383 research outputs found
Effect of short- and long-range scattering in the conductivity of graphene: Boltzmann approach vs tight-binding calculations
We present a comparative study of the density dependence of the conductivity
of graphene sheets calculated in the tight-binding (TB) Landauer approach and
on the basis of the Boltzmann theory. The TB calculations are found to give the
same density dependence of the conductivity, , for short-range
and long-range Gaussian scatterers. In the case of short-range scattering the
TB calculations are in agreement with the predictions of the Boltzmann theory
going beyond the Born approximation, but in qualitative and quantitative
disagreement with the standard Boltzmann approach within the Born
approximation, predicting const. Even for the long-range Gaussian
potential in a parameter range corresponding to realistic systems the standard
Boltzmann predictions are in quantitative and qualitative disagreement with the
TB results. This questions the applicability of the standard Boltzmann approach
within the Born approximation, commonly used for the interpretation of the
results of experimental studies of the transport in graphene.Comment: 5 page
Collisional properties of cold spin-polarized nitrogen gas: theory, experiment, and prospects as a sympathetic coolant for trapped atoms and molecules
We report a combined experimental and theoretical study of collision-induced
dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use
buffer gas cooling to create trapped samples of 14N and 15N atoms with
densities 5+/-2 x 10^{12} cm-3 and measure their magnetic relaxation rates at
milli-Kelvin temperatures. Rigorous quantum scattering calculations based on
accurate ab initio interaction potentials for the 7Sigma_u electronic state of
N2 demonstrate that dipolar relaxation in N + N collisions occurs at a slow
rate of ~10^{-13} cm3/s over a wide range of temperatures (1 mK to 1 K) and
magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are
insensitive to small variations of the interaction potential and to the
magnitude of the spin-exchange interaction, enabling the accurate calibration
of the measured N atom density. We find consistency between the calculated and
experimentally determined rates. Our results suggest that N atoms are promising
candidates for future experiments on sympathetic cooling of molecules.Comment: 48 pages, 17 figures, 3 table
Tangential View and Intraoperative Three-Dimensional Fluoroscopy for the Detection of Screw-Misplacements in Volar Plating of Distal Radius Fractures
Background: Volar locking plate fixation has become the gold standard in the treatment of unstable distal radius fractures. Juxta-articular screws should be placed as close as possible to the subchondral zone, in an optimized length to buttress the articular surface and address the contralateral cortical bone. On the other hand, intra-articular screw misplacements will promote osteoarthritis, while the penetration of the contralateral bone surface may result in tendon irritations and ruptures. The intraoperative control of fracture reduction and implant positioning is limited in the common postero-anterior and true lateral two-dimensional (2D)-fluoroscopic views. Therefore, additional 2D-fluoroscopic views in different projections and intraoperative three-dimensional (3D) fluoroscopy were recently reported. Nevertheless, their utility has issued controversies.
Objectives: The following questions should be answered in this study; 1) Are the additional tangential view and the intraoperative 3D fluoroscopy useful in the clinical routine to detect persistent fracture dislocations and screw misplacements, to prevent revision surgery? 2) Which is the most dangerous plate hole for screw misplacement?
Patients and Methods: A total of 48 patients (36 females and 13 males) with 49 unstable distal radius fractures (22 x 23 A; 2 x 23 B, and 25 x 23 C) were treated with a 2.4 mm variable angle LCP Two-Column volar distal radius plate (Synthes GmbH, Oberdorf, Switzerland) during a 10-month period. After final fixation, according to the manufactures' technique guide and control of implant placement in the two common perpendicular 2D-fluoroscopic images (postero-anterior and true lateral), an additional tangential view and intraoperative 3D fluoroscopic scan were performed to control the anatomic fracture reduction and screw placements. Intraoperative revision rates due to screw misplacements (intra-articular or overlength) were evaluated. Additionally, the number of surgeons, time and radiation-exposure, for each step of the operating procedure, were recorded.
Results: In the standard 2D-fluoroscopic views (postero-anterior and true lateral projection), 22 screw misplacements of 232 inserted screws were not detected. Based on the additional tangential view, 12 screws were exchanged, followed by further 10 screws after performing the 3D fluoroscopic scan. The most lateral screw position had the highest risk for screw misplacement (accounting for 45.5% of all exchanged screws). The mean number of images for the tangential view was 3 ± 2.5 images. The mean surgical time was extended by 10.02 ± 3.82 minutes for the 3D fluoroscopic scan. An additional radiation exposure of 4.4 ± 4.5seconds, with a dose area product of 39.2 ± 14.5 cGy/cm2 were necessary for the tangential view and 54.4 ± 20.9 seconds with a dose area product of 2.1 ± 2.2 cGy/cm2, for the 3D fluoroscopic scan.
Conclusions: We recommend the additional 2D-fluoroscopic tangential view for detection of screw misplacements caused by overlength, with penetration on the dorsal cortical surface of the distal radius, predominantly observed for the most lateral screw position. The use of intraoperative 3D fluoroscopy did not become accepted in our clinical routine, due to the technical demanding and time consuming procedure, with a limited image quality so far
Het Nieuwe Werken: een analyse van de verschillen in de implementatie binnen publieke en private organisaties
Transition from ballistic to diffusive behavior of graphene ribbons in the presence of warping and charged impurities
We study the effects of the long-range disorder potential and warping on the
conductivity and mobility of graphene ribbons using the Landauer formalism and
the tight-binding p-orbital Hamiltonian. We demonstrate that as the length of
the structure increases the system undergoes a transition from the ballistic to
the diffusive regime. This is reflected in the calculated electron density
dependencies of the conductivity and the mobility. In particular, we show that
the mobility of graphene ribbons varies as mu(n) n^(-lambda), with
0<lambda<0.5. The exponent lambda depends on the length of the system with
lambda=0.5 corresponding to short structures in the ballistic regime, whereas
the diffusive regime lambda=0 (when the mobility is independent on the electron
density) is reached for sufficiently long structures. Our results can be used
for the interpretation of experimental data when the value of lambda can be
used to distinguish the transport regime of the system (i.e. ballistic,
quasi-ballistic or diffusive). Based on our findings we discuss available
experimental results
Efficient chemical synthesis of human complement protein C3a
We report the total chemical synthesis of human C3a by one-pot native chemical ligation of three unprotected peptide segments, followed by efficient in vitro folding that yielded the anaphylatoxin C3a in high yield and excellent purity. Synthetic C3a was fully active and its crystal structure at 2.1 Å resolution showed 3 helices and a C-terminal turn motif
Far-infrared study of the Jahn-Teller distorted C60 monoanion in C60 tetraphenylphosphoniumiodide
We report high-resolution far-infrared transmission measurements on C(60)-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm(-1)), we assign intramolecular modes of the C(60) monoanion and identify low-frequency combination modes. The well-known F(1u)(1) and F(1u)(2) modes are split into doublers at room temperature, indicating a D(5d) or D(3d) distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the hall, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C(60)(-) and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening (with decreasing temperature) in several modes may correlate with the radial character of those vibrations. [S0163-1829(98)03245-7]
Evidence for a New Resonance from Polarized Neutron-Proton Scattering
Exclusive and kinematically complete high-statistics measurements of
quasifree polarized scattering have been performed in the energy
region of the narrow resonance structure with , 2380 MeV/ and 70 MeV observed recently in the
double-pionic fusion channels and .
The experiment was carried out with the WASA detector setup at COSY having a
polarized deuteron beam impinged on the hydrogen pellet target and utilizing
the quasifree process . That way the
analyzing power was measured over a large angular range. The obtained
angular distributions deviate systematically from the current SAID SP07
NN partial-wave solution. Incorporating the new data into the SAID
analysis produces a pole in the waves as expected from the
resonance hypothesis
Cross Sections of the Deuteron-Proton Breakup at 130 MeV:A Probe of Three-Nucleon System Dynamics
Three-nucleon system dynamics can be investigated quantitatively by comparing observables calculated with the use of Faddeev equations with results of precise measurements. Proper description of the experimental data can be achieved only if the dynamical models include, in addition to the nucleon-nucleon interaction, subtle effects of suppressed degrees of freedom, effectively introduced by means of genuine three-nucleon forces. A large set of high precision, exclusive cross-section data for the (1)H(d,pp)n breakup reaction at 130 MeV contributes significantly to constrain the physical assumptions underlying the theoretical interaction models. Comparison of nearly 1,800 cross section data points with the predictions using nuclear interactions generated in various ways, allowed to establish importance of including both, the three-nucleon and the Coulomb forces to significantly improve the description of the whole data set
Search for eta-mesic 4He in the dd->3He n pi0 and dd->3He p pi- reactions with the WASA-at-COSY facility
The search for 4He-eta bound states was performed with the WASA-at-COSY
facility via the measurement of the excitation function for the dd->3He n pi0
and dd->3He p pi- processes. The beam momentum was varied continuously between
2.127 GeV/c and 2.422 GeV/c, corresponding to the excess energy for the dd->4He
eta reaction ranging from Q=-70 MeV to Q=30 MeV. The luminosity was determined
based on the dd->3He n reaction and quasi-free proton-proton scattering via
dd->pp n_spectator n_spectator reactions. The excitation functions determined
independently for the measured reactions do not reveal a structure which could
be interpreted as a narrow mesic nucleus. Therefore, the upper limits of the
total cross sections for the bound state production and decay in
dd->(4He-eta)_bound->3He n pi0 and dd->(4He-eta)_bound->3He p pi- processes
were determined taking into account the isospin relation between both the
channels considered. The results of the analysis depend on the assumptions of
the N* momentum distribution in the anticipated mesic-4He. Assuming as in the
previous works, that this is identical with the distribution of nucleons bound
with 20 MeV in 4He, we determined that (for the mesic bound state width in the
range from 5 MeV to 50 MeV) the upper limits at 90% confidence level are about
3 nb and about 6 nb for npi0 and ppi- channels, respectively. However, based on
the recent theoretical findings of the N*(1535) momentum distribution in the
N*-3He nucleus bound by 3.6 MeV, we find that the WASA-at-COSY detector
acceptance decreases and hence the corresponding upper limits are 5 nb and 10
nb for npi0 and ppi- channels respectively.Comment: This article will be submitted to JHE
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