5,058 research outputs found
Functionalization of BN Honeycomb structure by Adsorption and Substitution of Foreign atoms
We carried out first-principles calculations within Density Functional Theory
to investigate the structural, electronic and magnetic properties of
boron-nitride (BN) honeycomb structure functionalized by adatom adsorption, as
well as by the substitution of foreign atoms for B and N atoms. For periodic
high density coverage, most of transition metal atoms and some of group
3A, 4A, and 6A elements are adsorbed with significant binding energy and modify
the electronic structure of bare BN monolayer. While bare BN monolayer is
nonmagnetic, wide band gap semiconductor, at high coverage of specific adatoms
it can achieve magnetic metallic, even half-metallic ground states. At low
coverage, the bands associated with adsorbed atoms are flat and the band
structure of parent BN is not affected significantly. Therefore, adatoms and
substitution of foreign atoms at low coverage are taken to be the
representative of impurity atoms yielding localized states in the band gap and
resonance states in the band continua. Notably, the substitution of C for B and
N yield donor and acceptor like magnetic states in the band gap. Localized
impurity states occurring in the gap give rise to interesting properties for
electronic and optical application of the single layer BN honeycomb structure.Comment: 10 pages, 6 figures, 4 table
Photoacoustic wave propagating from normal into superconductive phases in Pb single crystals
Photoacoustic (PA) wave has been examined in a superconductor of the first
kind, Pb single crystal. The PA wave is induced by optical excitation of
electronic state and propagates from normal into superconductive phases below
T. It is clearly shown by wavelet analysis that the measured PA wave
includes two different components. The high-frequency component is
MHz-ultrasonic and the relative low-frequency one is induced by thermal wave.
The latter is observed in a similar manner irrespective of T. On the
other hand, the MHz-frequency component is obviously enhanced below T. The behavior is reproduced by the change of attenuation of longitudinal
ultrasonic wave and is consistent with BCS theory.Comment: 5 pages, 5 figures (fig.3 is colored), RevTeX4; the text is modifie
Hopping and clustering of oxygen vacancies in SrTiO3 by anelastic relaxation
The complex elastic compliance s11(w,T) of SrTiO3-d has been measured as a
function of the O deficiency d < 0.01. The two main relaxation peaks in the
absorption are identified with hopping of isolated O vacancies over a barrier
of 0.60 eV and reorientation of pairs of vacancies involving a barrier of 1 eV.
The pair binding energy is ~0.2 eV and indications for additional clustering,
possibly into chains, is found already at d ~0.004. The anistropic component of
the elastic dipole of an O vacancy is Deltalambda = 0.026.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Fast mode of rotating atoms in one-dimensional lattice rings
We study the rotation of atoms in one-dimensional lattice rings. In
particular, the "fast mode", where the ground state atoms rotate faster than
the stirring rotating the atoms, is studied both analytically and numerically.
The conditions for the transition to the fast mode are found to be very
different from that in continuum rings. We argue that these transition
frequencies remain unchanged for bosonic condensates described in a mean field.
We show that Fermionic interaction and filling factor have a significant effect
on the transition to the fast mode, and Pauli principle may suppress it
altogether.Comment: 4 pages, 5 figure
Current-spin coupling for ferromagnetic domain walls in fine wires
The coupling between a current and a domain wall is examined. In the presence
of a finite current and the absence of a potential which breaks the
translational symmetry, there is a perfect transfer of angular momentum from
the conduction electrons to the wall. As a result, the ground state is in
uniform motion. This remains the case when relaxation is accounted for. This is
described by, appropriately modified, Landau-Lifshitz-Gilbert equations.Comment: 4 pqges, no figure
New Ideas on SUSY Searches at Future Linear Colliders
Several results obtained within the SUSY group of the ECFA/DESY linear
collider study are presented: (i) a possibility to determine tan beta and the
trilinear couplings A_f via polarisation in sfermion decays, (ii) the impact of
complex MSSM parameters on the third generation sfermion decays, (iii)
determination of CP violation in the complex MSSM via T-odd asymmetries in
neutralino production and decay, and (iv) an analysis of the chargino and
neutralino mass parameters at one-loop level.Comment: 3 pages, LaTeX, 5 eps figures; talk presented by S. Hesselbach at the
International Europhysics Conference on High Energy Physics (HEP 2003), 17-23
July 2003, Aachen, German
Report of the sensor cooler technology panel
The Sensor Cooler Technology Panel identified three major areas in which technology development must be supported in order to meet the system performance requirements for the Astrotech 21 mission set science objectives. They are: long life vibration free refrigerators; mechanical refrigeration for 2 K to 5 K; and flight testing of emerging prototype refrigerators. A development strategy and schedule were recommended for each of the three areas
Computational study of boron nitride nanotube synthesis: how catalyst morphology stabilizes the boron nitride bond
In an attempt to understand why catalytic methods for the growth of boron
nitride nanotubes work much worse than for their carbon counterparts, we use
first-principles calculations to study the energetics of elemental reactions
forming N2, B2 and BN molecules on an iron catalyst. We observe that in the
case of these small molecules, the catalytic activity is hindered by the
formation of B2 on the iron surface. We also observe that the local morphology
of a step edge present in our nanoparticle model stabilizes the boron nitride
molecule with respect to B2 due to the ability of the step edge to offer sites
with different coordination simultaneously for nitrogen and boron. Our results
emphasize the importance of atomic steps for a high yield chemical vapor
deposition growth of BN nanotubes and may outline new directions for improving
the efficiency of the method.Comment: submitted to physical review
Dynamic Kerr effect responses in the Terahertz-range
Dynamic Kerr effect measurements provide a simple realization of a nonlinear
experiment. We propose a field-off experiment where an electric field of one or
several sinusoidal cycles is applied to a sample in thermal equilibrium.
Afterwards, the evolution of the polarizability is measured. If such an
experiment is performed in the Terahertz-range it might provide valuable
information about the low-frequency dynamics in disordered systems. We treat
these dynamics in terms of a Brownian oscillator model and calculate the Kerr
effect response. It is shown that frequency-selective behaviour can be
expected. In the interesting case of underdamped vibrational motion we find
that the frequency-dependence of the phonon-damping can be determined from the
experiment. Also the behaviour of overdamped relaxational modes is discussed.
For typical glassy materials we estimate the magnitude of all relevant
quantities, which we believe to be helpful in experimental realizations.Comment: 26 pages incl. 5 figure
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