167 research outputs found
Two-gap superconductivity in MgB: clean or dirty?
A large number of experimental facts and theoretical arguments favor a
two-gap model for superconductivity in MgB. However, this model predicts
strong suppression of the critical temperature by interband impurity scattering
and, presumably, a strong correlation between the critical temperature and the
residual resistivity. No such correlation has been observed. We argue that this
fact can be understood if the band disparity of the electronic structure is
taken into account, not only in the superconducting state, but also in normal
transport
Effect of strain aging on physical and mechanical properties of austenitic chromium-nickel steel
The processes occurring in chromium-nickel steel 08Kh18N10T due to strain aging are studied. The tensile mechanical properties are determined. Relaxation tests and metallographic and x-ray diffraction analyses are performed. The internal energy state of the steel is estimated with the help of acoustic emissionyesBelgorod State Universit
Magnetic Field Effects on the Far-Infrared Absorption in Mn_12-acetate
We report the far-infrared spectra of the molecular nanomagnet Mn_12-acetate
(Mn_12) as a function of temperature (5-300 K) and magnetic field (0-17 T). The
large number of observed vibrational modes is related to the low symmetry of
the molecule, and they are grouped together in clusters. Analysis of the mode
character based on molecular dynamics simulations and model compound studies
shows that all vibrations are complex; motion from a majority of atoms in the
molecule contribute to most modes. Three features involving intramolecular
vibrations of the Mn_12 molecule centered at 284, 306 and 409 cm-1 show changes
with applied magnetic field. The structure near 284 cm displays the
largest deviation with field and is mainly intensity related. A comparison
between the temperature dependent absorption difference spectra, the gradual
low-temperature cluster framework distortion as assessed by neutron diffraction
data, and field dependent absorption difference spectra suggests that this mode
may involve Mn motion in the crown.Comment: 5 pages, 4 figures, PRB accepte
Multi-Gap Superconductivity in MgB2: Magneto-Raman Spectroscopy
Electronic Raman scattering studies on MgB2 single crystals as a function of
excitation and polarization have revealed three distinct superconducting
features: a clean gap below 37 cm-1 and two coherence peaks at 109 cm-1 and 78
cm-1 which we identify as the superconducting gaps in \pi- and \sigma-bands and
as the Leggett's collective mode arising from the fluctuation in the relative
phase between two superconducting condensates residing on corresponding bands.
The temperature and field dependencies of the superconducting features have
been established. A phononic Raman scattering study of the E2g boron stretching
mode anharmonicity and of superconductivity induced self-energy effects is
presented. We show that anharmonic two phonon decay is mainly responsible for
the unusually large linewidth of the E2g mode. We observe ~2.5% hardening of
the E2g phonon frequency upon cooling into the superconducting state and
estimate the electron-phonon coupling strength associated with this
renormalization.Comment: Invited review in a special issue "Superconductivity in MgB_2:
Physics and Applications
Multiband model for penetration depth in MgB2
The results of first principles calculations of the electronic structure and
the electron-phonon interaction in MgB2 are used to study theoretically the
temperature dependence and anisotropy of the magnetic field penetration depth.
The effects of impurity scattering are essential for a proper description of
the experimental results. We compare our results with experimental data and we
argue that the two-band model describes the data rather well.Comment: submitted to Phys. Rev.
Epidemiological Situation on Tick-Borne Borreliosis in the Russian Federation in 2021 and Forecast for 2022
3875 cases of tick-borne borreliosis (TBB) (2.65 per 100000 of population) were recorded in Russia in 2021. Compared to 2020, 61 out of 78 constituent entities experienced a decrease in the incidence rate in 2021. Over the past year, the largest number of cases was registered in the Central Federal District (CFD) – 1797 cases, which is 46.4 % of cases in Russia. Second in the rank by the number of cases of TBB comes the Siberian FD (SFD) – 616 cases (15.9 %), followed by Ural FD – 445 cases (11.5 %), the North-Western FD – 418 (10.8 %), and the Volga FD – 388 (10 %). 134 (3.5 %) and 60 (1.5 % of the total number of cases of TBB) cases were registered in the Far Eastern and Southern Federal Districts, respectively. The last place is occupied by the North Caucasus Federal District (NCFD), where 17 cases were registered, the share of which in the total structure of cases in Russia is 0.4 %. When assessing the long-term dynamics of TBB incidence, a significant trend towards a decrease in the intensity of the epidemic process has been revealed for the North-Western FD, UFD and VFD, as opposed to the CFD and Southern FD, where a significant upward trend was noted. For the Russian Federation on the whole, the Siberian FD, FEFD and NCFD the variation in the incidence rates within the confidence intervals of the long-term annual average values is most likely to be observed in the near future
Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors
Quasiclassic Uzadel equations for two-band superconductors in the dirty limit
with the account of both intraband and interband scattering by nonmagnetic
impurities are derived for any anisotropic Fermi surface. From these equations
the Ginzburg-Landau equations, and the critical temperature are obtained.
An equation for the upper critical field, which determines both the temperature
dependence of and the orientational dependence of
as a function of the angle between and the c-axis is
obtained. It is shown that the shape of the curve essentially
depends on the ratio of the intraband electron diffusivities and ,
and can be very different from the standard one-gap dirty limit theory. In
particular, the value can considerably exceed ,
which can have important consequences for applications of . A scaling
relation is proposed which enables one to obtain the angular dependence of
from the equation for at . It is shown
that, depending on the relation between and , the ratio of the upper
critical field for and can both increase and decrease as the temperature decreases. Implications
of the obtained results for are discussed
Review of the Epidemiological Situation on Ixodic Tick-Borne Borrelioses in the Russian Federation in 2013–2022 and Forecast for 2023
The aim of the review was to characterize the epidemiological situation on ixodic tick-borne borreliosis (ITBB) in the constituent entities of the Russian Federation in 2022, to forecast the development of the ITBB epidemic process in 2023 based on the analysis of its trends in the period of 2013–2022. In total, 7 264 cases of ITBB were registered in the Russian Federation in 2022, which is 1.9 times higher than the 2021-value. In all regions in 2022, after a sharp decrease in the level of registered morbidity during the “covid” phase of 2020–2021, there was an incidence raise to the pre-pandemic level. During 2013–2022, distinctive features of the trends in the development of the ITBB epidemic process in the territories of varying degree of epidemic hazard were retained. A downward tendency in the incidence rates was revealed in 14 out of 18 entities with a long-term average annual incidence (LTAA) of ITBB above 6.2 0 /0000, with the exception of Moscow and the Kaluga Region, where a further increase in the incidence of ITBB is possible. In 6 out of 16 regions with LTAA 2013–2022 between 3.31 and 6.2 0 /0000, an upward trend was observed, and in 8 – a decrease in morbidity. The trend towards an increase in morbidity was detected in 7 out of 21 entities with LTAA of ITBB from 1.21 to 3.3 0 /0000 and in 15 out of 26 regions with a low LTAA 2013–2022 (below 1.2 0 /0000). The analysis of trends in the development of the ITBB epidemic process against the trends in changes in the frequency of contacts of the population with ticks, as well as calculations of the probable incidence rate allowed us to conclude that the incidence rate of ITBB in 2023 will range within the confidence intervals of long-term average annual values for the majority of the federal districts, with the exception of the Central Federal District, where an increase in the number of cases of ixodic tick-borne borreliosis is likely to occur, provided that the existing volumes of preventive measures are maintained and abnormal climatic phenomena absent. Effective control of the epidemic situation on the ITBB is attainable through preservation, and for such areas as Moscow, the Moscow Region, Voronezh, Kaluga, Orel, Ryazan, Penza and Rostov Regions, the Republic of Buryatia – increase in the volume of preventive measures, strengthening zoological and entomological monitoring of activity and structure of natural foci. There is an obvious need to develop algorithms for molecular-genetic monitoring of pathogens circulating in natural foci, and to focus attention onto the problems of diagnosis and prophylaxis of tick-borne transmissible infections
Infrared optical properties of Pr2CuO4
The ab-plane reflectance of a Pr2CuO4 single crystal has been measured over a
wide frequency range at a variety of temperatures, and the optical properties
determined from a Kramers-Kronig analysis. Above ~ 250 K, the low frequency
conductivity increases quickly with temperature; the resistivity follows the
form e^(E_a/k_BT), where E_a ~ 0.17 eV is much less than the inferred optical
gap of ~ 1.2 eV. Transport measurements show that at low temperature the
resistivity deviates from activated behavior and follows the form
e^[(T_0/T)^1/4], indicating that the dc transport in this material is due to
variable-range hopping between localized states in the gap. The four
infrared-active Eu modes dominate the infrared optical properties. Below ~ 200
K, a striking new feature appears near the low-frequency Eu mode, and there is
additional new fine structure at high frequency. A normal coordinate analysis
has been performed and the detailed nature of the zone-center vibrations
determined. Only the low-frequency Eu mode has a significant Pr-Cu interaction.
Several possible mechanisms related to the antiferromagnetism in this material
are proposed to explain the sudden appearance of this and other new spectral
features at low temperature.Comment: 11 pages, 7 embedded EPS figures, REVTeX
The Tc amplification by quantum interference effects in diborides
The model of two (s and p) channel superconductivity known to be necessary to
explain the superconductivity in MgB2 has been applied to the Al1-xMgxB2
diborides by tuning x from MgB2 to AlMgB4. The evolution of the interband
coupling parameter (probing the strength of the interchannel pairing due to
quantum interference effects) and the two gaps in the s and p channel as a
function of x have been calculated. While in MgB2 the quantum interference
effects give an amplification of Tc by a factor of 1.5 in comparison with the
dominant intra s band single channel pairing, in AlMgB4 the amplification is
about 100, in comparison with the dominant intra p band single channel pairing.Comment: 7 pages, 6 figure
- …