2,008 research outputs found
Unbalanced superconductivity induced by the constant electron-phonon coupling on a square lattice
In the present paper, we analyze the properties of the unbalanced
superconducting state on a square lattice with the constant value of the
electron-phonon coupling function. We conduct our analysis in the framework of
the Eliashberg formalism, explicitly considering {\bf k}-dependence of the
electron and phonon dispersion relations. It is found that the balanced
superconducting state does not induce itself in the system due to the high
value of the electron effective mass. However, in the unbalanced case the
thermodynamic properties of the superconducting condensate can be distinctly
different from the predictions of the Bardeen-Cooper-Schieffer theory; when the
coupling constant value in the diagonal channel of the self-energy is
diminished comparing to the non-diagonal channel. This is due to the reduced
dimensionality of the tested system and the strong-coupling effects included in
the Eliashberg formalism.Comment: 7 pages, 4 figure
The unbalanced phonon-induced superconducting state on a square lattice beyond the static boundary
The paper presents our verification of induction of the superconducting state
on a square lattice by the linear electron-phonon interaction for values of the
unbalance parameter () less than
. Symbols and denote the values
of the coupling constant in the diagonal and the non-diagonal channel of the
self-energy. Calculations were carried out using the Eliashberg equations, in
which the order parameter () and the
wave function renormalising factor () depend
explicitly on the Matsubara frequency () and the wave vector (). The value of in the static boundary (), equal to (), is significantly greater
than the obtained limit value of . Values of the thermodynamic functions
of the superconducting state determined for our assumptions are significantly
different from the values calculated in accordance with the BCS theory. The
results were obtained for the electron-phonon interaction function explicitly
dependent on the momentum transfer between electron states.Comment: 12 pages, 9 figure
The Araucaria Project: The distance to the Small Magellanic Cloud from near infrared photometry of Type~II Cepheids
We have obtained deep near infrared J- and K-band observations of 14 BL
Herculis and 5 W Virginis SMC stars from the OGLE III survey with the ESO New
Technology Telescope equipped with the SOFI infrared camera. From these
observations, period-luminosity (P-L) relations in the J and Ks 2MASS bands
were derived. The slopes of the K and J band relations of -2.15 +- 0.19 and
-1.95 +- 0.24, respectively, agree very well with the corresponding slopes
derived previously for population II Cepheids in globular clusters, Galactic
bulge and in the Large Magellanic Cloud. The distance modulus to the SMC
obtained from our data using P-L relation derived for globular cluster Cepheids
equals 18.85 +- 0.07 (statistical) +- 0.07 (systematic without including
potential metallicity effect), which within the uncertainties agrees well with
the results obtained with other methods.Comment: Acta Astonomica in pres
Influence of strong-coupling and retardation effects on superconducting state in compound
In the framework of strong-coupling formalism, we have calculated the
thermodynamic parameters of superconducting state in the
compound. The values of critical temperature () are ~K and ~K
for the Coulomb pseudopotential and , respectively. In
the paper, we have determined the low temperature values of order parameter
(), specific heat jump at the critical temperature (), and thermodynamic critical field (). The dimensionless
thermodynamic ratios: , , and
are equal to:
,
, and
. Due to the
significant strong-coupling and retardation effects () those values highly deviate from the predictions of BCS theory.Comment: 5 pages, 4 figure
The effect of B and C addition on structure and properties of sintered Mn-Cr-Mo steels
The effect of boron and/or carbon additions on the microstructure and properties of sintered manganese steels was investigated. Thesesteels were produced based on following powders: pre-alloyed Astaloy CrL and ferromanganese. Sintering was carried out at 1150oC and1250oC in hydrogen atmosphere. The density, hardness and microstructure were investigated. The main objective of this work was to find correlation between sintering temperature and chemical composition of steel and theirs properties. It was shown that both alloying elements and sintering temperature influence the microstructure and properties of sintered manganese steels. When boron or graphite was added, density and also hardness of the Fe-Mn-Cr-Mo steels were increased. Significant change in hardness were related to changes in the microstructure. Boron has a beneficial effect on the densification process, due to faster and more efficient sintering kinetics. In general, it can be observed that by sintering at higher temperature the microstructure and properties were improved
Study of hydroxyapatite behaviour during sintering of 316L steel
316L stainless steel – hydroxyapatite composite biomaterials with different hydroxyapatite weight fraction in the composite wereinvestigated. Hydroxyapatite (HAp – Ca10(PO4)6(OH)2) is well known biomaterial. HAp reveals excellent chemical and biological affinitywith bony tissues. On the other hand hydroxyapatite shows low mechanical properties. The combination of very good biocompatibility of hydroxyapatite and high mechanical properties of stainless steel seems to be a good solution. In presented research natural originhydroxyapatite and 316L austenitic stainless steel were used. In this work, metal-ceramics composites were fabricated by the powdermetallurgy technology (involving pressing and sintering process). Sintering was carried out at 1250oC in hydrogen atmosphere. Thedensity, porosity and hardness were investigated. Metallographic microscope and SEM were carried out in order to investigate themicrostructure. The horizontal NETZSCH DIL 402E dilatometer was used to evaluate the dimensional changes and phenomena occurringduring sintering. The research displayed that physical properties of sintered 316L-HAp composites decrease with increase ofhydroxyapatite content. Microstructure of investigated composites consists of austenitic and probably inclusions of hydroxyapatite andheterogeneous eutectic occurring on the grain boundaries. It was shown that amount of hydroxyapatite in the powder mixtures influencethe dimensional changes occurring during sintering
Interaction of the hydrogen molecule with the environment: stability of the system
We study the stability of the hydrogen molecule interacting with the
environment according to the balanced gain and loss energy scheme. We
determined the properties of the molecule taking into account all electronic
interactions, where the parameters of the Hamiltonian have been computed by
using the variational method. The interaction of the hydrogen molecule with the
environment was modeled parametrically () with the help of the
non-hermitian operator. We have shown that the hydrogen molecule is dynamically
unstable. The dissociation time () decreases, if the parameter
increases (for , we get ). At
the dynamic instability of the hydrogen molecule overlaps its static
instability as the coupling constant increases. We observed the
decrease in the dissociation energy and the existence of the metastable state
of the molecule (~Ry). The hydrogen molecule is
statically unstable for ~Ry. One can also observed
the symmetry breaking effect for the electronic Hamiltonian
(~Ry). However, it does not affect the
properties of the hydrogen molecule, such as: the electronic Hamiltonian
parameters, the phonon and rotational energy, and the values of the
electron-phonon coupling constants.Comment: 16 pages, 8 figure
The Optical Gravitational Lensing Experiment. Additional Planetary and Low-Luminosity Object Transits from the OGLE 2001 and 2002 Observational Campaigns
The photometric data collected by OGLE-III during the 2001 and 2002
observational campaigns aiming at detection of planetary or low-luminosity
object transits were corrected for small scale systematic effects using the
data pipeline by Kruszewski and Semeniuk and searched again for low amplitude
transits. Sixteen new objects with small transiting companions, additional to
previously found samples, were discovered. Most of them are small amplitude
cases which remained undetected in the original data.
Several new objects seem to be very promising candidates for systems
containing substellar objects: extrasolar planets or brown dwarfs. Those
include OGLE-TR-122, OGLE-TR-125, OGLE-TR-130, OGLE-TR-131 and a few others.
Those objects are particularly worth spectroscopic follow-up observations for
radial velocity measurements and mass determination. With well known
photometric orbit only a few RV measurements should allow to confirm their
actual status.
All photometric data of presented objects are available to the astronomical
community from the OGLE Internet archive.Comment: 8 pages. Appendix in 'jpg' format. Minor changes. Full resolution
pages of Appendix and photometric data presented in the paper are available
from the OGLE Internet archive: See http://ogle.astrouw.edu.pl or its US
mirror, at http://bulge.princeton.edu/~ogl
The Optical Gravitational Lensing Experiment. Period-Luminosity Relations of Variable Red Giant Stars
Period-luminosity (PL) relations of variable red giants in the Large (LMC)
and Small Magellanic Clouds (SMC) are presented. The PL diagrams are plotted in
three planes: logP-K_S, logP-W_{JK}, and logP-W_I. Fourteen PL sequences are
distinguishable, and some of them consist of three closely spaced ridges. Each
of the sequences is fitted with a linear or quadratic function. The
similarities and differences between the PL relations in both galaxies are
discussed for four types of red giant variability: OGLE Small Amplitude Red
Giants (OSARGs), Miras and Semiregular Variables (SRVs), Long Secondary Periods
(LSPs) and ellipsoidal variables. We propose a new method of separating OSARGs
from non-variable stars and SRVs. The method employs the position in the
reddening-free PL diagrams and the characteristic period ratios of these
multiperiodic variables. The PL relations for the LMC OSARG are compared with
the calculated relations for RGB models along isochrones of relevant ages and
metallicities. We also compare measured periods and amplitudes of the OSARGs
with predictions based on the relations valid for less luminous solar-like
pulsators. Miras and SRVs seem to follow PL relation of the same slopes in the
LMC and SMC, while for LSP and ellipsoidal variables slopes in both galaxies
are different. The PL sequences defined by LSP variables and binary systems
overlap in the whole range of analyzed wavebands. We put forward new arguments
for the binary star scenario as an explanation of the LSP variability and
elaborate on it further. The measured pulsation to orbital period ratio implies
nearly constant ratio of the star radius to orbital distance, R/A=0.4, as we
find. Combined effect of tidal friction and mass loss enhanced by the low-mass
companion may explain why such a value is preferred.Comment: 25 pages, 8 figure
The Optical Gravitational Lensing Experiment. Small Amplitude Variable Red Giants in the Magellanic Clouds
We present analysis of the large sample of variable red giants from the Large
and Small Magellanic Clouds detected during the second phase of the Optical
Gravitational Lensing Experiment (OGLE-II) and supplemented with OGLE-III
photometry. Comparing pulsation properties of detected objects we find that
they constitute two groups with clearly distinct features. In this paper we
analyze in detail small amplitude variable red giants (about 15400 and 3000
objects in the LMC and SMC, respectively). The vast majority of these objects
are multi-periodic. At least 30% of them exhibit two modes closely spaced in
the power spectrum, what likely indicates non-radial oscillations. About 50%
exhibit additional so called Long Secondary Period.
To distinguish between AGB and RGB red giants we compare PL diagrams of
multi-periodic red giants located above and below the tip of the Red Giant
Branch (TRGB). The giants above the TRGB form four parallel ridges in the PL
diagram. Among much more numerous sample of giants below the TRGB we find
objects located on the low luminosity extensions of these ridges, but most of
the stars are located on the ridges slightly shifted in logP. We interpret the
former as the second ascent AGB red giants and the latter as the first ascent
RGB objects. Thus, we empirically show that the pulsating red giants fainter
than the TRGB are a mixture of RGB and AGB giants.
Finally, we compare the Petersen diagrams of the LMC, SMC and Galactic bulge
variable red giants and find that they are basically identical indicating that
the variable red giants in all these different stellar environments share
similar pulsation properties.Comment: 23 pages, 13 figures included in GIF forma
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