174 research outputs found
Imaging Flux Vortices in MgB2 using Transmission Electron Microscopy
We report the successful imaging of flux vortices in single crystal MgB2
using transmission electron microscopy. The specimen was thinned to electron
transparency (350 nm thickness) by focussed ion beam milling. An artefact of
the thinning process was the production of longitudinal thickness undulations
of height 1-2 nm in the sample which acted as pinning sites due to the energy
required for the vortices to cross them. These had a profound effect on the
patterns of vortex order observed which we examine here.
Supplementary information can be downloaded from
http://www-hrem.msm.cam.ac.uk/people/loudon/#publicationsComment: 3 pages, 2 figures to appear in Physica C. Supplementary information
can be downloaded from
http://www-hrem.msm.cam.ac.uk/people/loudon/#publications. The discussion of
the vortex-free region near the sample edge has been revised in response to
referees' comments. Changes have been made to clarify that the specimen
thickness is 250nm parallel to the c-axis but 350nm parallel to the electron
bea
Asymptotically Free Non-Abelian Gauge Theories With Fermions and Scalars As Alternatives to QCD
In this paper we construct non-Abelian gauge theories with fermions and
scalars that nevertheless possess asymptotic freedom.The scalars are taken to
be in a chiral multiplet transforming as under
and transforming as singlets under the colour SU(3) group. We consider two
distinct scenarios, one in which the additional scalars are light and another
in which they are heavier than half the Z-boson mass. It is shown that
asymptotic freedom is obtained without requiring that all additional couplings
keep fixed ratios with each other. It is also shown that both scenarios can not
be ruled out by what are considered standard tests of QCD like R- parameter,
g-2 for muons or deep inelastic phenomena. The light mass scenario is however
ruled out by high precision Z-width data (and only by that one data).The heavy
mass scenario is still viable and is shown to naturally pass the test of
flavour changing neutral currents. It also is not ruled out by precision
electroweak oblique parameters. Many distinctive experimental signatures of
these models are also discussed.Comment: 37 pages in LATEX with 10 fig
Kosterlitz Thouless Universality in Dimer Models
Using the monomer-dimer representation of strongly coupled U(N) lattice gauge
theories with staggered fermions, we study finite temperature chiral phase
transitions in (2+1) dimensions. A new cluster algorithm allows us to compute
monomer-monomer and dimer-dimer correlations at zero monomer density (chiral
limit) accurately on large lattices. This makes it possible to show
convincingly, for the first time, that these models undergo a finite
temperature phase transition which belongs to the Kosterlitz-Thouless
universality class. We find that this universality class is unaffected even in
the large N limit. This shows that the mean field analysis often used in this
limit breaks down in the critical region.Comment: 4 pages, 4 figure
The phase diagram of quantum systems: Heisenberg antiferromagnets
A novel approach for studying phase transitions in systems with quantum
degrees of freedom is discussed. Starting from the microscopic hamiltonian of a
quantum model, we first derive a set of exact differential equations for the
free energy and the correlation functions describing the effects of
fluctuations on the thermodynamics of the system. These equations reproduce the
full renormalization group structure in the neighborhood of a critical point
keeping, at the same time, full information on the non universal properties of
the model. As a concrete application we investigate the phase diagram of a
Heisenberg antiferromagnet in a staggered external magnetic field. At long
wavelengths the known relationship to the Quantum Non Linear Sigma Model
naturally emerges from our approach. By representing the two point function in
an approximate analytical form, we obtain a closed partial differential
equation which is then solved numerically. The results in three dimensions are
in good agreement with available Quantum Monte Carlo simulations and series
expansions. More refined approximations to the general framework presented here
and few applications to other models are briefly discussed.Comment: 17 pages, 7 figure
Active Brownian Particles. From Individual to Collective Stochastic Dynamics
We review theoretical models of individual motility as well as collective
dynamics and pattern formation of active particles. We focus on simple models
of active dynamics with a particular emphasis on nonlinear and stochastic
dynamics of such self-propelled entities in the framework of statistical
mechanics. Examples of such active units in complex physico-chemical and
biological systems are chemically powered nano-rods, localized patterns in
reaction-diffusion system, motile cells or macroscopic animals. Based on the
description of individual motion of point-like active particles by stochastic
differential equations, we discuss different velocity-dependent friction
functions, the impact of various types of fluctuations and calculate
characteristic observables such as stationary velocity distributions or
diffusion coefficients. Finally, we consider not only the free and confined
individual active dynamics but also different types of interaction between
active particles. The resulting collective dynamical behavior of large
assemblies and aggregates of active units is discussed and an overview over
some recent results on spatiotemporal pattern formation in such systems is
given.Comment: 161 pages, Review, Eur Phys J Special-Topics, accepte
Measurement of and charged current inclusive cross sections and their ratio with the T2K off-axis near detector
We report a measurement of cross section and the first measurements of the cross section
and their ratio
at (anti-)neutrino energies below 1.5
GeV. We determine the single momentum bin cross section measurements, averaged
over the T2K -flux, for the detector target material (mainly
Carbon, Oxygen, Hydrogen and Copper) with phase space restricted laboratory
frame kinematics of 500 MeV/c. The
results are and $\sigma(\nu)=\left( 2.41\
\pm0.022{\rm{(stat.)}}\pm0.231{\rm (syst.)}\ \right)\times10^{-39}^{2}R\left(\frac{\sigma(\bar{\nu})}{\sigma(\nu)}\right)=
0.373\pm0.012{\rm (stat.)}\pm0.015{\rm (syst.)}$.Comment: 18 pages, 8 figure
Search for Lorentz and CPT violation using sidereal time dependence of neutrino flavor transitions over a short baseline
A class of extensions of the Standard Model allows Lorentz and CPT violations, which can be identified
by the observation of sidereal modulations in the neutrino interaction rate. A search for such modulations
was performed using the T2K on-axis near detector. Two complementary methods were used in this study,
both of which resulted in no evidence of a signal. Limits on associated Lorentz and CPT-violating terms
from the Standard Model extension have been derived by taking into account their correlations in this
model for the first time. These results imply such symmetry violations are suppressed by a factor of more
than 10 20 at the GeV scale
Open data from the third observing run of LIGO, Virgo, KAGRA, and GEO
The global network of gravitational-wave observatories now includes five detectors, namely LIGO Hanford, LIGO Livingston, Virgo, KAGRA, and GEO 600. These detectors collected data during their third observing run, O3, composed of three phases: O3a starting in 2019 April and lasting six months, O3b starting in 2019 November and lasting five months, and O3GK starting in 2020 April and lasting two weeks. In this paper we describe these data and various other science products that can be freely accessed through the Gravitational Wave Open Science Center at https://gwosc.org. The main data set, consisting of the gravitational-wave strain time series that contains the astrophysical signals, is released together with supporting data useful for their analysis and documentation, tutorials, as well as analysis software packages
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