1,743 research outputs found
Jet engine powers large, high-temperature wind tunnel
Wind tunnel for large component testing uses a jet engine with afterburner to provide high temperatures /1200 degrees to 2000 degrees F/ and controlled high velocity gas. This economical wind tunnel can accommodate parts ten feet by ten feet or larger, and is a useful technique for qualitative information
Structural models for the Si(553)-Au atomic chain reconstruction
Recent photoemission experiments on the Si(553)-Au reconstruction show a
one-dimensional band with a peculiar ~1/4 filling. This band could provide an
opportunity for observing large spin-charge separation if electron-electron
interactions could be increased. To this end, it is necessary to understand in
detail the origin of this surface band. A first step is the determination of
the structure of the reconstruction. We present here a study of several
structural models using first-principles density functional calculations. Our
models are based on a plausible analogy with the similar and better known
Si(557)-Au surface, and compared against the sole structure proposed to date
for the Si(553)-Au system [Crain JN et al., 2004 Phys. Rev. B 69 125401 ].
Results for the energetics and the band structures are given. Lines for the
future investigation are also sketched
Asymptotic tunneling conductance in Luttinger liquids
Conductance through weak constrictions in Luttinger liquids is shown to
vanish with frequency as , where
is a dimensionless parameter characterizing the Luttinger liquid phase, and
and are nonuniversal constants. The first term arises from the ^^
Coulomb blockade' effect and dominates for , whereas the second
results from eliminating high-energy modes and dominates for .Comment: Latex file + one appended postcript figur
Reconstruction Mechanism of FCC Transition-Metal (001) Surfaces
The reconstruction mechanism of (001) fcc transition metal surfaces is
investigated using a full-potential all-electron electronic structure method
within density-functional theory. Total-energy supercell calculations confirm
the experimental finding that a close-packed quasi-hexagonal overlayer
reconstruction is possible for the late 5-metals Ir, Pt, and Au, while it is
disfavoured in the isovalent 4 metals (Rh, Pd, Ag). The reconstructive
behaviour is driven by the tensile surface stress of the unreconstructed
surfaces; the stress is significantly larger in the 5 metals than in 4
ones, and only in the former case it overcomes the substrate resistance to the
required geometric rearrangement. It is shown that the surface stress for these
systems is due to charge depletion from the surface layer, and that the
cause of the 4th-to-5th row stress difference is the importance of relativistic
effects in the 5 series.Comment: RevTeX 3.0, 12 pages, 1 PostScript figure available upon request] 23
May 199
Majorana: from atomic and molecular, to nuclear physics
In the centennial of Ettore Majorana's birth (1906-1938?), we re-examine some
aspects of his fundamental scientific production in atomic and molecular
physics, including a not well known short communication. There, Majorana
critically discusses Fermi's solution of the celebrated Thomas-Fermi equation
for electron screening in atoms and positive ions. We argue that some of
Majorana's seminal contributions in molecular physics already prelude to the
idea of exchange interactions (or Heisenberg-Majorana forces) in his later
workson theoretical nuclear physics. In all his papers, he tended to emphasize
the symmetries at the basis of a physical problem, as well as the limitations,
rather than the advantages, of the approximations of the method employed.Comment: to appear in Found. Phy
Halogen Bonding with Phosphine: Evidence for Mulliken Inner Complexes and the Importance of Relaxation Energy
Intermolecular halogen bonding in complexes of phosphine and dihalogens has been
theoretically investigated using explicitly correlated coupled cluster methods and symmetry
adapted perturbation theory. The complexes H3P· · · ClF, H3P· · · BrF and H3P· · ·IF
are demonstrated to possess unusually strong interactions that are accompanied by an
increase in the induction component of the interaction energy and significant elongation
of the X–Y halogen distance on complex formation. The combination of these factors
is indicative of Mulliken inner complexes and criteria for identifying this classification
are further developed. The importance of choosing an electronic structure method that
describes both dispersion and longer range interactions is demonstrated, along with the
need to account for the change in geometry on complexation formation via relaxation
energy and overall stabilisation energies
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
Hydrogen molecule in a magnetic field: The lowest states of the Pi manifold and the global ground state of the parallel configuration
The electronic structure of the hydrogen molecule in a magnetic field is
investigated for parallel internuclear and magnetic field axes. The lowest
states of the manifold are studied for spin singlet and triplet as well as gerade and ungerade parity for a broad range of field
strengths For both states with gerade parity we
observe a monotonous decrease in the dissociation energy with increasing field
strength up to and metastable states with respect to the
dissociation into two H atoms occur for a certain range of field strengths. For
both states with ungerade parity we observe a strong increase in the
dissociation energy with increasing field strength above some critical field
strength . As a major result we determine the transition field strengths
for the crossings among the lowest , and
states. The global ground state for is the strongly
bound state. The crossings of the with the
and state occur at and , respectively. The transition between the and
state occurs at Therefore, the global ground state of the
hydrogen molecule for the parallel configuration is the unbound
state for The ground state for is the strongly bound state. This result is of great
relevance to the chemistry in the atmospheres of magnetic white dwarfs and
neutron stars.Comment: submitted to Physical Review
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