340 research outputs found
Electrostatic potential profiles of molecular conductors
The electrostatic potential across a short ballistic molecular conductor
depends sensitively on the geometry of its environment, and can affect its
conduction significantly by influencing its energy levels and wave functions.
We illustrate some of the issues involved by evaluating the potential profiles
for a conducting gold wire and an aromatic phenyl dithiol molecule in various
geometries. The potential profile is obtained by solving Poisson's equation
with boundary conditions set by the contact electrochemical potentials and
coupling the result self-consistently with a nonequilibrium Green's function
(NEGF) formulation of transport. The overall shape of the potential profile
(ramp vs. flat) depends on the feasibility of transverse screening of electric
fields. Accordingly, the screening is better for a thick wire, a multiwalled
nanotube or a close-packed self-assembled monolayer (SAM), in comparison to a
thin wire, a single-walled nanotube or an isolated molecular conductor. The
electrostatic potential further governs the alignment or misalignment of
intramolecular levels, which can strongly influence the molecular I-V
characteristic. An external gate voltage can modify the overall potential
profile, changing the current-voltage (I-V) characteristic from a resonant
conducting to a saturating one. The degree of saturation and gate modulation
depends on the metal-induced-gap states (MIGS) and on the electrostatic gate
control parameter set by the ratio of the gate oxide thickness to the channel
length.Comment: to be published in Phys. Rev. B 69, No.3, 0353XX (2004
The role of affect-driven impulsivity in gambling cognitions: a convenience-sample study with a Spanish version of the Gambling-Related Cognitions Scale
Background and aims: abnormal cognitions are among the most salient domain-specific features of gambling disorder. The aims of this study were: (a) to examine and validate a Spanish version of the Gambling-Related Cognitions Scale (GRCS; Raylu & Oei, 2004) and (b) to examine associations between cognitive distortion levels, impulsivity, and gambling behavior. Methods: this study first recruited a convenience sample of 500 adults who had gambled during the previous year. Participants were assessed using the Spanish version of GRCS (GRCS-S) questionnaire, the UPPS-P impulsivity questionnaire, measures of gambling behavior, and potentially relevant confounders. Robust confirmatory factor analysis methods on half the sample were used to select the best models from a hypothesis-driven set. The best solutions were validated on the other half, and the resulting factors were later correlated with impulsivity dimensions (in the whole n = 500 factor analysis sample) and clinically relevant gambling indices (in a separate convenience sample of 137 disordered and non-disordered gamblers; validity sample). Results: this study supports the original five-factor model, suggests an alternative four-factor solution, and confirms the psychometric soundness of the GRCS-S. Importantly, cognitive distortions consistently correlated with affect-or motivation-driven aspects of impulsivity (urgency and sensation seeking), but not with cognitive impulsivity (lack of premeditation and lack of perseverance). Discussion and conclusions: our findings suggest that the GRCS-S is a valid and reliable instrument to identify gambling cognitions in Spanish samples. Our results expand upon previous research signaling specific associations between gambling-related distortions and affect-driven impulsivity in line with models of motivated reasoning
Mirror Manifolds in Higher Dimension
We describe mirror manifolds in dimensions different from the familiar case
of complex threefolds. We emphasize the simplifying features of dimension three
and supply more robust methods that do not rely on such special characteristics
and hence naturally generalize to other dimensions. The moduli spaces for
Calabi--Yau -folds are somewhat different from the ``special K\"ahler
manifolds'' which had occurred for , and we indicate the new geometrical
structures which arise. We formulate and apply procedures which allow for the
construction of mirror maps and the calculation of order-by-order instanton
corrections to Yukawa couplings. Mathematically, these corrections are expected
to correspond to calculating Chern classes of various parameter spaces (Hilbert
schemes) for rational curves on Calabi--Yau manifolds. Our results agree with
those obtained by more traditional mathematical methods in the limited number
of cases for which the latter analysis can be carried out. Finally, we make
explicit some striking relations between instanton corrections for various
Yukawa couplings, derived from the associativity of the operator product
algebra.Comment: 44 pages plus 3 tables using harvma
Modeling transport through single-molecule junctions
Non-equilibrium Green's functions (NEGF) formalism combined with extended
Huckel (EHT) and charging model are used to study electrical conduction through
single-molecule junctions. Analyzed molecular complex is composed of asymmetric
1,4-Bis((2'-para-mercaptophenyl)-ethinyl)-2-acetyl-amino-5-nitro-benzene
molecule symmetrically coupled to two gold electrodes [Reichert et al., Phys.
Rev. Lett. Vol.88 (2002), pp. 176804]. Owing to this model, the accurate values
of the current flowing through such junction can be obtained by utilizing basic
fundamentals and coherently deriving model parameters. Furthermore, the
influence of the charging effect on the transport characteristics is
emphasized. In particular, charging-induced reduction of conductance gap,
charging-induced rectification effect and charging-generated negative value of
the second derivative of the current with respect to voltage are observed and
examined for molecular complex.Comment: 8 pages, 3 figure
A Tamm plasmon-porous GaN distributed Bragg reflector cavity
This paper reports on design, measurement and optimisation of a Tamm plasmon metal-distributed Bragg reflector (DBR) cavity for use in the green part of the visible spectrum. It uses an optimised silver layer thickness and a porous DBR created using a novel electro-chemical etching technique. This device has applications in low-cost lasers, photodetectors, and photoconductive switches for the visible wavelength range
Correlates of Premenstrual Dysphoria in Help-seeking Women
Comparisons were made between the premenstrual changes reported by nontreatment-seekers (NTS) (n = 32) and those of treatment-seekers (TS) (n = 52). The Premenstrual Assessment Form Luteal Phase and Follicular Phase versions were completed and the Beck Depression Inventory, the Automatic Thoughts Questionnaire and the State-Trait Anxiety Inventory were completed at both the luteal and follicular phases. Prospective daily ratings were made for two treatment cycles on the Daily Ratings Form and TS were screened for a mood-disorder history. Using the commonly cited 30% decrease in dysphoric levels from the pre- to postmenstrual phases as the criterion of prospective confirmation, women with prospectively confirmed dysphoria (PMD +) were not significantly more symptomatic than those without prospective dysphoric confirmation (PMD -). However, TS were more symptomatic than NTS on measures of depression, anxiety and frequency of negative automatic thoughts but not on mood behaviour and physical changes reflected in the PAF scales. No demographic differences were found between TS and NTS. Results did not support the issue of requiring `confirmation' of self-reports within a help-seeking group or the use of the 30% criterion in particular. Findings further suggest that the 95-item PAF may be inadequate in differentiating TS from others
Direct Integration of the Topological String
We present a new method to solve the holomorphic anomaly equations governing
the free energies of type B topological strings. The method is based on direct
integration with respect to the non-holomorphic dependence of the amplitudes,
and relies on the interplay between non-holomorphicity and modularity
properties of the topological string amplitudes. We develop a formalism valid
for any Calabi-Yau manifold and we study in detail two examples, providing
closed expressions for the amplitudes at low genus, as well as a discussion of
the boundary conditions that fix the holomorphic ambiguity. The first example
is the non-compact Calabi-Yau underlying Seiberg-Witten theory and its
gravitational corrections. The second example is the Enriques Calabi-Yau, which
we solve in full generality up to genus six. We discuss various aspects of this
model: we obtain a new method to generate holomorphic automorphic forms on the
Enriques moduli space, we write down a new product formula for the fiber
amplitudes at all genus, and we analyze in detail the field theory limit. This
allows us to uncover the modularity properties of SU(2), N=2 super Yang-Mills
theory with four massless hypermultiplets.Comment: 75 pages, 3 figure
Theory of Current and Shot Noise Spectroscopy in Single-Molecular Quantum Dots with Phonon Mode
Using the Keldysh nonequilibrium Green function technique, we study the
current and shot noise spectroscopy of a single molecular quantum dot coupled
to a local phonon mode. It is found that in the presence of electron-phonon
coupling, in addition to the resonant peak associated with the single level of
the dot, satellite peaks with the separation set by the frequency of phonon
mode appear in the differential conductance. In the ``single level'' resonant
tunneling region, the differential shot noise power exhibit two split peaks.
However, only single peaks show up in the ``phonon assisted''
resonant-tunneling region. An experimental setup to test these predictions is
also proposed.Comment: 5 pages, 3 eps figures embedde
Cognitive and Biochemical Processes in Depressed Adult Outpatients: a Test of the Circular Process Model
The circular process model is a psychobiological model of depression, in which it is postulated that catecholamines and negative cognitions interact to influence depression. Since its publication, there have been no empirical tests to support or refute the model. This study tested the model in 92 depressed adult outpatients with non-bipolar non-psychotic depression. There were no significant bivariate correlations among the biochemical and cognitive measures. However, the interactive model was supported by results of two out of three hierarchical regression analyses, in which the biochemical-by-cognitive interactive terms significantly predicted depression after the main effects of each variable were accounted for. These findings show sufficient evidence in support of the Circular Process Model to warrant further testing over the treatment period
Programmable antivirals targeting critical conserved viral RNA secondary structures from influenza A virus and SARS-CoV-2
Influenza A virusâs (IAVâs) frequent genetic changes challenge vaccine strategies and engender resistance to current drugs. We sought to identify conserved and essential RNA secondary structures within IAVâs genome that are predicted to have greater constraints on mutation in response to therapeutic targeting. We identified and genetically validated an RNA structure (packaging stemâloop 2 (PSL2)) that mediates in vitro packaging and in vivo disease and is conserved across all known IAV isolates. A PSL2-targeting locked nucleic acid (LNA), administered 3 d after, or 14 d before, a lethal IAV inoculum provided 100% survival in mice, led to the development of strong immunity to rechallenge with a tenfold lethal inoculum, evaded attempts to select for resistance and retained full potency against neuraminidase inhibitor-resistant virus. Use of an analogous approach to target SARS-CoV-2, prophylactic administration of LNAs specific for highly conserved RNA structures in the viral genome, protected hamsters from efficient transmission of the SARS-CoV-2 USA_WA1/2020 variant. These findings highlight the potential applicability of this approach to any virus of interest via a process we term âprogrammable antiviralsâ, with implications for antiviral prophylaxis and post-exposure therapy
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