Magnetotransport in disordered two-dimensional topological insulators: signatures of charge puddles

In this numerical study we investigate the influence and interplay of disorder, spin-orbit coupling and magnetic field on the edge-transport in HgTe/CdTe quantum wells in the framework of coherent elastic scattering. We show that the edge states remain unaffected by the combined effect of moderate disorder and a weak magnetic field at realistic spin-orbit coupling strengths. Agreement with the experimentally observed linear magnetic field dependence for the conductance of long samples is obtained when considering the existence of charge puddles.Comment: 17 pages, 6 figure

Two-dimensional topological insulator edge state backscattering by dephasing

To understand the seemingly absent temperature dependence in the conductance of two-dimensional topological insulator edge states, we perform a numerical study which identifies the quantitative influence of the combined effect of dephasing and elastic scattering in charge puddles close to the edges. We show that this mechanism may be responsible for the experimental signatures in HgTe/CdTe quantum wells if the puddles in the samples are large and weakly coupled to the sample edges. We propose experiments on artificial puddles which allow to verify this hypothesis and to extract the real dephasing time scale using our predictions. In addition, we present a new method to include the effect of dephasing in wave-packet-time-evolution algorithms.Comment: 7 pages, 5 figure

Diagonalization of a bosonic quadratic form using CCM: Application on a system with two interpenetrating square lattice antiferromagnets

While the diagonalization of a quadratic bosonic form can always be done using a Bogoliubov transformation, the practical implementation for systems with a large number of different bosons is a tedious analytical task. Here we use the coupled cluster method (CCM) to exactly diagonalise such complicated quadratic forms. This yields to a straightforward algorithm which can easily be implemented using computer algebra even for a large number of different bosons. We apply this method on a Heisenberg system with two interpenetrating square lattice antiferromagnets, which is a model for the quasi 2D antiferromagnet Ba_2Cu_3O_4Cl_2. Using a four-magnon spin wave approximation we get a complicated Hamiltonian with four different bosons, which is treated with CCM. Results are presented for magnetic ground state correlations.Comment: 4 pages, 2 Postscript figures, to be published in acta physica polonica A (European Conference 'Physics of Magnetism 99'

A sub-determinant approach for pseudo-orbit expansions of spectral determinants in quantum maps and quantum graphs

We study implications of unitarity for pseudo-orbit expansions of the spectral determinants of quantum maps and quantum graphs. In particular, we advocate to group pseudo-orbits into sub-determinants. We show explicitly that the cancellation of long orbits is elegantly described on this level and that unitarity can be built in using a simple sub-determinant identity which has a non-trivial interpretation in terms of pseudo-orbits. This identity yields much more detailed relations between pseudo orbits of different length than known previously. We reformulate Newton identities and the spectral density in terms of sub-determinant expansions and point out the implications of the sub-determinant identity for these expressions. We analyse furthermore the effect of the identity on spectral correlation functions such as the auto-correlation and parametric cross correlation functions of the spectral determinant and the spectral form factor.Comment: 25 pages, one figur

A Poincar\'e section for the general heavy rigid body

A general recipe is developed for the study of rigid body dynamics in terms of Poincar\'e surfaces of section. A section condition is chosen which captures every trajectory on a given energy surface. The possible topological types of the corresponding surfaces of section are determined, and their 1:1 projection to a conveniently defined torus is proposed for graphical rendering.Comment: 25 pages, 10 figure

Does value-based management facilitate managerial decision-making? An analysis of divestiture decisions

While recent studies indicate that value-based management (VBM) helps owners in aligning managerial interests (i.e., decision-influencing role), little evidence is provided for its support in managerial decision-making (i.e., decision-facilitating role). We investigate whether the depth of VBM implementation and contextual factors may determine VBM’s decision-facilitating role. We investigate our research question on a dataset of 1,774 divestitures by European firms between 2005 and 2016. Divestitures allow for the analysis of managerial decision-making in situations where managerial self-interest is less pronounced and, thus, where VBM’s decision-facilitating role can be differentiated from its decision-influencing role. Our empirical results indicate that VBM implementation down to the business-unit level is positively associated with divestiture returns, while we find no such effect if VBM implementation is limited to the corporate level. Further empirical tests indicate that this positive association is contingent on a high dispersion of the costs of capital across a firm’s business portfolio. In sum, our study indicates that VBM can facilitate managerial decision-making when firms consider its depth of implementation and firm-specific information needs

The spin 1/2 Heisenberg star with frustration II: The influence of the embedding medium

We investigate the spin 1/2 Heisenberg star introduced in J. Richter and A. Voigt, J. Phys. A: Math. Gen. {\bf 27}, 1139 (1994). The model is defined by $H=J_1 \sum_{i=1}^{N}{{\bf s}_0{\bf s}_i} + J_2 H_{R}\{{\bf s}_i\}$ ; $J_1,J_2 \ge 0$ , $i=1,...,N$. In extension to the Ref. we consider a more general $H_{R}\{{\bf s}_i\}$ describing the properties of the spins surrounding the central spin ${\bf s}_0$. The Heisenberg star may be considered as an essential structure element of a lattice with frustration (namely a spin embedded in a magnetic matrix $H_R$) or, alternatively, as a magnetic system $H_R$ with a perturbation by an extra spin. We present some general features of the eigenvalues, the eigenfunctions as well as the spin correlation $\langle {\bf s}_0{\bf s}_i \rangle$ of the model. For $H_R$ being a linear chain, a square lattice or a Lieb-Mattis type system we present the ground state properties of the model in dependence on the frustration parameter $\alpha=J_2/J_1$. Furthermore the thermodynamic properties are calculated for $H_R$ being a Lieb--Mattis antiferromagnet.Comment: 16 pages, uuencoded compressed postscript file, accepted to J. Phys. A: Math. Ge

Agiles Entwerfen – Lektionen aus einem Experiment

Erfolg macht attraktiv – das gilt auch für die Prinzipien und Methoden der agilen Softwareentwicklung. Diese finden momentan eine weite Verbreitung, denn sie geben offenbar gute oder zumindest bessere Antworten auf die Probleme, mit denen die herkömmlichen Projektmethoden nicht zurande kommen (Royce 1970). Agile, manchmal auch »leichtgewichtig« genannte Methoden vermeiden übermäßige Planung und Spezifikation, sie ersetzen sie durch eine schnelle Abfolge von Zyklen aus Aktion-Reflexion und Neuausrichtung. Sie bevorzugen die Kollaboration unter gleichberechtigten Experten, die Kommunikation statt Weisung. Und sie beziehen den Kunden und späteren Nutzer bereits in die Entwicklungsarbeit mit ein, denn der Kunde ist die wichtigste Informationsquelle, er bestimmt, inwiefern das Produkt für ihn nützlich ist. Durch solche Prinzipien ist es möglich, Zeit- und Budgetüberschreitungen besser zu vermeiden, einen produktiven Umgang mit Ungewissheit und wechselnden Situationen zu entwickeln und schonender mit der menschlichen Arbeitskraft umzugehen (Abrahamsson et al. 2003). So hat sich das Konzept der »Agilität« auch auf andere Bereiche übertragen, z.B. auf die Gründung von Unternehmen (»Lean Start Up«, Ries 2011) oder die Gestaltung von Organisationsstrukturen (»Agile Organisation«, Richardson 2005). [... aus dem Text