850 research outputs found
Superconductors with Topological Order
We propose a mechanism of superconductivity in which the order of the ground
state does not arise from the usual Landau mechanism of spontaneous symmetry
breaking but is rather of topological origin. The low-energy effective theory
is formulated in terms of emerging gauge fields rather than a local order
parameter and the ground state is degenerate on topologically non-trivial
manifolds. The simplest example of this mechanism of superconductivty is
concretely realized as global superconductivty in Josephson junction arrays.Comment: 4 pages, no figure
Analytical bunch compression studies for a linac-based electron accelerator
The current paper deals with analytical bunch compression studies for FLUTE whose results are compared to simulations. FLUTE is a linac-based electron accelerator with a design energy of approximately 40 MeV currently being constructed at the Karlsruhe Institute of Technology. One of the goals of FLUTE is to generate electron bunches with their length lying in the femtosecond regime. In the first phase this will be accomplished using a magnetic bunch compressor. This compressor forms the subject of the studies presented. The paper is divided into two parts. The first part deals with pure geometric investigations of the bunch compressor where space charge effects and the backreaction of bunches with coherent synchrotron radiation are neglected. The second part is dedicated to the treatment of space charge effects. The upshot is that the analytical results in the two parts agree quite well with what is obtained from simulations. This paper shall form the basis for future analytical studies of the FLUTE bunch compressor and of bunch compression, in general
Density-functional embedding using a plane-wave basis
The constrained electron density method of embedding a Kohn-Sham system in a
substrate system (first described by P. Cortona, Phys. Rev. B {\bf 44}, 8454
(1991) and T.A. Wesolowski and A. Warshel, J. Phys. Chem {\bf 97}, 8050 (1993))
is applied with a plane-wave basis and both local and non-local
pseudopotentials. This method divides the electron density of the system into
substrate and embedded electron densities, the sum of which is the electron
density of the system of interest. Coupling between the substrate and embedded
systems is achieved via approximate kinetic energy functionals. Bulk aluminium
is examined as a test case for which there is a strong interaction between the
substrate and embedded systems. A number of approximations to the
kinetic-energy functional, both semi-local and non-local, are investigated. It
is found that Kohn-Sham results can be well reproduced using a non-local
kinetic energy functional, with the total energy accurate to better than 0.1 eV
per atom and good agreement between the electron densities.Comment: 11 pages, 4 figure
Multiscale simulations in simple metals: a density-functional based methodology
We present a formalism for coupling a density functional theory-based quantum
simulation to a classical simulation for the treatment of simple metallic
systems. The formalism is applicable to multiscale simulations in which the
part of the system requiring quantum-mechanical treatment is spatially confined
to a small region. Such situations often arise in physical systems where
chemical interactions in a small region can affect the macroscopic mechanical
properties of a metal. We describe how this coupled treatment can be
accomplished efficiently, and we present a coupled simulation for a bulk
aluminum system.Comment: 15 pages, 7 figure
A Novel Optical Beam Concept for Producing Coherent Synchrotron Radiation with Large Energy Spread Beams
Up to now two FEL concepts are known in conventional accelerators: 1.) In THz lasers an off-crest cavity adds a chirp to the bunch followed by a bunch compressor. Particles with different energies travel on different trajectories to the radiator. 2.) For EUV and X-ray FELs the beam enters an undulator which produces microbunches which then radiate. In this paper it is proposed to copy the THz laser scheme for EUV lasers. The incoming beam is chirped and a dogleg forces afterwards the particles with different energies to move on different parallel trajectories. Considering a detector plane perpendicular to the trajectories the particles with different energies arrive in general at different times. When in this plane for instance a TGU (Transverse Gradient Undulator) is positioned the emitted radiation in the TGU is monochromatic. If in addition chirp and dogleg are selected in such a way that the particles with different energies arrive at the same time at the entrance of the TGU the radiation is monochromatic and coherent similar to the THz laser concept
Dynamical Generation of Fermion Mass and Magnetic Field in Three-Dimensional QED with Chern-Simons Term
We study dynamical symmetry breaking in three-dimensional QED with a
Chern-Simons (CS) term, considering the screening effect of flavor
fermions. We find a new phase of the vacuum, in which both the fermion mass and
a magnetic field are dynamically generated, when the coefficient of the CS term
equals . The resultant vacuum becomes the finite-density
state half-filled by fermions. For , we find the fermion
remains massless and only the magnetic field is induced. For ,
spontaneous magnetization does not occur and should be regarded as an external
field.Comment: 8 pages, no figure, to be published in Phys. Rev. Let
Left ventricular T1-mapping in diastole versus systole in patients with mitral regurgitation
Cardiovascular magnetic resonance T1-mapping enables myocardial tissue characterisation, and is capable of quantifying both intracellular and extracellular volume. T1-mapping is conventionally performed in diastole, however, we hypothesised that systolic readout would reduce variability due to a reduction in myocardial blood volume. This study investigated whether T1-mapping in systole alters T1 values compared to diastole and whether reproducibility alters in atrial fibrillation compared to sinus rhythm. We prospectively identified 103 consecutive patients recruited to the Mitral FINDER study who had T1 mapping in systole and diastole. These patients had moderate or severe mitral regurgitation and a high incidence of ventricular dilatation and atrial fibrillation. T1, ECV and goodness-of-fit (R2) values of the T1 times were calculated offline using Circle cvi42 and in house-developed software. Systolic T1 mapping was associated with fewer myocardial segments being affected by artefact compared to diastolic T1 mapping [217/2472 (9%) vs 515/2472 (21%)]. Mean native T1 values were not significantly different when measured in systole and diastole (985 ± 26 ms vs 988 ± 29 respectively; p = 0.061) and mean post-contrast values showed similar good agreement (462 ± 32 ms vs 459 ± 33 respectively, p = 0.052). No clinically significant differences in ECV, native T1 and post-contrast T1 were identified between diastolic and systolic T1 maps in males versus females, or in patients with permanent atrial fibrillation versus sinus rhythm. A statistically significant improvement in R2 value was observed with systolic over diastolic T1 mapping in all analysed maps (n = 411) (96.2 ± 1.4% vs 96.0 ± 1.4%; p < 0.001) and in subgroup analyses [Sinus rhythm: 96.1 ± 1.4 vs 96.3 ± 1.4 (n = 327); p < 0.001. AF: 95.5 ± 1.3 vs 95.9 ± 1.2 (n = 80); p < 0.001] [Males: 95.8 ± 1.4 vs 96.1 ± 1.3 (n = 264); p < 0.001; Females: 96.2 ± 1.3 vs 96.4 ± 1.4 (n = 143); p = 0.009]. In conclusion, myocardial T1 mapping is associated with similar T1 and ECV values in systole and diastole. Furthermore, systolic acquisition is less prone to gating artefact in arrhythmia.</p
Higher Derivative CP(N) Model and Quantization of the Induced Chern-Simons Term
We consider higher derivative CP(N) model in 2+1 dimensions with the
Wess-Zumino-Witten term and the topological current density squared term. We
quantize the theory by using the auxiliary gauge field formulation in the path
integral method and prove that the extended model remains renormalizable in the
large N limit. We find that the Maxwell-Chern-Simons theory is dynamically
induced in the large N effective action at a nontrivial UV fixed point. The
quantization of the Chern-Simons term is also discussed.Comment: 8 pages, no figure, a minor change in abstract, added Comments on the
quantization of the Chern-Simons term whose coefficient is also corrected,
and some references are added. Some typos are corrected. Added a new
paragraph checking the equivalence between (3) and (5), and a related
referenc
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