982 research outputs found
Summary on Transverse Emittance Measurements and Instruments
Preservation of emittance and, as a prerequisite, emittance measurement, take on a particular importance in the long chain of accelerators and storage rings of big hadron colliders. This summary reports the advantages and disadvantages of the instruments quoted during the Workshop, and draws some general conclusions
The Effect of the Gaseous Environment on the Electrical Conductivity of Multi-Walled Carbon Nanotube Films over a Wide Temperature Range.
The surrounding gas atmosphere can have a significant influence on the electrical properties of multi-walled carbon nanotube (CNT) ensembles. In this study, we subjected CNT films to various gaseous environments or vacuum to observe how such factors alter the electrical resistance of networks at high temperatures. We showed that the removal of adsorbed water and other contaminants from the surface under reduced pressure significantly affects the electrical conductivity of the material. We also demonstrated that exposing the CNT films to the hydrogen atmosphere (as compared to a selection of gases of inert and oxidizing character) at elevated temperatures results in a notable reduction of electrical resistance. We believe that the observed sensitivity of the electrical properties of the CNT films to hydrogen or vacuum at elevated temperatures could be of practical importance
Copper-decorated CNTs as a possible electrode material in supercapacitors
Copper is probably one of the most important metal used in the broad range of electronic applications. It has been developed for many decades, and so it is very hard to make any further advances in its electrical and thermal performance by simply changing the manufacture to even more oxygen-free conditions. Carbon nanotubes (CNTs) due to their excellent electrical, thermal and mechanical properties seem like an ideal component to produce Cu-CNT composites of superior electrochemical performance. In this report we present whether Cu-CNT contact has a beneficial influence for manufacturing of a new type of carbon-based supercapacitor with embedded copper particles. The prepared electrode material was examined in symmetric cell configuration. The specific capacity and cyclability of composite were compared to parent CNT and oxidized CNT
Intrinsic Pinning in the High Field C-Phase of UPt_3
We report on the a.c. magnetic response of superconducting UPt_3 in a d.c.
magnetic field. At low fields (H < H^*), the in-phase susceptibility shows a
sharp drop at followed by a gradual decrease with decreasing temperature,
while the out-of-phase component shows a large peak at T_c followed by an
unusual broad peak. As the B-C phase line is crossed (H>H^*), however, both the
in-phase and out-of-phase susceptibilities resemble the zero-field Meissner
curves. We interpret these results in terms of a vortex pinning force which,
while comparatively small in the A/B-phases, becomes large enough to
effectively prevent vortex motion in the C-phase.Comment: Modified discussion, slight changes to figures, accepted in PRB Rapid
Communications. RevTex file, 5 figure
Quantum phases of hardcore bosons with repulsive dipolar density-density interactions on two-dimensional lattices
We analyse the ground-state quantum phase diagram of hardcore Bosons
interacting with repulsive dipolar potentials. The bosons dynamics is described
by the extended-Bose-Hubbard Hamiltonian on a two-dimensional lattice. The
ground state results from the interplay between the lattice geometry and the
long-range interactions, which we account for by means of a classical spin
mean-field approach limited by the size of the considered unit cells. This
extended classical spin mean-field theory accounts for the long-range
density-density interaction without truncation. We consider three different
lattice geometries: square, honeycomb, and triangular. In the limit of zero
hopping the ground state is always a devil's staircase of solid (gapped)
phases. Such crystalline phases with broken translational symmetry are robust
with respect to finite hopping amplitudes. At intermediate hopping amplitudes,
these gapped phases melt, giving rise to various lattice supersolid phases,
which can have exotic features with multiple sublattice densities. At
sufficiently large hoppings the ground state is a superfluid. The stability of
phases predicted by our approach is gauged by comparison to the known quantum
phase diagrams of the Bose-Hubbard model with nearest-neighbour interactions as
well as quantum Monte Carlo simulations for the dipolar case on the square
lattice. Our results are of immediate relevance for experimental realisations
of self-organised crystalline ordering patterns in analogue quantum simulators,
e.g., with ultracold dipolar atoms in an optical lattice.Comment: 31 pages, 9 figure
Order-by-disorder and long-range interactions in the antiferromagnetic transverse-field Ising model on the triangular lattice -- A perturbative point of view
We study the low-field ground-state (GS) properties of the antiferromagnetic
transverse-field Ising model with long-range interactions (afLRTFIM) on the
triangular lattice. We use the method of perturbative continuous unitary
transformations (pCUT) to derive an effective model for the degenerate GS space
of the antiferromagnetic nearest-neighbour (NN) Ising model on a finite system,
by treating the transverse-field (TF) and the long-range interactions (LRI) as
a perturbation. We determine a level-crossing between the plain stripe phase at
small TF and the clock-ordered phase at intermediate TF at for
, spins in order three perturbation theory. We discuss the
qualitative layout of the quantum phase diagram of the afLRTFIM on the
triangular lattice.Comment: 3 pages, 1 figur
Systematic Analysis of Crystalline Phases in Bosonic Lattice Models with Algebraically Decaying Density-Density Interactions
We propose a general approach to analyse diagonal ordering patterns in
bosonic lattice models with algebraically decaying density-density interactions
on arbitrary lattices. The key idea is a systematic search for the
energetically best order on all unit cells of the lattice up to a given extent.
Using resummed couplings we evaluate the energy of the ordering patterns in the
thermodynamic limit using finite unit cells. We apply the proposed approach to
the atomic limit of the extended Bose-Hubbard model on the triangular lattice
at fillings and . We investigate the ground-state properties of
the antiferromagnetic long-range Ising model on the triangular lattice and
determine a six-fold degenerate plain-stripe phase to be the ground state for
finite decay exponents. We also probe the classical limit of the
Fendley-Sengupta-Sachdev model describing Rydberg atom arrays. We focus on
arrangements where the atoms are placed on the sites or links of the Kagome
lattice. \changed{Our method provides a general framework to treat cristalline
structures resulting from long-range interactions.Comment: 35 pages, 11 figure
Comparative analysis of Wnt expression identifies a highly conserved developmental transition in flatworms
Background: Early developmental patterns of flatworms are extremely diverse and difficult to compare between distant groups. In parasitic flatworms, such as tapeworms, this is confounded by highly derived life cycles involving indirect development, and even the true orientation of the tapeworm antero-posterior (AP) axis has been a matter of controversy. In planarians, and metazoans generally, the AP axis is specified by the canonical Wnt pathway, and we hypothesized that it could also underpin axial formation during larval metamorphosis in tapeworms. Results: By comparative gene expression analysis of Wnt components and conserved AP markers in the tapeworms Echinococcus multilocularis and Hymenolepis microstoma, we found remarkable similarities between the early stages of larval metamorphosis in tapeworms and late embryonic and adult development in planarians. We demonstrate posterior expression of specific Wnt factors during larval metamorphosis and show that scolex formation is preceded by localized expression of Wnt inhibitors. In the highly derived larval form of E. multilocularis, which proliferates asexually within the mammalian host, we found ubiquitous expression of posterior Wnt factors combined with localized expression of Wnt inhibitors that correlates with the asexual budding of scoleces. As in planarians, muscle cells are shown to be a source of secreted Wnt ligands, providing an explanation for the retention of a muscle layer in the immotile E. multilocularis larva. Conclusions: The strong conservation of gene expression between larval metamorphosis in tapeworms and late embryonic development in planarians suggests, for the first time, a homologous developmental period across this diverse phylum. We postulate these to represent the phylotypic stages of these flatworm groups. Our results support the classical notion that the scolex is the true anterior end of tapeworms. Furthermore, the up-regulation of Wnt inhibitors during the specification of multiple anterior poles suggests a mechanism for the unique asexual reproduction of E. multilocularis larvae
Monte Carlo based techniques for quantum magnets with long-range interactions
Long-range interactions are relevant for a large variety of quantum systems
in quantum optics and condensed matter physics. In particular, the control of
quantum-optical platforms promises to gain deep insights in quantum-critical
properties induced by the long-range nature of interactions. From a theoretical
perspective, long-range interactions are notoriously complicated to treat.
Here, we give an overview of recent advancements to investigate quantum magnets
with long-range interactions focusing on two techniques based on Monte Carlo
integration. First, the method of perturbative continuous unitary
transformations where classical Monte Carlo integration is applied within the
embedding scheme of white graphs. This linked-cluster expansion allows to
extract high-order series expansions of energies and observables in the
thermodynamic limit. Second, stochastic series expansion quantum Monte Carlo
which enables calculations on large finite systems. Finite-size scaling can
then be used to determine physical properties of the infinite system. In recent
years, both techniques have been applied successfully to one- and
two-dimensional quantum magnets involving long-range Ising, XY, and Heisenberg
interactions on various bipartite and non-bipartite lattices. Here, we
summarise the obtained quantum-critical properties including critical exponents
for all these systems in a coherent way. Further, we review how long-range
interactions are used to study quantum phase transitions above the upper
critical dimension and the scaling techniques to extract these quantum critical
properties from the numerical calculations.Comment: 141 pages, 38 figure
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