2,364 research outputs found
The self-consistent field model for Fermi systems with account of three-body interactions
On the basis of a microscopic model of self-consistent field, the
thermodynamics of the many-particle Fermi system at finite temperatures with
account of three-body interactions is built and the quasiparticle equations of
motion are obtained. It is shown that the delta-like three-body interaction
gives no contribution into the self-consistent field, and the description of
three-body forces requires their nonlocality to be taken into account. The
spatially uniform system is considered in detail, and on the basis of the
developed microscopic approach general formulas are derived for the fermion's
effective mass and the system's equation of state with account of contribution
from three-body forces. The effective mass and pressure are numerically
calculated for the potential of "semi-transparent sphere" type at zero
temperature. Expansions of the effective mass and pressure in powers of density
are obtained. It is shown that, with account of only pair forces, the
interaction of repulsive character reduces the quasiparticle effective mass
relative to the mass of a free particle, and the attractive interaction raises
the effective mass. The question of thermodynamic stability of the Fermi system
is considered and the three-body repulsive interaction is shown to extend the
region of stability of the system with the interparticle pair attraction. The
quasiparticle energy spectrum is calculated with account of three-body forces.Comment: 21 pages, 5 figure
Super-Eddington accretion in the Q2237+0305 quasar?
The interband time lags between the flux variations of the Q2237+0305 quasar
have been determined from light curves in the Johnson-Cousins V, R, and I
spectral bands. The values of the time lags for filter pairs R-V, I-R, and I-V
are significantly higher than those predicted by the standard accretion disk
model by Shakura and Sunyaev. To explain the discrepancy, the idea of a
supercritical accretion regime in quasars considered in 1973 by Shakura and
Sunyaev is applied. This regime has been shown by them to cause an extended
scattering envelope around the accretion disk. The envelope efficiently
scatters and re-emits the radiation from the accretion disk and thus increases
the apparent disk size. We made use of analytical expressions for the envelope
radius and temperature derived by Shakura and Sunyaev in their analysis of
super-Eddington accretion and show that our results are consistent with the
existence of such an envelope. The corresponding parameters of the accretion
regime were calculated. They provide the radii of the envelope in the V, R, and
I spectral bands consistent with the inter-band time lags determined in our
work.Comment: 11 pages, 4 figure
Validity of abundances derived from spaxel spectra of the MaNGA survey
We measured the emission lines in the spaxel spectra of MaNGA galaxies in
order to determine the abundance distributions therein. It has been suggested
that the strength of the low-ionization lines, R_2, N_2, and S_2 may be
increased (relative to Balmer lines) in (some) spaxel spectra of the MaNGA
survey due to a contribution of the radiation of the diffuse ionized gas.
Consequently, the abundances derived from the spaxel spectra through
strong-line methods may suffer from large errors. We examined this expectation
by comparing the behaviour of the line intensities and the abundances estimated
through different calibrations for slit spectra of HII regions in nearby
galaxies, for fibre spectra from the SDSS, and for spaxel spectra of the MaNGA
survey. We found that the S_2 strength is increased significantly in the fibre
and spaxel spectra. The mean enhancement changes with metallicity and can be as
large as a factor of 2. The mean distortion of R_2 and N_2 is less than a
factor of 1.3. This suggests that Kaufmann et al.'s demarcation line between
AGNs and HII regions in the BPT diagram is a useful criterion to reject spectra
with significantly distorted strengths of the N_2 and R_2 lines. We find that
the three-dimensional R calibration, which uses the N_2 and R_2 lines, produces
reliable abundances in the MaNGA galaxies. The one-dimensional N2 calibration
produces either reliable or wrong abundances depending on whether excitation
and N/O abundance ratio in the target region (spaxel) are close to or differ
from those parameters in the calibrating points located close to the
calibration relation. We then determined abundance distributions within the
optical radii in the discs of 47 MaNGA galaxies. The optical radii of the
galaxies were estimated from the surface brightness profiles constructed based
on the MaNGA observations.Comment: 19 pages, 15 figures, accepted for publication in A&
Specific heat of MgB_2 after irradiation
We studied the effect of disorder on the superconducting properties of
polycrystalline MgB_2 by specific-heat measurements. In the pristine state,
these measurements give a bulk confirmation of the presence of two
superconducting gaps with 2 Delta 0 / k_B T_c = 1.3 and 3.9 with nearly equal
weights. The scattering introduced by irradiation suppresses T_c and tends to
average the two gaps although less than predicted by theory. We also found that
by a suitable irradiation process by fast neutrons, a substantial bulk increase
of dH_{c2}/dT at T_c can be obtained without sacrificing more than a few
degrees in T_c. The upper critical field of the sample after irradiation
exceeds 28 T at T goes to 0 K.Comment: 11 pages text, 6 figures, accepted by Journal of Physics: Condensed
Matte
Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals
Directional point-contact spectroscopy measurements were performed for the
first time in state-of-the-art MgB_2 single crystals. The selective suppression
of the superconductivity in the "pi" band by means of a suitable magnetic field
allowed separating the partial contribution of each band to the total
point-contact conductance. By fitting the partial conductance curves
sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two
gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental
uncertainty. Their temperature dependence was found to agree well with the
predictions of the two-band models for MgB_2.Comment: 6 pages, 4 eps figures. References added, abstract rewritten, text
slightly changed. Proceedings of the BOROMAG Conference, June 17-19, Genoa,
Ital
On the Ginzburg-Landau Analysis of the Upper Critical Field Hc2 in MgB2
Temperature dependence of the upper critical field Hc2 (T) for the
superconducting magnesium diboride, MgB2, is studied in the vicinity of Tc by
using a two-band Ginzburg-Landau (G-L) theory. The temperature dependence of
Hc2 (T) near Tc exhibits a positive curvature. In addition, the calculated
temperature dependence and its higher order derivatives are also shown to be in
a good agreement with the experimental data. In analogy with the multi-band
character of Eliashberg microscopic theory, the positive curvature of Hc2 (T)
is described reasonably by solving the two-band of G-L theory.Comment: 14 pages, 2 figures, submitted to SUST November 200
Infrared and optical properties of pure and cobalt-doped LuNi_2B_2C
We present optical conductivity data for Lu(NiCo)BC over
a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials
show evidence of being good Drude metals with the infrared data in reasonable
agreement with dc resistivity measurements at low frequencies. An absorption
threshold is seen at approximately 700 cm-1. In the cobalt-doped material we
see a superconducting gap in the conductivity spectrum with an absorption onset
at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong
coupling. The pure material is in the clean limit and no gap can be seen. We
discuss the data in terms of the electron-phonon interaction and find that it
can be fit below 600 cm-1 with a plasma frequency of 3.3 eV and an
electron-phonon coupling constant lambda_{tr}=0.33 using an alpha^{2}F(omega)
spectrum fit to the resistivity.Comment: 10 pages with 10 embedded figures, submitted to PR
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