well and magnetic field parallel to the growth direction. In acceptor‐doped samples, the Hall electric field plays the role of the gate voltage. It is shown that at magnetic fields as weak as 1.5 T (or higher), the drain current reaches a constant value independent of the drain voltage. This phenomenon is due to the electron localization resulting in the decrease of conducting electron density in the crossed‐field configuration. The above special behavior of acceptor‐doped GaAs/GaAlAs heterostructures is exploited to realize a device called Magnetic‐TEGFET (Magnetic Two‐dimensional Electron Gas Field Effect Transistor) operating at low temperatures. The elimination of the gate in the studied transistor suppresses the gate‐to‐drain leakage current, which, in the standard TEGFETs, results in the electronic shot noise.

le M-TEGFET, est un nouveau type de transistor, sans grille, fonctionnant grâce à un effet magnétique.

We study experimentally low-temperature current-voltage characteristics of n-type GaAs/GaAlAs modulation doped quantum wells under the influence of an external magnetic field. In particular, we use samples doped additionally in the well with Be acceptors. As showed previously, negatively charged acceptor ions can localize conduction electrons by a joint effect of a quantum well and an external magnetic field. It is found that, in the acceptor-doped samples, the Hall field resulting from the presence of magnetic field plays the role of gate voltage. At sufficiently high magnetic fields the drain current has a constant value independent of the drain voltage. It is argued that the above phenomenon is due to the electron localization with the resulting decrease of conducting electron density in the crossed-field configuration. We propose to exploit the observed unusual behaviour as a new device called “magnetic two-dimensional field effect transistor” (M-TEGFET) operating at low temperatures.

We study room-temperature performance of micro-Hall magnetic sensors based on pseudomorphic GaAlAs/GaInAs heterostructures. Active areas of our sensors range from 1 to 80 microns. We focus on the smallest detectable magnetic fields in small sensors and perform a systematic study of noise at room temperature in the frequency range between 1 Hz and 100 kHz. Our data are interpreted by the mobility fluctuation model. The Hooge parameter is determined for the applied technology. We show that, independently of the experimental frequency, the ratio of sensitivity to noise is proportional to characteristic length of the sensor. The resolution of $1\, miliGauss/ \sqrt{Hertz}$ is achievable in a $3 \mu$m sensor at room temperature .