44,175 research outputs found
Orbital magnetoelectric coupling in band insulators
Magnetoelectric responses are a fundamental characteristic of materials that
break time-reversal and inversion symmetries (notably multiferroics) and,
remarkably, of "topological insulators" in which those symmetries are unbroken.
Previous work has shown how to compute spin and lattice contributions to the
magnetoelectric tensor. Here we solve the problem of orbital contributions by
computing the frozen-lattice electronic polarization induced by a magnetic
field. One part of this response (the "Chern-Simons term") can appear even in
time-reversal-symmetric materials and has been previously shown to be quantized
in topological insulators. In general materials there are additional orbital
contributions to all parts of the magnetoelectric tensor; these vanish in
topological insulators by symmetry and also vanish in several simplified models
without time-reversal and inversion those magnetoelectric couplings were
studied before. We give two derivations of the response formula, one based on a
uniform magnetic field and one based on extrapolation of a long-wavelength
magnetic field, and discuss some of the consequences of this formula.Comment: 13 page
Legal pluralism and science and technology studies: exploring sources of the legal pluriverse
Predicted efficiency of Si wire array solar cells
Solar cells based on arrays of CVD-grown Si nano- or micro-wires have attracted interest as potentially low-cost alternatives to conventional wafer-based Si photovoltaics [1-6], and single-wire solar cells have been reported with efficiencies of up to 3.4% [7]. We recently presented device physics simulations which predicted efficiencies exceeding 17%, based on experimentally observed diffusion lengths within our wires [8]. However, this model did not take into account the optical properties of a wire array device - in particular the inherently low packing fraction of wires within CVD-grown wire arrays, which might limit their ability to fully absorb incident sunlight. For this reason, we have combined a device physics model of Si wire solar cells with FDTD simulations of light absorption within wire arrays to investigate the potential photovoltaic efficiency of this cell geometry. We have found that even a sparsely packed array (14%) is expected to absorb moderate (66%) amounts of above-bandgap solar energy, yielding a simulated photovoltaic efficiency of 14.5%. Because the wire array comprises such a small volume of Si, the observed absorption represents an effective optical concentration, which enables greater operating voltages than previously predicted for Si wire array solar cells
X-Ray Spectral Study of AGN Sources Content in Some Deep Extragalactic XMM-Newton Fields
We undertake a spectral study of a sample of bright X-ray sources taken from
six XMM-Newton fields at high galactic latitudes, where AGN are the most
populous class. These six fields were chosen such that the observation had an
exposure time more than 60 ksec, had data from the EPIC-pn detector in the
full-Frame mode and lying at high galactic latitude . The analysis
started by fitting the spectra of all sources with an absorbed power-law model,
and then we fitted all the spectra with an absorbed power-law with a low energy
black-body component model.The sources for which we added a black body gave an
F-test probability of 0.01 or less (i.e. at 99% confidence level), were
recognized as sources that display soft excess. We perform a comparative
analysis of soft excess spectral parameters with respect to the underlying
power-law one for sources that satisfy this criterion. Those sources, that do
not show evidence for a soft excess, based on the F-test probability at a 99%
confidence level, were also fitted with the absorbed power-law with a low
energy black-body component model with the black-body temperature fixed at 0.1
and 0.2 keV. We establish upper limits on the soft excess flux for those
sources at these two temperatures. Finally we have made use of Aladdin
interactive sky atlas and matching with NASA/IPAC Extragalactic Database (NED)
to identify the X-ray sources in our sample. For those sources which are
identified in the NED catalogue, we make a comparative study of the soft excess
phenomenon for different types of systems
Structure of deformed silicon and implications for low cost solar cells
The microstructure and minority carrier lifetime of silicon were investigated in uniaxially compressed silicon samples. The objective of the investigation was to determine if it is feasible to produce silicon solar cells from sheet formed by high temperature rolling. The initial structure of the silicon samples ranged from single crystal to fine-grained polycrystals. The samples had been deformed at strain rates of 0.1 to 8.5/sec and temperatures of 1270-1380 C with subsequent annealing at 1270-1380 C. The results suggest that high temperature rolling of silicon to produce sheet for cells of high efficiency is not practical
Perceptions of the intergroup structure and anti-Asian prejudice amongst white Australians
Proof oSubjective intergroup beliefs and authoritarianism were assessed in a field study (N= 255) of White Australians’ anti-Asian stereotyping and prejudice. A social identity analysis of intergroup prejudice was adopted, such that perceptions of the intergroup structure (instability, permeability, legitimacy and higher ingroup status) were proposed as predictors of higher prejudice (blatant and covert) and less favorable stereotyping. Consistent with the social identity approach, both independent and interacting roles for sociostructural predictors of Anti-Asian bias were observed, even after demographic and personality variables were controlled. For example, perceived legitimacy was associated with higher prejudice when White Australians’ status position relative to Asian Australians was valued. Moreover, when participants evaluated Whites’ position as unstable and high status or legitimate, perceptions of permeable intergroup boundaries were associated with anti-Asian bias. The present findings demonstrate status protection responses in advantaged group members in a field setting, lending weight to the contention that perceptions of sociostructural threat interact to predict outgroup derogation. Implications for theories of intergroup relations are discussed
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