1,007 research outputs found

### Soft x-ray spectroscopy experiments on the near K-edge of B in MB2 (M=Mg, Al, Ta, and Nb)

Soft X-ray absorption and emission measurements are performed for the K- edge
of B in MB$_2$ (M=Mg, Al, Ta and Nb). Unique feature of MgB$_2$ with a high
density of B 2$p_{xy}(\sigma)$-state below and above the Fermi edge, which
extends to 1 eV above the edge, is confirmed. In contrast, the B 2$p$ density
of states in AlB$_2$ and TaB$_2$, both of occupied and unoccupied states,
decreased linearly towards the Fermi energy and showed a dip at the Fermi
energy. Furthermore, there is a broadening of the peaks with
$p\sigma$-character in XES and XAS of AlB$_2$, which is due to the increase of
three dimensionality in the $p\sigma$-band in AlB$_2$. The DOS of NbB$_2$ has a
dip just below the Fermi energy. The present results indicate that the large
DOS of B-2$p\sigma$ states near the Fermi energy are crucial for the
superconductivity of MgB$_2$.Comment: 3 pages text and 4 pages figures. accepted for publication to Phys.
Rev.

### Effects of Al doping on the structural and electronic properties of Mg(1-x)Al(x)B2

We have studied the structural and electronic properties of Mg(1-x)Al(x)B2
within the Virtual Crystal Approximation (VCA) by means of first-principles
total-energy calculations. Results for the lattice parameters, the electronic
band structure, and the Fermi surface as a function of Al doping for 0<x<0.6
are presented. The ab initio VCA calculations are in excellent agreement with
the experimentally observed change in the lattice parameters of Al doped MgB2.
The calculations show that the Fermi surface associated with holes a the boron
planes collapses gradually with aluminum doping and vanishes for x=0.56. In
addition, an abrupt topological change in the sigma-band Fermi surface was
found for x=0.3. The calculated hole density correlates closely with existing
experimental data for Tc(x), indicating that the observed loss of
superconductivity in Mg(1-x)Al(x)B2 is a result of hole bands filling.Comment: 4 pages (revtex) and 4 figures (postscript

### Structural and Superconducting Transitions in Mg_{1-x}Al_{x}B_2

From systematic ab initio calculations of the alloy system Mg_{1-x}Al_{x}B_2,
we find a strong tendency for the formation of a superstructure characterized
by Al-rich layers. We also present a simple model, based on calculated energies
and an estimate of the configurational entropy, which suggests that the alloy
has two separate concentration regimes of phase separation, with critical
points near x = 0.25 and x = 0.75. These results, together with calculations of
electronic densities of states in several ionic arrangements, give a
qualitative explanation for the observed structural instabilities, as well as
the x-dependence of the superconducting T_c for x<0.6.Comment: 4 pp./4 figs.; revisions in responce to Referee comment

### Dielectric functions and collective excitations in MgB_2

The frequency- and momentum-dependent dielectric function $\epsilon{(\bf
q,\omega)}$ as well as the energy loss function Im[-$\epsilon^{-1}{(\bf
q,\omega)}$\protect{]} are calculated for intermetallic superconductor $MgB_2$
by using two {\it ab initio} methods: the plane-wave pseudopotential method and
the tight-binding version of the LMTO method. We find two plasmon modes
dispersing at energies $\sim 2$-8 eV and $\sim 18$-22 eV. The high energy
plasmon results from a free electron like plasmon mode while the low energy
collective excitation has its origin in a peculiar character of the band
structure. Both plasmon modes demonstrate clearly anisotropic behaviour of both
the peak position and the peak width. In particular, the low energy collective
excitation has practically zero width in the direction perpendicular to boron
layers and broadens in other directions.Comment: 3 pages with 10 postscript figures. Submitted to PRB on May 14 200

### Neuropsychological constraints to human data production on a global scale

Which are the factors underlying human information production on a global
level? In order to gain an insight into this question we study a corpus of
252-633 Million publicly available data files on the Internet corresponding to
an overall storage volume of 284-675 Terabytes. Analyzing the file size
distribution for several distinct data types we find indications that the
neuropsychological capacity of the human brain to process and record
information may constitute the dominant limiting factor for the overall growth
of globally stored information, with real-world economic constraints having
only a negligible influence. This supposition draws support from the
observation that the files size distributions follow a power law for data
without a time component, like images, and a log-normal distribution for
multimedia files, for which time is a defining qualia.Comment: to be published in: European Physical Journal

### Dicyclic Horizontal Symmetry and Supersymmetric Grand Unification

It is shown how to use as horizontal symmetry the dicyclic group $Q_6 \subset
SU(2)$ in a supersymmetric unification $SU(5)\otimes SU(5)\otimes SU(2)$ where
one $SU(5)$ acts on the first and second families, in a horizontal doublet, and
the other acts on the third. This can lead to acceptable quark masses and
mixings, with an economic choice of matter supermultiplets, and charged lepton
masses can be accommodated.Comment: 10 pages, LaTe

### Effects of C, Cu and Be substitutions in superconducting MgB2

Density functional calculations are used to investigate the effects of
partial substitutional alloying of the B site in MgB2 with C and Be alone and
combined with alloying of the Mg site with Cu. The effect of such substitutions
on the electronic structure, electron phonon coupling and superconductivity are
discussed. We find that Be substitution for B is unfavorable for
superconductivity as it leads to a softer lattice and weaker electron-phonon
couplings. Replacement of Mg by Cu leads to an increase in the stiffness and
doping level at the same time, while the carrier concentration can be
controlled by partial replacement of B by C. We estimate that with full
replacement of Mg by Cu and fractional substitution of B by C, Tc values of 50K
may be attainable.Comment: 5 pages, 4 figure

### QED Effective Action at Finite Temperature: Two-Loop Dominance

We calculate the two-loop effective action of QED for arbitrary constant
electromagnetic fields at finite temperature T in the limit of T much smaller
than the electron mass. It is shown that in this regime the two-loop
contribution always exceeds the influence of the one-loop part due to the
thermal excitation of the internal photon. As an application, we study light
propagation and photon splitting in the presence of a magnetic background field
at low temperature. We furthermore discover a thermally induced contribution to
pair production in electric fields.Comment: 34 pages, 4 figures, LaTe

### Horizontal Symmetry for Quark and Squark Masses in Supersymmetric SU(5)

Recent interest in horizontal symmetry model building has been driven mainly
by the large top mass and hence strong hierarchy in quark masses, and the
possibility of appropriately constrained soft squark mass matrices, in place of
an assumed universality condition, for satisfying the relevant FCNC
constraints. Here we present the first successful SUSY-$SU(5)$ model that has
such a feature. The horizontal symmetry is a gauged $(Q_{12} \times U(1))_H$
($\subset (SU(2) \times U(1))_H$). All nonrenormalizable terms compatible with
the symmetry are allowed in the mass matrix constructions. Charged lepton
masses can also be accommodated.Comment: 15 pages, latex, 1 latex figure included version to be published in
Phys. Rev. Lett. ; some small changes in notations and presentation, a small
paragragh and 3 references adde

### MSSM Higgs sector CP violation at photon colliders: Revisited

We present a comprehensive analysis on the MSSM Higgs sector CP violation at
photon colliders including the chargino contributions as well as the
contributions of other charged particles. The chargino loop contributions can
be important for the would-be CP odd Higgs production at photon colliders.
Polarization asymmetries are indispensable in determining the CP properties of
neutral Higgs bosons.Comment: 24 pages, 40 figure

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