2,409 research outputs found
Accurate molecular energies by extrapolation of atomic energies using an analytic quantum mechanical model
Using a new analytic quantum mechanical method based on Slater's Xalpha
method, we show that a fairly accurate estimate of the total energy of a
molecule can be obtained from the exact energies of its constituent atoms. The
mean absolute error in the total energies thus determined for the G2 set of 56
molecules is about 16 kcal/mol, comparable to or better than some popular pure
and hybrid density functional models.Comment: 5 pages, REVTE
Single molecule analysis of DNA wrapping and looping by a circular 14mer wheel of the bacteriophage 186 CI repressor
The lytic–lysogenic decision in bacteriophage 186 is governed by the 186 CI repressor protein in a unique way. The 186 CI is proposed to form a wheel-like oligomer that can mediate either wrapped or looped nucleoprotein complexes to provide the cooperative and competitive interactions needed for regulation. Although consistent with structural, biochemical and gene expression data, many aspects of this model are based on inference. Here, we use atomic force microscopy (AFM) to reveal the various predicted wrapped and looped species, and new ones, for CI regulation of lytic and lysogenic transcription. Automated AFM analysis showed CI particles of the predicted dimensions on the DNA, with CI multimerization favoured by DNA binding. Measurement of the length of the wrapped DNA segments indicated that CI may move on the DNA, wrapping or releasing DNA on either side of the wheel. Tethered particle motion experiments were consistent with wrapping and looping of DNA by CI in solution, where in contrast to λ repressor, the looped species were exceptionally stable. The CI regulatory system provides an intriguing comparison with that of nucleosomes, which share the ability to wrap and release similar sized segments of DNA.Haowei Wang, Ian B. Dodd, David D. Dunlap, Keith E. Shearwin, and Laura Finz
Time evolution of models described by one-dimensional discrete nonlinear Schr\"odinger equation
The dynamics of models described by a one-dimensional discrete nonlinear
Schr\"odinger equation is studied. The nonlinearity in these models appears due
to the coupling of the electronic motion to optical oscillators which are
treated in adiabatic approximation. First, various sizes of nonlinear cluster
embedded in an infinite linear chain are considered. The initial excitation is
applied either at the end-site or at the middle-site of the cluster. In both
the cases we obtain two kinds of transition: (i) a cluster-trapping transition
and (ii) a self-trapping transition. The dynamics of the quasiparticle with the
end-site initial excitation are found to exhibit, (i) a sharp self-trapping
transition, (ii) an amplitude-transition in the site-probabilities and (iii)
propagating soliton-like waves in large clusters. Ballistic propagation is
observed in random nonlinear systems. The effect of nonlinear impurities on the
superdiffusive behavior of random-dimer model is also studied.Comment: 16 pages, REVTEX, 9 figures available upon request, To appear in
Physical Review
On the role of a new type of correlated disorder in extended electronic states in the Thue-Morse lattice
A new type of correlated disorder is shown to be responsible for the
appearance of extended electronic states in one-dimensional aperiodic systems
like the Thue-Morse lattice. Our analysis leads to an understanding of the
underlying reason for the extended states in this system, for which only
numerical evidence is available in the literature so far. The present work also
sheds light on the restrictive conditions under which the extended states are
supported by this lattice.Comment: 11 pages, LaTeX V2.09, 1 figure (available on request), to appear in
Physical Review Letter
Effect of nonlinearity on the dynamics of a particle in dc field-induced systems
Dynamics of a particle in a perfect chain with one nonlinear impurity and in
a perfect nonlinear chain under the action of dc field is studied numerically.
The nonlinearity appears due to the coupling of the electronic motion to
optical oscillators which are treated in adiabatic approximation.
We study for both the low and high values of field strength. Three different
range of nonlinearity is obtained where the dynamics is different. In low and
intermediate range of nonlinearity, it reduces the localization. In fact in the
intermediate range subdiffusive behavior in the perfect nonlinear chain is
obtained for a long time. In all the cases a critical value of nonlinear
strength exists where self-trapping transition takes place. This critical value
depends on the system and the field strength. Beyond the self-trapping
transition nonlinearity enhances the localization.Comment: 9 pages, Revtex, 6 ps figures include
College Students’ Use of Social Media and E-Cigarettes: How Correctly Identifying Platform Type Influences Findings
The purpose of this study was to examine how college students post or share JUUL-related content on social media. Using a sequential explanatory mixed-methods design, current JUUL users (n = 667) completed a cross-sectional survey in March of 2019, then 51 participants completed in-person follow-up interviews in April of 2019. Survey questions asked about JUUL-related social media postings and commenting history as well as demographic questions. Interview participants were asked to explain their survey responses and were then shown the survey results and asked for reasons why they and others did not post or comment about JUUL on social media. Qualitative data were coded independently by two coders using NVivo, and analyzed for themes. Survey participant (ages 18-24, mean age 20 years, 50.5% female, and 80.6% white) responses showed 81% had not posted a JUUL-focused comment on social media and had not posted a picture of themselves JUULing in the past year. However, interviewees reported they had continued to post about JUUL on social media but moved away from more public social media accounts (e.g., Facebook); private Instagram and Snapchat accounts were used to post both JUUL use and JUUL-focused content without risk of damaging their personal image to family or potential employers. How social media use questions are asked is critical for understanding college student promotion of JUULing and social norms. Young adults protect their social media presence by not including themselves in JUUL-focused content; thus, the spread of JUULing through private social media like Snapchat or Finstas may not be identified and young adults normalize JUUL use through memes or images
Frenkel Excitons in Random Systems With Correlated Gaussian Disorder
Optical absorption spectra of Frenkel excitons in random one-dimensional
systems are presented. Two models of inhomogeneous broadening, arising from a
Gaussian distribution of on-site energies, are considered. In one case the
on-site energies are uncorrelated variables whereas in the second model the
on-site energies are pairwise correlated (dimers). We observe a red shift and a
broadening of the absorption line on increasing the width of the Gaussian
distribution. In the two cases we find that the shift is the same, within our
numerical accuracy, whereas the broadening is larger when dimers are
introduced. The increase of the width of the Gaussian distribution leads to
larger differences between uncorrelated and correlated disordered models. We
suggest that this higher broadening is due to stronger scattering effects from
dimers.Comment: 9 pages, REVTeX 3.0, 3 ps figures. To appear in Physical Review
Quantum tunneling Sb-heterostructure millimeter-wave diodes
We have developed a new zero bias millimeter wave diode based on quantum tunneling in an InAs/AlSb/GaSb nanostructure. It is ideal for square law radiometry and passive millimeter wave imaging. Excellent sensitivity has been demonstrated at present up to 110 GHz, with higher bandwidth predicted for smaller area diodes
Vibrational signatures for low-energy intermediate-sized Si clusters
We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size
Periodic features in the Dynamic Structure Factor of the Quasiperiodic Period-doubling Lattice
We present an exact real-space renormalization group (RSRG) method for
evaluating the dynamic structure factor of an infinite one-dimensional
quasiperiodic period-doubling (PD) lattice. We observe that for every normal
mode frequency of the chain, the dynamic structure factor always
exhibits periodicity with respect to the wave vector and the presence of
such periodicity even in absence of translational invariance in the system is
quite surprising. Our analysis shows that this periodicity in
actually indicates the presence of delocalized phonon modes in the PD chain.
The Brillouin Zones of the lattice are found to have a hierarchical structure
and the dispersion relation gives both the acoustic as well as optical
branches. The phonon dispersion curves have a nested structure and we have
shown that it is actually the superposition of the dispersion curves of an
infinite set of periodic lattices.Comment: 9 pages, 3 postscript figures, REVTeX, To appear in Phys. Rev. B (1
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