238 research outputs found
Tight binding description of the STM image of molecular chains
A tight binding model for scanning tunneling microscopy images of a molecule
adsorbed on a metal surface is described. The model is similar in spirit to
that used to analyze conduction along molecular wires connecting two metal
leads and makes it possible to relate these two measurements and the
information that may be gleaned from the corresponding results. In particular,
the dependence of molecular conduction properties along and across a molecular
chain on the chain length, intersite electronic coupling strength and on
thermal and disorder effects are discussed and contrasted. It is noted that
structural or chemical defects that may affect drastically the conduction along
a molecular chain have a relatively modest influence on conduction across the
molecular wire in the transversal direction.Comment: 22 pages, 9 figures, Israel J Chemistry, in pres
Polariton-assisted Singlet Fission in Acene Aggregates
Singlet fission is an important candidate to increase energy conversion
efficiency in organic photovoltaics by providing a pathway to increase the
quantum yield of excitons per photon absorbed in select materials. We
investigate the dependence of exciton quantum yield for acenes in the strong
light-matter interaction (polariton) regime, where the materials are embedded
in optical microcavities. Starting from an open-quantum-systems approach, we
build a kinetic model for time-evolution of species of interest in the presence
of quenchers and show that polaritons can decrease or increase exciton quantum
yields compared to the cavity-free case. In particular, we find that hexacene,
a typically poor singlet-fission candidate, can feature a higher yield than
cavity-free pentacene when assisted by polaritonic effects. Similarly, we show
that pentacene yield can be increased when assisted by polariton states.
Finally, we address how various relaxation processes between bright and dark
states in lossy microcavities affect polariton photochemistry. Our results also
provide insights on how to choose microcavities to enhance similarly related
chemical processes.Comment: 12 pages, 4 figure
Upregulation of Neurotrophic Factors Selectively in Frontal Cortex in Response to Olfactory Discrimination Learning
We have previously shown that olfactory discrimination learning is accompanied by several forms of long-term enhancement in
synaptic connections between layer II pyramidal neurons selectively in the piriform cortex. This study sought to examine whether
the previously demonstrated olfactory-learning-task-induced modifications are preceded by suitable changes in the expression of
mRNA for neurotrophic factors and in which brain areas this occurs. Rats were trained to discriminate positive cues in pair of odors
for a water reward. The relationship between the learning task and local levels of mRNA for brain-derived neurotrophic factor,
tyrosine kinase B, nerve growth factor, and neurotrophin-3 in the frontal cortex, hippocampal subregions, and other regions were
assessed 24 hours post olfactory learning. The olfactory discrimination learning activated production of endogenous neurotrophic
factors and induced their signal transduction in the frontal cortex, but not in other brain areas. These findings suggest that different
brain areas may be preferentially involved in different learning/memory tasks
Screen-Printed Flexible Circular and Rectangular Silver Spirals for Planar Electrodynamic Loudspeakers: A Comparative Study of Pressure Frequency Response
We present the fabrication and characterization of flexible planar electrodynamic loudspeakers. Conductive spirals are fabricated on a flexible and transparent polyethylene terephthalate substrate via screen printing. Different geometries (circular and rectangular) and sizes of the conductive spirals are investigated to understand their impact on the
performance. The optimized circular spiral allows achieving an average sound pressure level of 63 dB at 1m distance in 2kHz-20kHz band, proving the suitability of these devices as high-frequency loudspeaker drivers
Quantum master equation for electron transport through quantum dots and single molecules
A quantum master equation (QME) is derived for the many-body density matrix
of an open current-carrying system weakly coupled to two metal leads. The
dynamics and the steady-state properties of the system for arbitrary bias are
studied using projection operator techniques, which keep track of number of
electrons in the system. We show that coherences between system states with
different number of electrons, n, (Fock space coherences) do not contribute to
the transport to second order in system-lead coupling.
However, coherences between states with the same n may effect transport
properties when the damping rate is of the order or faster then the system Bohr
frequencies.
For large bias, when all the system many-body states lie between the chemical
potentials of the two leads, we recover previous results. In the rotating wave
approximation (when the damping is slow compared to the Bohr frequencies of the
system), the dynamics of populations and the coherences in the system
eigenbasis are decoupled. The QME then reduces to a birth and death master
equation for populations.Comment: 22 pages, 8 figures, paper accepted in Phys. Rev.
Process-based framework for precise neuromodulation
Functional MRI neurofeedback (NF) allows humans to self-modulate neural patterns in specific brain areas. This technique is regarded as a promising tool to translate neuroscientific knowledge into brain-guided psychiatric interventions. However, its clinical implementation is restricted by unstandardized methodological practices, by clinical definitions that are poorly grounded in neurobiology, and by lack of a unifying framework that dictates experimental choices. Here we put forward a new framework, termed ‘process-based NF’, which endorses a process-oriented characterization of mental dysfunctions to form precise and effective psychiatric treatments. This framework relies on targeting specific dysfunctional mental processes by modifying their underlying neural mechanisms and on applying process-specific contextual feedback interfaces. Finally, process-based NF offers designs and a control condition that address the methodological shortcomings of current approaches, thus paving the way for a precise and personalized neuromodulation
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