5,240 research outputs found
On the geometry of moduli spaces of coherent systems on algebraic curves
Let be an algebraic curve of genus . A coherent system on consists
of a pair , where is an algebraic vector bundle over of rank
and degree and is a subspace of dimension of the space of sections
of . The stability of the coherent system depends on a parameter .
We study the geometry of the moduli space of coherent systems for different
values of when and the variation of the moduli spaces when
we vary . As a consequence, for sufficiently large , we compute
the Picard groups and the first and second homotopy groups of the moduli spaces
of coherent systems in almost all cases, describe the moduli space for the case
explicitly, and give the Poincar\'e polynomials for the case .Comment: 38 pages; v3. Appendix and new references added; v4. minor
corrections, two added references; v5. final version, one typo corrected and
one reference delete
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
Isoscaling and the high Temperature limit
This study shows that isoscaling, usually studied in nuclear reactions, is a
phenomenon common to all cases of fair sampling. Exact expressions for the
yield ratio and approximate expressions for the isoscaling parameters
and are obtained and compared to experimental results. It is
concluded that nuclear isoscaling is bound to contain a component due to
sampling and, thus, a words of caution is issued to those interested in
extracting information about the nuclear equation of state from isoscaling.Comment: 7 pages, 1 figur
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
Magnetic and vibrational properties of high-entropy alloys
The magnetic properties of high-entropy alloys based on equimolar FeCoCrNi were investigated using vibrating sample magnetometry to determine their usefulness in high-temperature magnetic applications. Nuclear resonant inelastic x-ray scattering measurements were performed to evaluate the vibrational entropy of the ^(57)Fe atoms and to infer chemical order. The configurational and vibrational entropy of alloying are discussed as they apply to these high-entropy alloys
Critical wetting of a class of nonequilibrium interfaces: A mean-field picture
A self-consistent mean-field method is used to study critical wetting
transitions under nonequilibrium conditions by analyzing Kardar-Parisi-Zhang
(KPZ) interfaces in the presence of a bounding substrate. In the case of
positive KPZ nonlinearity a single (Gaussian) regime is found. On the contrary,
interfaces corresponding to negative nonlinearities lead to three different
regimes of critical behavior for the surface order-parameter: (i) a trivial
Gaussian regime, (ii) a weak-fluctuation regime with a trivially located
critical point and nontrivial exponents, and (iii) a highly non-trivial
strong-fluctuation regime, for which we provide a full solution by finding the
zeros of parabolic-cylinder functions. These analytical results are also
verified by solving numerically the self-consistent equation in each case.
Analogies with and differences from equilibrium critical wetting as well as
nonequilibrium complete wetting are also discussed.Comment: 11 pages, 2 figure
A model to assess customer alignment through customer experience concepts
Business and Information Technology Alignment (BITA) has been one of the main
concerns of IT and Business executives and directors due to its importance to
overall company performance, especially today in the age of digital
transformation. For BITA has been developed several models which in general has
focused in the implementation of alignment strategies for the internal
operation of the organizations and in the measurement of this internal
alignment, but, there is still a big gap in measurement models of the alignment
with the external environment of the organizations. In this paper is presented
the design and application of a maturity measurement model for BITA with the
customers, where the customers are actors of the external environment of the
companies. The proposed model involves evaluation criteria and business
practices which the companies ideally do for improve the relationship with
their customers.Comment: 12 pages, Preprint version, BIS 2019 International Workshops,
Seville, Spain, June 26 to 28, 2019, Revised Paper
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
Effects of chemical composition and B2 order on phonons in bcc FeâCo alloys
The phonon density of states (DOS) gives insight into interatomic forces and provides the vibrational entropy, making it a key thermodynamic function for understanding alloy phase transformations. Nuclear resonant inelastic x-ray scattering and inelastic neutron scattering were used to measure the chemical dependence of the DOS of bcc FeâCo alloys. For the equiatomic alloy, the A2âB2 (chemically disorderedâchemically ordered) phase transformation caused measurable changes in the phonon spectrum. The measured change in vibrational entropy upon ordering was â0.02±0.02âk_B/atom, suggesting that vibrational entropy results in a reduction in the orderâdisorder transition temperature by 60±60âK. The ConnollyâWilliams cluster inversion method was used to obtain interaction DOS (IDOS) curves that show how point and pair variables altered the phonon DOS of disordered bcc FeâCo alloys. These IDOS curves accurately captured the change in the phonon DOS and vibrational entropy of the B2 ordering transition
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