233 research outputs found
Ab-initio study of the thermopower of biphenyl-based single-molecule junctions
Employing ab-initio electronic structure calculations combined with the
non-equilibrium Green's function technique, we study the dependence of the
thermopower Q on the conformation in biphenyl-based single-molecule junctions.
For the series of experimentally available biphenyl molecules, alkyl side
chains allow us to gradually adjust the torsion angle \phi\ between the two
phenyl rings from 0 to 90{\deg} and to control in this way the degree of
\pi-electron conjugation. Studying different anchoring groups and binding
positions, our theory predicts that the absolute values of the thermopower
decrease slightly towards larger torsion angles, following an a+b*cos^{2}\phi\
dependence. The anchoring group determines the sign of Q and a,b,
simultaneously. Sulfur and amine groups give rise to Q,a,b>0, while for cyano
Q,a,b<0. The different binding positions can lead to substantial variations of
the thermopower mostly due to changes in the alignment of the frontier
molecular orbital levels and the Fermi energy. We explain our ab-initio results
in terms of a \pi-orbital tight-binding model and a minimal two-level model,
which describes the pair of hybridizing frontier orbital states on the two
phenyl rings. The variations of the thermopower with \phi\ seem to be within
experimental resolution.Comment: 8 pages, 4 figues, 3 table
A taxonomic revision of the whitefish of lakes Brienz and Thun, Switzerland, with descriptions of four new species (Teleostei, Coregonidae)
The alpha taxonomy of the endemic whitefish of lakes Brienz and Thun, Switzerland, is revised. We evaluate the status of seven known species: Coregonus steinmanni sp. nov., Coregonus profundussp. nov.and Coregonus acrinasussp. nov.are endemic to Lake Thun; Coregonusbrienzii sp. nov. is endemic to Lake Brienz; and C. alpinus, C. albellus, and C. fatioi from lakes Brienz and Thun are redescribed. One of these species, C. alpinus, is revised, since the lectotype for this species is incongruent with the species descrip-tion given by Kottelat (1997) and Kottelat and Freyhof (2007). The name C. alpinus is thus retained for the lectotype designated by Kottelat (1997) and a new description of this taxon provided. For the species otherwise described by Kottelat (1997) and Kottelat and Freyhof (2007) as C.alpinus the new name C. profundus is designated. Coregonusacrinasus is genetically partially of allochthonous origin, closely related to the radiation of Lake Constance, and we therefore compare it to all recognized species of Lake Constance, C. wartmanni, C.macrophthalmus, C. arenicolus,and C. gutturosus
Graviton production in anti-de Sitter braneworld cosmology: A fully consistent treatment of the boundary condition
In recent work by two of us, [Durrer & Ruser, PRL 99, 071601 (2007); Ruser &
Durrer PRD 76, 104014 (2007)], graviton production due to a moving spacetime
boundary (braneworld) in a five dimensional bulk has been considered. In the
same way as the presence of a conducting plate modifies the electromagnetic
vacuum, the presence of a brane modifies the graviton vacuum. As the brane
moves, the time dependence of the vacuum leads to particle creation via the so
called 'dynamical Casimir effect'. In our previous work a term in the boundary
condition which is linear in the brane velocity has been neglected. In this
work we develop a new approach which overcomes this approximation. We show that
the previous results are not modified if the brane velocity is low.Comment: 13 pages, 6 figures, added a clarifying paragraph about the setup,
the brane motion adapted w.r.t the version published in PR
Diagnostic performance of CT with Valsalva maneuver for the diagnosis and characterization of inguinal hernias
PURPOSE
Inguinal hernias are mainly diagnosed clinically, but imaging can aid in equivocal cases or for treatment planning. The purpose of this study was to evaluate the diagnostic performance of CT with Valsalva maneuver for the diagnosis and characterization of inguinal hernias.
METHODS
This single-center retrospective study reviewed all consecutive Valsalva-CT studies between 2018 and 2019. A composite clinical reference standard including surgery was used. Three blinded, independent readers (readers 1-3) reviewed the CT images and scored the presence and type of inguinal hernia. A fourth reader measured hernia size. Interreader agreement was quantified with Krippendorff's α coefficients. Sensitivity, specificity, and accuracy of Valsalva-CT for the detection of inguinal hernias was computed for each reader.
RESULTS
The final study population included 351 patients (99 women) with median age 52.2 years (interquartile range (IQR), 47.2, 68.9). A total of 381 inguinal hernias were present in 221 patients. Sensitivity, specificity, and accuracy were 85.8%, 98.1%, and 91.5% for reader 1, 72.7%, 92.5%, and 81.8% for reader 2, and 68.2%, 96.3%, and 81.1% for reader 3. Hernia neck size was significantly larger in cases correctly detected by all three readers (19.0 mm, IQR 13, 25), compared to those missed by all readers (7.0 mm, IQR, 5, 9; p < 0.001). Interreader agreement was substantial (α = 0.723) for the diagnosis of hernia and moderate (α = 0.522) for the type of hernia.
CONCLUSION
Valsalva-CT shows very high specificity and high accuracy for the diagnosis of inguinal hernia. Sensitivity is only moderate which is associated with missed smaller hernias
Weak Ferromagnetism and Excitonic Condensates
We investigate a model of excitonic ordering (i.e electron-hole pair
condensation) appropriate for the divalent hexaborides. We show that the
inclusion of imperfectly nested electron hole Fermi surfaces can lead to the
formation of an undoped excitonic metal phase. In addition, we find that weak
ferromagnetism with compensated moments arises as a result of gapless
excitations. We study the effect of the low lying excitations on the density of
states, Fermi surface topology and optical conductivity and compare to
available experimental data.Comment: 10 Pages, 8 Figures, RevTe
Do primordial Lithium abundances imply there's no Dark Energy?
Explaining the well established observation that the expansion rate of the
universe is apparently accelerating is one of the defining scientific problems
of our age. Within the standard model of cosmology, the repulsive 'dark energy'
supposedly responsible has no explanation at a fundamental level, despite many
varied attempts. A further important dilemma in the standard model is the
Lithium problem, which is the substantial mismatch between the theoretical
prediction for 7-Li from Big Bang Nucleosynthesis and the value that we observe
today. This observation is one of the very few we have from along our past
worldline as opposed to our past lightcone. By releasing the untested
assumption that the universe is homogeneous on very large scales, both apparent
acceleration and the Lithium problem can be easily accounted for as different
aspects of cosmic inhomogeneity, without causing problems for other
cosmological phenomena such as the cosmic microwave background. We illustrate
this in the context of a void model.Comment: 14 pages, 4 figures. v2: minor rearrangements in the text, comments
and references expanded, results unchange
Signals of the cosmological reionization in the radio sky through C and O fine structure lines
We study the excitation of fine structure levels of C I, C II and O I by
ultraviolet (UV) photons around strong UV sources which are also ionizing
sources of the cosmological reionization at redshift of . The
evolutions of ionized regions around a point source are calculated by solving
rate equations for non-equilibrium chemistry. Signals of UV photons through the
fine structure lines are considered to be stronger at locations of more
abundant chemical species of C I, C II and O I. Such environments would be
realized where strong fluxes of non-ionizing UV line photons available for the
pumping up of fine structure levels exist, and simultaneously ionizing UV
photons are effectively shielded by dense H I regions. Signals from H I regions
of moderately large densities induced by redshifted UV photons emitted at the
point sources are found to be dominantly large over those of others. We discuss
the detectability of the signals, and show that signals from idealized
environments will be possibly detected by radio observations with
next-generation arrays to come after the Atacama Large Millimeter/submillimeter
Array (ALMA).Comment: 23 pages, 21 figures, significant revisions in Secs. 2, 3, and 4, and
Appendix, main conclusions unchanged, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
The Cosmic Microwave Background in an Inhomogeneous Universe - why void models of dark energy are only weakly constrained by the CMB
The dimming of Type Ia supernovae could be the result of Hubble-scale
inhomogeneity in the matter and spatial curvature, rather than signaling the
presence of a dark energy component. A key challenge for such models is to fit
the detailed spectrum of the cosmic microwave background (CMB). We present a
detailed discussion of the small-scale CMB in an inhomogeneous universe,
focusing on spherically symmetric `void' models. We allow for the dynamical
effects of radiation while analyzing the problem, in contrast to other work
which inadvertently fine tunes its spatial profile. This is a surprisingly
important effect and we reach substantially different conclusions. Models which
are open at CMB distances fit the CMB power spectrum without fine tuning; these
models also fit the supernovae and local Hubble rate data which favours a high
expansion rate. Asymptotically flat models may fit the CMB, but require some
extra assumptions. We argue that a full treatment of the radiation in these
models is necessary if we are to understand the correct constraints from the
CMB, as well as other observations which rely on it, such as spectral
distortions of the black body spectrum, the kinematic Sunyaev-Zeldovich effect
or the Baryon Acoustic Oscillations.Comment: 23 pages with 14 figures. v2 has considerably extended discussion and
analysis, but the basic results are unchanged. v3 is the final versio
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