The structural dependence of the effective mass and Luttinger parameters in semiconductor quantum wells

A detailed comparison of the empirical pseudopotential method with single and multiple band calculations based on the envelope function and effective mass approximations are presented. It is shown that, in order to give agreement with the more rigorous microscopic approach of the pseudopotential method, structural dependent effective masses and Luttinger parameters must be invoked. The CdTe/Cd(1 – x)Mn(x)Te system has been employed as an example, and the first pseudopotential calculations of quantum wells and superlattices in this material are presented. It is shown that the electron, light- and heavy-hole effective masses tend towards twice their bulk values in the limit of narrow quantum wells. © 1997 American Institute of Physics

Double crystal x-ray diffraction simulations of diffusion in semiconductor microstructures

Diffusion in group IV, III-V and II-VI semiconductors is an interesting problem not only from a fundamental physics viewpoint but also in practical terms, since it could determine the useful lifetime of a device. Any attempt to control the amount of diffusion in a semiconductor device, whether it be a quantum well structure or not, requires an accurate determination of the diffusion coefficient. The present theoretical study shows that this could be achieved via x-ray diffraction studies in quantum well structures. It is demonstrated that the rocking curves of single quantum wells are not sensitive to diffusion. However the intensity of the first order satellite, which is characteristic of superlattice rocking curves, is strongly dependent upon diffusion and it is proposed that this technique could be used to measure the diffusion coefficient D. © 1998 American Institute of Physics

Electric-field controlled ferromagnetism in MnGe magnetic quantum dots

Electric-field control of ferromagnetism in magnetic semiconductors at room temperature has been actively pursued as one of the important approaches to realize practical spintronics and non-volatile logic devices. While Mn-doped III-V semiconductors were considered as potential candidates for achieving this controllability, the search for an ideal material with high Curie temperature (Tc>300 K) and controllable ferromagnetism at room temperature has continued for nearly a decade. Among various dilute magnetic semiconductors (DMSs), materials derived from group IV elements such as Si and Ge are the ideal candidates for such materials due to their excellent compatibility with the conventional complementary metal-oxide-semiconductor (CMOS) technology. Here, we review recent reports on the development of high-Curie temperature Mn0.05Ge0.95 quantum dots (QDs) and successfully demonstrate electric-field control of ferromagnetism in the Mn0.05Ge0.95 quantum dots up to 300 K. Upon the application of gate-bias to a metal-oxide-semiconductor (MOS) capacitor, the ferromagnetism of the channel layer (i.e. the Mn0.05Ge0.95 quantum dots) was modulated as a function of the hole concentration. Finally, a theoretical model based upon the formation of magnetic polarons has been proposed to explain the observed field controlled ferromagnetism

Utopianism and social change: materialism, conflict and pluralism

This article discusses criticisms that utopia and utopianism undermine social change. It outlines two types of utopia, future and current. It argues against claims that utopianism is idealist and steps aside from material and conflictual dimensions of society and so undermines change, proposing that utopias are material and conflictual and contribute to change. Against liberal and pluralist criticisms that utopianism is end-ist and totalitarian and terminates diversity and change it argues that utopianism can encompass liberal and pluralist dimensions and be dynamic rather than static. It is proposed that criticisms create false conflations and dichotomies. Critical perspectives, rather than being rejected, are answered on their own terms. Utopianism, it is argued, is part of change, materially, now and in the future

Theory of excitons and magnetic polarons in diluted magnetic semiconductor quantum well structures

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Fast Neutrino Flavor Conversions: Stability Analysis in the Linear Regime

Due to their special properties neutrinos are valuable messengers of physical information. In particular the low interaction rate in matter allows us to receive neutrino signals from otherwise inaccessible regions, like the interior of stars. For the correct interpretation of these signals a profound comprehension of associated processes is necessary. One of them is the conversion of neutrino flavours caused by neutrino mixing. When neutrinos propagate in a medium, the interaction with the environment can influence the mixing, i.e. enhance or suppress flavour conversions. At high neutrino densities, as they occur for example in core-collapse supernovae or neutron-star mergers, neutrinos themselves are the medium and neutrino-neutrino interactions can dominate the flavour evolution, which leads to the appearance of collective flavour modes. One class of collective phenomena is called fast flavour conversion because the length scale at which the oscillation takes place is much shorter than the scale of other oscillation effects. The conversion occurs when the initially small flavour coherence becomes unstable and grows exponentially. The existence of unstable fast-flavour modes depends crucially on the angular lepton-number distribution. This thesis is dedicated to investigate the theoretical origin of fast flavour conversions. For this purpose the formalism based on the matrix of flavour densities is applied and the corresponding equation of motion is derived including refraction terms from the interaction with a medium. The result is used to calculate the dispersion relation for the flavour correlation function as long as its equation of motion can be linearised. The connection between instabilities in the correlation function and crossings in the angular lepton number distribution is proven for axially symmetric systems by deriving the full set of eigenfunctions and matching them for collective and non-collective modes. In this context the difference between the stability of symmetry preserving and breaking modes is demonstrated. Finally the assumption of axial symmetry is dropped and examples are studied for general settings, i.e. with arbitrary wave vectors and non-symmetric lepton-number distributions