3,848 research outputs found
Frequency-dependent fluctuational conductivity above Tc in anisotropic superconductors: effects of a short wavelength cutoff
We discuss the excess conductivity at nonzero frequencies in a superconductor
above T_c within the gaussian approximation. We focus the attention on the
temperature range not too close to T_c: within a time-dependent Ginzburg-Landau
formulation, we phenomenologically introduce a short wavelength cutoff (of the
order of the inverse coherence length) in the fluctuational spectrum to
suppress high momentum modes. We treat the general cases of thin wires,
anisotropic thin films and anisotropic bulk samples. We obtain in all cases
explicit expressions for the finite frequency fluctuational conductivity. The
dc case directly follows. Close to T_c the cutoff has no effect, and the known
results for Gaussian fluctuations are recovered. Above T_c, and already for
epsilon = ln(T/T_c) > 10^{-2}, we find strong suppression of the
paraconductivity as compared to the gaussian prediction, in particular in the
real part of the paraconductivity. At high epsilon the cutoff effects are
dominant. We discuss our results in comparison with data on high-T_c
superconductors.Comment: Changed RevTe
Analysis of the measurements of anisotropic a.c. vortex resistivity in tilted magnetic fields
Measurements of the high-frequency complex resistivity in superconductors are
a tool often used to obtain the vortex parameters, such as the vortex
viscosity, the pinning constant and the depinning frequency. In anisotropic
superconductors, the extraction of these quantities from the measurements faces
new difficulties due to the tensor nature of the electromagnetic problem. The
problem is specifically intricate when the magnetic field is tilted with
respect to the crystallographic axes. Partial solutions exist in the
free-flux-flow (no pinning) and Campbell (pinning dominated) regimes. In this
paper we develop a full tensor model for the vortex motion complex resistivity,
including flux-flow, pinning, and creep. We give explicit expressions for the
tensors involved. We obtain that, despite the complexity of the physics, some
parameters remain scalar in nature. We show that under specific circumstances
the directly measured quantities do not reflect the true vortex parameters, and
we give procedures to derive the true vortex parameters from measurements taken
with arbitrary field orientations. Finally, we discuss the applicability of the
angular scaling properties to the measured and transformed vortex parameters
and we exploit these properties as a tool to unveil the existence of
directional pinning.Comment: 21 pages, 3 figures. arXiv admin note: text overlap with
arXiv:1402.316
Vortex pinning and flux flow microwave studies of coated conductors
Demanding microwave applications in a magnetic field require the material
optimization not only in zero-field but, more important, in the in-field flux
motion dominated regime. However, the effect of artificial pinning centers
(APC) remains unclear at high frequency. Moreover, in coated conductors the
evaluation of the high frequency material properties is difficult due to the
complicated electromagnetic problem of a thin superconducting film on a
buffered metal substrate. In this paper we present an experimental study at 48
GHz of 150-200 nm YBaCuO coated conductors, with and without
APCs, on buffered Ni-5at%W tapes. By properly addressing the electromagnetic
problem of the extraction of the superconductor parameters from the measured
overall surface impedance , we are able to extract and to comment on the
London penetration depth, the flux flow resistivity and the pinning constant,
highlighting the effect of artificial pinning centers in these samples.Comment: 5 pages, IEEE Trans. Appl. Supercond., accepted for publication
(2019
Surface impedance measurements on NbSn at high magnetic fields
NbSn is a superconductor of great relevance for perspective RF
applications. We present for the first time surface impedance
measurements at 15 GHz and low RF field amplitude on NbSn in high
magnetic fields up to 12 T, with the aim of increasing the knowledge of
NbSn behavior in such conditions. is a fundamental material
parameter that directly gives useful information about the dissipative and
reactive phenomena when the superconductor is subjected to high-frequency
excitations. Therefore, we present an analysis of the measured with the
aim of extracting interesting data about pinning in NbSn at high
frequencies. From we extract the vortex motion complex resistivity to
obtain the -parameter and the depinning frequency in high magnetic
fields. The comparison of the results with the literature shows that the
measured on bulk NbSn is several times greater than that of pure
Nb. This demonstrates how NbSn can be a good candidate for RF
technological applications, also in high magnetic fields.Comment: ASC 2018 conference, accepted in IEEE Trans Appl Supercon
Measurements of microwave vortex response in dc magnetic fields in TlBaCaCuO films
There is a renewed interest in superconductors for high-frequency
applications, leading to a reconsideration of already known low- and
high- materials. In this view, we present an experimental investigation of
the millimeter-wave response in moderate magnetic fields of
TlBaCaCuO superconducting films with the aim of identifying
the mechanisms of the vortex-motion-induced response. We measure the dc
magnetic-field-dependent change of the surface impedance, at 48 GHz by means of the dielectric resonator
method. We find that the overall response is made up of several contributions,
with different weights depending on the temperature and field: a possible
contribution from Josephson or Abrikosov-Josephson fluxons at low fields; a
seemingly conventional vortex dynamics at higher fields; a significant pair
breaking in the temperature region close to . We extract the vortex motion
depinning frequency , which attains surprisingly high values. However, by
exploiting the generalized model for relaxational dynamics we show that this
result come from a combination of a pinning constant arising from
moderate pinning, and a vortex viscosity with anomalously small values.
This latter fact, implying large dissipation, is likely a result from a
peculiar microscopic structure and thus poses severe limits to the application
of TlBaCaCuO in a magnetic field.Comment: Presented at Applied Superconductivity Conference, Seattle (US) 2018.
Accepted for publication on IEEE Trans. Appl. Supercon
Chlamydophila pneumoniae Infection and Its Role in Neurological Disorders
Chlamydophila pneumoniae is an intracellular pathogen responsible for a number of different acute and chronic infections. The recent deepening of knowledge on the biology and the use of increasingly more sensitive and
specific molecular techniques has allowed demonstration of C. pneumoniae in
a large number of persons suffering from different diseases including cardiovascular (atherosclerosis and stroke) and central nervous system (CNS) disorders. Despite this, many important issues remain unanswered with regard to the role that C. pneumoniae may play in initiating atheroma or in the progression of the disease. A growing body of evidence concerns the involvement of this pathogen in chronic neurological disorders and particularly in Alzheimer's disease (AD) and Multiple Sclerosis (MS). Monocytes may traffic C. pneumoniae across the blood-brain-barrier, shed the organism in the
CNS and induce neuroinflammation. The demonstration of C. pneumoniae by
histopathological, molecular and culture techniques in the late-onset AD dementia has suggested a relationship between CNS infection with C. pneumoniae and the AD neuropathogenesis. In particular subsets of MS patients, C. pneumoniae could induce a chronic persistent brain infection acting as a cofactor in the development of the disease. The role of Chlamydia in the pathogenesis of mental or neurobehavioral disorders including schizophrenia and autism is uncertain and fragmentary and will require further
confirmation
Frequency span optimization for asymmetric resonance curve fitting
The wide application of the modern resonant measurement techniques makes all
the steps of the measuring process, including data acquisition more efficient
and reliable. Here we investigate the multidimensional space of the parameters
to determine the optimum span for resonant measurements. The study concentrated
on experimental systems with standard performance and capabilities. We
determine the range of the optimum span for the resonant frequency and quality
factor by simulating and fitting resonant curves with different levels of
asymmetry.Comment: 7 pages, 5 figures, Accepted to IEEE I2MTC 2021 conferenc
Earthquake loss estimation for the Kathmandu Valley
The capital city, Kathmandu, is the most developed and populated place in Nepal. The majority of the
administrative offices, headquarters, numerous historical monuments, and eight World Heritages sites
are in the Kathmandu Valley. However, this region is geologically located on lacustrine sediment
basin, characterized by a long history of destructive earthquakes. The past events resulted in great
damage of structures, losses of human life’s and property, and interrupted the social development.
Therefore, earthquake disaster management is one of the most serious issues in highly seismically
active regions such as the Kathmandu Valley. In recent years, the earthquake risk in this area has
significantly increased due to uncontrolled development, poor construction practices with no
earthquake safety consideration, and lack of awareness amongst the general public and government
authorities. In this context, this study explores the realistic situation of earthquake losses due to future
earthquakes in Kathmandu Valley. To this end, three municipalities: (a) Kathmandu metropolitan city
(KMC), (b) Lalitpur Sub-Metropolitan City (LSMC) and (c) Bhaktapur Municipality (BMC) are
selected for study. The earthquake loss estimation in the selected municipalities is performed through
the combination of seismic hazard, structural vulnerability, and exposure data. For what concerns the
seismic input, various earthquake scenarios considering four seismic sources in Nepal were adopted.
Regarding the exposure, data about the type of existing buildings, population, and ward level
distribution of building typologies is estimated from the recent national census survey of 2011. The
economic losses due to the scenario earthquakes are determined using fragility functions. The
commonly used standard fragility curves are adopted for adobe, brick/stone with mud mortar
buildings, and brick/stone with cement mortar buildings. For the reinforced concrete structures, a new
fragility model was derived considering four construction typologies: i) current construction practices
(CCP), ii) structures according to the Nepal buildings code (NBC), iii) structures according to the
modified Nepal building code (NBC+) and iv) well designed structures (WDS). In this study, a set of
fragility functions is converted into a vulnerability model through a consequences model. Finally, the
ward level distribution of damage for each building typology, building losses and the corresponding
economic loss for each scenario earthquake is obtained using the OpenQuake-engine. The distribution
of damage within the Kathmandu Valley is currently being employing in the development of a shelter
model for the region, involving various local authorities and decision makers
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