600 research outputs found
Nonlocal effects in the shot noise of diffusive superconductor - normal-metal systems
A cross-shaped diffusive system with two superconducting and two normal
electrodes is considered. A voltage is applied between the normal
leads. Even in the absence of average current through the superconducting
electrodes their presence increases the shot noise at the normal electrodes and
doubles it in the case of a strong coupling to the superconductors. The
nonequilibrium noise at the superconducting electrodes remains finite even in
the case of a vanishingly small transport current due to the absence of energy
transfer into the superconductors. This noise is suppressed by
electron-electron scattering at sufficiently high voltages.Comment: 4 pages, RevTeX, 2 eps figure
Optical Response of Solid CO as a Tool for the Determination of the High Pressure Phase
We report first-principles calculations of the frequency dependent linear and
second-order optical properties of the two probable extended-solid phases of
CO--V, i.e. and . Compared to the parent
phase the linear optical susceptibility of both phases is much smaller. We find
that and differ substantially in their linear optical
response in the higher energy regime. The nonlinear optical responses of the
two possible crystal structures differ by roughly a factor of five. Since the
differences in the nonlinear optical spectra are pronounced in the low energy
regime, i.e. below the band gap of diamond, measurements with the sample inside
the diamond anvil cell are feasible. We therefore suggest optical experiments
in comparison with our calculated data as a tool for the unambiguous
identification of the high pressure phase of CO.Comment: 4 pages 2 fig
Oscillation of the tunnel splitting in nanospin systems within the particle mapping formalism
The oscillation of tunnel splitting in the biaxial spin system within
magnetic field along the anisotropy axis is analyzed within the particle
mapping approach, rather than in the (\theta-\phi) spin coherent-state
representation. In our mapping procedure, the spin system is transformed into a
particle moving in the restricted geometry whose wave function subjects
to the boundary condition involving additional phase shift. We obtain the new
topological phase that plays the same role as the Wess-Zumino action in spin
coherent-state representation. Considering the interference of two possible
trajectories, instanton and anti-instanton, we get the identical condition for
the field at which tunneling is quenched, with the previous result within spin
coherent-state representation.Comment: 11 pages, 1 figure; Some typographical errors have been correcte
Substructures in lens galaxies: PG1115+080 and B1555+375, two fold configurations
We study the anomalous flux ratio which is observed in some four-image lens
systems, where the source lies close to a fold caustic. In this case two of the
images are close to the critical curve and their flux ratio should be equal to
unity, instead in several cases the observed value differs significantly. The
most plausible solution is to invoke the presence of substructures, as for
instance predicted by the Cold Dark Matter scenario, located near the two
images. In particular, we analyze the two fold lens systems PG1115+080 and
B1555+375, for which there are not yet satisfactory models which explain the
observed anomalous flux ratios. We add to a smooth lens model, which reproduces
well the positions of the images but not the anomalous fluxes, one or two
substructures described as singular isothermal spheres. For PG1115+080 we
consider a smooth model with the influence of the group of galaxies described
by a SIS and a substructure with mass as well as a
smooth model with an external shear and one substructure with mass . For B1555+375 either a strong external shear or two substructures
with mass reproduce the data quite well.Comment: 26 pages, updated bibliography, Accepted for publication in
Astrophysics & Space Scienc
Material-Specific Investigations of Correlated Electron Systems
We present the results of numerical studies for selected materials with
strongly correlated electrons using a combination of the local-density
approximation and dynamical mean-field theory (DMFT). For the solution of the
DMFT equations a continuous-time quantum Monte-Carlo algorithm was employed.
All simulations were performed on the supercomputer HLRB II at the Leibniz
Rechenzentrum in Munich. Specifically we have analyzed the pressure induced
metal-insulator transitions in Fe2O3 and NiS2, the charge susceptibility of the
fluctuating-valence elemental metal Yb, and the spectral properties of a
covalent band-insulator model which includes local electronic correlations.Comment: 14 pages, 7 figures, to appear in "High Performance Computing in
Science and Engineering, Garching 2009" (Springer
Absorption of electromagnetic and gravitational waves by Kerr black holes
We calculate the absorption cross section for planar waves incident upon Kerr black holes, and present
a unified picture for scalar, electromagnetic and gravitational waves. We highlight the spin-helicity effect
that arises from a coupling between the rotation of the black hole and the helicity of a circularlypolarized
wave. For the case of on-axis incidence, we introduce an extended ‘sinc approximation’ to
quantify the spin-helicity effect in the strong-field regime
Iron under Earth's core conditions: Liquid-state thermodynamics and high-pressure melting curve
{\em Ab initio} techniques based on density functional theory in the
projector-augmented-wave implementation are used to calculate the free energy
and a range of other thermodynamic properties of liquid iron at high pressures
and temperatures relevant to the Earth's core. The {\em ab initio} free energy
is obtained by using thermodynamic integration to calculate the change of free
energy on going from a simple reference system to the {\em ab initio} system,
with thermal averages computed by {\em ab initio} molecular dynamics
simulation. The reference system consists of the inverse-power pair-potential
model used in previous work. The liquid-state free energy is combined with the
free energy of hexagonal close packed Fe calculated earlier using identical
{\em ab initio} techniques to obtain the melting curve and volume and entropy
of melting. Comparisons of the calculated melting properties with experimental
measurement and with other recent {\em ab initio} predictions are presented.
Experiment-theory comparisons are also presented for the pressures at which the
solid and liquid Hugoniot curves cross the melting line, and the sound speed
and Gr\"{u}neisen parameter along the Hugoniot. Additional comparisons are made
with a commonly used equation of state for high-pressure/high-temperature Fe
based on experimental data.Comment: 16 pages including 6 figures and 5 table
Dynamical mean-field approach to materials with strong electronic correlations
We review recent results on the properties of materials with correlated
electrons obtained within the LDA+DMFT approach, a combination of a
conventional band structure approach based on the local density approximation
(LDA) and the dynamical mean-field theory (DMFT). The application to four
outstanding problems in this field is discussed: (i) we compute the full
valence band structure of the charge-transfer insulator NiO by explicitly
including the p-d hybridization, (ii) we explain the origin for the
simultaneously occuring metal-insulator transition and collapse of the magnetic
moment in MnO and Fe2O3, (iii) we describe a novel GGA+DMFT scheme in terms of
plane-wave pseudopotentials which allows us to compute the orbital order and
cooperative Jahn-Teller distortion in KCuF3 and LaMnO3, and (iv) we provide a
general explanation for the appearance of kinks in the effective dispersion of
correlated electrons in systems with a pronounced three-peak spectral function
without having to resort to the coupling of electrons to bosonic excitations.
These results provide a considerable progress in the fully microscopic
investigations of correlated electron materials.Comment: 24 pages, 14 figures, final version, submitted to Eur. Phys. J. for
publication in the Special Topics volume "Cooperative Phenomena in Solids:
Metal-Insulator Transitions and Ordering of Microscopic Degrees of Freedom
Starch analysis using hydrodynamic chromatography with a mixed-bed particle column
Columns packed with commercial glass beads 5 and 19 lm average size and a mixture of both (0.7 volume
fraction of large particles) were used to analyse starch composition by hydrodynamic chromatography
(HDC), applying water as mobile phase. To obviate retrogradation, experiments were carried out at
column temperatures of 15 and 3 °C and several types of starch were assayed. In what concerns amylopectin
and amylose separation, a better resolution and a lower pressure drop were obtained for the mixed
binary packing when compared with the packing containing uniform 5 lm glass beads. A more efficient
cooling of the mobile phase was also obtained with the mixed packing, which was determinant for
improving resolution. For the Hylon VII starch the relative retention times (RRT) were 0.777 and 0.964
for amylopectin and amylose, respectively, while for the Tapioca starch the obtained RRTs were 0.799
and 0.923. Application of unbound glass beads as column packing not only might reduce equipment
and running costs in preparative scale separations, but also proved to be useful as a fast and reliable
method to monitor the amylose and amylopectin content of starch samples of different sources.FEDERThe authors wish to thank FCT for the grant provided to Dr. Alexander Yelshin (Yelshyn). This work was developed under the framework of the project POCI-EQU-58337/2004, partially funded by FEDER
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