The metal-nonmetal transition in GaAs-Gai "Al"As heterostructures has been investigated using electrical conductivity and Hall measurements in the presence of a magnetic field and hydrostatic pressure. The binding energy of a magnetodonor, composed of donor atoms and electrons separated from each other by a spacer, has been measured as a function of magnetic field for different surface densities controlled by the pressure. A simple model is presented which accounts qualitatively for the observed effects.Properties of impurities in quasi-two-dimensional structures have lately become the subject of numerous investigations because of their interesting physical properties as well as their important applications. As in other cases of "manmade" quantum structures, the geometry of a given situation is decisive for its behavior. Until the present, the most frequently investigated cases were those of impurities in the quantum well, either at its center or at the edge. ' This is a situation in which an impurity atom is constrained from the outside, i.e. , the electron is kept closer to the impurity center than it should be according to its "natural" behavior.In modulation-doped structures with a spacer, a different situation is possible, in which the impurity atom is constrained from the inside, i.e. , the electron is kept farther from the impurity center than it~ould be according to its behavior without constraint.It is well known from investigations on bulk semiconductors that the presence of an external magnetic field shrinks the atomic orbit and enhances the binding energy of the impurity atom.~This results in a magnetic freezeout of conduction electrons on the impurity level. 9 The effect represents a magnetic-field-induced metal-nonmetal transition. ' Magneto-optical investigations of impurities in quantum wells show that, in this case also, the presence of a magnetic field enhances the impurity energy. ""In this Rapid Communication we report on transport investigations of two-dimensional (2D) structures in which the impurity atom and the electron are separated from each other by a spacer. The effect of an external magnetic field and of hydrostatic pressure on impurity behavior is examl.ned.In order to reach the ultraquantum limit with available magnetic fields, it is necessary to deal with a 20 electron gas of sufficiently low density. In this case only, the lowest Landau level is occupied. In our experiments such a condition was realized by applying hydrostatic pressure. It has been shown' that one can diminish the surface electron density n, in GaAs-Ga~"Al"As heterostructures by the effect of pressure on the Si donor in Gal "A1"As. This deep-lying level, located in the energy gap of Gal "Al"As, shifts rapidly downward with respect to the conduction-band minimum.As a consequence, donor deionization takes place and n, is reduced. At temperatures lower than 100 K the deep-lying level is characterized by metastable occupation. ' Because of lattice relaxation effects, the surface density can be slightly modified for a ...
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A testing device has been developed to study the dynamic compressive behaviour of a quasi-brittle material under confining pressure. At the opposite of similar studies conducted with SHPB tests, this one is achieved with a crossbow system. This direct impact device is composed of a hurled mass and a measuring output bar. Moreover, a specific confinement system is developed. The specimen is confined laterally by a thin metallic sleeve and axially between two metallic plugs [1,2]. A metallic part gathers the confining cell and the output bar together, and guides the whole during compression. High-speed camera is used to follow the cell compression, and an image post-treatment is realised. The axial strain is consequently obtained from the displacement between the input and the output plugs. In addition, the confining pressure is calculated using the ring expansion and the material constitutive law. Finally, strain gauges on the output bar are used to determine specimen axial stress. Different projectile masses, specimen diameters and ring thicknesses were tested in order to get specimen strain up to failure associated for different constant confinements. Finally, the device allows obtaining different strain rates with various confining pressures.
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