449 research outputs found
New spectral functions of the near-ground albedo derived from aircraft diffraction spectrometer observations
The airborne spectral observations of the upward and downward irradiances are
revisited to investigate the dependence of the near-ground albedo as a
function of wavelength in the entire solar spectrum for different surfaces
(sand, water, snow) and under different conditions (clear or cloudy sky). The
radiative upward and downward fluxes were determined by a diffraction
spectrometer flown on a research aircraft that was performing multiple flight
paths near the ground. The results obtained show that the near-ground albedo
does not generally increase with increasing wavelengths for all kinds of
surfaces as is widely believed today. Particularly, in the case of water
surfaces it was found that the albedo in the ultraviolet region is more or
less independent of the wavelength on a long-term basis. Interestingly, in
the visible and near-infrared spectra the water albedo obeys an almost
constant power-law relationship with wavelength. In the case of sand surfaces
it was found that the sand albedo is a quadratic function of wavelength,
which becomes more accurate if the ultraviolet wavelengths are neglected.
Finally, it was found that the spectral dependence of snow albedo behaves
similarly to that of water, i.e. both decrease from the ultraviolet to the
near-infrared wavelengths by 20–50%, despite the fact that their values
differ by one order of magnitude (water albedo being lower). In addition, the
snow albedo vs. ultraviolet wavelength is almost constant, while in the
visible near-infrared spectrum the best simulation is achieved by a
second-order polynomial, as in the case of sand, but with opposite slopes
Coherent phenomena in mesoscopic systems
A mesoscopic system of cylindrical geometry made of a metal or a
semiconductor is shown to exhibit features of a quantum coherent state. It is
shown that magnetostatic interaction can play an important role in mesoscopic
systems leading to an ordered ground state. The temperature below the
system exhibits long-range order is determined. The self-consistent mean field
approximation of the magnetostatic interaction is performed giving the
effective Hamiltonian from which the self-sustaining currents can be obtained.
The relation of quantum coherent state in mesoscopic cylinders to other
coherent systems like superconductors is discussed.Comment: REVTeX, 4 figures, in print in Supercond. Sci. Techno
Conductance of a tunnel point-contact of noble metals in the presence of a single defect
In paper [1] (Avotina et al. Phys. Rev. B,74, 085411 (2006)) the effect of
Fermi surface anisotropy to the conductance of a tunnel point contact, in the
vicinity of which a single point-like defect is situated, has been investigated
theoretically. The oscillatory dependence of the conductance on the distance
between the contact and the defect has been found for a general Fermi surface
geometry. In this paper we apply the method developed in [1] to the calculation
of the conductance of noble metal contacts. An original algorithm, which
enables the computation of the conductance for any parametrically given Fermi
surface, is proposed. On this basis a pattern of the conductance oscillations,
which can be observed by the method of scanning tunneling microscopy, is
obtained for different orientations of the surface for the noble metals.Comment: 8 pages, 5 figure
Theory of oscillations in the STM conductance resulting from subsurface defects (Review Article)
In this review we present recent theoretical results concerning
investigations of single subsurface defects by means of a scanning tunneling
microscope (STM). These investigations are based on the effect of quantum
interference between the electron partial waves that are directly transmitted
through the contact and the partial waves scattered by the defect. In
particular, we have shown the possibility imaging the defect position below a
metal surface by means of STM. Different types of subsurface defects have been
discussed: point-like magnetic and non-magnetic defects, magnetic clusters in a
nonmagnetic host metal, and non-magnetic defects in a s-wave superconductor.
The effect of Fermi surface anisotropy has been analyzed. Also, results of
investigations of the effect of a strong magnetic field to the STM conductance
of a tunnel point contact in the presence of a single defect has been
presented.Comment: 31 pages, 10 figuers Submitted to Low. Temp. Phy
Fermi Surface of the Electron-doped Cuprate Superconductor Nd_{2-x}Ce_xCuO_{4} Probed by High-Field Magnetotransport
We report on the study of the Fermi surface of the electron-doped cuprate
superconductor NdCeCuO by measuring the interlayer
magnetoresistance as a function of the strength and orientation of the applied
magnetic field. We performed experiments in both steady and pulsed magnetic
fields on high-quality single crystals with Ce concentrations of to
0.17. In the overdoped regime of we found both semiclassical
angle-dependent magnetoresistance oscillations (AMRO) and Shubnikov-de Haas
(SdH) oscillations. The combined AMRO and SdH data clearly show that the
appearance of fast SdH oscillations in strongly overdoped samples is caused by
magnetic breakdown. This observation provides clear evidence for a
reconstructed multiply-connected Fermi surface up to the very end of the
overdoped regime at . The strength of the superlattice potential
responsible for the reconstructed Fermi surface is found to decrease with
increasing doping level and likely vanishes at the same carrier concentration
as superconductivity, suggesting a close relation between translational
symmetry breaking and superconducting pairing. A detailed analysis of the
high-resolution SdH data allowed us to determine the effective cyclotron mass
and Dingle temperature, as well as to estimate the magnetic breakdown field in
the overdoped regime.Comment: 23 pages, 8 figure
Complex magnetic ordering in the oxide selenide Sr2Fe3Se2O3
Sr2Fe3Se2O3 is a localised-moment iron oxide selenide in which two unusual coordinations for Fe2+
ions form two sublattices in a 2:1 ratio. In the paramagnetic region at room temperature the
compound adopts the crystal structure first reported for Sr2Co3S2O3, crystallising in space group
Pbam with a = 7.8121 Å, b = 10.2375 Å, c = 3.9939 Å and Z = 2. The sublattice occupied by two thirds
of the iron ions (Fe2 site) is formed by a network of distorted mer-[FeSe3O3] octahedra linked via
shared Se2 edges and O vertices forming layers, which connect to other layers by shared Se vertices.
As shown by magnetometry, neutron powder diffraction and Mössbauer spectroscopy
measurements, these moments undergo long range magnetic ordering below TN1 = 118 K, initially
adopting a magnetic structure with a propagation vector (½–δ, 0, ½) (0 ≤ ≤ 0.1) which is
incommensurate with the nuclear structure and described in the Pbam1’(a01/2)000s magnetic
superspace group, until at 92 K (TINC) there is a first order lock-in transition to a structure in which
these Fe2 moments form a magnetic structure with a propagation vector (½ , 0, ½) which may be
modelled using a 2a × b × 2c expansion of the nuclear cell in space group 36.178 Bab21m (BNS
notation). Below TN2 = 52 K the remaining third of the Fe2+ moments (Fe1 site) which are in a
compressed trans-[FeSe4O2] octahedral environment undergo long range ordering, as is evident
from the magnetometry, the Mössbauer spectra and the appearance of new magnetic Bragg peaks
in the neutron diffractograms. The ordering of the second set of moments on the Fe1 sites results in
a slight re-orientation of the majority moments on the Fe2 sites. The magnetic structure at 1.5 K is
described by a 2a × 2b × 2c expansion of the nuclear cell in space group 9.40 Iab (BNS notation)
Little groups of irreps of O(3), SO(3), and the infinite axial subgroups
Little groups are enumerated for the irreps and their components in any basis
of O(3) and SO(3) up to rank 9, and for all irreps of C, C, C, D and D. The results are obtained
by a new chain criterion, which distinguishes massive (rotationally
inequivalent) irrep basis functions and allows for multiple branching paths,
and are verified by inspection. These results are relevant to the determination
of the symmetry of a material from its linear and nonlinear optical properties
and to the choices of order parameters for symmetry breaking in liquid
crystals.Comment: 28 pages and 3 figure
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