461 research outputs found
Thermodynamics of Two - Band Superconductors: The Case of MgB
Thermodynamic properties of the multiband superconductor MgB have often
been described using a simple sum of the standard BCS expressions corresponding
to - and -bands. Although, it is \textit{a priori} not clear if
this approach is working always adequately, in particular in cases of strong
interband scattering. Here we compare the often used approach of a sum of two
independent bands using BCS-like -model expressions for the specific
heat, entropy and free energy to the solution of the full Eliashberg equations.
The superconducting energy gaps, the free energy, the entropy and the heat
capacity for varying interband scattering rates are calculated within the
framework of two-band Eliashberg theory. We obtain good agreement between the
phenomenological two-band -model with the Eliashberg results, which
delivers for the first time the theoretical verification to use the
-model as a useful tool for a reliable analysis of heat capacity data.
For the thermodynamic potential and the entropy we demonstrate that only the
sum over the contributions of the two bands has physical meaning.Comment: 27 pages, 10 figures, 1 table, submitted to Phys. Rev.
Frustrated spin model as a hard-sphere liquid
We show that one-dimensional topological objects (kinks) are natural degrees
of freedom for an antiferromagnetic Ising model on a triangular lattice. Its
ground states and the coexistence of spin ordering with an extensive
zero-temperature entropy can be easily understood in terms of kinks forming a
hard-sphere liquid. Using this picture we explain effects of quantum spin
dynamics on that frustrated model, which we also study numerically.Comment: 5 pages, 3 figure
The turbulent spectrum created by non-Abelian plasma instabilities
Recent numerical work on the fate of plasma instabilities in weakly-coupled
non-Abelian gauge theory has shown the development of a cascade of energy from
long to short wavelengths. This cascade has a steady-state spectrum, analogous
to the Kolmogorov spectrum for turbulence in hydrodynamics or for energy
cascades in other systems. In this paper, we theoretically analyze processes
responsible for this cascade and find a steady-state spectrum f_k ~ k^-2, where
f_k is the phase-space density of particles with momentum k. The exponent -2 is
consistent with results from numerical simulations. We also discuss
implications of the emerging picture of instability development on the
"bottom-up" thermalization scenario for (extremely high energy) heavy ion
collisions, emphasizing fundamental questions that remain to be answered.Comment: 17 pages, 5 figure
Observation of anisotropic effect of antiferromagnetic ordering on the superconducting gap in ErNi2B2C
The point-contact (PC) spectra of the Andreev reflection dV/dI curves of the
superconducting rare-earth nickel borocarbide ErNi2B2C (Tc=11 K) have been
analyzed in the "one-gap" and "two-gap" approximations using the generalized
Blonder-Tinkham-Klapwijk (GBTK) model and the Beloborod'ko (BB) model allowing
for the pair-breaking effect of magnetic impurities. Experimental and
calculated curves have been compared not only in shape, but in magnitude as
well, which provide more reliable data for determining the temperature
dependence of the energy gap (or superconducting order parameter) \Delta(T).
The anisotropic effect of antiferromagnetic ordering at T_N =6 K on the
superconducting gap/order parameter has been determined: as the temperature is
lowered, \Delta(T) decreases by 25% in the c-direction and only by 4% in the
ab-plane. It is found that the pair-breaking parameter increases in the
vicinity of the magnetic transitions, the increase being more pronounced in the
c-direction. The efficiency of the models was tested for providing \Delta(T)
data for ErNi2B2C from Andreev reflection spectra.Comment: 16 two column pages, 20 figs., will be published in Fiz. Nizk. Temp.
N10, 2010; V2: added - "Acknowledgement" & "Note added in proof
Localized states and interaction induced delocalization in Bose gases with quenched disorder
Very diluted Bose gas placed into a disordered environment falls into a
fragmented localized state. At some critical density the repulsion between
particles overcomes the disorder. The gas transits into a coherent superfluid
state. In this article the geometrical and energetic characteristics of the
localized state at zero temperature and the critical density at which the
quantum phase transition from the localized to the superfluid state proceeds
are found.Comment: 17 pages, 5 figur
Berry phase of magnons in textured ferromagnets
We study the energy spectrum of magnons in a ferromagnet with topologically
nontrivial magnetization profile. In the case of inhomogeneous magnetization
corresponding to a metastable state of ferromagnet, the spin-wave equation of
motion acquires a gauge potential leading to a Berry phase for the magnons
propagating along a closed contour. The effect of magnetic anisotropy is
crucial for the Berry phase: we show that the anisotropy suppresses its
magnitude, which makes the Berry phase observable in some cases, similar to the
Aharonov-Bohm effect for electrons. For example, it can be observed in the
interference of spin waves propagating in mesoscopic rings. We discuss the
effect of domain walls on the interference in ferromagnetic rings, and propose
some experiments with a certain geometry of magnetization. We also show that
the nonvanishing average topological field acts on the magnons like a uniform
magnetic field on electrons. It leads to the quantization of the magnon
spectrum in the topological field.Comment: 8 pages, 5 figure
Origin of elemental carbon in snow from western Siberia and northwestern European Russia during winter-spring 2014, 2015 and 2016
Short-lived climate forcers have been proven important both for the climate and human health. In particular, black carbon (BC) is an important climate forcer both as an aerosol and when deposited on snow and ice surface because of its strong light absorption. This paper presents measurements of elemental carbon (EC; a measurement-based definition of BC) in snow collected from western Siberia and northwestern European Russia during 2014, 2015 and 2016. The Russian Arctic is of great interest to the scientific community due to the large uncertainty of emission sources there. We have determined the major contributing sources of BC in snow in western Siberia and northwestern European Russia using a Lagrangian atmospheric transport model. For the first time, we use a recently developed feature that calculates deposition in backward (so-called retroplume) simulations allowing estimation of the specific locations of sources that contribute to the deposited mass
Geochemical groundwater peculiarities of Paleogene sediments in S-E Western Siberia artesian basin
The geochemical peculiarities of groundwater in Paleogene deposits in southeastern part of Western Siberia artesian basin are considered in the paper. Landscape, climate, geostructural and hydrogeological conditions define the water composition and quality peculiarities in this region. It has been established that ion-saline composition, mineralization and water quality changes arre governed by the horizontal zonal distribution. Groundwater of taiga landscapes generally is in equilibrium with kaolinite and quartz, mainly involving Ca- and Mg-montmorillonite, illite, carbonate minerals, sometimes barite. Groundwater in woodland grass and grassland, together with previously mentioned minerals, is usually in equilibrium with barite, colestine, and particularly, fluorite and gypsum. As a result, all relevant elements are removed from the groundwater and their accumulation level is restricted
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