22,647 research outputs found
Site specific spin dynamics in BaFe2As2: tuning the ground state by orbital differentiation
The role of orbital differentiation on the emergence of superconductivity in
the Fe-based superconductors remains an open question to the scientific
community. In this investigation, we employ a suitable microscopic spin probe
technique, namely Electron Spin Resonance (ESR), to investigate this issue on
selected chemically substituted BaFeAs single crystals. As the
spin-density wave (SDW) phase is suppressed, we observe a clear increase of the
Fe 3 bands anisotropy along with their localization at the FeAs plane. Such
an increase of the planar orbital content interestingly occurs independently on
the chemical substitution responsible for suppressing the SDW phase. As a
consequence, the magnetic fluctuations combined with the resultant particular
symmetry of the Fe 3 bands are propitious ingredients to the emergence of
superconductivity in this class of materials.Comment: 6 pages, 5 figure
Constraints on the Primordial Magnetic Field from
A primordial magnetic field (PMF) can affect the evolution of density field
fluctuations in the early universe.In this paper we constrain the PMF amplitude
and power spectral index by comparing calculated
density field fluctuations with observational data, i.e. the number density
fluctuation of galaxies.We show that the observational constraints on
cosmological density fluctuations, as parameterized by , lead to
strong constraints on the amplitude and spectral index of the PMF.Comment: 11 pages, 1 figure, accepted for publication as Phys. Rev.
Bulk Viscosity, Decaying Dark Matter, and the Cosmic Acceleration
We discuss a cosmology in which cold dark-matter particles decay into
relativistic particles. We argue that such decays could lead naturally to a
bulk viscosity in the cosmic fluid. For decay lifetimes comparable to the
present hubble age, this bulk viscosity enters the cosmic energy equation as an
effective negative pressure. We investigate whether this negative pressure is
of sufficient magnitude to account fo the observed cosmic acceleration. We show
that a single decaying species in a flat, dark-matter dominated cosmology
without a cosmological constant cannot reproduce the observed
magnitude-redshift relation from Type Ia supernovae. However, a delayed bulk
viscosity, possibly due to a cascade of decaying particles may be able to
account for a significant fraction of the apparent cosmic acceleration.
Possible candidate nonrelativistic particles for this scenario include sterile
neutrinos or gauge-mediated decaying supersymmetric particles.Comment: 7 pages, 4 figure
A semi-empirical model for streamwise vortex intensification
Vortex intensification plays an important role in a wide range of flows of engineering interest. One scenario of interest is when a streamwise vortex passes through the contracting streamtube of an aircraft intake. There is, however, limited experimental data of flows of this type to reveal the dominant flow physics and to guide the development of vortex models. To this end, the evolution of wing-tip vortices inside a range of streamtube contractions has been measured using stereoscopic particle image velocimetry. A semi-empirical model has been applied to provide new insight on the role of vorticity diffusion during the intensification process. The analysis demonstrates that for mild flow contractions, vorticity diffusion has a negligible influence due to the low rates of diffusion in the vortex flow prior to intensification and the short convective times associated with the streamtube contraction. As the contraction levels increase, there is a substantial increase in the rates of diffusion which is driven by the greater levels of vorticity in the vortex core. A new semi-empirical relationship, as a function of the local streamtube contraction levels and vortex Reynolds number, has been developed. The model comprises a simple correction to vortex filament theory and provides a significant improvement in the estimation of vortex characteristics in contracting flows. For the range of contractions investigated, errors in the estimation of vortex core radius, peak tangential velocity and vorticity are reduced by an order of magnitude. The model can be applied to estimate the change in vortex characteristics for a range of flows with intense axial strain, such as contracting intake streamtubes and swirling flows in turbomachinery
Theory of Orbital Magnetization in Solids
In this review article, we survey the relatively new theory of orbital
magnetization in solids-often referred to as the "modern theory of orbital
magnetization"-and its applications. Surprisingly, while the calculation of the
orbital magnetization in finite systems such as atoms and molecules is straight
forward, in extended systems or solids it has long eluded calculations owing to
the fact that the position operator is ill-defined in such a context.
Approaches that overcome this problem were first developed in 2005 and in the
first part of this review we present the main ideas reaching from a Wannier
function approach to semi-classical and finite-temperature formalisms. In the
second part, we describe practical aspects of calculating the orbital
magnetization, such as taking k-space derivatives, a formalism for
pseudopotentials, a single k-point derivation, a Wannier interpolation scheme,
and DFT specific aspects. We then show results of recent calculations on Fe,
Co, and Ni. In the last part of this review, we focus on direct applications of
the orbital magnetization. In particular, we will review how properties such as
the nuclear magnetic resonance shielding tensor and the electron paramagnetic
resonance g-tensor can elegantly be calculated in terms of a derivative of the
orbital magnetization
Conformity and controversies in the diagnosis, staging and follow-up evaluation of canine nodal lymphoma: a systematic review of the last 15 years of published literature
Diagnostic methods used in the initial and post-treatment evaluation of canine lymphoma are heterogeneous and can vary within countries and institutions. Accurate reporting of clinical stage and response assessment is crucial in determining the treatment efficacy and predicting prognosis. This study comprises a systematic review of all available canine multicentric lymphoma studies published over 15 years. Data concerning diagnosis, clinical stage evaluation and response assessment procedures were extracted and compared. Sixty-three studies met the eligibility criteria. Fifty-five (87.3%) studies were non-randomized prospective or retrospective studies. The survey results also expose variations in diagnostic criteria and treatment response assessment in canine multicentric lymphoma. Variations in staging procedures performed and recorded led to an unquantifiable heterogeneity among patients in and between studies, making it difficult to compare treatment efficacies. Awareness of this inconsistency of procedure and reporting may help in the design of future clinical trials
Sphalerons with CP-Violating Higgs Potentials
We investigate the effect on the sphaleron in the two Higgs doublet
electroweak theory of including CP violation in the Higgs potential. To have
better control over the relation between the sphaleron energy and the physical
quantities in the theory, we show how to parametrize the Higgs potential in
terms of physical masses and mixing angles, one of which causes CP violation.
By altering this CP violating angle (and keeping the other physical quantities
fixed) the sphaleron energy increases by up to 10%. We also calculate the
static minimum energy path between adjacent vacua as a function of Chern-Simons
number, using the method of gradient flow. The only effect CP violation has on
the barrier is the change in height. As a by-product of our work on
parametrization of the potential, we demonstrate that CP violation in the Higgs
sector favours nearly degenerate light Higgs masses.Comment: 13pp LaTeX2e, 2 eps figs, uses graphicx, a
On the formation and decay of a molecular ultracold plasma
Double-resonant photoexcitation of nitric oxide in a molecular beam creates a
dense ensemble of Rydberg states, which evolves to form a plasma of
free electrons trapped in the potential well of an NO spacecharge. The
plasma travels at the velocity of the molecular beam, and, on passing through a
grounded grid, yields an electron time-of-flight signal that gauges the plasma
size and quantity of trapped electrons. This plasma expands at a rate that fits
with an electron temperature as low as 5 K, colder that typically observed for
atomic ultracold plasmas. The recombination of molecular NO cations with
electrons forms neutral molecules excited by more than twice the energy of the
NO chemical bond, and the question arises whether neutral fragmentation plays a
role in shaping the redistribution of energy and particle density that directs
the short-time evolution from Rydberg gas to plasma. To explore this question,
we adapt a coupled rate-equations model established for atomic ultracold
plasmas to describe the energy-grained avalanche of electron-Rydberg and
electron-ion collisions in our system. Adding channels of Rydberg
predissociation and two-body, electron- cation dissociative recombination to
the atomic formalism, we investigate the kinetics by which this relaxation
distributes particle density and energy over Rydberg states, free electrons and
neutral fragments. The results of this investigation suggest some mechanisms by
which molecular fragmentation channels can affect the state of the plasma
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