286 research outputs found
Electron correlations, spontaneous magnetization and momentum density in quantum dots
The magnetization of quantum dots is discussed in terms of a relatively
simple but exactly solvable model Hamiltonian. The model predicts oscillations
in spin polarization as a function of dot radius for a fixed electron density.
These oscillations in magnetization are shown to yield distinct signature in
the momentum density of the electron gas, suggesting the usefulness of momentum
resolved spectroscopies for investigating the magnetization of dot systems. We
also present variational quantum Monte Carlo calculations on a square dot
containing 12 electrons in order to gain insight into correlation effects on
the interactions between like and unlike spins in a quantum dot.Comment: 6 pages, 4 figure
Nesting properties and anomalous band effect in MgB2
First principle FLAPW band calculations of the new superconductor MgB2 were
performed and the polarization function P12(Q) between the two p-bands mainly
formed of boron pz-orbital was calculated. We found that P12(Q) is
substantially enhanced around Q=(0,0,p/c), which supports the two-band
mechanism of superconductivity for MgB2. P12(Q) peaks at Qz ~ 0.3(2p/c) and Qz
\~ 0.5(2p/c). These two peaks are related to the nesting of these Fermi
surfaces, but significantly deviates from the position expected from the
simplest tight-binding bands for the p-bands. From the calculations for
different lattice parameters, we have found significant dependences on the
isotopic species of B and on the pressure effect of the polarization function
in accordance with the respective changes of Tc in the above-mentioned
framework.Comment: 15 pages, 7 graphs. to be published in J. Phys. Soc. Jpn. 70_, No.
Design definition study of a NASA/Navy lift/cruise fan technology V/STOL airplane: Risk assessment addendum to the final report
An assessment of risk, in terms of delivery delays, cost overrun, and performance achievement, associated with the V/STOL technology airplane is presented. The risk is discussed in terms of weight, structure, aerodynamics, propulsion, mechanical drive, and flight controls. The analysis ensures that risks associated with the design and development of the airplane will be eliminated in the course of the program and a useful technology airplane that meets the predicted cost, schedule, and performance can be produced
Electronic structure, magnetism and superconductivity of MgCNi
The electronic structure of the newly discovered superconducting perovskite
MgCNi is calculated using the LMTO and KKR methods. The states near the
Fermi energy are found to be dominated by Ni-d. The Stoner factor is low while
the electron-phonon coupling constant is estimated to be about 0.7, which
suggests that the material is a conventional type of superconductor where T
is not affected by magnetic interactions. However, the proximity of the Fermi
energy to a large peak in the density of states in conjunction with the
reported non-stoichiometry of the compound, has consequences for the stability
of the results.Comment: 3 pages, 4 figure
A critical assessment of the Self-Interaction Corrected Local Density Functional method and its algorithmic implementation
We calculate the electronic structure of several atoms and small molecules by
direct minimization of the Self-Interaction Corrected Local Density
Approximation (SIC-LDA) functional. To do this we first derive an expression
for the gradient of this functional under the constraint that the orbitals be
orthogonal and show that previously given expressions do not correctly
incorporate this constraint. In our atomic calculations the SIC-LDA yields
total energies, ionization energies and charge densities that are superior to
results obtained with the Local Density Approximation (LDA). However, for
molecules SIC-LDA gives bond lengths and reaction energies that are inferior to
those obtained from LDA. The nonlocal BLYP functional, which we include as a
representative GGA functional, outperforms both LDA and SIC-LDA for all ground
state properties we considered.Comment: 14 pages, 5 figure
Nonlinear effects in E Jahn-Teller model: Variational approach with excited phonon states and mode correlations
Interplay of nonlinear and quantum effects in the ground state of the
E Jahn-Teller model was investigated by the {\it variational
approach and exact numerical simulations}. They result in the recognition of
(i) importance of the admixture of {\it the first excited state of the
displaced harmonic oscillator} of the symmetric phonon mode in the ground state
of the system in the selftrapping-dominated regime; (ii) existence of {\it the
region of localized -undisplaced oscillator states} in the
tunneling-dominated regime. The effect (i) occurs owing to significant decrease
of the ground state energy on account of the overlapping contribution of the
symmetric phonon mode between the states of the same parity. This contribution
considerably improves variational results especially in the
selftrapping-dominated regime. Close to the Ee limit, the nonlinear
effects of {\it two-mode correlations} turn to be effective due to the
rotational symmetry of this case. In the tunneling-dominated regime the phonon
wave functions behave like the strongly localized harmonic oscillator ground
state and the effect (i) looses its significance.Comment: 28 pages,6 figure
Nonradiative Electronic Deexcitation Time Scales in Metal Clusters
The life-times due to Auger-electron emission for a hole on a deep electronic
shell of neutral and charged sodium clusters are studied for different sizes.
We consider spherical clusters and calculate the Auger-transition probabilities
using the energy levels and wave functions calculated in the
Local-Density-Approximation (LDA).
We obtain that Auger emission processes are energetically not allowed for
neutral and positively charged sodium clusters. In general, the Auger
probabilities in small Na clusters are remarkably different from the
atomic ones and exhibit a rich size dependence.
The Auger decay times of most of the cluster sizes studied are orders of
magnitude larger than in atoms and might be comparable with typical
fragmentation times.Comment: 11 pages, 4 figures. Accepted for publication in Phys. Rev.
A complex storm system in Saturn’s north polar atmosphere in 2018
Producción CientíficaSaturn’s convective storms usually fall in two categories. One consists of mid-sized storms ∼2,000 km wide, appearing as irregular bright cloud systems that evolve rapidly, on scales of a few days. The other includes the Great White Spots, planetary-scale giant storms ten times larger than the mid-sized ones, which disturb a full latitude band, enduring several months, and have been observed only seven times since 1876. Here we report a new intermediate type, observed in 2018 in the north polar region. Four large storms with east–west lengths ∼4,000–8,000 km (the first one lasting longer than 200 days) formed sequentially in close latitudes, experiencing mutual encounters and leading to zonal disturbances affecting a full latitude band ∼8,000 km wide, during at least eight months. Dynamical simulations indicate that each storm required energies around ten times larger than mid-sized storms but ∼100 times smaller than those necessary for a Great White Spot. This event occurred at about the same latitude and season as the Great White Spot in 1960, in close correspondence with the cycle of approximately 60 years hypothesized for equatorial Great White Spots.Ministerio de Economía, Industria y Competitividad - Fondo Europeo de Desarrollo Regional (project AYA2015-65041-P)Gobierno Vasco (project IT-366-19
Electronic structure of undoped and potassium doped coronene investigated by electron energy-loss spectroscopy
We performed electron energy-loss spectroscopy studies in transmission in
order to obtain insight into the electronic properties of potassium
intercalated coronene, a recently discovered superconductor with a rather high
transition temperature of about 15\,K. A comparison of the loss function of
undoped and potassium intercalated coronene shows the appearance of several new
peaks in the optical gap upon potassium addition. Furthermore, our core level
excitation data clearly signal filling of the conduction bands with electrons.Comment: 15 pages, 5 figures. arXiv admin note: substantial text overlap with
arXiv:1102.328
Mott transitions in correlated electron systems with orbital degrees of freedom
Mott metal-insulator transitions in an M-fold orbitally degenerate Hubbard
model are studied by means of a generalization of the linearized dynamical
mean-field theory. The method allows for an efficient and reliable
determination of the critical interaction U_c for any integer filling n and
different M at zero temperature. For half-filling a linear dependence of U_c on
M is found. Inclusion of the (full) Hund's rule exchange J results in a strong
reduction of U_c. The transition turns out to change qualitatively from
continuous for J=0 to discontinuous for any finite J
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