5,124 research outputs found
Vector constants of motion for time-dependent Kepler and isotropic harmonic oscillator potentials
A method of obtaining vector constants of motion for time-independent as well
as time-dependent central fields is discussed. Some well-established results
are rederived in this alternative way and new ones obtained.Comment: 18 pages, no figures, regular Latex article forma
Evolution of spin correlations in SrDy2O4 in an applied magnetic field
The development of short- and long-range magnetic order induced in a
frustrated zig-zag ladder compound SrDy2O4 by an applied field is studied using
neutron diffraction techniques. In zero field, SrDy2O4 lacks long-range
magnetic order down to temperatures as low as 60 mK, and the observed powder
neutron diffraction (PND) patterns are dominated by very broad diffuse
scattering peaks. Single crystal neutron diffraction reveals that the
zero-field magnetic structure consists of a collection of antiferromagnetic
chains running along the c axis and that there is very little correlation
between the chains in the ab plane. In an applied magnetic field, the broad
diffuse scattering features in PND are gradually replaced by much sharper
peaks, however, the pattern remains rather complex, reflecting the highly
anisotropic nature of SrDy2O4. Single crystal neutron diffraction shows that a
moderate field applied along the b axis induces an up-up-down magnetic order
associated with a 1/3-magnetisation plateau, in which magnetic correlation
length in the ab plane is significantly increased, but it nevertheless remains
finite. The resolution limited k = 0 peaks associated with a ferromagnetic
arrangement appear in powder and single crystal neutron diffraction patterns in
fields of 2.5 T and above.Comment: 10 pages, 11 figure
Decoherence-protected memory for a single-photon qubit
The long-lived, efficient storage and retrieval of a qubit encoded on a
photon is an important ingredient for future quantum networks. Although systems
with intrinsically long coherence times have been demonstrated, the combination
with an efficient light-matter interface remains an outstanding challenge. In
fact, the coherence times of memories for photonic qubits are currently limited
to a few milliseconds. Here we report on a qubit memory based on a single atom
coupled to a high-finesse optical resonator. By mapping and remapping the qubit
between a basis used for light-matter interfacing and a basis which is less
susceptible to decoherence, a coherence time exceeding 100 ms has been measured
with a time-independant storage-and-retrieval efficiency of 22%. This
demonstrates the first photonic qubit memory with a coherence time that exceeds
the lower bound needed for teleporting qubits in a global quantum internet.Comment: 3 pages, 4 figure
Growing Faults in the Lab: Insights into the Scale Dependence of the Fault Zone Evolution Process
Analog sandbox experiments are a widely used method to investigate tectonic processes that cannot be resolved from natural data alone, such as strain localization and the formation of fault zones. Despite this, it is still unclear, to which extent the dynamics of strain localization and fault zone formation seen in sandbox experiments can be extrapolated to a natural prototype. Of paramount importance for dynamic similarity is the proper scaling of the work required to create the fault system, Wprop. Using analog sandbox experiments of strike-slip deformation, we show Wprop to scale approximately with the square of the fault system length, l, which is consistent with the theory of fault growth in nature. Through quantitative measurements of both Wprop and strain distribution we are able to show that Wprop is mainly spent on diffuse deformation prior to localization, which we therefore regard as analogous to distributed deformation on small-scale faults below seismic resolution in natural fault networks. Finally, we compare our data to estimates of the work consumed by natural fault zones to verify that analog sandbox experiments scale properly with respect to energy, i. e. that they scale truly dynamically
Indomethacin decreases viscosity of gallbladder bile in patients with cholesterol gallstone disease
There is experimental evidence that inhibition of cyclooxygenase with nonsteroidal anti-inflammatory drugs may decrease cholesterol gall-stone formation and mitigate biliary pain in gall-stone patients. The mechanisms by which NSAIDs exert these effect are unclear. In a prospective, controlled clinical trial we examined the effects of oral indomethacin on the composition of human gall-bladder bile. The study included 28 patients with symptomatic cholesterol or mixed gallstones. Of these, 8 were treated with 3 × 25 mg indomethacin daily for 7 days prior to elective cholecystectomy while 20 received no treatment and served as controls. Bile and tissue samples from the gallbladder were obtained during cholecystectomy. Indomethacin tissue levels in the gallbladder mucosa, as assessed by HPLC, were 1.05±0.4 ng/mg wet weight, a concentration known to inhibit effectively cyclooxygenase activity. Nevertheless, no differences between the treated and untreated groups were found in the concentrations of biliary mucus glycoprotein (0.94±0.27 versus 0.93±0.32 mg/ml) or total protein (5.8±0.9 versus 6.4±1.3 mg/ml), cholesterol saturation (1.3±0.2 versus 1.5±0.2), or nucleation time (2.0±3.0 versus 1.5±2.0 days). However, biliary viscosity, measured using a low-shear rotation viscosimeter, was significantly lower in patients receiving indomethacin treatment (2.9±0.6 versus 5.6±1.2 mPa.s; P < 0.02). In conclusion, in man oral indomethacin decreases bile viscosity without alteration of bile lithogenicity or biliary mucus glycoprotein content. Since mucus glycoproteins are major determinants of bile viscosity, an alteration in mucin macromolecular composition may conceivably cause the indomethacin-induced decrease in biliary viscosity and explain the beneficial effects of nonsteroidal anti-inflammatory drugs in gallstone disease
A non-apoptotic role for caspase-9 in muscle differentiation
Caspases, a family of cysteine proteases most often investigated for their roles in apoptosis, have also been demonstrated to have functions that are vital for the efficient execution of cell differentiation. One such role that has been described is the requirement of caspase-3 for the differentiation of skeletal myoblasts into myotubes but, as yet, the mechanism leading to caspase-3 activation in this case remains elusive. Here, we demonstrate that caspase-9, an initiator caspase in the mitochondrial death pathway, is responsible for the activation of caspase-3 in differentiating C2C12 cells. Reduction of caspase-9 levels, using an shRNA construct, prevented caspase-3 activation and inhibited myoblast fusion. Myosin-heavy-chain expression, which accompanies myoblastic differentiation, was not caspase-dependent. Overexpression of Bcl-xL, a protein that inhibits caspase-9 activation, had the same effect on muscle differentiation as knockdown of caspase-9. These data suggest that the mitochondrial pathway is required for differentiation; however, the release of cytochrome c or Smac (Diablo) could not be detected, raising the possibility of a novel mechanism of caspase-9 activation during muscle differentiation.</jats:p
Graphene on Si(111)7x7
We demonstrate that it is possible to mechanically exfoliate graphene under
ultra high vacuum conditions on the atomically well defined surface of single
crystalline silicon. The flakes are several hundred nanometers in lateral size
and their optical contrast is very faint in agreement with calculated data.
Single layer graphene is investigated by Raman mapping. The G and 2D peaks are
shifted and narrowed compared to undoped graphene. With spatially resolved
Kelvin probe measurements we show that this is due to p-type doping with hole
densities of n_h \simeq 6x10^{12} cm^{-2}. The in vacuo preparation technique
presented here should open up new possibilities to influence the properties of
graphene by introducing adsorbates in a controlled way.Comment: 8 pages, 7 figure
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A high-wavenumber boundary-element method for an acoustic scattering problem
In this paper we show stability and convergence for a novel Galerkin boundary element method approach to the impedance boundary value problem for the Helmholtz equation in a half-plane with piecewise constant boundary data. This problem models, for example, outdoor sound propagation over inhomogeneous flat terrain. To achieve a good approximation with a relatively low number of degrees of freedom we employ a graded mesh with smaller elements adjacent to discontinuities in impedance, and a special set of basis functions for the Galerkin method so that, on each element, the approximation space consists of polynomials (of degree ) multiplied by traces of plane waves on the boundary. In the case where the impedance is constant outside an interval , which only requires the discretization of , we show theoretically and experimentally that the error in computing the acoustic field on is , where is the number of degrees of freedom and is the wavenumber. This indicates that the proposed method is especially commendable for large intervals or a high wavenumber. In a final section we sketch how the same methodology extends to more general scattering problems
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