866 research outputs found
Screening of Coulomb interactions in transition metals
We discuss different methods of calculation of the screened Coulomb
interaction in transition metals and compare the constraint local-density
approximation (LDA) with the GW approach. We clarify that they offer
complementary methods of treating the screening and should serve for different
purposes. In the GW method, the renormalization of bare on-site Coulomb
interactions between 3d electrons occurs mainly through the screening by the
same 3d electrons, treated in the random phase approximation (RPA). The basic
difference of the constraint-LDA method is that it deals with the neutral
processes, where the Coulomb interactions are additionally screened by the
``excited'' electron, since it continues to stay in the system. This is the
main channel of screening by the itinerant () electrons, which is
especially strong in the case of transition metals and missing in the GW
approach, although the details of this screening may be affected by additional
approximations, which typically supplement these two methods. The major
drawback of the conventional constraint-LDA method is that it does not allow to
treat the energy-dependence of . We propose a promising approximation based
on the combination of these two methods. First, we take into account the
screening of Coulomb interactions in the 3d-electron-line bands located near
the Fermi level by the states from the subspace being orthogonal to these
bands, using the constraint-LDA methods. The obtained interactions are further
renormalized within the bands near the Fermi level in RPA. This allows the
energy-dependent screening by electrons near the Fermi level including the same
3d electrons.Comment: 25 pages, 5 figures, 2 table
Doping-driven Mott transition in La_{1-x}Sr_xTiO_3 via simultaneous electron and hole doping of t2g subbands
The insulator to metal transition in LaTiO_3 induced by La substitution via
Sr is studied within multi-band exact diagonalization dynamical mean field
theory at finite temperatures. It is shown that weak hole doping triggers a
large interorbital charge transfer, with simultaneous electron and hole doping
of t2g subbands. The transition is first-order and exhibits phase separation
between insulator and metal. In the metallic phase, subband compressibilities
become very large and have opposite signs. Electron doping gives rise to an
interorbital charge flow in the same direction as hole doping. These results
can be understood in terms of a strong orbital depolarization.Comment: 4 pages, 5 figure
`Island Surfing' Mechanism of Electron Acceleration During Magnetic Reconnection
One of the key unresolved problems in the study of space plasmas is to
explain the production of energetic electrons as magnetic field lines
`reconnect' and release energy in a exposive manner. Recent observations
suggest possible roles played by small scale magnetic islands in the
reconnection region, but their precise roles and the exact mechanism of
electron energization have remained unclear. Here we show that secondary
islands generated in the reconnection region are indeed efficient electron
accelerators. We found that, when electrons are trapped inside the islands,
they are energized continuously by the reconnection electric field prevalent in
the reconnection diffusion region. The size and the propagation speed of the
secondary islands are similar to those of islands observed in the magnetotail
containing energertic electrons.Comment: 5 pages, 4 figures, submitted to J. Geophys. Res
Ground State Properties and Optical Conductivity of the Transition Metal Oxide
Combining first-principles calculations with a technique for many-body
problems, we investigate properties of the transition metal oxide from the microscopic point of view. By using the local density
approximation (LDA), the high-energy band structure is obtained, while screened
Coulomb interactions are derived from the constrained LDA and the GW method.
The renormalization of the kinetic energy is determined from the GW method. By
these downfolding procedures, an effective Hamiltonian at low energies is
derived. Applying the path integral renormalization group method to this
Hamiltonian, we obtain ground state properties such as the magnetic and orbital
orders. Obtained results are consistent with experiments within available data.
We find that is close to the metal-insulator transition.
Furthermore, because of the coexistence and competition of ferromagnetic and
antiferromgnetic exchange interactions in this system, an antiferromagnetic and
orbital-ordered state with a nontrivial and large unit cell structure is
predicted in the ground state. The calculated optical conductivity shows
characteristic shoulder structure in agreement with the experimental results.
This suggests an orbital selective reduction of the Mott gap.Comment: 38pages, 22figure
1D Modeling for Temperature-Dependent Upflow in the Dimming Region Observed by Hinode/EIS
We have previously found a temperature-dependent upflow in the dimming region
following a coronal mass ejection (CME) observed by the {\it Hinode} EUV
Imaging Spectrometer (EIS). In this paper, we reanalyzed the observations along
with previous work on this event, and provided boundary conditions for
modeling. We found that the intensity in the dimming region dramatically drops
within 30 minutes from the flare onset, and the dimming region reaches the
equilibrium stage after 1 hour later. The temperature-dependent upflows
were observed during the equilibrium stage by EIS. The cross sectional area of
the fluxtube in the dimming region does not appear to expand significantly.
From the observational constraints, we reconstructed the temperature-dependent
upflow by using a new method which considers the mass and momentum conservation
law, and demonstrated the height variation of plasma conditions in the dimming
region. We found that a super radial expansion of the cross sectional area is
required to satisfy the mass conservation and momentum equations. There is a
steep temperature and velocity gradient of around 7 Mm from the solar surface.
This result may suggest that the strong heating occurred above 7 Mm from the
solar surface in the dimming region. We also showed that the ionization
equilibrium assumption in the dimming region is violated especially in the
higher temperature range.Comment: accepted for publication in The Astrophysical Journa
Frequency-dependent local interactions and low-energy effective models from electronic structure calculations
We propose a systematic procedure for constructing effective models of
strongly correlated materials. The parameters, in particular the on-site
screened Coulomb interaction U, are calculated from first principles, using the
GW approximation. We derive an expression for the frequency-dependent U and
show that its high frequency part has significant influence on the spectral
functions. We propose a scheme for taking into account the energy dependence of
U, so that a model with an energy-independent local interaction can still be
used for low-energy properties.Comment: 16 pages, 5 figure
Perbandingan Aktivitas Antioksidan Ekstrak Etanol 70% Daun Binahong (Anredera Cordifolia, (Ten.) Steenis) Segar Dan Daun Binahong (Anredera Cordifolia, (Ten.) Steenis) Kering Dengan Metode Dpph (2,2-diphenil-1-picryl-hydrazyl)
Ibuprofen is a nonsteroid anti-inflammatory drug (NSAID) that can inhibit activities of siklooksigenase-1 and siklooksigenase-2 isoenzyme, so thats with inhibiting the changing of arachidonic acid to prostaglandin can be decreased. Ibuprofen belongs to BCS class II, because it has high permeability and low solubility. The goals of this research are to determine the pKa of ibuprofen and to know the influence of the complexe formation of ibuprofen with β-cyclodextrin on ibuprofen solubility. Determinnation of the pKa value of ibuprofen has been conducted by measuring absorbance at wavelength 266 nm. The optimization of complexes formation has been worked using factorial design method. Then, the complex of ibuprofen and β-cyclodextrin evaluated using infrared spectrophotometry, and optimized with using software design expert version: 7.1.3. The result of this research showeds that the pKa value of ibuprofen wasis 4, 37 ± 0,07 and 5, 24 ± 0,07. The complexes ibuprofen with β--siklodekstrin was evaluated using infrared spectrophotometry there was no peak at 1721 that indicated that there was no functional group of –C=O. Based on simplex lattice design calculation, the best complexes consisted of ibuprofen and β-cyclodextrin in ratio 120 and 180. The solubility data showed that complexes of ibuprofen with β-cyclodextrin was 2,30 ± 0,12 times at pH 1,30 ± 0,16 times; 1,74 ± 0,06 times , and 0,99 time ± 0,02 at pH 7,2; respectively at pH of 2,39; 6,39; and 7,2. Based on factorial design data, it was known that the combination of ibuprofen (100 mg) and β-cyclodextrin (200 mg) has an optimum solubility in codition pH 2,39; 6,39 and temperature 30°C. In codition, pH of 3,2 this mixture has an optimum solubility at temperature 45°C
Metal-insulator transition in Ca_{1-x}Li_xPd_3O_4
Metal-insulator transition in Ca_{1-x}Li_xPd_3O_4 has been studied through
charge transport measurements. The resistivity, the Seebeck coefficient, and
the Hall coefficient are consistently explained in terms of a simple one-band
picture, where a hole with a moderately enhanced mass is itinerant
three-dimensionally. Contrary to the theoretical prediction [Phys. Rev. B62,
13426 (2000)], CaPd_3O_4 is unlikely to be an excitonic insulator, and holds a
finite carrier concentration down to 4.2 K. Thus the metal-insulator transition
in this system is basically driven by localization effects.Comment: RevTeX4 format, 4 pages, 5 eps figure
Magnetic Reconnection in Non-Equilibrium Ionization Plasma
We have studied the effect of time-dependent ionization and recombination
processes on magnetic reconnection in the solar corona. Petschek-type steady
reconnection, in which model the magnetic energy is mainly converted at the
slow-mode shocks, was assumed. We carried out the time-dependent ionization
calculation in the magnetic reconnection structure. We only calculated the
transient ionization of iron; the other species were assumed to be in
ionization equilibrium. The intensity of line emissions at specific wavelengths
were also calculated for comparison with {\it Hinode} or other observations in
future. What we found is as follows: (1) iron is mostly in non-equilibrium
ionization in the reconnection region, (2) the intensity of line emission
estimated by the time-dependent ionization calculation is significantly
different from that with the ionization equilibrium assumption, (3) the effect
of time-dependent ionization is sensitive to the electron density in the case
that the electron density is less than cm, (4) the effect of
thermal conduction lessens the time-dependent ionization effect, (5) the effect
of radiative cooling is negligibly small even if we take into account
time-dependent ionization.Comment: accepted for publication in The Astrophysical Journa
Photometry of VS0329+1250: A New, Short-Period SU Ursae Majoris Star
Time-resolved CCD photometry is presented of the recently-discovered (V~15 at
maximum light) eruptive variable star in Taurus, which we dub VS0329+1250. A
total of ~20 hr of data obtained over six nights reveals superhumps in the
light curves, confirming the star as a member of the SU UMa class of dwarf
novae. The superhumps recur with a mean period of 0.053394(7) days (76.89 min),
which represents the shortest superhump period known in a classical SU UMa
star. A quadratic fit to the timings of superhump maxima reveals that the
superhump period was increasing at a rate given by dP/dt ~ (2.1 +/- 0.8) x
10^{-5} over the course of our observations. An empirical relation between
orbital period and the absolute visual magnitude of dwarf novae at maximum
light, suggests that VS0329+1250 lies at a distance of ~1.2 +/- 0.2 kpc.Comment: V2 - The paper has been modified to incorporate the referee's
comments, and has now been accepted for publication in the PASP. The most
significant change is that we are now able to confirm that the superhump
period was increasing during the course of our observation
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