1,460 research outputs found
Role of inertia in two-dimensional deformation and breakup of a droplet
We investigate by Lattice Boltzmann methods the effect of inertia on the
deformation and break-up of a two-dimensional fluid droplet surrounded by fluid
of equal viscosity (in a confined geometry) whose shear rate is increased very
slowly. We give evidence that in two dimensions inertia is {\em necessary} for
break-up, so that at zero Reynolds number the droplet deforms indefinitely
without breaking. We identify two different routes to breakup via two-lobed and
three-lobed structures respectively, and give evidence for a sharp transition
between these routes as parameters are varied.Comment: 4 pages, 4 figure
Duality in Superstring Compactifications with Magnetic Field Backgrounds
Motivated by the work of Polchinski and Strominger on type IIA theory, where
the effect of non-trivial field strengths for p-form potentials on a Calabi-Yau
space was discussed, we study four-dimensional heterotic string theory in the
presence of magnetic field on a 2-cycle in the internal manifold, for both N=4
and N=2 cases. We show that at special points in the moduli space, certain
perturbative charged states become tachyonic and stabilize the vacuum by
acquiring vacuum expectation values, thereby restoring supersymmetry. We
discuss both the cases where the tachyons appear with a tower of Landau levels,
which become light in the limit of large volume of the 2-cycle, and the case
where such Landau levels are not present. In the latter case it is sufficient
to restrict the analysis to the quartic potential for the tachyon. On the other
hand, in the former case it is necessary to include the Landau levels in the
analysis of the potential; for toroidal and orbifold examples, we give an
explicit CFT description of the new supersymmetric vacuum. The resulting new
vacuum turns out to be in the same class as the original supersymmetric one.
Finally, using duality, we discuss the role of the Landau levels on the type
IIA side.Comment: 26 pages, LaTeX, 1 figure; Nucl. Phys. B version with typos corrected
and references adde
N=2 Type II- Heterotic duality and Higher derivative F-terms
We test the recently conjectured duality between supersymmetric type II
and heterotic string models by analysing a class of higher dimensional
interactions in the respective low-energy Lagrangians. These are -terms of
the form where is the gravitational superfield. On the type II
side these terms are generated at the -loop level and in fact are given by
topological partition functions of the twisted Calabi-Yau sigma model. We show
that on the heterotic side these terms arise at the one-loop level. We study in
detail a rank 3 example and show that the corresponding couplings satisfy
the same holomorphic anomaly equations as in the type II case. Moreover we
study the leading singularities of 's on the heterotic side, near the
enhanced symmetry point and show that they are universal poles of order
with coefficients that are given by the Euler number of the moduli
space of genus- Riemann surfaces. This confirms a recent conjecture that the
physics near conifold singularity is governed by string theory at the
self-dual point.Comment: 33 pages, latex, no figure
Open string topological amplitudes and gaugino masses
We discuss the moduli-dependent couplings of the higher derivative F-terms
(\Tr W^2)^{h-1}, where is the gauge N=1 chiral superfield. They are
determined by the genus zero topological partition function , on a
world-sheet with boundaries. By string duality, these terms are also
related to heterotic topological amplitudes studied in the past, with the
topological twist applied only in the left-moving supersymmetric sector of the
internal superconformal field theory. The holomorphic anomaly of
these couplings relates them to terms of the form ,
where 's represent chiral projections of non-holomorphic functions of
chiral superfields. An important property of these couplings is that they
violate R-symmetry for . As a result, once supersymmetry is broken by
D-term expectation values, (\Tr W^2)^2 generates gaugino masses that can be
hierarchically smaller than the scalar masses, behaving as
in string units. Similarly, generates Dirac masses for
non-chiral brane fermions, of the same order of magnitude. This mechanism can
be used for instance to obtain fermion masses at the TeV scale for scalar
masses as high as GeV. We present explicit examples
in toroidal string compactifications with intersecting D-branes.Comment: 57 pages, 6 figures; Abstract and references correcte
Topological Amplitudes in String Theory
We show that certain type II string amplitudes at genus are given by the
topological partition function discussed recently by Bershadsky, Cecotti,
Ooguri and Vafa. These amplitudes give rise to a term in the four-dimensional
effective action of the form , where is the chiral
superfield of supergravitational multiplet. The holomorphic anomaly of
is related to non-localities of the effective action due to the
propagation of massless states. This result generalizes the holomorphic anomaly
of the one loop case which is known to lead to non-harmonic gravitational
couplings.Comment: 31 pages (LATEX) + 2 figures (available upon request as PS file or
hardcopy), NUB-3071 - IC/93/202 - CPTH-A258.079
Perturbative Prepotential and Monodromies in N=2 Heterotic Superstring
We discuss the prepotential describing the effective field theory of N=2
heterotic superstring models. At the one loop-level the prepotential develops
logarithmic singularities due to the appearance of charged massless states at
particular surfaces in the moduli space of vector multiplets. These
singularities modify the classical duality symmetry group which now becomes a
representation of the fundamental group of the moduli space minus the singular
surfaces. For the simplest two-moduli case, this fundamental group turns out to
be a certain braid group and we determine the resulting full duality
transformations of the prepotential, which are exact in perturbation theory.Comment: 39 pages, LaTeX, no figures; section 6 expanded to include explicit
construction of the monodromy group in a (4,0) orbifold exampl
Effects of Electron-Electron and Electron-Phonon Interactions in Weakly Disordered Conductors and Heterostuctures
We investigate quantum corrections to the conductivity due to the
interference of electron-electron (electron-phonon) scattering and elastic
electron scattering in weakly disordered conductors. The electron-electron
interaction results in a negative -correction in a 3D conductor. In
a quasi-two-dimensional conductor, ( is the thickness, is
the Fermi velocity), with 3D electron spectrum this correction is linear in
temperature and differs from that for 2D electrons (G. Zala et. al., Phys.
Rev.B {\bf 64}, 214204 (2001)) by a numerical factor. In a
quasi-one-dimensional conductor, temperature-dependent correction is
proportional to . The electron interaction via exchange of virtual phonons
also gives -correction. The contribution of thermal phonons interacting
with electrons via the screened deformation potential results in -term and
via unscreened deformation potential results in -term. The interference
contributions dominate over pure electron-phonon scattering in a wide
temperature range, which extends with increasing disorder.Comment: 6 pages, 2figure
Topological Amplitudes in Heterotic Superstring Theory
We show that certain heterotic string amplitudes are given in terms of
correlators of the twisted topological (2,0) SCFT, corresponding to the
internal sector of the N=1 spacetime supersymmetric background. The genus g
topological partition function corresponds to a term in the effective
action of the form , where W is the gauge or gravitational superfield.
We study also recursion relations related to holomorphic anomalies, showing
that, contrary to the type II case, they involve correlators of anti-chiral
superfields. The corresponding terms in the effective action are of the form
, where is a chiral superfield obtained by chiral projection
of a general superfield. We observe that the structure of the recursion
relations is that of N=1 spacetime supersymmetry Ward identity. We give also a
solution of the tree level recursion relations and discuss orbifold examples.Comment: LaTeX, 60 pages, 2 PS figure
Molecular transport calculations with Wannier functions
We present a scheme for calculating coherent electron transport in
atomic-scale contacts. The method combines a formally exact Green's function
formalism with a mean-field description of the electronic structure based on
the Kohn-Sham scheme of density functional theory. We use an accurate
plane-wave electronic structure method to calculate the eigenstates which are
subsequently transformed into a set of localized Wannier functions (WFs). The
WFs provide a highly efficient basis set which at the same time is well suited
for analysis due to the chemical information contained in the WFs. The method
is applied to a hydrogen molecule in an infinite Pt wire and a benzene-dithiol
(BDT) molecule between Au(111) surfaces. We show that the transmission function
of BDT in a wide energy window around the Fermi level can be completely
accounted for by only two molecular orbitals.Comment: 15 pages, 12 figures, submitted to Chemical Physic
Non-Equilibrium Electron Transport in Two-Dimensional Nano-Structures Modeled by Green's Functions and the Finite-Element Method
We use the effective-mass approximation and the density-functional theory
with the local-density approximation for modeling two-dimensional
nano-structures connected phase-coherently to two infinite leads. Using the
non-equilibrium Green's function method the electron density and the current
are calculated under a bias voltage. The problem of solving for the Green's
functions numerically is formulated using the finite-element method (FEM). The
Green's functions have non-reflecting open boundary conditions to take care of
the infinite size of the system. We show how these boundary conditions are
formulated in the FEM. The scheme is tested by calculating transmission
probabilities for simple model potentials. The potential of the scheme is
demonstrated by determining non-linear current-voltage behaviors of resonant
tunneling structures.Comment: 13 pages,15 figure
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