Biomedical subjects
Publications and source records attributed to A Mascarenhas.
We report a giant bowing of the spin-orbit splitting energy Delta0 in the dilute GaAs1-xBix alloy for Bi concentrations ranging from 0% to 1.8%. This is the first observation of a large relativistic correction to the host electronic band structure induced by just a few percent of isoelectronic doping in a semiconductor material. It opens up the possibility of tailoring the spin-orbit splitting in semiconductors for spintronic applications.
The temperature dependence of the band gap of semiconducting carbon nanotubes was measured for ten different nanotube species. The unprecedented effectiveness in avoiding the effect of external strain, or any other effects originating from the surrounding environment, lead to an accurate measurement of the band gap temperature dependence, giving fundamental insight into the nanotube electron-phonon interaction. Small but reproducible energy shifts of the emission lines with temperature were observed, showing a moderate chirality dependence, well in agreement with recent theoretical calculations. In addition to the energy shift, a substantial narrowing of the emission lines was also observed. The removal of the temperature shift of the band gap allows the precise measurement of the effect of external strain on carbon nanotubes in different environments.
We discuss theoretically a novel approach to tailoring the properties of a new family of organic-inorganic hybrid superlattices, using two isostructural materials, ZnSe(en)0.5 and ZnTe(en)0.5, as examples. Replacing Se with Te leads to a number of nontrivial changes: the conduction band parity, singularity type, conductivity in the superlattice direction, and the p-type dopability. Experimentally, we report the first unambiguous observation of exciton-polariton emission in a hybrid semiconductor, i.e., ZnTe(en)0.5 . The band-edge excitonic transitions in both emission and absorption are explained by the calculated electronic structures.
Using optical spectroscopy with diffraction limited spatial resolution, the possibility of measuring the luminescence from single impurity centers in a semiconductor is demonstrated. Selectively studying individual centers that are formed by two neighboring nitrogen atoms in GaAs makes it possible to unveil their otherwise concealed polarization anisotropy, analyze their selection rules, identify their particular configuration, map their spatial distribution, and demonstrate the presence of a diversity of local environments. Circumventing the limitation imposed by ensemble averaging and the ability to discriminate the individual electronic responses from discrete emitters provides an unprecedented perspective on the nanoscience of impurities.
It is found that there exists a category of material interfaces, readily available, that not only can provide total refraction (i.e., zero reflection) but can also give rise to amphoteric refraction (i.e., both positive and negative refraction) for electromagnetic waves in any frequency domain as well as for ballistic electron waves. These two unusual phenomena are demonstrated experimentally for the propagation of light through such an interface.
We report studies of (GaAs)(n)/(AlAs)(n) ultrashort-period superlattices using synchrotron x-ray scattering. In particular, we demonstrate that interfaces of these superlattices contain features on two different length scales: namely, random atomic mixture and ordered mesoscopic domains. Both features are asymmetric on the two interfaces (AlAs-on-GaAs and GaAs-on-AlAs) for n>2. Periodic compositional stacking faults, arising from the intrinsic nature of molecular-beam epitaxy, are found in the superlattices. In addition, the effect of growth interruption on the interfacial structure is discussed. The relevant scattering theory is developed to give excellent fits to the data.
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