Energy spectra of donors in GaAs-Ga_(1-x)Al_(x)As quantum well structures in the effective mass approximation

We present the results of a study of the energy spectrum of the ground state and the low-lying excited states for shallow donors in quantum well structures consisting of a single slab of GaAs sandwiched between two semi-infinite layers of Ga_(1-x)Al_(x)As. The effect of the position of the impurity atom within central GaAs slab is investigated for different slab thicknesses and alloy compositions. Two limiting cases are presented: one in which the impurity atom is located at the center of the quantum well (on-center impurity), the other in which the impurity atom is located at the edge of the quantum well (on-edge impurity). Both the on-center and the on-edge donor ground state are bound for all values of GaAs slab thicknesses and alloy compositions. The alloy composition x is varied between 0.1 and 0.4. In this composition range, Ga_(1-x)Al_(x)As is direct, and the single-valley effective-mass theory is a valid technique for treating shallow donor states. Calculations are carried out in the case of finite potential barriers determined by realistic conduction-band offsets

Transport characteristics of L-point and Г-point electrons through GaAs-Ga_(1-x)Ai_xAs-GaAs(111} double heterojunctions

We present here a study on the transport characteristics of L‐point and Γ‐point derived electrons through abrupt GaAs–Ga_(1−x)Al_xAs–GaAs(111) double heterojunctions. The use of complex‐k band structures in the tight‐binding approximation and transfer matrices provide a reasonably accurate description of the wave function at the GaAs–Ga_(1−x)Al_xAs interface. A representation of the wave function in terms of bulk complex‐k Bloch states is used in the GaAs regions where the potential is bulklike. A representation of the wave function in terms of planar orbitals is used in the central Ga_(1−x)Al_xAs region where the potential deviates from its bulk value (i.e., interfacial region). Within this theoretical framework, realistic band structure effects are taken into account and no artificial rules regarding the connection of the wave function across the interface are introduced. The ten‐band tight‐binding model includes admixture in the total wave function of states derived from different extrema of the GaAs conduction band. States derived from the same extremum of the conduction band appear to couple strongly to each other, whereas states derived from different extrema are found to couple weakly. Transport characteristics of incoming L‐point and Γ‐point Bloch states are examined as a function of the energy of the incoming state, thickness of the Ga_(1−x)Al_xAs barrier, and alloy composition x. Transmission through the Ga_(1−x)Al_xAs barrier is either tunneling or propagating depending on the nature of the Bloch states available for strong coupling in the alloy. Since Bloch states derived from different extrema of the conduction band appear to couple weakly to each other, it seems possible to reflect the low velocity L‐point component of the current while transmitting the high velocity Γ‐point component

Les recherches en psychologie sociale au Canada français (1946-1962)

Commentair

Small band gap superlattices as intrinsic long wavelength infrared detector materials

Intrinsic long wavelength (lambda greater than or equal to 10 microns) infrared (IR) detectors are currently made from the alloy (Hg, Cd)Te. There is one parameter, the alloy composition, which can be varied to control the properties of this material. The parameter is chosen to set the band gap (cut-off wavelength). The (Hg, Cd)Te alloy has the zincblend crystal structure. Consequently, the electron and light-hole effective masses are essentially inversely proportional to the band gap. As a result, the electron and light-hole effective masses are very small (M sub(exp asterisk)/M sub o approx. M sub Ih/M sub o approx. less than 0.01) whereas the heavy-hole effective mass is ordinary size (M sub hh(exp asterisk)/M sub o approx. 0.4) for the alloy compositions required for intrinsic long wavelength IR detection. This combination of effective masses leads to rather easy tunneling and relatively large Auger transition rates. These are undesirable characteristics, which must be designed around, of an IR detector material. They follow directly from the fact that (Hg, Cd)Te has the zincblend crystal structure and a small band gap. In small band gap superlattices, such as HgTe/CdTe, In(As, Sb)/InSb and InAs/(Ga,In)Sb, the band gap is determined by the superlattice layer thicknesses as well as by the alloy composition (for superlattices containing an alloy). The effective masses are not directly related to the band gap and can be separately varied. In addition, both strain and quantum confinement can be used to split the light-hole band away from the valence band maximum. These band structure engineering options can be used to reduce tunneling probabilities and Auger transition rates compared with a small band gap zincblend structure material. Researchers discuss the different band structure engineering options for the various classes of small band gap superlattices

Assessment in Action: A Journey through Campus Collaboration, a Learning Community, and Research Design

Members of the first cohort (2014) of the Association of College and Research Libraries’ (ACRL) Assessment in Action (AiA) learning community share the impact of the AiA program on library and university assessment initiatives. This article shares brief examples of effective and challenging cross-campus collaborative assessment projects and the five best practices the authors developed through the year-long experience of examining student success in three different academic library environments

Vanderbilt Visions: An Exercise in Collaboration

Five months before the Fall 2007 semester the Committee on Undergraduate Information Literacy (CUIL) and Vanderbilt Libraries were notified their proposal to present one of the weekly sessions for Vanderbilt Visions, the university’s first-year orientation program, was accepted. Through a collaborative effort, over 30 staff from 5 libraries, the Center for Teaching, the Writing Studio and the Learning Resource Center worked together to design and present a session that focused on intellectual engagement at Vanderbilt and explored the differences between college and high school research. This was the first opportunity for the Libraries to work with our campus partners on such a large-scale project. Several challenges the planning and development group faced included: * Developing a multi-media program that would meet cross-disciplinary needs for students enrolled in 4 undergraduate schools * Ensuring that the diversity of the Vanderbilt community was reflected in the presentation * Determining the best way to present this session to 1700 students utilizing available staff resources * Introducing staff with differing levels of technical expertise to new technologies * Asking staff to take on a new, large-scale project scheduled for the 3rd week of fall semester - a time that is traditionally very busy This presentation will focus on how we met these challenges, what we learned from working with our campus partners, what we would do differently next time, and the unexpected benefits to our experience

Autorité et tâches dans les petits groupes

L'expérimentation en psychologie sociale nous a appris à distinguer entre groupes étendus et groupes restreints, entre groupes centrés sur la tâche et groupes centrés sur le groupe. Il ne s'agira ici que de groupes de travail et de petits groupes de travail. Nous tenterons de dégager les conditionnements psychologiques de l'exercice de l'autorité dans ce contexte précis : des groupes restreints se consacrant à résoudre des problèmes ou à accomplir des tâches. Plusieurs optiques nous paraissent valables pour rendre ce phénomène intelligible, cependant nous nous limiterons délibérément à ses seuls aspects et à ses seules dimensions psychologiques. Nous tenterons spécifiquement de définir les types de comportements, d'attitudes, d'interactions et de motivations les plus favorables à l'exercice de l'autorité dans les petits groupes de travail.Nombre d'écrits ont paru sur ce thème exposant les résultats des multiples recherches de ces dernières années. Ces écrits sont de très inégale valeur. Nous ne retiendrons ici que les données les plus concluantes, celles qui nous paraissent posséder une valeur scientifique. Nous tenterons de systématiser ces données, en y intégrant ce que nos propres recherches nous ont appris sur ce problème. Nous serons ainsi amenés à pratiquer une autopsie des façons arbitraires d'exercer l'autorité qui, trop souvent, prévalent dans nos petits groupes de travail et les vouent fatalement à la sclérose et à la stérilité, quand ce n'est pas à la nécrose. Mais avant tout, cette étude voudrait constituer une anatomie du leadership fonctionnel en groupe de travail, c'est-à-dire définir en termes opératoires le style de leadership que devraient adopter les petits groupes de nos divers milieux pour devenir plus créateurs et plus inventifs en s'acquittant de leur tâche. Le jour où, au Canada français, les petits groupes s'inspireront de ces données scientifiques, non seulement l'autorité apprendra à s'y exercer de façon plus démocratique mais aussi leur dynamique de groupe aura chance de devenir plus fonctionnelle et leurs membres plus créateurs

k⋅p theory of semiconductor superlattice electronic structure in an applied magnetic field

We present a k⋅p theory of semiconductor superlattices in an applied magnetic field. We consider superlattices with a [001] growth axis and the magnetic field along the growth axis. A single-basis set for the constituent materials is provided by a zone-center pseudopotential calculation with a reference Hamiltonian. The Γ15 valence and Γ1 conduction states are coupled with a spinor and treated explicitly. Nearby energy states are treated in Löwdin perturbation theory with the k⋅p operator and the difference between the material pseudopotential and the reference pseudopotential as the perturbation. The calculation is carried out consistently to first order in wave functions and second order in energies. Magnetic, exchange (in semimagnetic materials), spin-orbit, and strain (in strained-layer superlattices) interactions are included between the explicitly included states. When inversion-asymmetry and warping terms are dropped in the Hamiltonian, a Landau index becomes a good quantum number. Bloch and evanescent states are computed for a fixed Landau index in each material. Interface matching of the constituent-material bulk eigenfunctions is accomplished with use of results derived for the normal component of the current density operator. The Landau indices are not mixed by the interface matching. Superlattice translational symmetry is used to derive an eigenvalue equation for the superlattice wave vectors and eigenfunctions. The numerical implementation of the formal results is described and used to investigate a nonmagnetic superlattice Ga0.47In0.53As/Al0.48In0.52As and a semimagnetic superlattice Hg0.95Mn0.05Te/Cd0.78Mn0.22Te