Hopping conductivity in heavily doped n-type GaAs layers in the quantum Hall effect regime

We investigate the magnetoresistance of epitaxially grown, heavily doped n-type GaAs layers with thickness (40-50 nm) larger than the electronic mean free path (23 nm). The temperature dependence of the dissipative resistance R_{xx} in the quantum Hall effect regime can be well described by a hopping law (R_{xx} \propto exp{-(T_0/T)^p}) with p=0.6. We discuss this result in terms of variable range hopping in a Coulomb gap together with a dependence of the electron localization length on the energy in the gap. The value of the exponent p>0.5 shows that electron-electron interactions have to be taken into account in order to explain the occurrence of the quantum Hall effect in these samples, which have a three-dimensional single electron density of states.Comment: 5 pages, 2 figures, 1 tabl

Magneto-optical study of electron occupation and hole wave functions in stacked self-assembled InP quantum dots

We have studied the magnetophotoluminescence of doubly stacked layers of self-assembled InP quantum dots in a GaInP matrix. 4.0±0.1 monolayers of InP were deposited in the lower layer of each sample, whereas in the upper layer 3.9, 3.4, and 3.0 monolayers were used. Low-temperature photoluminescence measurements in zero magnetic field are used to show that, in each case, only one layer of dots is occupied by an electron, and imply that when the amount of InP in both layers is the same, the dots in the upper layer are larger. High-field photoluminescence data reveal that the position and extent of the hole wave function are strongly dependent on the amount of InP in the stack. ©2001 American Institute of Physics

Improved full one-loop corrections to A^0 -> \sf_1 \sf_2 and \sf_2 -> \sf_1 A^0

We calculate the full electroweak one-loop corrections to the decay of the CP-odd Higgs boson A^0 into scalar fermions in the minimal supersymmetric extension of the Standard Model. For this purpose many parameters of the MSSM have to be properly renormalized in the on-shell renormalization scheme. We have also included the SUSY-QCD corrections. For the decay into bottom squarks and tau sleptons, especially for large \tan\b, the corrections can be very large making the perturbation expansion unreliable. We solve this problem by an appropriate definition of the tree-level coupling in terms of running fermion masses and running trilinear couplings A_f. We also discuss the decay of heavy scalar fermions into light scalar fermions and A^0. We find that the corrections can be sizeable and therefore cannot be neglected.Comment: 42 pages, 20 figures (23 eps-files

Evidence of breakdown of the spin symmetry in diluted 2D electron gases

Recent claims of an experimental demonstration of spontaneous spin polarisation in dilute electron gases \cite{young99} revived long standing theoretical discussions \cite{ceper99,bloch}. In two dimensions, the stabilisation of a ferromagnetic fluid might be hindered by the occurrence of the metal-insulator transition at low densities \cite{abra79}. To circumvent localisation in the two-dimensional electron gas (2DEG) we investigated the low populated second electron subband, where the disorder potential is mainly screened by the high density of the first subband. This letter reports on the breakdown of the spin symmetry in a 2DEG, revealed by the abrupt enhancement of the exchange and correlation terms of the Coulomb interaction, as determined from the energies of the collective charge and spin excitations. Inelastic light scattering experiments and calculations within the time-dependent local spin-density approximation give strong evidence for the existence of a ferromagnetic ground state in the diluted regime.Comment: 4 pages, 4 figures, Revte

Direct control of the tunnel splitting in a one-electron double quantum dot

Quasi-static transport measurements are employed on a laterally defined tunnel-coupled double quantum dot. A nearby quantum point contact allows us to track the charge as added to the device. If charged with only up to one electron, the low-energy spectrum of the double quantum dot is characterized by its quantum mechanical interdot tunnel splitting. We directly measure its magnitude by utilizing particular anticrossing features in the stability diagram at finite source-drain bias. By modification of gate voltages defining the confinement potential as well as by variation of a perpendicular magnetic field we demonstrate the tunability of the coherent tunnel coupling.Comment: High resolution pdf file available at http://www2.nano.physik.uni-muenchen.de/~huettel/research/anticrossing.pd

Qualitative Interviews with Irregular Migrants in Times of COVID-19: Recourse to Remote Interview Techniques as a Possible Methodological Adjustment

Forschungsdesigns erfordern Flexibilität. Wichtig ist aber, dass Anpassungen nicht immer ausschließlich mit Nachteilen verbunden sein müssen. In dieser Forschungsnotiz möchten wir unsere Überlegungen zu den Auswirkungen von COVID-19 auf die Durchführung von qualitativen Interviews mit irregulären Migrant*innen veranschaulichen. Die Ausführungen wurden in Anlehnung an eines unserer eigenen Projekte entwickelt, bei dem sich die Feldarbeit derzeit in der Planungsphase befindet. Aufgrund ihrer möglichen Relevanz für ähnliche Projekte möchten wir unsere methodischen Überlegungen teilen. Wir liefern Anmerkungen zur aktuellen Situation irregulärer Migrant*innen in verschiedenen (europäischen) Ländern sowie eine Einschätzung der methodischen Durchführbarkeit von qualitativen Face-to-Face-Interviews mit irregulären Migrant*innen und möglicher Alternativen zu dieser Methode (insbesondere verschiedener Formen von Ferninterviews). Abschließend kommen wir auf unsere Entscheidung zu sprechen, mit einem Mixed-Mode-Ansatz zu arbeiten, der es uns erlaubt, verschiedene Fernbefragungsmodi zu nutzen, und damit die nötige Flexibilität zur Anpassung an den Verlauf derartiger gesundheitlicher und gesellschaftlicher Krisen bietet.Research designs require flexibility, and adjustments made to the designs do not always have to lead exclusively to disadvantages. In this research note, we would like to share our reflections on the impact of COVID-19 on the conduct of qualitative interviews with irregular migrants. Since these considerations were developed in close connection with one of our own projects, in which fieldwork is currently in the planning phase, we believe they may be relevant to similar projects. We include a brief remark on the current situation of irregular migrants in different (European) countries, as well as an assessment of the methodological feasibility of qualitative face-to-face interviews with irregular migrants and possible alternatives to this method such as remote and online interview formats. We conclude with insights on our recommendation to rely on a mixed-mode approach, which allows us to use various remote interview modes, thus providing the necessary flexibility to adapt to profound health and social crises such as COVID-19

Safety of clonidine used for long-term sedation in paediatric intensive care: A systematic review

AIM: Although not approved, the α-adrenoceptor agonist clonidine is considered an option for long-term sedation protocols in paediatric intensive care. We reviewed adverse effects of clonidine occurring in this indication. METHODS: Relevant literature was systematically identified from PubMed and Embase. We included interventional and observational studies on paediatric patients admitted to intensive care units and systemically long-term sedated with clonidine-containing regimes. In duplicates, we conducted standardised and independent full-text assessment and extraction of safety data. RESULTS: Data from 11 studies with 909 patients were analysed. The studies were heterogeneous regarding patient characteristics (age groups, comorbidity, or comedication) and sedation regimes (dosage, route, duration, or concomitant sedatives). Just four randomised controlled trials (RCTs) and one observational study had comparison groups, using placebo or midazolam. For safety outcomes, our validity evaluation showed low risk of bias only in three studies. All studies focused on haemodynamic problems, particularly bradycardia and hypotension. Observed incidences or subsequent interventions never caused concerns. However, only two RCTs allowed meaningful comparisons with control groups. Odds ratios showed no significant difference between the groups, but small sample sizes (50 and 125 patients) must be considered; pooled analyses were not reasonable. CONCLUSION: All evaluated studies concluded that the use of clonidine in paediatric intensive care units is safe. However, a valid characterisation of the safety profile remains challenging due to limited, biased and heterogeneous data and missing investigation of long-term effects. This evaluation demonstrates the lack of data, which prevents reliable conclusions on the safety of clonidine for long-term sedation in critically ill children. For an evidence-based use, further studies are needed

Discontentment trumps Euphoria: Interacting with European Politicians' migration-related messages on social media

We investigate user engagement with politicians' migration discourses on social media. In particular, we study the effects of message framing and support base attitudes on interactions on Facebook and Twitter in five European countries. Enriching automated analysis of social media content with survey data in a multilevel negative binomial regression approach, findings show that migration-related messages tend to elicit more interactions than other kinds of messages. Furthermore, the presence of a security frame in a migration-related message positively relates to user engagement. However, additional analyses suggest that the relevance of these frames differ between different political parties. In fact, a message gets an even higher number of interactions, when the dimension of the migration issue included in those framed messages is perceived more negatively by a party's support base. The findings have important implications for communication strategies of political actors and the state of migration discourses on social media