165 research outputs found
Shock waves in superconducting cosmic strings: growth of current
Intrinsic equations of motion of superconducting cosmic string may admit
solutions in the shock-wave form that implies discontinuity of the current term
\chi. The hypersurface of discontinuity propagates at finite velocity
determined by finite increment \Delta \chi =\chi_+ -\chi_-. The current
increases \chi_+>\chi_- in stable shocks but transition between spacelike (\chi
>0) and timelike (\chi<0) currents is impossible.Comment: 13 pages, 3 figure
Methods and models of decision making in information systems in quality assessment of software systems
Conventional superconductivity at 203 K at high pressures
A superconductor is a material that can conduct electricity with no
resistance below its critical temperature (Tc). The highest Tc that has been
achieved in cuprates1 is 133 K at ambient pressure2 and 164 K at high
pressures3. As the nature of superconductivity in these materials has still not
been explained, the prospects for a higher Tc are not clear. In contrast, the
Bardeen-Cooper-Schrieffer (BCS) theory gives a guide for achieving high Tc and
does not put bounds on Tc, all that is needed is a favorable combination of
high frequency phonons, strong electron-phonon coupling, and a high density of
states. These conditions can be fulfilled for metallic hydrogen and covalent
compounds dominated by hydrogen4,5. Numerous calculations support this idea and
predict Tc of 50-235 K for many hydrides6 but only moderate Tc=17 K has been
observed experimentally7. Here we studied sulfur hydride8 where a Tc~80 K was
predicted9. We found that it transforms to a metal at pressure ~90 GPa. With
cooling superconductivity was found deduced from a sharp drop of the
resistivity to zero and a decrease of Tc with magnetic field. The pronounce
isotope shift of Tc in D2S is evidence of an electron-phonon mechanism of
superconductivity that is consistent with the BCS scenario. The
superconductivity has been confirmed by magnetic susceptibility measurements
with Tc=203K. The high Tc superconductivity most likely is due to H3S which is
formed from H2S under its decomposition under pressure. Even higher Tc, room
temperature superconductivity, can be expected in other hydrogen-based
materials since hydrogen atoms provide the high frequency phonon modes as well
as the strong electron-phonon coupling
Quasi Two-dimensional Vortex Matter in ThH Superhydride
A comprehensive study of the vortex phases and vortex dynamics is presented
for a recently discovered high-temperature superconductor ThH with
= 153 K at 170 GPa. The obtained results strongly suggest a
quasi two-dimensional (2D) character of the vortex glass phase transition in
ThH. The activation energy yields a logarithmic dependence
ln() on magnetic field in a low field
region and a power law dependence ~ in a
high field region, signaling a crossover from 2D regime to 3D collective
pinning regime, respectively. Additionally, a pinning force field dependence
showcases dominance of surface-type pinning in the vicinity of
. Thermal activation energy (), derived within
thermally activated flux flow (TAFF) theory, takes very high values above
210 K together with the Ginzburg number = 0.039 -
0.085, which is lower only than those of BiSrCaCuO cuprates and 10-3-8 family
of iron based superconductor. This indicates the enormous role of thermal
fluctuations in the dynamics of the vortex lattice of superhydrides, the
physics of which is similar to the physics of unconventional high-temperature
superconductors
Melting Point and Lattice Parameter Shifts in Supported Metal Nanoclusters
The dependencies of the melting point and the lattice parameter of supported
metal nanoclusters as functions of clusters height are theoretically
investigated in the framework of the uniform approach. The vacancy mechanism
describing the melting point and the lattice parameter shifts in nanoclusters
with decrease of their size is proposed. It is shown that under the high vacuum
conditions (p<10^-7 torr) the essential role in clusters melting point and
lattice parameter shifts is played by the van der Waals forces of
cluster-substrate interation. The proposed model satisfactorily accounts for
the experimental data.Comment: 6 pages, 3 figures, 1 tabl
Off-shell scattering amplitudes in the double-logarithmic approximation
When scattering amplitudes are calculated in the double-logarithmic
approximation, it is possible to relate the double-logarithmic on-shell and
off-shell amplitudes. Explicit relations are obtained for scattering amplitudes
in QED, QCD, and the ElectroWeak Standard Model. The off-shell amplitudes are
considered in the hard and the Regge kinematic limits. We compare our results
in both the Feynman and Coulomb gauges.Comment: 15 pages, 3 figures; RevTeX
Synthesis of technetium hydride TcH at 27 GPa
In this work, we synthesize and investigate lower technetium hydrides at
pressures up to 45 GPa using the synchrotron X-ray diffraction, reflectance
spectroscopy, and ab initio calculations. In the Tc-H system, the hydrogen
content in TcH phases increases when the pressure rises, and at 27 GPa we
found a new hexagonal (hcp) nonstoichiometric hydride TcH. The
formation of technetium hydrides is also confirmed by the emergence of a new
reflective band at 450-600 nm in the reflectance spectra of TcH samples
synthesized at 45 GPa. On the basis of the theoretical analysis, we proposed
crystal structures for the TcH (TcH) and
TcH (TcH) phases previously obtained at 1-2 GPa. The
calculations of the electron-phonon interaction show that technetium hydrides
TcH do not possess superconducting properties due to the low
electron-phonon interaction parameter ()
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