Charles University in Prague

About: Charles University in Prague is a education organization based out in Prague, Czechia. It is known for research contribution in the topics: Population & Large Hadron Collider. The organization has 32392 authors who have published 74435 publications receiving 1804208 citations.

Infrared and terahertz studies of polar phonons and magnetodielectric effect in multiferroic Bi Fe O 3 ceramics

Abstract: $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ ceramics were investigated by means of infrared reflectivity and time domain terahertz transmission spectroscopy at temperatures $20\char21{}950\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the magnetodielectric effect was studied at $10\char21{}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the magnetic field up to $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Below $175\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the sum of polar phonon contributions to the permittivity corresponds to the value of measured permittivity below $1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. At higher temperatures, a giant low-frequency permittivity was observed, obviously due to the enhanced conductivity and possible Maxwell-Wagner contribution. Above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the observed magnetodielectric effect is caused essentially through the combination of magnetoresistance and the Maxwell-Wagner effect, as recently predicted by Catalan [Appl. Phys. Lett. 88, 102902 (2006)]. Since the magnetodielectric effect does not occur due to a coupling of polarization and magnetization as expected in magnetoferroelectrics, we call it an improper magnetodielectric effect. Below $175\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the magnetodielectric effect is by several orders of magnitude lower due to the decreased conductivity. Several phonons exhibit gradual softening with increasing temperature, which explains the previously observed high-frequency permittivity increase on heating. The observed noncomplete phonon softening seems to be the consequence of the first-order nature of the ferroelectric transition.

Charge Transport in Disordered Graphene-Based Low Dimensional Materials

Abstract: Two-dimensional graphene, carbon nanotubes and graphene nanoribbons represent a novel class of low dimensional materials that could serve as building blocks for future carbon-based nanoelectronics. Although these systems share a similar underlying electronic structure, whose exact details depend on confinement effects, crucial differences emerge when disorder comes into play. In this short review, we consider the transport properties of these materials, with particular emphasis to the case of graphene nanoribbons. After summarizing the electronic and transport properties of defect-free systems, we focus on the effects of a model disorder potential (Anderson-type), and illustrate how transport properties are sensitive to the underlying symmetry. We provide analytical expressions for the elastic mean free path of carbon nanotubes and graphene nanoribbons, and discuss the onset of weak and strong localization regimes, which are genuinely dependent on the transport dimensionality. We also consider the effects of edge disorder and roughness for graphene nanoribbons in relation to their armchair or zigzag orientation.

Factors associated with FEV1 decline in cystic fibrosis: analysis of the ECFS patient registry.

Abstract: Pulmonary insufficiency is the main cause of death in cystic fibrosis (CF). We analysed forced expiratory volume in 1 s (FEV1) data of 14,732 patients registered in the European Cystic Fibrosis Society Patient Registry (ECFSPR) database in 2007. We used linear and logistic regressions to investigate associations between FEV1 % predicted and clinical outcomes. Body mass index (BMI), chronic infection by Pseudomonas aeruginosa, pancreatic status and CF-related diabetes (CFRD) showed a statistically significant (all p<0.0001) and clinically relevant effect on FEV1 % pred after adjusting for age. Patients with a lower BMI experience a six-fold increased odds ratio (95% CI 5.0-7.3) of having severe lung disease (FEV1 <40% pred) compared to patients with normal BMI. Being chronically infected with P. aeruginosa increases the odds ratio of severe lung disease by 2.4 (95% CI 2.0-2.7), and patients with pancreatic insufficiency experience a 2.0-fold increased odds ratio (95% CI 1.6-2.5) of severe lung disease compared to pancreatic sufficient patients. Patients with CFRD have a 1.8-fold increased odds ratio (95% CI 1.6-2.2) compared to patients not affected. These potential risk factors for pulmonary disease in patients with CF are to some degree preventable or treatable. We emphasise the importance of their early identification through frequent routine tests, the implementation of infection control measures, and a timely initiation of relevant therapies.

Dielectric, infrared, and Raman response of undoped SrTiO 3 ceramics: Evidence of polar grain boundaries

Abstract: Thorough Raman and infrared (IR) reflectivity investigations of nominally pure ${\mathrm{SrTiO}}_{3}$ ceramics in the 10--300 K range have revealed a clear presence of the polar phase whose manifestation steeply increases on cooling. The Raman strengths of the Raman-forbidden IR modes are proportional to ${\ensuremath{\omega}}_{\mathrm{TO}1}^{\ensuremath{-}\ensuremath{\alpha}}(\ensuremath{\alpha}\ensuremath{\approx}1.6)$ where ${\ensuremath{\omega}}_{\mathrm{TO}1}$ is the polar soft mode frequency. No pronounced permittivity dispersion is observed below the soft mode frequency so that, as in single crystals, the static permittivity is essentially determined by the soft mode contribution. A theory is suggested which assumes a frozen dipole moment connected with the grain boundaries which induces the polar phase in the grain bulk in correlation with the bulk soft-mode frequency. This stiffens slightly the effective soft mode response and reduces the low-temperature permittivity compared to that of single crystals. Moreover, the polar soft mode strongly couples to the ${E}_{g}$ component of the structural soft doublet showing that the polar axis is perpendicular to the tetragonal axis below the structural transition which is shifted to 132 K in our ceramics. Whereas the ${\mathrm{TiO}}_{6}$ octahedra tilt (primary order parameter) below the structural transition corresponds to that in single crystals, much smaller ${A}_{1g}\ensuremath{-}{E}_{g}$ splitting of the structural soft doublet shows that the tetragonal deformation (secondary order parameter) is nearly 10 times smaller, apparently due to the grain volume clamping in ceramics.