Renormalization of the Optical Response of Semiconductors by Electron-Phonon Interaction

In the past five years enormous progress has been made in the ab initio calculations of the optical response of electrons in semiconductors. The calculations include the Coulomb interaction between the excited electron and the hole left behind, as well as local field effects. However, they are performed under the assumption that the atoms occupy fixed equilibrium positions and do not include effects of the interaction of the lattice vibrations with the electronic states (electron-phonon interaction). This interaction shifts and broadens the energies at which structure in the optical spectra is observed, the corresponding shifts being of the order of the accuracy claimed for the ab initio calculations. These shifts and broadenings can be calculated with various degrees of reliability using a number of semiempirical and ab initio techniques, but no full calculations of the optical spectra including electron-phonon interaction are available to date. This article discusses experimental and theoretical aspects of the renormalization of optical response functions by electron-phonon interaction, including both temperature and isotopic mass effects. Some of the theoretical techniques used can also be applied to analyze the renormalization of other response functions, such as the phonon spectral functions, the lattice parameters, and the elastic constants.Comment: Latex 2.09, 28 pages, 13 Figs., 2 Tables, submitted to Phys. Stat. Sol.

Mutationsspektrum, Entstehungsmechanismen und Genotyp-Phänotyp-Beziehungen

Mutationen des RB1-Gens sind ursächlich für die Entstehung des Retinoblastoms, des häufigsten malignen Tumors des Kindesalters. Die Bestimmung der krankheitsursächlichen Veränderung ist bei jedem Patienten für die optimale Betreuung seiner Familie erforderlich. Um Mutationen vom Typ gross deletion effizient zu erkennen, wurde in der vorliegenden Arbeit die quantitative Multiplex-PCR etabliert und mit hier neu entwickelten Methoden zur Bestätigung kombiniert. Die genomische Realtime-PCR und die Longrange-PCR bewährten sich hierbei als geeignete Verfahren. Wir konnten mittels quantitativer Multiplex-PCR bei 61 Patienten gross deletions im RB1-Gen identifizieren (33 konstitutionelle Mutationen und 28 Mutationen in Tumormaterial). Somit konnten wir die Anzahl der publizierten Mutationen dieser Art annähernd verdoppeln. In unserem Patientenkollektiv haben gross deletions einen Anteil von 15% von 443 der konstitutionellen Mutationen bei Patienten mit bilateralem oder familiärem Retinoblastom. Bei isoliert unilateral Betroffenen sind konstitutionelle gross deletions bei 6,1% von 262 festzustellen. Durch die Analyse der Genotyp-Phänotyp-Beziehungen (mit Einschluss von zuvor publizierten Mutationen) konnte hier gezeigt werden, dass gross deletions, die zu vorzeitigen Stopp Kodons führen, die A/B Pocket-Domänen betreffen oder nur einen der Bruchpunkte innerhalb des Gens haben, zu einem schwereren Krankheitsbild führen (größere Zahl von Tumoren) als Ganzgendeletionen oder in-Frame Deletionen ohne Beteiligung der A/B Pocket-Domänen. Eine mögliche Ursache für die mildere phänotypische Ausprägung bei Patienten mit konstitutionellen Ganzgendeletionen – im Vergleich zu Deletionen mit einem Bruch-punkt im RB1-Gen – könnte die Beteiligung von Genen in direkter Nähe zum RB1 sein: Die homozygote Deletion von benachbarten Genen in 5’- und in 3’-Richtung des RB1-Gens könnte zum Zelltod führen und so das Entstehen eines Tumors unterbinden. Die Analyse der Bruchpunktlokalisationen der von uns identifizierten und der zuvor ver-öffentlichten Mutationen zeigte vier Deletions-Bruchpunkt-Cluster im RB1-Gen: Intron 23, 24, 13 und 16. Durch Sequenzanalysen konnten wir zeigen, dass vermutlich Scaffold/Matrix Attached Regions (S/MARs) in Bezug auf die Ursache dieser Bruchpunkthäufungen eine Rolle spielen. Die Integration der hier entwickelten Methodik in die Routine der molekulargenetischen Analyse bei Retinoblastom führt nicht nur zu einer höheren Mutations-Finderate sondern auch zu einer Verkürzug der molekulargenetischen Befunderhebung. Daher bleiben Angehörigen mit Risikoausschluss belastende Untersuchungen früher erspart

Harnessing spatial homogeneity of neuroimaging data: patch individual filter layers for CNNs

Neuroimaging data, e.g. obtained from magnetic resonance imaging (MRI), is comparably homogeneous due to (1) the uniform structure of the brain and (2) additional efforts to spatially normalize the data to a standard template using linear and non-linear transformations. Convolutional neural networks (CNNs), in contrast, have been specifically designed for highly heterogeneous data, such as natural images, by sliding convolutional filters over different positions in an image. Here, we suggest a new CNN architecture that combines the idea of hierarchical abstraction in neural networks with a prior on the spatial homogeneity of neuroimaging data: Whereas early layers are trained globally using standard convolutional layers, we introduce for higher, more abstract layers patch individual filters (PIF). By learning filters in individual image regions (patches) without sharing weights, PIF layers can learn abstract features faster and with fewer samples. We thoroughly evaluated PIF layers for three different tasks and data sets, namely sex classification on UK Biobank data, Alzheimer's disease detection on ADNI data and multiple sclerosis detection on private hospital data. We demonstrate that CNNs using PIF layers result in higher accuracies, especially in low sample size settings, and need fewer training epochs for convergence. To the best of our knowledge, this is the first study which introduces a prior on brain MRI for CNN learning

Modulation of N^N′-bidentate chelating pyridyl–pyridylidene amide ligands offers mechanistic insights into Pd-catalysed ethylene/methyl acrylate copolymerisation

The efficient copolymerisation of functionalised olefins with alkenes continues to offer considerable challenges to catalyst design. Based on recent work using palladium complexes containing a dissymmetric NN '-bidentate pyridyl-PYA ligand (PYA = pyridylidene amide), which showed a high propensity to insert methyl acrylate, we have here modified this catalyst structure by inserting shielding groups either into the pyridyl fragment, or the PYA unit, or both to avoid fast beta-hydrogen elimination. While a phenyl substituent at the pyridyl side impedes catalytic activity completely and leads to an off-cycle cyclometallation, the introduction of an ortho-methyl group on the PYA side of the NN '-ligand was more prolific and doubled the catalytic productivity. Mechanistic investigations with this ligand system indicated the stabilisation of a 4-membered metallacycle intermediate at room temperature, which has previously been postulated and detected only at 173 K, but never observed at ambient temperature so far. This intermediate was characterised by solution NMR spectroscopy and rationalises, in part, the formation of alpha,beta-unsaturated esters under catalytic conditions, thus providing useful principles for optimised catalyst design

Yambo: an \textit{ab initio} tool for excited state calculations

{\tt yambo} is an {\it ab initio} code for calculating quasiparticle energies and optical properties of electronic systems within the framework of many-body perturbation theory and time-dependent density functional theory. Quasiparticle energies are calculated within the $GW$ approximation for the self-energy. Optical properties are evaluated either by solving the Bethe--Salpeter equation or by using the adiabatic local density approximation. {\tt yambo} is a plane-wave code that, although particularly suited for calculations of periodic bulk systems, has been applied to a large variety of physical systems. {\tt yambo} relies on efficient numerical techniques devised to treat systems with reduced dimensionality, or with a large number of degrees of freedom. The code has a user-friendly command-line based interface, flexible I/O procedures and is interfaced to several publicly available density functional ground-state codes.Comment: This paper describes the features of the Yambo code, whose source is available under the GPL license at www.yambo-code.or

Impaired evoked and resting-state brain oscillations in patients with liver cirrhosis as revealed by magnetoencephalography

AbstractA number of studies suggest that the clinical manifestation of neurological deficits in hepatic encephalopathy results from pathologically synchronized neuronal oscillations and altered oscillatory coupling. In the present study spontaneous and evoked oscillatory brain activities were analyzed jointly with established behavioral measures of altered visual oscillatory processing. Critical flicker and fusion frequencies (CFF, FUF) were measured in 25 patients diagnosed with liver cirrhosis and 30 healthy controls. Magnetoencephalography (MEG) data were collected at rest and during a visual task employing repetitive stimulation. Resting MEG and evoked fields were analyzed. CFF and FUF were found to be reduced in patients, providing behavioral evidence for deficits in visual oscillatory processing. These alterations were found to be related to resting brain activity in patients, namely that the lower the dominant MEG frequency at rest, the lower the CFF and FUF. An analysis of evoked fields at sensor level indicated that in comparison to normal controls, patients were not able to dynamically adapt to flickering visual stimulation. Evoked activity was also analyzed based on independent components (ICs) derived by independent component analysis. The similarity between the shape of each IC and an artificial sine function representing the stimulation frequency was tested via magnitude squared coherence. In controls, we observed a small number of components that correlated strongly with the sine function and a high number of ICs that did not correlate with the sine function. Interestingly, patient data were characterized by a high number of moderately correlating components. Taken together, these results indicate a fundamental divergence of the cerebral resonance activity in cirrhotic patients