25,809 research outputs found
Vector magnetic hysteresis of hard superconductors
Critical state problems which incorporate more than one component for the
magnetization vector of hard superconductors are investigated. The theory is
based on the minimization of a cost functional
which weighs the changes of the magnetic field vector within the sample. We
show that Bean's simplest prescription of choosing the correct sign for the
critical current density in one dimensional problems is just a particular
case of finding the components of the vector . is
determined by minimizing under the constraint , with a bounded set. Upon the selection of
different sets we discuss existing crossed field measurements and
predict new observable features. It is shown that a complex behavior in the
magnetization curves may be controlled by a single external parameter, i.e.:
the maximum value of the applied magnetic field .Comment: 10 pages, 9 figures, accepted in Phys. Rev.
Ferrocenyl gold complexes as efficient catalysts
More than half a century after the discovery of the ferrocene structure in 1951, it remains as a suitable building block in many research areas, including catalysis with the development of key chiral catalysts. On the other hand, gold-mediated catalysis has been raised in recent years, allowing the creation of a great variety of C-C bonds and C-heteroatom bonds. In this context, this review covers the recent advances made with the combination of these two iconic figures in the organometallic chemistry field, since the first gold catalyzed reaction using a ferrocene ligand reported in 1986. The combination of the excellent properties and versatility of this metallocene, has allowed the obtainment of a plethora of ligands for metal catalysis, although their use joined to gold catalysis is still scarcely explored
Simulation of many-qubit quantum computation with matrix product states
Matrix product states provide a natural entanglement basis to represent a
quantum register and operate quantum gates on it. This scheme can be
materialized to simulate a quantum adiabatic algorithm solving hard instances
of a NP-Complete problem. Errors inherent to truncations of the exact action of
interacting gates are controlled by the size of the matrices in the
representation. The property of finding the right solution for an instance and
the expected value of the energy are found to be remarkably robust against
these errors. As a symbolic example, we simulate the algorithm solving a
100-qubit hard instance, that is, finding the correct product state out of ~
10^30 possibilities. Accumulated statistics for up to 60 qubits point at a slow
growth of the average minimum time to solve hard instances with
highly-truncated simulations of adiabatic quantum evolution.Comment: 5 pages, 4 figures, final versio
The nature of the ISM in galaxies during the star-formation activity peak of the Universe
We combine a semi-analytic model of galaxy formation, tracking atomic and
molecular phases of cold gas, with a three-dimensional radiative-transfer and
line tracing code to study the sub-mm emission from atomic and molecular
species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that
drives the formation of stars at the epoch of peak star formation (SF) in the
Universe (z = 2.0) with that in local galaxies. We find that normal
star-forming galaxies at high redshift have much higher CO-excitation peaks
than their local counterparts and that CO cooling takes place at higher
excitation levels. CO line ratios increase with redshift as a function of
galaxy star-formation rate, but are well correlated with H2 surface density
independent of redshift. We find an increase in the [OI]/[CII] line ratio in
typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to
counterparts at z = 0. Our model results suggest that typical star-forming
galaxies at high redshift consist of much denser and warmer star-forming clouds
than their local counterparts. Galaxies belonging to the tail of the SF
activity peak at z = 1.2 are already less dense and cooler than counterparts
during the actual peak of SF activity (z = 2.0). We use our results to discuss
how future ALMA surveys can best confront our predictions and constrain models
of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA
Optical amplification enhancement in photonic crystals
Improving and controlling the efficiency of a gain medium is one of the most
challenging problems of laser research. By measuring the gain length in an opal
based photonic crystal doped with laser dye, we demonstrate that optical
amplification is more than twenty-fold enhanced along the Gamma-K symmetry
directions of the face centered cubic photonic crystal. These results are
theoretically explained by directional variations of the density of states,
providing a quantitative connection between density of the states and light
amplification
Inner Triplet Corrector Package MQSXA for the LHC
The eight Inner Triplets of the LHC will each house a combined corrector magnet assembly, MQSXA, which comprises a skew quadrupole (MQSX) in line with nested skew octupole (MCOSX), octupole (MCOX), and skew sextupole (MCSSX) windings. These superconducting single-aperture magnet assemblies have a bore of 70 mm diameter, and the complete MQSXA assemblies are 530 mm long, have an outer diameter of 180 mm and an approximate mass of 90 kg. In the Inner Triplets the MQSXA assemblies are flanged to the end plate of the high gradient quadrupoles (MQX). This paper presents the main design features of the MQSXA and the experience gained with the prototype of the nested part of this magnet assembly, which has been built at CERN. The results of the training tests at 4.3 K and 1.9 K together with the cold magnetic measurements are given
Ga-induced atom wire formation and passivation of stepped Si(112)
We present an in-depth analysis of the atomic and electronic structure of the
quasi one-dimensional (1D) surface reconstruction of Ga on Si(112) based on
Scanning Tunneling Microscopy and Spectroscopy (STM and STS), Rutherford
Backscattering Spectrometry (RBS) and Density Functional Theory (DFT)
calculations. A new structural model of the Si(112)6 x 1-Ga surface is
inferred. It consists of Ga zig-zag chains that are intersected by
quasi-periodic vacancy lines or misfit dislocations. The experimentally
observed meandering of the vacancy lines is caused by the co-existence of
competing 6 x 1 and 5 x 1 unit cells and by the orientational disorder of
symmetry breaking Si-Ga dimers inside the vacancy lines. The Ga atoms are fully
coordinated, and the surface is chemically passivated. STS data reveal a
semiconducting surface and show excellent agreement with calculated Local
Density of States (LDOS) and STS curves. The energy gain obtained by fully
passivating the surface calls the idea of step-edge decoration as a viable
growth method toward 1D metallic structures into question.Comment: Submitted, 13 pages, accepted in Phys. Rev. B, notational change in
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