10,775 research outputs found
Brauer group of moduli spaces of pairs
We show that the Brauer group of any moduli space of stable pairs with fixed
determinant over a curve is zero.Comment: 12 pages. Final version, accepted in Communications in Algebr
Adiabatic Electron-Phonon Interaction and High-Temperature Thermodynamics of A15 Compounds
Inelastic neutron scattering was used to measure the phonon densities of states of the A15 compounds V_3Si, V_3Ge, and V_3Co at temperatures from 10 to 1273 K. It was found that phonons in V_3Si and V_3Ge, which are superconducting at low temperatures, exhibit an anomalous stiffening with increasing temperature, whereas phonons in V_3Co have a normal softening behavior. First-principles calculations show that this anomalous increase in phonon frequencies at high temperatures originates with an adiabatic electron-phonon coupling mechanism. The anomaly is caused by the thermally induced broadening of sharp peaks in the electronic density of states of V_3Si and V_3Ge, which tends to decrease the electronic density at the Fermi level. These results show that the adiabatic electron-phonon coupling can influence the phonon thermodynamics at temperatures exceeding 1000 K
Investigation of HNCO isomers formation in ice mantles by UV and thermal processing: an experimental approach
Current gas phase models do not account for the abundances of HNCO isomers
detected in various environments, suggesting a formation in icy grain mantles.
We attempted to study a formation channel of HNCO and its possible isomers by
vacuum-UV photoprocessing of interstellar ice analogues containing HO,
NH, CO, HCN, CHOH, CH, and N followed by warm-up, under
astrophysically relevant conditions. Only the HO:NH:CO and HO:HCN
ice mixtures led to the production of HNCO species. The possible isomerization
of HNCO to its higher energy tautomers following irradiation or due to ice
warm-up has been scrutinized. The photochemistry and thermal chemistry of
HO:NH:CO and HO:HCN ices was simulated using the Interstellar
Astrochemistry Chamber (ISAC), a state-of-the-art ultra-high-vacuum setup. The
ice was monitored in situ by Fourier transform mid-infrared spectroscopy in
transmittance. A quadrupole mass spectrometer (QMS) detected the desorption of
the molecules in the gas phase. UV-photoprocessing of
HO:NH:CO/HO:HCN ices lead to the formation of OCN as main
product in the solid state and a minor amount of HNCO. The second isomer HOCN
has been tentatively identified. Despite its low efficiency, the formation of
HNCO and the HOCN isomers by UV-photoprocessing of realistic simulated ice
mantles, might explain the observed abundances of these species in PDRs, hot
cores, and dark clouds
The very faint hard state of the persistent neutron star X-ray binary SLX 1737-282 near the Galactic centre
We report on a detailed study of the spectral and temporal properties of the
neutron star low mass X-ray binary SLX 1737-282, which is located only ~1degr
away from Sgr A. The system is expected to have a short orbital period, even
within the ultra-compact regime, given its persistent nature at low X-ray
luminosities and the long duration thermonuclear burst that it has displayed.
We have analysed a Suzaku (18 ks) observation and an XMM-Newton (39 ks)
observation taken 7 years apart. We infer (0.5-10 keV) X-ray luminosities in
the range 3-6 x10^35erg s-1, in agreement with previous findings. The spectra
are well described by a relatively cool (kTbb = 0.5 keV) black body component
plus a Comptonized emission component with {\Gamma} ~1.5-1.7. These values are
consistent with the source being in a faint hard state, as confirmed by the ~
20 per cent fractional root mean square amplitude of the fast variability (0.1
- 7 Hz) inferred from the XMM-Newton data. The electron temperature of the
corona is >7 keV for the Suzaku observation, but it is measured to be as low as
~2 keV in the XMM-Newton data at higher flux. The latter is significantly lower
than expected for systems in the hard state. We searched for X-ray pulsations
and imposed an upper limit to their semi-amplitude of 2 per cent (0.001 - 7
Hz). Finally, we investigated the origin of the low frequency variability
emission present in the XMM-Newton data and ruled out an absorption dip origin.
This constraint the orbital inclination of the system to 65 degr unless the
orbital period is longer than 11 hr (i.e. the length of the XMM-Newton
observation).Comment: 7 pages, 4 figures, 1 table. Accepted for publication in MNRA
Electron-phonon interactions and high-temperature thermodynamics of vanadium and its alloys
Inelastic neutron scattering was used to measure the phonon densities of states (DOSs) for pure V and solid solutions of V with 6 to 7at% of Co, Nb, and Pt, at temperatures from 10 K to 1323 K. Ancillary measurements of heat capacity and thermal expansion are reported on V and V-7at%Co and used to help identify the different sources of entropy. Pure V exhibits an anomalous anharmonic stiffening of phonons with increasing temperature. This anharmonicity is suppressed by Co and Pt, but not by isoelectronic Nb solutes. The changes in phonon frequency with alloying and with temperature both correlate to the decrease in electron density of states (DOS) at the Fermi level as calculated using density functional theory. The effects of both temperature and alloying can be understood in terms of an adiabatic electron-phonon interaction (EPI), which broadens sharp features in the electron DOS. These results show that the adiabatic EPI can influence the phonon thermodynamics at temperatures exceeding 1000 K, and that thermal trends of phonons may help assess the strength of the EPI
Langevin equations for reaction-diffusion processes
For reaction-diffusion processes with at most bimolecular reactants, we
derive well-behaved, numerically tractable, exact Langevin equations that
govern a stochastic variable related to the response field in field theory.
Using duality relations, we show how the particle number and other quantities
of interest can be computed. Our work clarifies long-standing conceptual issues
encountered in field-theoretical approaches and paves the way for systematic
numerical and theoretical analyses of reaction-diffusion problems.Comment: 5 pages + 6 pages supplemental materia
Sharp values for the constants in the polynomial Bohnenblust-Hille inequality
In this paper we prove that the complex polynomial Bohnenblust-Hille constant
for -homogeneous polynomials in is exactly
. We also give the exact value of the real polynomial
Bohnenblust-Hille constant for -homogeneous polynomials in .
Finally, we provide lower estimates for the real polynomial Bohnenblust-Hille
constant for polynomials in of higher degrees.Comment: 16 page
Nonharmonic phonons in MgB_2 at elevated temperatures
Inelastic neutron scattering was used to measure phonon spectra in MgB_2 and Mg_(0.75)Al_(0.25)B_2 from 7 to 750 K to investigate anharmonicity and adiabatic electron-phonon coupling. First-principles calculations of phonons with a linear response method were performed at multiple unit cell volumes, and the Helmholtz free energy was minimized to obtain the lattice parameters and phonon dynamics at elevated temperature in the quasiharmonic approximation. Most of the temperature dependence of the phonon density of states could be understood with the quasiharmonic approximation, although there was also significant thermal broadening of the phonon spectra. In comparison to Mg_(0.75)Al_(0.25)B_2, in the energy range of 60 to 80 meV the experimental phonon spectra from MgB_2 showed a nonmonotonic change with temperature around 500 K. This may originate from a change with temperature of the adiabatic electron-phonon coupling
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