Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

We present a comparative study of ultrafast photo-conversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is activated and temperature-independent respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C$_{60}$-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.Comment:

Neural responses for evaluating self and mother traits in adolescence depend on mother-adolescent relationships

An important task in adolescence is to achieve autonomy while preserving a positive relationship with parents. Previous fMRI studies showed largely overlapping activation in medial prefrontal cortex (mPFC) for evaluating self and close-other traits but separable activation for self and non-close other. Possibly, more similar mPFC activation ref lects closeness or warmth in relationships. We investigated neural indicators of the mother–adolescent relationship in adolescents between 11 and 21 years (N = 143). Mother–adolescent relationship was measured using (i) mothers’ and adolescents’ trait evaluations about each other, (ii) observations of warmth, negativity and emotional support in mother–adolescent conversation and (iii) similarity in adolescents’ neural activation for evaluating self vs mother traits. Results showed relatively more similar mPFC activation in adolescents who evaluated their mothers’ traits more positively, suggesting that this is possibly a neural indicator of mother–adolescent relationship quality. Furthermore, mid-adolescence was characterized by more negative mother–adolescent interaction compared to early and late adolescence. This effect co-occurred with mid-adolescent peaks in dorsal striatum, dorsal anterior cingulate cortex and superior temporal sulcus activation in evaluating traits of self vs mother. These results suggest more negative relationships and stronger self-focus in mid-adolescenc

Observational reinforcement learning in children and young adults

Observational learning is essential for the acquisition of new behavior in educational practices and daily life and serves as an important mechanism for human cognitive and social-emotional development. However, we know little about its underlying neurocomputational mechanisms from a developmental perspective. In this study we used model-based fMRI to investigate differences in observational learning and individual learning between children and younger adults. Prediction errors (PE), the difference between experienced and predicted outcomes, related positively to striatal and ventral medial prefrontal cortex activation during individual learning and showed no age-related differences. PE-related activation during observational learning was more pronounced when outcomes were worse than predicted. Particularly, negative PE-coding in the dorsal medial prefrontal cortex was stronger in adults compared to children and was associated with improved observational learning in children and adults. The current findings pave the way to better understand observational learning challenges across development and educational settings

Neural responses for evaluating self and mother traits in adolescence depend on mother-adolescent relationships

An important task in adolescence is to achieve autonomy while preserving a positive relationship with parents. Previous fMRI studies showed largely overlapping activation in medial prefrontal cortex (mPFC) for evaluating self and close-other traits but separable activation for self and non-close other. Possibly, more similar mPFC activation reflects closeness or warmth in relationships. We investigated neural indicators of the mother-adolescent relationship in adolescents between 11 and 21years (N=143). Mother-adolescent relationship was measured using (i) mothers' and adolescents' trait evaluations about each other, (ii) observations of warmth, negativity and emotional support in mother-adolescent conversation and (iii) similarity in adolescents' neural activation for evaluating self vs mother traits. Results showed relatively more similar mPFC activation in adolescents who evaluated their mothers' traits more positively, suggesting that this is possibly a neural indicator of mother-adolescent relationship quality. Furthermore, mid-adolescence was characterized by more negative mother-adolescent interaction compared to early and late adolescence. This effect co-occurred with mid-adolescent peaks in dorsal striatum, dorsal anterior cingulate cortex and superior temporal sulcus activation in evaluating traits of self vs mother. These results suggest more negative relationships and stronger self-focus in mid-adolescence.Pathways through Adolescenc

Ictal propagation of high frequency activity is recapitulated in interictal recordings: effective connectivity of epileptogenic networks recorded with intracranial EEG

Seizures are increasingly understood to arise from epileptogenic networks across which ictal activity is propagated and sustained. In patients undergoing invasive monitoring for epilepsy surgery, high frequency oscillations have been observed within the seizure onset zone during both ictal and interictal intervals. We hypothesized that the patterns by which high frequency activity is propagated would help elucidate epileptogenic networks and thereby identify network nodes relevant for surgical planning. Intracranial EEG recordings were analyzed with a multivariate autoregressive modeling technique (short-time direct directed transfer function--SdDTF), based on the concept of Granger causality, to estimate the directionality and intensity of propagation of high frequency activity (70-175 Hz) during ictal and interictal recordings. These analyses revealed prominent divergence and convergence of high frequency activity propagation at sites identified by epileptologists as part of the ictal onset zone. In contrast, relatively little propagation of this activity was observed among the other analyzed sites. This pattern was observed in both subdural and depth electrode recordings of patients with focal ictal onset, but not in patients with a widely distributed ictal onset. In patients with focal ictal onsets, the patterns of propagation recorded during pre-ictal (up to 5 min immediately preceding ictal onset) and interictal (more than 24h before and after seizures) intervals were very similar to those recorded during seizures. The ability to characterize epileptogenic networks from interictal recordings could have important clinical implications for epilepsy surgery planning by reducing the need for prolonged invasive monitoring to record spontaneous seizures

The velocity peaks in the cold dark matter spectrum on Earth

The cold dark matter spectrum on earth is expected to have peaks in velocity space. We obtain estimates for the sizes and locations of these peaks. To this end we have generalized the secondary infall model of galactic halo formation to include angular momentum of the dark matter particles. This new model is still spherically symmetric and it has self-similar solutions. Our results are relevant to direct dark matter search experiments.Comment: 12 pages including 1 table and 4 figures, LaTeX, REVTEX 3.0 versio

A Guide to Localized Frames and Applications to Galerkin-like Representations of Operators

This chapter offers a detailed survey on intrinsically localized frames and the corresponding matrix representation of operators. We re-investigate the properties of localized frames and the associated Banach spaces in full detail. We investigate the representation of operators using localized frames in a Galerkin-type scheme. We show how the boundedness and the invertibility of matrices and operators are linked and give some sufficient and necessary conditions for the boundedness of operators between the associated Banach spaces.Comment: 32 page

Nanotechnology and Society: A discussion-based undergraduate course

Nanotechnology has emerged as a broad, exciting, yet ill-defined field of scientific research and technological innovation. There are important questions about the technology's potential economic, social, and environmental implications. We discuss an undergraduate course on nanoscience and nanotechnology for students from a wide range of disciplines, including the natural and social sciences, the humanities, and engineering. The course explores these questions and the broader place of technology in contemporary societies. The course is built around active learning methods and seeks to develop the students' critical thinking skills, written and verbal communication abilities, and general knowledge of nanoscience and nanoengineering concepts. Continuous assessment was used to gain information about the effectiveness of class discussions and enhancement of student understanding of the interaction between nanotechnology and society.Comment: 7 pages, 1 figure. Edited and shortened for readability. Visit http://www.tahan.com/charlie/nanosociety/course201/ for more informatio

A New Paradigm for Large Earthquakes in Stable Continental Plate Interiors

Large earthquakes within stable continental regions (SCR) show that significant amounts of elastic strain can be released on geological structures far from plate boundary faults, where the vast majority of the Earth's seismic activity takes place. SCR earthquakes show spatial and temporal patterns that differ from those at plate boundaries and occur in regions where tectonic loading rates are negligible. However, in the absence of a more appropriate model, they are traditionally viewed as analogous to their plate boundary counterparts, occuring when the accrual of tectonic stress localized at long-lived active faults reaches failure threshold. Here we argue that SCR earthquakes are better explained by transient perturbations of local stress or fault strength that release elastic energy from a pre-stressed lithosphere. As a result, SCR earthquakes can occur in regions with no previous seismicity and no surface evidence for strain accumulation. They need not repeat, since the tectonic loading rate is close to zero. Therefore, concepts of recurrence time or fault slip rate do not apply. As a consequence, seismic hazard in SCRs is likely more spatially distributed than indicated by paleoearthquakes, current seismicity, or geodetic strain rates