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 YBa$_2$Cu$_3$O$_{7-x}$ 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 $Z$, 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 Nb3_{3}Sn at high magnetic fields

Nb$_{3}$Sn is a superconductor of great relevance for perspective RF applications. We present for the first time surface impedance $Z_s$ measurements at 15 GHz and low RF field amplitude on Nb$_{3}$Sn in high magnetic fields up to 12 T, with the aim of increasing the knowledge of Nb$_{3}$Sn behavior in such conditions. $Z_s$ 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 $Z_s$ with the aim of extracting interesting data about pinning in Nb$_{3}$Sn at high frequencies. From $Z_s$ we extract the vortex motion complex resistivity to obtain the $r$-parameter and the depinning frequency $\nu_p$ in high magnetic fields. The comparison of the results with the literature shows that the measured $\nu_p$ on bulk Nb$_{3}$Sn is several times greater than that of pure Nb. This demonstrates how Nb$_{3}$Sn 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 Tl2_2Ba2_2CaCu2_2O8+x_{8+x} films

There is a renewed interest in superconductors for high-frequency applications, leading to a reconsideration of already known low-$T_c$ and high-$T_c$ materials. In this view, we present an experimental investigation of the millimeter-wave response in moderate magnetic fields of Tl$_2$Ba$_2$CaCu$_2$O$_{8+x}$ 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, $\Delta Z_s(H) = \Delta R_s(H) + i\Delta X_s(H)$ 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 $T_c$. We extract the vortex motion depinning frequency $f_p$, 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 $k_p$ arising from moderate pinning, and a vortex viscosity $\eta$ 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 Tl$_2$Ba$_2$CaCu$_2$O$_{8+x}$ 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