1,012 research outputs found
First Principles Calculations of Charge and Spin Density Waves of sqr3-Adsorbates on Semiconductors
We present ab-initio electronic structure results on the surface of sqr3
adsorbates. In particular, we address the issue of metal-insulator
instabilities, charge-density-waves (CDWs) or spin-density-waves (SDWs), driven
by partly filled surface states and their 2D Fermi surface, and/or by the onset
of magnetic instabilities. The focus is both on the newly discovered
commensurate CDW transitions in the Pb/Ge(111) and Sn/Ge(111) structures, and
on the puzzling semiconducting behavior of the Pb/Ge(111), K/Si(111):B and
SiC(0001) surfaces. In all cases, the main factor driving the instability
appears to be an extremely narrow surface state band. We have carried out so
far preliminary calculations for the Si/Si(111) surface, chosen as our model
system, within the gradient corrected local density (LDA+GC) and local spin
density (LSD+GC) approximations, with the aim of understanding the possible
interplay between 2D Fermi surface and electron correlations in the surface +
adsorbate system. Our spin- unrestricted results show that the sqr3
paramagnetic surface is unstable towards a commensurate SDW with periodicity
3x3 and magnetization 1/3.Comment: 9 pages, 4 Postscript figures, to be published in Surf. Sc
Wigner crystallization in quantum electron bilayers
The phase diagram of quantum electron bilayers in zero magnetic field is
obtained using density functional theory. For large electron densities the
system is in the liquid phase, while for smaller densities the liquid may
freeze (Wigner crystallization) into four different crystalline phases; the
lattice symmetry and the critical density depend on the the inter-layer
distance. The phase boundaries between different Wigner crystals consist of
both first and second order transitions, depending on the phases involved, and
join the freezing curve at three different triple points.Comment: To appear in Europhys. Lett. (11 pages in REVTEX + 2 figures in
postscript
X-ray Observation of SS 433 with RXTE
Apart from regular monitoring by ASM, the compact object SS 433 was observed
with RXTE several times last two/three years. We present the first analysis of
these observations. We also include the results of the recent exciting TOO
campaign made during donour inferior (orbital phase ) and superior
() conjunctions which took place on Oct. 2nd, 2003, and on March
13th, 2004 respectively, when the jet itself was directly pointing towards us
(i.e., precessional phase ). Generally, we found that two distinct
lines fit the spectra taken on all these days. We present some of the
light-curves and the X-ray spectra, and show that the Doppler shifts of the
emitted lines roughly match those predicted by the kinematic model for the
jets. We find that the line with a higher energy can be best identified with a
FeXXVI Ly- transition while the line with lower energy can be
identified with a FeXXV (1s2p - 1s) transition. We observe that the X-ray
flux on March 13th, 2004 (when the base of the jet is exposed) is more than
twice compared to that on Oct. 2nd, 2003 (when the base is covered by the
companion). We find the flux to continue to remain high at least till another
orbital period. We believe that this is because SS 433 was undergoing a weak
flaring activity during the recent observation.Comment: 15 pages, 6 figures, submitted for publication in MNRAS (April, 2004
The mechanism for the 3 x 3 distortion of Sn/ge (111)
We show that two distinct ground states, one nonmagnetic,
metallic, and distorted, the other magnetic, semimetallic (or insulating) and
undistorted, compete in -phase adsorbates on semiconductor (111)
surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with
experiment, that the distorted metallic ground state prevails. The reason for
stability of this state is analysed, and is traced to a sort of bond density
wave, specifically a modulation of the antibonding state filling between the
adatom and a Ge-Ge bond directly underneath
Electron-vibration coupling constants in positively charged fullerene
Recent experiments have shown that C60 can be positively field-doped. In that
state, fullerene exhibits a higher resistivity and a higher superconducting
temperature than the corresponding negatively doped state. A strong
intramolecular hole-phonon coupling, connected with the Jahn-Teller effect of
the isolated positive ion, is expected to be important for both properties, but
the actual coupling strengths are so far unknown. Based on density functional
calculations, we determine the linear couplings of the two a_g, six g_g, and
eight h_g vibrational modes to the H_u HOMO level of the C60 molecule. The
couplings predict a D_5 distortion, and an H_u vibronic ground state for C60^+.
They are also used to generate the dimensionless coupling constant
which controls the superconductivity and the phonon contribution to the
electrical resistivity in the crystalline phase. We find that is 1.4
times larger in positively-charged C60 than in the negatively-doped case. These
results are discussed in the context of the available transport data and
superconducting temperatures. The role of higher orbital degeneracy in
superconductivity is also addressed.Comment: 22 pages - 3 figures. This revision includes few punctuation
corrections from proofreadin
First observations of the X-ray transient EXO 2030+375 with IBIS/ISGRI
We present a first INTEGRAL observation of the 42s transient X-ray pulsar EXO
2030+375 with IBIS/ISGRI. The source was detected during Cyg X-1 observations
in December 2002. We analyzed observations during the outburst period from 9 to
21 December 2002 with a total exposure time of ~770 kiloseconds. EXO 2030+375
was almost always detected during single ~30 minute exposures in the 18-45
energy bands. The source light curve shows the characteristic outburst shape
observed in this source.Comment: 4 pages, 3 figures (1 in CMYK color), accepted by Astronomy and
Astrophysics, INTEGRAL special issue, 200
XMM-Newton Observation of the Black Hole Microquasar GRS 1758-258
The XMM-Newton X-ray observatory pointed the galactic black hole candidate
and microquasar GRS 1758-258 in September 2000 for about 10 ks during a program
devoted to the scan of the Galactic Center regions. Preliminary results from
EPIC MOS camera data are presented here. The data indicate that the source
underwent a state transition from its standard low-hard state to an
intermediate state. For the first time in this source the ultra-soft component
of the accretion disk, which black hole binaries display in intermediate or
high-soft states, was clearly detected and measured thanks to the high spectral
capabilities of XMM-Newton.Comment: To appear in the Proc. of the Gamma-Ray Astrophysics 2001 Symposium,
4-6 April 2001, Baltimore, Maryland, U.S.A.. American Institute of Physics
(AIP) series: 5 pages, 6 PS figures, latex, uses aipproc.cls aipproc.st
The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
We present for the first time a study of the 3-200 keV broad band spectra of
the bursting atoll source 4U 1728-34 (GX 354-0) along its hardness intensity
diagram. The analysis was done using the INTEGRAL public and Galactic Center
deep exposure data ranging from February 2003 to October 2004. The spectra are
well described by a thermal Comptonization model with an electron temperature
from 35 keV to 3 keV and Thomson optical depth, tau_T, from 0.5 to 5 in a slab
geometry. The source undergoes a transition from an intermediate/hard to a soft
state where the source luminosity increases from 2 to 12% of Eddington. We have
also detected 36 type I X-ray bursts two of which show photospheric radius
expansion. The energetic bursts with photospheric radius expansion occurred at
an inferred low mass accretion rate per unit area of \dot m ~ 1.7x10E3 g/cm2/s,
while the others at a higher one between 2.4x10E3 - 9.4x10E3 g/cm2/s. For
4U1728-34 the bursts' total fluence, and the bursts' peak flux are
anti-correlated with the mass accretion rate. The type I X-ray bursts involve
pure helium burning either during the hard state, or during the soft state of
the source.Comment: 11 pages, 7 figures, and 2 tables. Accepted for publication in A&
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