18,650 research outputs found
Anomalous temperature dependence of the band-gap in Black Phosphorus
Black Phosphorus (BP) has gained renewed attention due to its singular
anisotropic electronic and optical properties that might be exploited for a
wide range of technological applications. In this respect, the thermal
properties are particularly important both to predict its room temperature
operation and to determine its thermoelectric potential. From this point of
view, one of the most spectacular and poorly understood phenomena is, indeed,
the BP temperature-induced band-gap opening: when temperature is increased the
fundamental band-gap increases instead of decreasing. This anomalous thermal
dependence has also been observed, recently, in its monolayer counterpart. In
this work, based on \textit{ab-initio} calculations, we present an explanation
for this long known, and yet not fully explained, effect. We show that it
arises from a combination of harmonic and lattice thermal expansion
contributions, which are, in fact, highly interwined. We clearly narrow down
the mechanisms that cause this gap opening by identifying the peculiar atomic
vibrations that drive the anomaly. The final picture we give explains both the
BP anomalous band-gap opening and the frequency increase with increasing volume
(tension effect).Comment: Published in Nano Letter
Non-Associativity in the Clifford Bundle on the Parallelizable Torsion 7-Sphere
In this paper we discuss generalized properties of non-associativity in
Clifford bundles on the 7-sphere S7. Novel and prominent properties inherited
from the non-associative structure of the Clifford bundle on S7 are
demonstrated. They naturally lead to general transformations of the spinor
fields on S7 and have dramatic consequences for the associated Kac-Moody
current algebras. All additional properties concerning the non-associative
structure in the Clifford bundle on S7 are considered. We further discuss and
explore their applications.Comment: 16 page
Mimicking Nanoribbon Behavior Using a Graphene Layer on SiC
We propose a natural way to create quantum-confined regions in graphene in a
system that allows large-scale device integration. We show, using
first-principles calculations, that a single graphene layer on a trenched
region of mimics i)the energy bands around the Fermi level
and ii) the magnetic properties of free-standing graphene nanoribbons.
Depending on the trench direction, either zigzag or armchair nanoribbons are
mimicked. This behavior occurs because a single graphene layer over a
surface loses the graphene-like properties, which are restored solely over the
trenches, providing in this way a confined strip region.Comment: 4 pages, 4 figure
False Vacuum Transitions - Analytical Solutions and Decay Rate Values
In this work we show a class of oscillating configurations for the evolution
of the domain walls in Euclidean space. The solutions are obtained
analytically. Phase transitions are achieved from the associated fluctuation
determinant, by the decay rates of the false vacuum.Comment: 6 pages, improved to match the final version to appear in EP
Eisenstein Series and String Thresholds
We investigate the relevance of Eisenstein series for representing certain
-invariant string theory amplitudes which receive corrections from BPS
states only. may stand for any of the mapping class, T-duality and
U-duality groups , or respectively.
Using -invariant mass formulae, we construct invariant modular functions
on the symmetric space of non-compact type, with the
maximal compact subgroup of , that generalize the standard
non-holomorphic Eisenstein series arising in harmonic analysis on the
fundamental domain of the Poincar\'e upper half-plane. Comparing the
asymptotics and eigenvalues of the Eisenstein series under second order
differential operators with quantities arising in one- and -loop string
amplitudes, we obtain a manifestly T-duality invariant representation of the
latter, conjecture their non-perturbative U-duality invariant extension, and
analyze the resulting non-perturbative effects. This includes the and
couplings in toroidal compactifications of M-theory to any
dimension and respectively.Comment: Latex2e, 60 pages; v2: Appendix A.4 extended, 2 refs added, thms
renumbered, plus minor corrections; v3: relation (1.7) to math Eis series
clarified, eq (3.3) and minor typos corrected, final version to appear in
Comm. Math. Phys; v4: misprints and Eq C.13,C.24 corrected, see note adde
Gravastars and Black Holes of Anisotropic Dark Energy
Dynamical models of prototype gravastars made of anisotropic dark energy are
constructed, in which an infinitely thin spherical shell of a perfect fluid
with the equation of state divides the whole spacetime
into two regions, the internal region filled with a dark energy fluid, and the
external Schwarzschild region. The models represent "bounded excursion" stable
gravastars, where the thin shell is oscillating between two finite radii, while
in other cases they collapse until the formation of black holes. Here we show,
for the first time in the literature, a model of gravastar and formation of
black hole with both interior and thin shell constituted exclusively of dark
energy. Besides, the sign of the parameter of anisotropy () seems to
be relevant to the gravastar formation. The formation is favored when the
tangential pressure is greater than the radial pressure, at least in the
neighborhood of the isotropic case ().Comment: 16 pages, 8 figures. Accepted for publication in Gen. Rel. Gra
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