The massless single off-shell scalar box integral -- branch cut structure and all-order epsilon expansion

We investigate the single off-shell scalar box integral with massless internal lines in dimensional regularization. A special emphasis is given to higher orders in the dimensional regularization parameter epsilon, its branch cut structure, and kinematic limits. Common representations of the box integral introduce superficial branch cuts, which we eliminate to all orders in the epsilon expansion. In the literature so far a satisfactory solution for this issue only exists up to finite order in the epsilon expansion. However, for calculations at NNLO in perturbation theory, higher orders in epsilon of this integral are required. In this paper, we present results to all orders in epsilon in terms of single-valued polylogarithms and explicitly determine the real and imaginary part of the box integral in all kinematic regions.Comment: 29 pages, 5 figure

Konzeption und technische Realisierung einer mobilen Feedback-App zur Unterstützung schwangerer Frauen am Beispiel des Android Betriebssystems

Immer mehr Menschen nutzen Apps auf ihren Smartphones nicht nur um Spiele zu spielen oder für diverse Kommunikations- und Social-Media-Dienste. Es kommen immer mehr Apps auf den Markt um bestimmte Alltagssituationen zu erleichtern oder auch zur Förderung der Gesundheit. Diese Arbeit soll zeigen wie eine solche App zur Unterstützung und Begleitung von Schwangeren während ihrer Schwangerschaft entwickelt werden kann. Jede Schwangerschaft ist eine besondere Phase im Leben einer Frau. Dabei hat sie mit vielen neuen Situationen zu kämpfen und muss mit vielen körperlichen Veränderungen und Umstellungen klarkommen. Die heutige Forschung zeigt, dass bestimmte psychosoziale Einflüsse, wie zum Beispiel Lebenssituation, Stress oder Beziehungsprobleme sich auf die Schwangerschaft und die Entwicklung des Kindes auswirken können. In dieser Arbeit wird gezeigt wie es möglich ist diese verschiedenen Umstände im Leben einer Schwangeren zu erfassen und ihr dann konstruktives Feedback zu geben für eine positive Beeinflussung ihrer Schwangerschaft. Dabei wird gezeigt wie eine solche App architektonisch zu realisieren ist und es wird auf verschiedene Bereiche der Implementierung genauer eingegangen wie z.B. die Benachrichtigungen für das Beantworten der Fragebögen

Konzeption und Realisierung eines Patienten-Edukationsmoduls für eine multizentrische und multinationale mHealth-App für eine paneuropäische Tinnitus-Studie

Diese Arbeit soll zeigen, wie ein Edukationsmodul einer multizentrischen und multinationalen mHealth-App entwickelt werden kann, um Tinnitus-Patienten zu unterstützen. Mithilfe diesem und weiteren Modulen sollen im Rahmen des europäischen UNITI-Projektes Studien durchgeführt werden, um mögliche weitere Behandlungsmethoden zu identifizieren

Coherent control of correlated nanodevices: A hybrid time-dependent numerical renormalization-group approach to periodic switching

The time-dependent numerical renormalization-group approach (TD-NRG), originally devised for tracking the real-time dynamics of quantum-impurity systems following a single quantum quench, is extended to multiple switching events. This generalization of the TD-NRG encompasses the possibility of periodic switching, allowing for coherent control of strongly correlated systems by an external time-dependent field. To this end, we have embedded the TD-NRG in a hybrid framework that combines the outstanding capabilities of the numerical renormalization group to systematically construct the effective low-energy Hamiltonian of the system with the prowess of complementary approaches for calculating the real-time dynamics derived from this Hamiltonian. We demonstrate the power of our approach by hybridizing the TD-NRG with the Chebyshev expansion technique in order to investigate periodic switching in the interacting resonant-level model. Although the interacting model shares the same low-energy fixed point as its noninteracting counterpart, we surprisingly find the gradual emergence of damped oscillations as the interaction strength is increased. Focusing on a single quantum quench and using a strong-coupling analysis, we reveal the origin of these interaction-induced oscillations and provide an analytical estimate for their frequency. The latter agrees well with the numerical results.Comment: 20 pager, Revtex, 10 figures, submitted to Physical Review

I-V curves of Fe/MgO (001) single- and double-barrier tunnel junctions

In this work, we calculate with ab initio methods the current-voltage characteristics for ideal single- and double-barrier Fe/MgO (001) magnetic tunnel junctions. The current is calculated in the phase-coherent limit by using the recently developed SMEAGOL code, combining the nonequilibrium Green function formalism with density-functional theory. In general we find that double-barrier junctions display a larger magnetoresistance, which decays with bias at a slower pace than their single-barrier counterparts. This is explained in terms of enhanced spin filtering from the middle Fe layer sandwiched in between the two MgO barriers. In addition, for double-barrier tunnel junctions, we find a well defined peak in the magnetoresistance at a voltage of V=0.1 V. This is the signature of resonant tunneling across a majority quantum well state. Our findings are discussed in relation to recent experiments

Time dependent numerical model for the emission of radiation from relativistic plasma

We describe a numerical model constructed for the study of the emission of radiation from relativistic plasma under conditions characteristic, e.g., to gamma-ray bursts (GRB's) and active galactic nuclei (AGN's). The model solves self consistently the kinetic equations for e^\pm and photons, describing cyclo-synchrotron emission, direct Compton and inverse Compton scattering, pair production and annihilation, including the evolution of high energy electromagnetic cascades. The code allows calculations over a wide range of particle energies, spanning more than 15 orders of magnitude in energy and time scales. Our unique algorithm, which enables to follow the particle distributions over a wide energy range, allows to accurately derive spectra at high energies, >100 \TeV. We present the kinetic equations that are being solved, detailed description of the equations describing the various physical processes, the solution method, and several examples of numerical results. Excellent agreement with analytical results of the synchrotron-SSC model is found for parameter space regions in which this approximation is valid, and several examples are presented of calculations for parameter space regions where analytic results are not available.Comment: Minor changes; References added, discussion on observational status added. Accepted for publication in Ap.

Kinetics of electron-positron pair plasmas using an adaptive Monte Carlo method

A new algorithm for implementing the adaptive Monte Carlo method is given. It is used to solve the relativistic Boltzmann equations that describe the time evolution of a nonequilibrium electron-positron pair plasma containing high-energy photons and pairs. The collision kernels for the photons as well as pairs are constructed for Compton scattering, pair annihilation and creation, bremsstrahlung, and Bhabha & Moller scattering. For a homogeneous and isotropic plasma, analytical equilibrium solutions are obtained in terms of the initial conditions. For two non-equilibrium models, the time evolution of the photon and pair spectra is determined using the new method. The asymptotic numerical solutions are found to be in a good agreement with the analytical equilibrium states. Astrophysical applications of this scheme are discussed.Comment: 43 pages, 7 postscript figures, to appear in the Astrophysical Journa

Hole spin relaxation in intrinsic and pp-type bulk GaAs

We investigate hole spin relaxation in intrinsic and $p$-type bulk GaAs from a fully microscopic kinetic spin Bloch equation approach. In contrast to the previous study on hole spin dynamics, we explicitly include the intraband coherence and the nonpolar hole-optical-phonon interaction, both of which are demonstrated to be of great importance to the hole spin relaxation. The relative contributions of the D'yakonov-Perel' and Elliott-Yafet mechanisms on hole spin relaxation are also analyzed. In our calculation, the screening constant, playing an important role in the hole spin relaxation, is treated with the random phase approximation. In intrinsic GaAs, our result shows good agreement with the experiment data at room temperature, where the hole spin relaxation is demonstrated to be dominated by the Elliott-Yafet mechanism. We also find that the hole spin relaxation strongly depends on the temperature and predict a valley in the density dependence of the hole spin relaxation time at low temperature due to the hole-electron scattering. In $p$-type GaAs, we predict a peak in the spin relaxation time against the hole density at low temperature, which originates from the distinct behaviors of the screening in the degenerate and nondegenerate regimes. The competition between the screening and the momentum exchange during scattering events can also lead to a valley in the density dependence of the hole spin relaxation time in the low density regime. At high temperature, the effect of the screening is suppressed due to the small screening constant. Moreover, we predict a nonmonotonic dependence of the hole spin relaxation time on temperature associated with the screening together with the hole-phonon scattering. Finally, we find that the D'yakonov-Perel' mechanism can markedly contribute to the .... (omitted due to the limit of space)Comment: 11 pages, 7 figures, Phys. Rev. B, in pres

D'yakonov-Perel' spin relaxation for degenerate electrons in the electron-hole liquid

We present an analytical study of the D'yakonov-Perel' spin relaxation time for degenerate electrons in a photo-excited electron-hole liquid in intrinsic semiconductors exhibiting a spin-split band structure. The D'yakonov-Perel' spin relaxation of electrons in these materials is controlled by electron-hole scattering, with small corrections from electron-electron scattering and virtually none from electron-impurity scattering. We derive simple expressions (one-dimensional and two-dimensional integrals respectively) for the effective electron-hole and electron-electron scattering rates which enter the spin relaxation time calculation. The electron-hole scattering rate is found to be comparable to the scattering rates from impurities in the electron liquid - a common model for n-type doped semiconductors. As the density of electron-hole pairs decreases (within the degenerate regime), a strong enhancement of the scattering rates and a corresponding slowing down of spin relaxation is predicted due to exchange and correlation effects in the electron-hole liquid. In the opposite limit of high density, the original D'yakonov-Perel' model fails due to decreasing scattering rates and is eventually superseded by free precession of individual quasiparticle spins.Comment: 16 pages, 5 figure

Kinetics of spin coherence of electrons in nn-type InAs quantum wells under intense terahertz laser fields

Spin kinetics in $n$-type InAs quantum wells under intense terahertz laser fields is investigated by developing fully microscopic kinetic spin Bloch equations via the Floquet-Markov theory and the nonequilibrium Green's function approach, with all the relevant scattering, such as the electron-impurity, electron-phonon, and electron-electron Coulomb scattering explicitly included. We find that a {\em finite} steady-state terahertz spin polarization induced by the terahertz laser field, first predicted by Cheng and Wu [Appl. Phys. Lett. {\bf 86}, 032107 (2005)] in the absence of dissipation, exists even in the presence of all the scattering. We further discuss the effects of the terahertz laser fields on the spin relaxation and the steady-state spin polarization. It is found that the terahertz laser fields can {\em strongly} affect the spin relaxation via hot-electron effect and the terahertz-field-induced effective magnetic field in the presence of spin-orbit coupling. The two effects compete with each other, giving rise to {\em non-monotonic} dependence of the spin relaxation time as well as the amplitude of the steady state spin polarization on the terahertz field strength and frequency. The terahertz field dependences of these quantities are investigated for various impurity densities, lattice temperatures, and strengths of the spin-orbit coupling. Finally, the importance of the electron-electron Coulomb scattering on spin kinetics is also addressed.Comment: 17 pages, 16 figures, Phys. Rev. B 78, 2008, in pres