7,207 research outputs found
Enhancement of Jc by Hf -Doping in the Superconductor MgB2: A Hyperfine Interaction Study
Measurements of the critical current density (Jc) by magnetization and the
upper critical field (Hc2) by magnetoresistance have been performed for
hafnium-doped MgB2. There has been a remarkable enhancement of Jc as compared
to that by ion irradiation without any appreciable decrease in Tc, which is
beneficial from the point of view of applications. The irreversibility line
extracted from Jc shows an upward shift. In addition, there has been an
increase in the upper critical field which indicates that Hf partially
substitutes for Mg. Hyperfine interaction parameters obtained from time
differential perturbed angular correlation (TDPAC) measurements revealed the
formation of HfB and HfB2 phases along with the substitution of Hf. A possible
explanation is given for the role of these species in the enhancement of Jc in
MgB2 superconductor
A one-dimensional tunable magnetic metamaterial
We present experimental data on a one-dimensional superconducting
metamaterial that is tunable over a broad frequency band. The basic building
block of this magnetic thin-film medium is a single-junction (rf-)
superconducting quantum interference device (SQUID). Due to the nonlinear
inductance of such an element, its resonance frequency is tunable in situ by
applying a dc magnetic field. We demonstrate that this results in tunable
effective parameters of our metamaterial consisting of 54 SQUIDs. In order to
obtain the effective magnetic permeability from the measured data, we employ a
technique that uses only the complex transmission coefficient S21
The role of hydrogen in room-temperature ferromagnetism at graphite surfaces
We present a x-ray dichroism study of graphite surfaces that addresses the
origin and magnitude of ferromagnetism in metal-free carbon. We find that, in
addition to carbon states, also hydrogen-mediated electronic states
exhibit a net spin polarization with significant magnetic remanence at room
temperature. The observed magnetism is restricted to the top 10 nm of
the irradiated sample where the actual magnetization reaches emu/g
at room temperature. We prove that the ferromagnetism found in metal-free
untreated graphite is intrinsic and has a similar origin as the one found in
proton bombarded graphite.Comment: 10 pages, 5 figures, 1 table, submitted to New Journal of Physic
Disentangling chlorophyll fluorescence from atmospheric scattering effects in O_2 Aâband spectra of reflected sunâlight
Global retrieval of solar induced fluorescence emitted by terrestrial vegetation can provide an unprecedented measure for photosynthetic efficiency. The GOSAT (JAXA, launched Feb. 2009) and OCO-2 (NASA, to be launched 2013) satellites record high-resolution spectra in the O_2 A-band region, overlapping part of the chlorophyll fluorescence spectrum. We show that fluorescence cannot be unambiguously discriminated from atmospheric scattering effects using O_2 absorption lines. This can cause systematic biases in retrieved scattering parameters (aerosol optical thickness, aerosol height, surface pressure, surface albedo) if fluorescence is neglected. Hence, we demonstrate an efficient alternative fluorescence least-squares retrieval method based solely on strong Fraunhofer lines in the vicinity of the O_2 A-band, disentangling fluorescence from scattering effects. Not only does the Fraunhofer line fit produce a more accurate estimate of fluorescence emission, but it also allows improved retrievals of atmospheric aerosols from the O_2 A-band
Ab-initio study of the relation between electric polarization and electric field gradients in ferroelectrics
The hyperfine interaction between the quadrupole moment of atomic nuclei and
the electric field gradient (EFG) provides information on the electronic charge
distribution close to a given atomic site. In ferroelectric materials, the loss
of inversion symmetry of the electronic charge distribution is necessary for
the appearance of the electric polarization. We present first-principles
density functional theory calculations of ferroelectrics such as BaTiO3, KNbO3,
PbTiO3 and other oxides with perovskite structures, by focusing on both EFG
tensors and polarization. We analyze the EFG tensor properties such as
orientation and correlation between components and their link with electric
polarization. This work supports previous studies of ferroelectric materials
where a relation between EFG tensors and polarization was observed, which may
be exploited to study ferroelectric order when standard techniques to measure
polarization are not easily applied.Comment: 9 pages, 6 figures, 5 tables, corrected typos, as published in Phys.
Rev.
Using ocean-glint scattered sunlight as a diagnostic tool for satellite remote sensing of greenhouse gases
International audienceSpectroscopic measurements of sunlight backscattered by the Earth's surface is a technique widely used for remote sensing of atmospheric constituent concentrations from space. Thereby, remote sensing of greenhouse gases poses particularly challenging accuracy requirements for instrumentation and retrieval algorithms which, in general, suffer from various error sources. Here, we investigate a method that helps disentangle sources of error for observations of sunlight backscattered from the glint spot on the ocean surface. The method exploits the backscattering characteristics of the ocean surface, which is bright for glint geometry but dark for off-glint angles. This property allows for identifying a set of clean scenes where light scattering due to particles in the atmosphere is negligible such that uncertain knowledge of the lightpath can be excluded as a source of error. We apply the method to more than 3 yr of ocean-glint measurements by the Thermal And Near infrared Sensor for carbon Observation (TANSO) Fourier Transform Spectrometer (FTS) onboard the Greenhouse Gases Observing Satellite (GOSAT), which aims at measuring carbon dioxide (CO2) and methane (CH4) concentrations. The proposed method is able to clearly monitor recent improvements in the instrument calibration of the oxygen (O2) A-band channel and suggests some residual uncertainty in our knowledge about the instrument. We further assess the consistency of CO2 retrievals from several absorption bands between 6400 cm-1(1565 nm) and 4800 cm-1(2100 nm) and find that the absorption bands commonly used for monitoring of CO2 dry air mole fractions from GOSAT allow for consistency better than 1.5 ppm. Usage of other bands reveals significant inconsistency among retrieved CO2 concentrations pointing at inconsistency of spectroscopic parameters. © 2013 Author(s)
Interethnic Interactions: Expectancies, Emotions, and Behavioral Intentions
Two studies examine Hispanic and non-Hispanic White people's responses to interethnic interactions. Consistent with previous findings regarding White/Black interactions, participants who had negative expectations about intergroup interactions reported more anger and anxiety about interethnic interactions. These negative emotional responses, in turn, were associated with negative behavioral intentions such as the desire to avoid interethnic interactions and the externalization of blame if an interethnic interaction did not go well. Across the studies, White participants who were angry about interethnic interactions wanted to avoid these interactions, whereas anxiety was the key predictor of avoidance for the Hispanic participants. Results are discussed in terms of their implications for developing models of intergroup interactions and designing approaches to improve these interactions
Dynamical phase transition for a quantum particle source
We analyze the time evolution describing a quantum source for noninteracting
particles, either bosons or fermions. The growth behaviour of the particle
number (trace of the density matrix) is investigated, leading to spectral
criteria for sublinear or linear growth in the fermionic case, but also
establishing the possibility of exponential growth for bosons. We further study
the local convergence of the density matrix in the long time limit and prove
the semiclassical limit.Comment: 24 pages; In the new version, we added several references concerning
open quantum systems and present an extended result on linear particle
production in the fermionic cas
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