titles
stringlengths 1
382
| abstracts
stringlengths 6
6.09k
|
|---|---|
On universality of critical behaviour in the focusing nonlinear
Schr\"odinger equation, elliptic umbilic catastrophe and the {\it
tritronqu\'ee} solution to the Painlev\'e-I equation | We argue that the critical behaviour near the point of ``gradient
catastrophe" of the solution to the Cauchy problem for the focusing nonlinear
Schr\"odinger equation $ i\epsilon \psi_t +\frac{\epsilon^2}2\psi_{xx}+
|\psi|^2 \psi =0$ with analytic initial data of the form $\psi(x,0;\epsilon)
=A(x) e^{\frac{i}{\epsilon} S(x)}$ is approximately described by a particular
solution to the Painlev\'e-I equation.
|
Generic representations of orthogonal groups: projective functors in the
category Fquad | In this paper, we continue the study of the category of functors Fquad,
associated to F_2-vector spaces equipped with a nondegenerate quadratic form,
initiated in two previous papers of the author. We define a filtration of the
standard projective objects in Fquad; this refines to give a decomposition into
indecomposable factors of the two first standard projective objects in Fquad.
As an application of these two decompositions, we give a complete description
of the polynomial functors of the category Fquad.
|
Manifolds admitting a $\tilde G_2$-structure | We find a necessary and sufficient condition for a compact 7-manifold to
admit a $\tilde G_2$-structure. As a result we find a sufficient condition for
an open 7-manifold to admit a closed 3-form of $\tilde G_2$-type.
|
Compatibility of Exotic States with Neutron Star Observation | We consider the effect of hard core repulsion in the baryon-baryon
interaction at short distance to the properties of a neutron star. We obtain
that, even with hyperons in the interior of a neutron star, the neutron star
mass can be as large as $\sim 2 M_\odot$.
|
Exact Solutions of Einstein-Yang-Mills Theory with Higher-Derivative
Coupling | We construct a classical solution of an Einstein-Yang-Mills system with a
fourth order term with respect to the field strength of the Yang-Mills field.
The solution provides a spontaneous compactification proposed by Cremmer and
Scherk; ten-dimensional space-time with a cosmological constant is compactified
to the four-dimensional Minkowski space with a six-dimensional sphere S^6 on
which an instanton solution exists. The radius of the sphere is not a modulus
but is determined by the gauge coupling and the four-derivative coupling
constants and the Newton's constant. We also construct a solution of
ten-dimensional theory without a cosmological constant compactified to AdS_4 x
S^6.
|
Dimensional crossover of quantum critical behavior in CeCoIn$_5$ | The nature of quantum criticality in CeCoIn$_5$ is studied by low-temperature
thermal expansion $\alpha(T)$. At the field-induced quantum critical point at
H=5 T a crossover scale $T^\star\approx 0.3$ K is observed, separating
$\alpha(T)/T\propto T^{-1}$ from a weaker $T^{-1/2}$ divergence. We ascribe
this change to a crossover in the dimensionality of the critical fluctuations
which may be coupled to a change from unconventional to conventional quantum
criticality. Disorder, whose effect on quantum criticality is studied in
CeCoIn$_{5-x}$Sn$_x$ ($0\leq x\leq 0.18$), shifts $T^\star$ towards higher
temperatures.
|
E_6 and the bipartite entanglement of three qutrits | Recent investigations have established an analogy between the entropy of
four-dimensional supersymmetric black holes in string theory and entanglement
in quantum information theory. Examples include: (1) N=2 STU black holes and
the tripartite entanglement of three qubits (2-state systems), where the common
symmetry is [SL(2)]^3 and (2) N=8 black holes and the tripartite entanglement
of seven qubits where the common symmetry is E_7 which contains [SL(2)]^7. Here
we present another example: N=8 black holes (or black strings) in five
dimensions and the bipartite entanglement of three qutrits (3-state systems),
where the common symmetry is E_6 which contains [SL(3)]^3. Both the black hole
(or black string) entropy and the entanglement measure are provided by the
Cartan cubic E_6 invariant. Similar analogies exist for ``magic'' N=2
supergravity black holes in both four and five dimensions.
|
Invariance principle for additive functionals of Markov chains | We consider a sequence of additive functionals {\phi_n}, set on a sequence of
Markov chains {X_n} that weakly converges to a Markov process X. We give
sufficient condition for such a sequence to converge in distribution,
formulated in terms of the characteristics of the additive functionals, and
related to the Dynkin's theorem on the convergence of W-functionals. As an
application of the main theorem, the general sufficient condition for
convergence of additive functionals in terms of transition probabilities of the
chains X_n is proved.
|
Dissipative backward stochastic differential equations with locally
Lipschitz nonlinearity | In this paper we study a class of backward stochastic differential equations
(BSDEs) of the form dY(t)= -AY(t)dt -f_0(t,Y(t))dt -f_1(t,Y(t),Z(t))dt +
Z(t)dW(t) on the interval [0,T], with given final condition at time T, in an
infinite dimensional Hilbert space H. The unbounded operator A is sectorial and
dissipative and the nonlinearity f_0(t,y) is dissipative and defined for y only
taking values in a subspace of H. A typical example is provided by the
so-called polynomial nonlinearities. Applications are given to stochastic
partial differential equations and spin systems.
|
Axino warm dark matter and $\Omega_b - \Omega_{DM}$ coincidence | We show that axinos, which are dominantly generated by the decay of the
next-to-lightest supersymmetric particles produced from the leptonic $Q$-ball
($L$-ball), become warm dark matter suitable for the solution of the missing
satellite problem and the cusp problem. In addition, $\Omega_b - \Omega_{DM}$
coincidence is naturally explained in this scenario.
|
A unified approach to SIC-POVMs and MUBs | A unified approach to (symmetric informationally complete) positive operator
valued measures and mutually unbiased bases is developed in this article. The
approach is based on the use of operator equivalents expanded in the enveloping
algebra of SU(2). Emphasis is put on similarities and differences between
SIC-POVMs and MUBs.
|
Stable oscillations of a predator-prey probabilistic cellular automaton:
a mean-field approach | We analyze a probabilistic cellular automaton describing the dynamics of
coexistence of a predator-prey system. The individuals of each species are
localized over the sites of a lattice and the local stochastic updating rules
are inspired on the processes of the Lotka-Volterra model. Two levels of
mean-field approximations are set up. The simple approximation is equivalent to
an extended patch model, a simple metapopulation model with patches colonized
by prey, patches colonized by predators and empty patches. This approximation
is capable of describing the limited available space for species occupancy. The
pair approximation is moreover able to describe two types of coexistence of
prey and predators: one where population densities are constant in time and
another displaying self-sustained time-oscillations of the population
densities. The oscillations are associated with limit cycles and arise through
a Hopf bifurcation. They are stable against changes in the initial conditions
and, in this sense, they differ from the Lotka-Volterra cycles which depend on
initial conditions. In this respect, the present model is biologically more
realistic than the Lotka-Volterra model.
|
SDSS J233325.92+152222.1 and the evolution of intermediate polars | Intermediate polars (IPs) are cataclysmic variables which contain magnetic
white dwarfs with a rotational period shorter than the binary orbital period.
Evolutionary theory predicts that IPs with long orbital periods evolve through
the 2-3 hour period gap, but it is very uncertain what the properties of the
resulting objects are. Whilst a relatively large number of long-period IPs are
known, very few of these have short orbital periods. We present phase-resolved
spectroscopy and photometry of SDSS J233325.92+152222.1 and classify it as the
IP with the shortest known orbital period (83.12 +/- 0.09 min), which contains
a white dwarf with a relatively long spin period (41.66 +/- 0.13 min). We
estimate the white dwarf's magnetic moment to be mu(WD) \approx 2 x 10^33 G
cm^3, which is not only similar to three of the other four confirmed
short-period IPs but also to those of many of the long-period IPs. We suggest
that long-period IPs conserve their magnetic moment as they evolve towards
shorter orbital periods. Therefore the dominant population of long-period IPs,
which have white dwarf spin periods roughly ten times shorter than their
orbital periods, will likely end up as short-period IPs like SDSS J2333, with
spin periods a large fraction of their orbital periods.
|
Complexity Considerations, cSAT Lower Bound | This article discusses completeness of Boolean Algebra as First Order Theory
in Goedel's meaning. If Theory is complete then any possible transformation is
equivalent to some transformation using axioms, predicates etc. defined for
this theory. If formula is to be proved (or disproved) then it has to be
reduced to axioms. If every transformation is deducible then also optimal
transformation is deducible. If every transformation is exponential then
optimal one is too, what allows to define lower bound for discussed problem to
be exponential (outside P). Then we show algorithm for NDTM solving the same
problem in O(n^c) (so problem is in NP), what proves that P \neq NP.
Article proves also that result of relativisation of P=NP question and oracle
shown by Baker-Gill-Solovay distinguish between deterministic and
non-deterministic calculation models. If there exists oracle A for which
P^A=NP^A then A consists of infinite number of algorithms, DTMs, axioms and
predicates, or like NDTM infinite number of simultaneous states.
|
Temperature dependence of Coulomb drag between finite-length quantum
wires | We evaluate the Coulomb drag current in two finite-length
Tomonaga-Luttinger-liquid wires coupled by an electrostatic backscattering
interaction. The drag current in one wire shows oscillations as a function of
the bias voltage applied to the other wire, reflecting interferences of the
plasmon standing waves in the interacting wires. In agreement with this
picture, the amplitude of the current oscillations is reduced with increasing
temperature. This is a clear signature of non-Fermi-liquid physics because for
coupled Fermi liquids the drag resistance is always expected to increase as the
temperature is raised.
|
Effects of Imperfect Gate Operations in Shor's Prime Factorization
Algorithm | The effects of imperfect gate operations in implementation of Shor's prime
factorization algorithm are investigated. The gate imperfections may be
classified into three categories: the systematic error, the random error, and
the one with combined errors. It is found that Shor's algorithm is robust
against the systematic errors but is vulnerable to the random errors. Error
threshold is given to the algorithm for a given number $N$ to be factorized.
|
Using decomposed household food acquisitions as inputs of a Kinetic
Dietary Exposure Model | Foods naturally contain a number of contaminants that may have different and
long term toxic effects. This paper introduces a novel approach for the
assessment of such chronic food risk that integrates the pharmacokinetic
properties of a given contaminant. The estimation of such a Kinetic Dietary
Exposure Model (KDEM) should be based on long term consumption data which, for
the moment, can only be provided by Household Budget Surveys such as the
SECODIP panel in France. A semi parametric model is proposed to decompose a
series of household quantities into individual quantities which are then used
as inputs of the KDEM. As an illustration, the risk assessment related to the
presence of methyl mercury in seafood is revisited using this novel approach.
|
Dust and gas emission in the prototypical hot core G29.96-0.02 at
sub-arcsecond resolution | Aiming at a better understand of the physical and chemical processes in the
hot molecular core stage of high-mass star formation, we observed the
prototypical hot core G29.96-0.02 in the 862mu band with the Submillimeter
Array (SMA) at sub-arcsecond spatial resolution. The observations resolved the
hot molecular core into six submm continuum sources with the finest spatial
resolution of 0.36''x0.25'' (~1800AU) achieved so far. Four of them located
within 7800(AU)^2 comprise a proto-Trapezium system with estimated protostellar
densities of 1.4x0^5 protostars/pc^3. The plethora of ~80 spectral lines allows
us to study the molecular outflow(s), the core kinematics, the temperature
structure of the region as well as chemical effects. The derived hot core
temperatures are of the order 300K. We find interesting chemical spatial
differentiations, e.g., C34S is deficient toward the hot core and is enhanced
at the UCHII/hot core interface, which may be explained by temperature
sensitive desorption from grains and following gas phase chemistry. The
SiO(8-7) emission outlines likely two molecular outflows emanating from this
hot core region. Emission from most other molecules peaks centrally on the hot
core and is not dominated by any individual submm peak. Potential reasons for
that are discussed. A few spectral lines that are associated with the main
submm continuum source, show a velocity gradient perpendicular to the
large-scale outflow. Since this velocity structure comprises three of the
central protostellar sources, this is not a Keplerian disk. While the data are
consistent with a gas core that may rotate and/or collapse, we cannot exclude
the outflow(s) and/or nearby expanding UCHII region as possible alternative
causes of this velocity pattern.
|
Hamilton-Jacobi Fractional Sequential Mechanics | As a continuation of Rabei et al. work [11], the Hamilton- Jacobi partial
differential equation is generalized to be applicable for systems containing
fractional derivatives. The Hamilton- Jacobi function in configuration space is
obtained in a similar manner to the usual mechanics. Two problems are
considered to demonstrate the application of the formalism. The result found to
be in exact agreement with Agrawal's formalism.
|
A critical theory of quantum entanglement for the Hydrogen molecule | In this paper we investigate some entanglement properties for the Hydrogen
molecule considered as a two interacting spin 1/2 (qubit) model. The
entanglement related to the $H_{2}$ molecule is evaluated both using the von
Neumann entropy and the Concurrence and it is compared with the corresponding
quantities for the two interacting spin system. Many aspects of these functions
are examinated employing in part analytical and, essentially, numerical
techniques. We have compared analogous results obtained by Huang and Kais a few
years ago. In this respect, some possible controversial situations are
presented and discussed.
|
Fractionally charged excitations on frustrated lattices | Systems of strongly correlated fermions on certain geometrically frustrated
lattices at particular filling factors support excitations with fractional
charges $\pm e/2$. We calculate quantum mechanical ground states, low--lying
excitations and spectral functions of finite lattices by means of numerical
diagonalization. The ground state of the most thoroughfully studied case, the
criss-crossed checkerboard lattice, is degenerate and shows long--range order.
Static fractional charges are confined by a weak linear force, most probably
leading to bound states of large spatial extent. Consequently, the
quasi-particle weight is reduced, which reflects the internal dynamics of the
fractionally charged excitations. By using an additional parameter, we
fine--tune the system to a special point at which fractional charges are
manifestly deconfined--the so--called Rokhsar--Kivelson point. For a deeper
understanding of the low--energy physics of these models and for numerical
advantages, several conserved quantum numbers are identified.
|
Measurement of Decay Amplitudes of B -->(c cbar) Kstar with an Angular
Analysis, for (c cbar)=J/psi, psi(2S) and chi_c1 | We perform the first three-dimensional measurement of the amplitudes of $B\to
\psi(2S) K^*$ and $B\to \chi_{c1} K^*$ decays and update our previous
measurement for $B\to J/\psi K^*$. We use a data sample collected with the
BaBar detector at the PEP2 storage ring, corresponding to 232 million $B\bar B$
pairs. The longitudinal polarization of decays involving a $J^{PC}=1^{++}$
$\chi_{c1}$ meson is found to be larger than that with a $1^{--}$ $J/\psi$ or
$\psi(2S)$ meson. No direct {\it CP}-violating charge asymmetry is observed.
|
Quantum superpositions and entanglement of thermal states at high
temperatures and their applications to quantum information processing | We study characteristics of superpositions and entanglement of thermal states
at high temperatures and discuss their applications to quantum information
processing. We introduce thermal-state qubits and thermal-Bell states, which
are a generalization of pure-state qubits and Bell states to thermal mixtures.
A scheme is then presented to discriminate between the four thermal-Bell states
without photon number resolving detection but with Kerr nonlinear interactions
and two single-photon detectors. This enables one to perform quantum
teleportation and gate operations for quantum computation with thermal-state
qubits.
|
Optimal control of stochastic differential equations with dynamical
boundary conditions | In this paper we investigate the optimal control problem for a class of
stochastic Cauchy evolution problem with non standard boundary dynamic and
control. The model is composed by an infinite dimensional dynamical system
coupled with a finite dimensional dynamics, which describes the boundary
conditions of the internal system. In other terms, we are concerned with non
standard boundary conditions, as the value at the boundary is governed by a
different stochastic differential equation.
|
On the Energy-Momentum Problem in Static Einstein Universe | This paper has been removed by arXiv administrators because it plagiarizes
gr-qc/0410004, gr-qc/0603075, and others.
This paper also has excessive overlap with the following papers also written
by the authors or their collaborators: gr-qc/0608111, and others.
|
Fractional WKB Approximation | Wentzel, Kramers, Brillouin (WKB) approximation for fractional systems is
investigated in this paper using the fractional calculus. In the fractional
case the wave function is constructed such that the phase factor is the same as
the Hamilton's principle function "S". To demonstrate our proposed approach two
examples are investigated in details.
|
Towards Skyrmion Stars: Large Baryon Configurations in the
Einstein-Skyrme Model | We investigate the large baryon number sector of the Einstein-Skyrme model as
a possible model for baryon stars. Gravitating hedgehog skyrmions have been
investigated previously and the existence of stable solitonic stars excluded
due to energy considerations. However, in this paper we demonstrate that by
generating gravitating skyrmions using rational maps, we can achieve
multi-baryon bound states whilst recovering spherical symmetry in the limit
where B becomes large.
|
Many-to-One Throughput Capacity of IEEE 802.11 Multi-hop Wireless
Networks | This paper investigates the many-to-one throughput capacity (and by symmetry,
one-to-many throughput capacity) of IEEE 802.11 multi-hop networks. It has
generally been assumed in prior studies that the many-to-one throughput
capacity is upper-bounded by the link capacity L. Throughput capacity L is not
achievable under 802.11. This paper introduces the notion of "canonical
networks", which is a class of regularly-structured networks whose capacities
can be analyzed more easily than unstructured networks. We show that the
throughput capacity of canonical networks under 802.11 has an analytical upper
bound of 3L/4 when the source nodes are two or more hops away from the sink;
and simulated throughputs of 0.690L (0.740L) when the source nodes are many
hops away. We conjecture that 3L/4 is also the upper bound for general
networks. When all links have equal length, 2L/3 can be shown to be the upper
bound for general networks. Our simulations show that 802.11 networks with
random topologies operated with AODV routing can only achieve throughputs far
below the upper bounds. Fortunately, by properly selecting routes near the
gateway (or by properly positioning the relay nodes leading to the gateway) to
fashion after the structure of canonical networks, the throughput can be
improved significantly by more than 150%. Indeed, in a dense network, it is
worthwhile to deactivate some of the relay nodes near the sink judiciously.
|
Scanning Tunneling Spectroscopy in the Superconducting State and Vortex
Cores of the beta-pyrochlore KOs2O6 | We performed the first scanning tunneling spectroscopy measurements on the
pyrochlore superconductor KOs2O6 (Tc = 9.6 K) in both zero magnetic field and
the vortex state at several temperatures above 1.95 K. This material presents
atomically flat surfaces, yielding spatially homogeneous spectra which reveal
fully-gapped superconductivity with a gap anisotropy of 30%. Measurements
performed at fields of 2 and 6 T display a hexagonal Abrikosov flux line
lattice. From the shape of the vortex cores, we extract a coherence length of
31-40 {\AA}, in agreement with the value derived from the upper critical field
Hc2. We observe a reduction in size of the vortex cores (and hence the
coherence length) with increasing field which is consistent with the
unexpectedly high and unsaturated upper critical field reported.
|
Noncommutative Solitons in a Supersymmetric Chiral Model in 2+1
Dimensions | We consider a supersymmetric Bogomolny-type model in 2+1 dimensions
originating from twistor string theory. By a gauge fixing this model is reduced
to a modified U(n) chiral model with N<=8 supersymmetries in 2+1 dimensions.
After a Moyal-type deformation of the model, we employ the dressing method to
explicitly construct multi-soliton configurations on noncommutative R^{2,1} and
analyze some of their properties.
|
Gravitational Duality Transformations on (A)dS4 | We discuss the implementation of electric-magnetic duality transformations in
four-dimensional gravity linearized around Minkowski or (A)dS4 backgrounds. In
the presence of a cosmological constant duality generically modifies the
Hamiltonian, nevertheless the bulk dynamics is unchanged. We pay particular
attention to the boundary terms generated by the duality transformations and
discuss their implications for holography.
|
Effect of transition-metal elements on the electronic properties of
quasicrystals and complex aluminides | In this paper, we briefly present our work on the role of transition-metal
element in electronic structure and transport properties of quasicrystals and
related complex phases. Several Parts of these works have been done or
initiated in collaboration with Prof. T. Fujiwara.
|
Non-resonant and Resonant X-ray Scattering Studies on Multiferroic
TbMn2O5 | Comprehensive x-ray scattering studies, including resonant scattering at Mn
L-edge, Tb L- and M-edges, were performed on single crystals of TbMn2O5. X-ray
intensities were observed at a forbidden Bragg position in the ferroelectric
phases, in addition to the lattice and the magnetic modulation peaks.
Temperature dependences of their intensities and the relation between the
modulation wave vectors provide direct evidences of exchange striction induced
ferroelectricity. Resonant x-ray scattering results demonstrate the presence of
multiple magnetic orders by exhibiting their different temperature dependences.
The commensurate-to-incommensurate phase transition around 24 K is attributed
to discommensuration through phase slipping of the magnetic orders in spin
frustrated geometries. We proposed that the low temperature incommensurate
phase consists of the commensurate magnetic domains separated by anti-phase
domain walls which reduce spontaneous polarizations abruptly at the transition.
|
Self-diffusion and Interdiffusion in Al80Ni20 Melts: Simulation and
Experiment | A combination of experimental techniques and molecular dynamics (MD) computer
simulation is used to investigate the diffusion dynamics in Al80Ni20 melts.
Experimentally, the self-diffusion coefficient of Ni is measured by the
long-capillary (LC) method and by quasielastic neutron scattering. The LC
method yields also the interdiffusion coefficient. Whereas the experiments were
done in the normal liquid state, the simulations provided the determination of
both self-diffusion and interdiffusion constants in the undercooled regime as
well. The simulation results show good agreement with the experimental data. In
the temperature range 3000 K >= T >= 715 K, the interdiffusion coefficient is
larger than the self-diffusion constants. Furthermore the simulation shows that
this difference becomes larger in the undercooled regime. This result can be
refered to a relatively strong temperature dependence of the thermodynamic
factor \Phi, which describes the thermodynamic driving force for
interdiffusion. The simulations also indicate that the Darken equation is a
good approximation, even in the undercooled regime. This implies that dynamic
cross correlations play a minor role for the temperature range under
consideration.
|
The Formation of Globular Cluster Systems in Massive Elliptical
Galaxies: Globular Cluster Multimodality from Radial Variation of Stellar
Populations | The most massive elliptical galaxies show a prominent multi-modality in their
globular cluster system color distributions. Understanding the mechanisms which
lead to multiple globular cluster sub-populations is essential for a complete
picture of massive galaxy formation. By assuming that globular cluster
formation traces the total star formation and taking into account the radial
variations in the composite stellar populations predicted by the Pipino &
Matteucci (2004) multi-zone photo-chemical evolution code, we compute the
distribution of globular cluster properties as a function of galactocentric
radius. We compare our results to the spectroscopic measurements of globular
clusters in nearby early-type galaxies by Puzia et al. (2006) and show that the
observed multi-modality in globular cluster systems of massive ellipticals can
be, at least partly, ascribed to the radial variation in the mix of stellar
populations. Our model predicts the presence of a super-metal-rich population
of globular clusters in the most massive elliptical galaxies, which is in very
good agreement with the spectroscopic observations. Furthermore, we investigate
the impact of other non-linear mechanisms that shape the metallicity
distribution of globular cluster systems, in particular the role of
merger-induced globular cluster formation and a non-linear color-metallicity
transformation, and discuss their influence in the context of our model
(abridged)
|
Binaries, microquasars and GLAST | Radio and X-ray observations of the relativistic jets of microquasars show
evidence for the acceleration of particles to very high energies. Signatures of
non-thermal processes occurring closer in to the compact object can also be
found. In addition, three binaries are now established emitters of high (> 100
MeV) and/or very high (> 100GeV) energy gamma-rays. High-energy emission can
originate from a microquasar jet (accretion-powered) or from a shocked pulsar
wind (rotation-powered). I discuss the impact GLAST will have in the very near
future on studies of such binaries. GLAST is expected to shed new light on the
link between accretion and ejection in microquasars and to enable to probe
pulsar winds on small scales in rotation-powered binaries.
|
Linearisation of finite abelian subgroups of the Cremona group of the
plane | This article gives the proof of results announced in [J. Blanc, Finite
Abelian subgroups of the Cremona group of the plane, C.R. Acad. Sci. Paris,
S\'er. I 344 (2007), 21-26.] and some description of automorphisms of rational
surfaces.
Given a finite Abelian subgroup of the Cremona group of the plane, we provide
a way to decide whether it is birationally conjugate to a group of
automorphisms of a minimal surface.
In particular, we prove that a finite cyclic group of birational
transformations of the plane is linearisable if and only if none of its
non-trivial elements fix a curve of positive genus. For finite Abelian groups,
there exists only one surprising exception, a group isomorphic to Z/2ZxZ/4Z,
whose non-trivial elements do not fix a curve of positive genus but which is
not conjugate to a group of automorphisms of a minimal rational surface.
We also give some descriptions of automorphisms (not necessarily of finite
order) of del Pezzo surfaces and conic bundles.
|
Oriented growth of pentacene films on vacuum-deposited
polytetrafluoroethylene layers aligned by rubbing technique | We investigated structure and morphology of PTFE layers deposited by vacuum
process in dependence on deposition parameters: deposition rate, deposition
temperature, electron activation energy and activation current. Pentacene (PnC)
layers deposited on top of those PTFE films are used as a tool to demonstrate
the orienting ability of the PTFE layers. The molecular structure of the PTFE
films was investigated by use of infrared spectroscopy. By means of
ellipsometry, values of refractive index between 1.33 and 1.36 have been
obtained for PTFE films in dependence on deposition conditions. Using the cold
friction technique orienting PTFE layers with unidirectional grooves are
obtained. On top of these PTFE films oriented PnC layers were grown. The
obtained order depends both on the PTFE layer thickness and on PnC growth
temperature.
|
Integral representations for convolutions of non-central multivariate
gamma distributions | Three types of integral representations for the cumulative distribution
functions of convolutions of non-central p-variate gamma distributions are
given by integration of elementary complex functions over the p-cube Cp =
(-pi,pi]x...x(-pi,pi]. In particular, the joint distribution of the diagonal
elements of a generalized quadratic form XAX' with n independent normally
distributed column vectors in X is obtained. For a single p-variate gamma
distribution function (p-1)-variate integrals over Cp-1 are derived. The
integrals are numerically more favourable than integrals obtained from the
Fourier or laplace inversion formula.
|
On the Achievable Rate Regions for Interference Channels with Degraded
Message Sets | The interference channel with degraded message sets (IC-DMS) refers to a
communication model in which two senders attempt to communicate with their
respective receivers simultaneously through a common medium, and one of the
senders has complete and a priori (non-causal) knowledge about the message
being transmitted by the other. A coding scheme that collectively has
advantages of cooperative coding, collaborative coding, and dirty paper coding,
is developed for such a channel. With resorting to this coding scheme,
achievable rate regions of the IC-DMS in both discrete memoryless and Gaussian
cases are derived, which, in general, include several previously known rate
regions. Numerical examples for the Gaussian case demonstrate that in the
high-interference-gain regime, the derived achievable rate regions offer
considerable improvements over these existing results.
|
On complete subsets of the cyclic group | A subset $X$ of an abelian $G$ is said to be {\em complete} if every element
of the subgroup generated by $X$ can be expressed as a nonempty sum of distinct
elements from $X$.
Let $A\subset \Z_n$ be such that all the elements of $A$ are coprime with
$n$. Solving a conjecture of Erd\H{o}s and Heilbronn, Olson proved that
$A$ is complete if $n$ is a prime and if $|A|>2\sqrt{n}.$
Recently Vu proved that there is an absolute constant $c$, such that for an
arbitrary large $n$, $A$ is complete if $|A|\ge c\sqrt{n},$ and conjectured
that 2 is essentially the right value of $c$. We show that $A$ is complete if
$|A|> 1+2\sqrt{n-4}$, thus proving the last conjecture.
|
Hilbert functions of points on Schubert varieties in Orthogonal
Grassmannians | A solution is given to the following problem: how to compute the
multiplicity, or more generally the Hilbert function, at a point on a Schubert
variety in an orthogonal Grassmannian. Standard monomial theory is applied to
translate the problem from geometry to combinatorics. The solution of the
resulting combinatorial problem forms the bulk of the paper. This approach has
been followed earlier to solve the same problem for the Grassmannian and the
symplectic Grassmannian.
As an application, we present an interpretation of the multiplicity as the
number of non-intersecting lattice paths of a certain kind.
Taking the Schubert variety to be of a special kind and the point to be the
"identity coset," our problem specializes to a problem about Pfaffian ideals
treatments of which by different methods exist in the literature. Also
available in the literature is a geometric solution when the point is a
"generic singularity."
|
Swift/XRT observes the fifth outburst of the periodic Supergiant Fast
X-ray Transient IGR J11215-5952 | IGR J11215-5952 is a hard X-ray transient source discovered in April 2005
with INTEGRAL and a confirmed member of the new class of High Mass X-ray
Binaries, the Supergiant Fast X-ray Transients (SFXTs). Archival INTEGRAL data
and RXTE observations showed that the outbursts occur with a periodicity of
~330 days. Thus, IGR J11215-5952 is the first SFXT displaying periodic
outbursts, possibly related to the orbital period. We performed a Target of
Opportunity observation with Swift with the main aim of monitoring the source
behaviour around the time of the fifth outburst, expected on 2007 Feb 9. The
source field was observed with Swift twice a day (2ks/day) starting from 4th
February, 2007, until the fifth outburst, and then for ~5 ks a day afterwards,
during a monitoring campaign that lasted 23 days for a total on-source exposure
of ~73 ks. This is the most complete monitoring campaign of an outburst from a
SFXT. The spectrum during the brightest flares is well described by an absorbed
power law with a photon index of 1 and N_H~1 10^22 cm^-2. A 1-10 keV peak
luminosity of ~10^36 erg s^-1 was derived (assuming 6.2 kpc, the distance of
the optical counterpart). These Swift observations are a unique data-set for an
outburst of a SFXT, thanks to the combination of sensitivity and time coverage,
and they allowed a study of IGR J11215-5952 from outburst onset to almost
quiescence. We find that the accretion phase lasts longer than previously
thought on the basis of lower sensitivity instruments observing only the
brightest flares. The observed phenomenology is consistent with a smoothly
increasing flux triggered at the periastron passage in a wide eccentric orbit
with many flares superimposed, possibly due to episodic or inhomogeneous
accretion.
|
Crossover behavior in fluids with Coulomb interactions | According to extensive experimental findings, the Ginzburg temperature
$t_{G}$ for ionic fluids differs substantially from that of nonionic fluids
[Schr\"oer W., Weig\"{a}rtner H. 2004 {\it Pure Appl. Chem.} {\bf 76} 19]. A
theoretical investigation of this outcome is proposed here by a mean field
analysis of the interplay of short and long range interactions on the value of
$t_{G}$. We consider a quite general continuous charge-asymmetric model made of
charged hard spheres with additional short-range interactions (without
electrostatic interactions the model belongs to the same universality class as
the 3D Ising model). The effective Landau-Ginzburg Hamiltonian of the full
system near its gas-liquid critical point is derived from which the Ginzburg
temperature is calculated as a function of the ionicity. The results obtained
in this way for $t_{G}$ are in good qualitative and sufficient quantitative
agreement with available experimental data.
|
Reply to Comment of Kenzelmann and Harris | In response to the comment of Kenzelmann and Harris I show how the continuum
theory of spiral multiferroics can be modified to describe general magnetic
orders and discuss why the microscopic mechanism of magnetically-induced
ferroelectricity usually makes such modifications unnecessary. This explains
why the simple model with a single vector order parameter successfully
describes thermodynamics and magnetoelectric properties of many spiral
multiferroics.
|
Persistent Currents in Superconducting Quantum Interference Devices | Starting from the reduced dynamical model of a two-junction quantum
interference device, a quantum analog of the system has been exhibited, in
order to extend the well known properties of this device to the quantum regime.
By finding eigenvalues of the corresponding Hamiltonian operator, the
persistent currents flowing in the ring have been obtained. The resulting
quantum analog of the overdamped two-junction quantum interference device can
be seen as a supercurrent qubit operating in the limit of negligible
capacitance and finite inductance.
|
Mid-Infrared Fine Structure Line Ratios in Active Galactic Nuclei
Observed with Spitzer IRS: Evidence for Extinction by the Torus | We present the first systematic investigation of the [NeV] (14um/24um) and
[SIII] (18um/33um) infrared line flux ratios, traditionally used to estimate
the density of the ionized gas, in a sample of 41 Type 1 and Type 2 active
galactic nuclei (AGNs) observed with the Infrared Spectrograph on board
Spitzer. The majority of galaxies with both [NeV] lines detected have observed
[NeV] line flux ratios consistent with or below the theoretical low density
limit, based on calculations using currently available collision strengths and
ignoring absorption and stimulated emission. We find that Type 2 AGNs have
lower line flux ratios than Type 1 AGNs and that all of the galaxies with line
flux ratios below the low density limit are Type 2 AGNs. We argue that
differential infrared extinction to the [NeV] emitting region due to dust in
the obscuring torus is responsible for the ratios below the low density limit
and we suggest that the ratio may be a tracer of the inclination angle of the
torus to our line of sight. Because the temperature of the gas, the amount of
extinction, and the effect of absorption and stimulated emission on the line
ratios are all unknown, we are not able to determine the electron densities
associated with the [NeV] line flux ratios for the objects in our sample. We
also find that the [SIII] emission from the galaxies in our sample is extended
and originates primarily in star forming regions. Since the emission from
low-ionization species is extended, any analysis using line flux ratios from
such species obtained from slits of different sizes is invalid for most nearby
galaxies.
|
Neutrinos and Non-proliferation in Europe | Triggered by the demand of the IAEA, neutrino physicists in Europe involved
with the Double Chooz experiment are studying the potential of neutrino
detection to monitor nuclear reactors. In particular a new set of experiments
at the ILL is planned to improve the knowledge of the neutrino spectrum emitted
in the fission of 235U and 239Pu.
|
Kinks and Particles in Non-integrable Quantum Field Theories | In this talk we discuss an elementary derivation of the semi-classical
spectrum of neutral particles in two field theories with kink excitations. We
also show that, in the non-integrable cases, each vacuum state cannot
generically support more than two stable particles, since all other neutral
exitations are resonances, which will eventually decay.
|
The old open clusters Berkeley 32 and King 11 | We have obtained CCD BVI imaging of the old open clusters Berkeley 32 and
King 11. Using the synthetic colour-magnitude diagram method with three
different sets of stellar evolution models of various metallicities, with and
without overshooting, we have determined their age, distance, reddening, and
indicative metallicity, as well as distance from the Galactic centre and height
from the Galactic plane. The best parameters derived for Berkeley 32 are:
subsolar metallicity (Z=0.008 represents the best choice, Z=0.006 or 0.01 are
more marginally acceptable), age = 5.0-5.5 Gyr (models with overshooting;
without overshooting the age is 4.2-4.4 Gyr with poorer agreement),
(m-M)_0=12.4-12.6, E(B-V)=0.12-0.18 (with the lower value being more probable
because it corresponds to the best metallicity), Rgc ~ 10.7-11 kpc, and |Z| ~
231-254 pc. The best parameters for King 11 are: Z=0.01, age=3.5-4.75 Gyr,
(m-M)_0=11.67-11.75, E(B-V)=1.03-1.06, Rgc ~ 9.2-10 kpc, and |Z| ~ 253-387 pc.
|
The Genetic Programming Collaboration Network and its Communities | Useful information about scientific collaboration structures and patterns can
be inferred from computer databases of published papers. The genetic
programming bibliography is the most complete reference of papers on GP\@. In
addition to locating publications, it contains coauthor and coeditor
relationships from which a more complete picture of the field emerges. We treat
these relationships as undirected small world graphs whose study reveals the
community structure of the GP collaborative social network. Automatic analysis
discovers new communities and highlights new facets of them. The investigation
reveals many similarities between GP and coauthorship networks in other
scientific fields but also some subtle differences such as a smaller central
network component and a high clustering.
|
The Expanding Photosphere Method: Progress and Problems | Distances to well-observed Type II-P SNe are determined from an updated
version of the Expanding Photosphere Method (EPM), based on recent theoretical
models. The new EPM distances show good agreement with other independent
distances to the host galaxies without any significant systematic bias,
contrary to earlier results in the literature. The accuracy of the method is
comparable with that of the distance measurements for Type Ia SNe.
|
Spontaneous Lorentz Violation: Non-Abelian Gauge Fields as
Pseudo-Goldstone Vector Bosons | We argue that non-Abelian gauge fields can be treated as the pseudo-Goldstone
vector bosons caused by spontaneous Lorentz invariance violation (SLIV). To
this end, the SLIV which evolves in a general Yang-Mills type theory with the
nonlinear vector field constraint $Tr(% \boldsymbol{A}_{\mu
}\boldsymbol{A}^{\mu})=\pm M^{2}$ ($M$ is a proposed SLIV scale) imposed is
considered in detail. With an internal symmetry group $G$ having $D$ generators
not only the pure Lorentz symmetry SO(1,3), but the larger accidental symmetry
$SO(D,3D)$ of the SLIV constraint in itself appears to be spontaneously broken
as well. As a result, while the pure Lorentz violation still generates only one
genuine Goldstone vector boson, the accompanying pseudo-Goldstone vector bosons
related to the $SO(D,3D)$ breaking also come into play in the final arrangement
of the entire Goldstone vector field multiplet. Remarkably, they remain
strictly massless, being protected by gauge invariance of the Yang-Mills theory
involved. We show that, although this theory contains a plethora of Lorentz and
$CPT$ violating couplings, they do not lead to physical SLIV effects which turn
out to be strictly cancelled in all the lowest order processes considered.
However, the physical Lorentz violation could appear if the internal gauge
invariance were slightly broken at very small distances influenced by gravity.
For the SLIV scale comparable with the Planck one the Lorentz violation could
become directly observable at low energies.
|
In-medium effects on particle production in heavy ion collisions | The effect of possible in-medium modifications of nucleon-nucleon ($NN$)
cross sections on particle production is investigated in heavy ion collisions
($HIC$) at intermediate energies. In particular, using a fully covariant
relativistic transport approach, we see that the density dependence of the {\it
inelastic} cross sections appreciably affects the pion and kaon yields and
their rapidity distributions. However, the $(\pi^{-}/\pi^{+})$- and
$(K^{0}/K^{+})$-ratios depend only moderately on the in-medium behavior of the
inelastic cross sections. This is particularly true for kaon yield ratios,
since kaons are more uniformly produced in high density regions. Kaon
potentials are also suitably evaluated in two schemes, a chiral perturbative
approach and an effective meson-quark coupling method, with consistent results
showing a similar repulsive contribution for $K^{+}$ and $K^{0}$. As a
consequence we expect rather reduced effects on the yield ratios. We conclude
that particle ratios appear to be robust observables for probing the nuclear
equation of state ($EoS$) at high baryon density and, particularly, its
isovector sector.
|
The Graham conjecture implies the Erdos-Turan conjecture | Erd\"{o}s and Tur\'{a}n once conjectured that any set $A\subset\mathbb{N}$
with $\sum_{a\in A}{1}/{a}=\infty$ should contain infinitely many progressions
of arbitrary length $k\geq3$. For the two-dimensional case Graham conjectured
that if $B\subset \mathbb{N}\times\mathbb{N}$ satisfies $$\sum\limits_{(x,y)\in
B}\frac{1}{x^2+y^2}=\infty,$$ then for any $s\geq2$, $B$ contains an $s\times
s$ axes-parallel grid. In this paper it is shown that if the Graham conjecture
is true for some $s\geq2$, then the Erd\"{o}s-Tur\'{a}n conjecture is true for
$k=2s-1$.
|
Effective conservation of energy and momentum algorithm using switching
potentials suitable for molecular dynamics simulation of thermodynamical
systems | During a crossover via a switching mechanism from one 2-body potential to
another as might be applied in modeling (chemical) reactions in the vicinity of
bond formation, energy violations would occur due to finite step size which
determines the trajectory of the particles relative to the potential
interactions of the unbonded state by numerical (e.g. Verlet) integration. This
problem is overcome by an algorithm which preserves the coordinates of the
system for each move, but corrects for energy discrepancies by ensuring both
energy and momentum conservation in the dynamics. The algorithm is tested for a
hysteresis loop reaction model with an without the implementation of the
algorithm. The tests involve checking the rate of energy flow out of the MD
simulation box; in the equilibrium state, no net rate of flows within
experimental error should be observed. The temperature and pressure of the box
should also be invariant within the range of fluctuation of these quantities.
It is demonstrated that the algorithm satisfies these criteria.
|
Mixed chemistry phenomenon during late stages of stellar evolution | We discuss phenomenon of simultaneous presence of O- and C-based material in
surroundings of evolutionary advanced stars. We concentrate on silicate carbon
stars and present observations that directly confirm the binary model scenario
for them. We discuss also class of C-stars with OH emission detected, to which
some [WR] planetary nebulae do belong.
|
M-regularity of the Fano surface | Let $(A,\Theta)$ be a principally polarised abelian variety, and let Y be a
subvariety. Pareschi and Popa conjectured that Y has minimal cohomology class
if and only if the structure sheaf of Y satisfies a property that they call
M-regularity.
Let now X be a smooth cubic threefold. By a classical result due to Clemens
and Griffiths, its intermediate Jacobian J(X) is a principally polarised
abelian variety; furthermore the Fano surface of lines on X can be embedded in
J(X) and has minimal cohomology class. In this short note we show that its
structure sheaf is M-regular.
|
Signal for space-time noncommutativity: the Z -> gamma gamma decay in
the renormalizable gauge sector of the theta-expanded NCSM | We propose the Z -> gamma gamma decay, a process strictly forbidden in the
standard model, as a signal suitable for the search of noncommutativity of
coordinates at very short distances. We compute the Z -> gamma gamma partial
widthin the framework of the recently proposed renormalizable gauge sector of
the noncommutative standard model. The one-loop renormalizability is obtained
for the model containing the usual six representations of matter fields of the
first generation. Even more, the noncommutative part is finite or free of
divergences, showing that perhaps new interaction symmetry exists in the
noncommutative gauge sector of the model. Discovery of such symmetry would be
of tremendous importance in further search for the violation of the Lorentz
invariance at very high energies. Experimental possibilities of Z -> gamma
gamma decay are analyzed and a firm bound to the scale of the noncommutativity
parameter is set around 1 TeV.
|
Laser spectroscopy of hyperfine structure in highly-charged ions: a test
of QED at high fields | An overview is presented of laser spectroscopy experiments with cold,
trapped, highly-charged ions, which will be performed at the HITRAP facility at
GSI in Darmstadt (Germany). These high-resolution measurements of ground state
hyperfine splittings will be three orders of magnitude more precise than
previous measurements. Moreover, from a comparison of measurements of the
hyperfine splittings in hydrogen- and lithium-like ions of the same isotope,
QED effects at high electromagnetic fields can be determined within a few
percent. Several candidate ions suited for these laser spectroscopy studies are
presented.
|
Confinement into a state with persistent current by thermal quenching of
loop of Josephson junctions | We study a loop of Josephson junctions that is quenched through its critical
temperature. For three or more junctions, symmetry breaking states can be
achieved without thermal activation, in spite of the fact that the relaxation
time is practically constant when the critical temperature is approached from
above. The probability for these states decreases with quenching time, but the
dependence is not allometric. For large number of junctions, cooling does not
have to be fast. For this case, we evaluate the standard deviation of the
induced flux. Our results are consistent with the available experimental data.
|
Frequency modulation Fourier transform spectroscopy | A new method, FM-FTS, combining Frequency Modulation heterodyne laser
spectroscopy and Fourier Transform Spectroscopy is presented. It provides
simultaneous sensitive measurement of absorption and dispersion profiles with
broadband spectral coverage capabilities. Experimental demonstration is made on
the overtone spectrum of C2H2 in the 1.5 $\mu$m region.
|
Universe Without Singularities. A Group Approach to De Sitter Cosmology | In the last years the traditional scenario of Big Bang has been deeply
modified by the study of the quantum features of the Universe evolution,
proposing again the problem of using local physical laws on cosmic scale, with
particular regard to the cosmological constant role. The group extention method
shows that the De Sitter group univocally generalizes the Poincare group,
formally justifies the cosmological constant use and suggests a new
interpretation for Hartle-Hawking boundary conditions in Quantum Cosmology.
|
Spectral action on noncommutative torus | The spectral action on noncommutative torus is obtained, using a
Chamseddine--Connes formula via computations of zeta functions. The importance
of a Diophantine condition is outlined. Several results on holomorphic
continuation of series of holomorphic functions are obtained in this context.
|
The Lifshitz-Slyozov-Wagner equation for reaction-controlled kinetics | We rigorously derive a weak form of the Lifshitz-Slyozov-Wagner equation as
the homogenization limit of a Stefan-type problem describing
reaction-controlled coarsening of a large number of small spherical particles.
Moreover, we deduce that the effective mean-field description holds true in the
particular limit of vanishing surface-area density of particles.
|
Canonical singular hermitian metrics on relative canonical bundles | We introduce a new class of canonical AZD's (called the supercanonical AZD's)
on the canonical bundles of smooth projective varieties with pseudoeffective
canonical classes. We study the variation of the supercanonical AZD
$\hat{h}_{can}$ under projective deformations and give a new proof of the
invariance of plurigenera.
|
Yield Curve Shapes and the Asymptotic Short Rate Distribution in Affine
One-Factor Models | We consider a model for interest rates, where the short rate is given by a
time-homogenous, one-dimensional affine process in the sense of Duffie,
Filipovic and Schachermayer. We show that in such a model yield curves can only
be normal, inverse or humped (i.e. endowed with a single local maximum). Each
case can be characterized by simple conditions on the present short rate. We
give conditions under which the short rate process will converge to a limit
distribution and describe the limit distribution in terms of its cumulant
generating function. We apply our results to the Vasicek model, the CIR model,
a CIR model with added jumps and a model of Ornstein-Uhlenbeck type.
|
Thermally Stimulated Luminescence and Current in new heterocyclic
materials for Organic field transistors and organic light emitting diodes | The present work is focused on theoretical and experimental study of
localised levels in organic materials suitable for light-emitting devices and
field effect transistors by means of thermal techniques. In our work we focused
on low molecular compounds as well as on polymers, especially of two classes of
materials: oxadiazoles and quinoxalines. Both organic compounds are well know
as electron transport materials in OLEDs.
|
A microfluidic device based on droplet storage for screening solubility
diagrams | This work describes a new microfluidic device developed for rapid screening
of solubility diagrams. In several parallel channels, hundreds of
nanoliter-volume droplets of a given solution are first stored with a gradual
variation in the solute concentration. Then, the application of a temperature
gradient along these channels enables us to read directly and quantitatively
phase diagrams, concentration vs. temperature. We show, using a solution of
adipic acid, that we can measure ten points of the solubility curve in less
than 1 hr and with only 250 $\mu$L of solution.
|
Composite fermion wave functions as conformal field theory correlators | It is known that a subset of fractional quantum Hall wave functions has been
expressed as conformal field theory (CFT) correlators, notably the Laughlin
wave function at filling factor $\nu=1/m$ ($m$ odd) and its quasiholes, and the
Pfaffian wave function at $\nu=1/2$ and its quasiholes. We develop a general
scheme for constructing composite-fermion (CF) wave functions from conformal
field theory. Quasiparticles at $\nu=1/m$ are created by inserting anyonic
vertex operators, $P_{\frac{1}{m}}(z)$, that replace a subset of the electron
operators in the correlator. The one-quasiparticle wave function is identical
to the corresponding CF wave function, and the two-quasiparticle wave function
has correct fractional charge and statistics and is numerically almost
identical to the corresponding CF wave function. We further show how to exactly
represent the CF wavefunctions in the Jain series $\nu = s/(2sp+1)$ as the CFT
correlators of a new type of fermionic vertex operators, $V_{p,n}(z)$,
constructed from $n$ free compactified bosons; these operators provide the CFT
representation of composite fermions carrying $2p$ flux quanta in the $n^{\rm
th}$ CF Landau level. We also construct the corresponding quasiparticle- and
quasihole operators and argue that they have the expected fractional charge and
statistics. For filling fractions 2/5 and 3/7 we show that the chiral CFTs that
describe the bulk wave functions are identical to those given by Wen's general
classification of quantum Hall states in terms of $K$-matrices and $l$- and
$t$-vectors, and we propose that to be generally true. Our results suggest a
general procedure for constructing quasiparticle wave functions for other
fractional Hall states, as well as for constructing ground states at filling
fractions not contained in the principal Jain series.
|
$\Bz\to\pip\pim\piz$ Time Dependent Dalitz analysis at BaBar | I present here results of a time-dependent analysis of the Dalitz structure
of neutral $B$ meson decays to
$\pip\pim\piz$ from a dataset of 346 million $B \bar B$ pairs collected at
the $\Upsilon(4S)$ center of mass energy by the BaBar detector at the SLAC
PEP-II $e^+e^-$ accelerator. No significant CP violation effects are observed
and 68% confidence interval is derived on the weak angle $\alpha$ to be
[75$^o$,152$^o$]
|
New Organic thermally stable materials for optoelectronics devices - A
linear spectroscopy study | Thermally stable polymers have attracted a lot of interest due to their
potential use as the active component in electronic, optical and optoelectronic
applications, such as light-emitting diodes, light emitting electrochemical
cells, photodiodes, photovoltaic cells, field effect transistors, optocouplers
and optically pumped lasers in solution and solid state.We report results of
investigations into the use of thermal treatment of poly(p-phenylene vinylene)
(PPV) films grown on a variety of substrates (quartz and glass). Film
thickness, morphology and structural properties were investigated by a range of
techniques in particular: atomic force microscope - AFM, DEKTAK method,
Ellipsometry and UV-VIS spectroscopy.
|
Elativistic treatment in}$D$ - Dimensions to a spin-zero particle with
noncentral equal scalar and vector ring-shaped Kratzer potential | The Klein-Gordon equation in D-dimensions for a recently proposed Kratzer
potential plus ring-shaped potential is solved analytically by means of the
conventional Nikiforov-Uvarov method. The exact energy bound-states and the
corresponding wave functions of the Klein-Gordon are obtained in the presence
of the noncentral equal scalar and vector potentials. The results obtained in
this work are more general and can be reduced to the standard forms in
three-dimensions given by other works.
|
On the potential of transit surveys in star clusters: Impact of
correlated noise and radial velocity follow-up | We present an extension of the formalism recently proposed by Pepper & Gaudi
to evaluate the yield of transit surveys in homogeneous stellar systems,
incorporating the impact of correlated noise on transit time-scales on the
detectability of transits, and simultaneously incorporating the magnitude
limits imposed by the need for radial velocity follow-up of transit candidates.
New expressions are derived for the different contributions to the noise budget
on transit time-scales and the least-squares detection statistic for box-shaped
transits, and their behaviour as a function of stellar mass is re-examined.
Correlated noise that is constant with apparent stellar magnitude implies a
steep decrease in detection probability at the high mass end which, when
considered jointly with the radial velocity requirements, can severely limit
the potential of otherwise promising surveys in star clusters. However, we find
that small-aperture, wide field surveys may detect hot Neptunes whose radial
velocity signal can be measured with present-day instrumentation in very nearby
(<100 pc) clusters.
|
A non-perturbative proof of Bertrand's theorem | We discuss an alternative non-perturbative proof of Bertrand's theorem that
leads in a concise way directly to the two allowed fields: the newtonian and
the isotropic harmonic oscillator central fields.
|
Neutron-Capture Elements in the Double-Enhanced Star HE 1305-0007: a New
s- and r-Process Paradigm | The star HE 1305-0007 is a metal-poor double-enhanced star with metallicity
[Fe/H] $=-2.0$, which is just at the upper limit of the metallicity for the
observed double-enhanced stars. Using a parametric model, we find that almost
all s-elements were made in a single neutron exposure. This star should be a
member of a post-common-envelope binary. After the s-process material has
experienced only one neutron exposure in the nucleosynthesis region and is
dredged-up to its envelope, the AGB evolution is terminated by the onset of
common-envelope evolution. Based on the high radial-velocity of HE 1305-0007,
we speculate that the star could be a runaway star from a binary system, in
which the AIC event has occurred and produced the r-process elements.
|
Membrane in M5-branes Background | In this paper, we investigate the properties of a membrane in the M5-brane
background. Through solving the classical equations of motion of the membrane,
we can understand the classical dynamics of the membrane in this background.
|
3D photospheric velocity field of a Supergranular cell | We investigate the plasma flow properties inside a Supergranular (SG) cell,
in particular its interaction with small scale magnetic field structures. The
SG cell has been identified using the magnetic network (CaII wing brightness)
as proxy, applying the Two-Level Structure Tracking (TST) to high spatial,
spectral and temporal resolution observations obtained by IBIS. The full 3D
velocity vector field for the SG has been reconstructed at two different
photospheric heights. In order to strengthen our findings, we also computed the
mean radial flow of the SG by means of cork tracing. We also studied the
behaviour of the horizontal and Line of Sight plasma flow cospatial with
cluster of bright CaII structures of magnetic origin to better understand the
interaction between photospheric convection and small scale magnetic features.
The SG cell we investigated seems to be organized with an almost radial flow
from its centre to the border. The large scale divergence structure is probably
created by a compact region of constant up-flow close to the cell centre. On
the edge of the SG, isolated regions of strong convergent flow are nearby or
cospatial with extended clusters of bright CaII wing features forming the knots
of the magnetic network.
|
Substructures in WINGS clusters | We search for and characterize substructures in the projected distribution of
galaxies observed in the wide field CCD images of the 77 nearby clusters of the
WIde-field Nearby Galaxy-cluster Survey (WINGS). This sample is complete in
X-ray flux in the redshift range 0.04<z<0.07. We search for substructures in
WINGS clusters with DEDICA, an adaptive-kernel procedure. We test the procedure
on Monte-Carlo simulations of the observed frames and determine the reliability
for the detected structures. DEDICA identifies at least one reliable structure
in the field of 55 clusters. 40 of these clusters have a total of 69
substructures at the same redshift of the cluster (redshift estimates of
substructures are from color-magnitude diagrams). The fraction of clusters with
subclusters (73%) is higher than in most studies. The presence of subclusters
affects the relative luminosities of the brightest cluster galaxies (BCGs).
Down to L ~ 10^11.2 L_Sun, our observed differential distribution of subcluster
luminosities is consistent with the theoretical prediction of the differential
mass function of substructures in cosmological simulations.
|
Ising-like dynamics and frozen states in systems of ultrafine magnetic
particles | We use Monte-Carlo simulations to study aging phenomena and the occurence of
spinglass phases in systems of single-domain ferromagnetic nanoparticles under
the combined influence of dipolar interaction and anisotropy energy, for
different combinations of positional and orientational disorder. We find that
the magnetic moments oriente themselves preferably parallel to their anisotropy
axes and changes of the total magnetization are solely achieved by 180 degree
flips of the magnetic moments, as in Ising systems. Since the dipolar
interaction favorizes the formation of antiparallel chain-like structures,
antiparallel chain-like patterns are frozen in at low temperatures, leading to
aging phenomena characteristic for spin-glasses. Contrary to the intuition,
these aging effects are more pronounced in ordered than in disordered
structures.
|
Counting characters in linear group actions | Let $G$ be a finite group and $V$ be a finite $G$--module. We present upper
bounds for the cardinalities of certain subsets of $\Irr(GV)$, such as the set
of those $\chi\in\Irr(GV)$ such that, for a fixed $v\in V$, the restriction of
$\chi$ to $<v>$ is not a multiple of the regular character of $<v>$. These
results might be useful in attacking the non--coprime $k(GV)$--problem.
|
Continuous interfaces with disorder: Even strong pinning is too weak in
2 dimensions | We consider statistical mechanics models of continuous height effective
interfaces in the presence of a delta-pinning at height zero. There is a
detailed mathematical understanding of the depinning transition in 2 dimensions
without disorder. Then the variance of the interface height w.r.t. the Gibbs
measure stays bounded uniformly in the volume for any positive pinning force
and diverges like the logarithm of the pinning force when it tends to zero.
How does the presence of a quenched disorder term in the Hamiltonian modify
this transition? We show that an arbitarily weak random field term is enough to
beat an arbitrarily strong delta-pinning in 2 dimensions and will cause
delocalization. The proof is based on a rigorous lower bound for the overlap
between local magnetizations and random fields in finite volume. In 2
dimensions it implies growth faster than the volume which is a contradiction to
localization. We also derive a simple complementary inequality which shows that
in higher dimensions the fraction of pinned sites converges to one when the
pinning force tends to infinity.
|
On the KK-theory of strongly self-absorbing C*-algebras | Let $\Dh$ and $A$ be unital and separable $C^{*}$-algebras; let $\Dh$ be
strongly self-absorbing. It is known that any two unital $^*$-homomorphisms
from $\Dh$ to $A \otimes \Dh$ are approximately unitarily equivalent. We show
that, if $\Dh$ is also $K_{1}$-injective, they are even asymptotically
unitarily equivalent. This in particular implies that any unital endomorphism
of $\Dh$ is asymptotically inner. Moreover, the space of automorphisms of $\Dh$
is compactly-contractible (in the point-norm topology) in the sense that for
any compact Hausdorff space $X$, the set of homotopy classes $[X,\Aut(\Dh)]$
reduces to a point. The respective statement holds for the space of unital
endomorphisms of $\Dh$. As an application, we give a description of the
Kasparov group $KK(\Dh, A\ot \Dh)$ in terms of $^*$-homomorphisms and
asymptotic unitary equivalence. Along the way, we show that the Kasparov group
$KK(\Dh, A\ot \Dh)$ is isomorphic to $K_0(A\ot \Dh)$.
|
Effective interactions from q-deformed inspired transformations | From the mass term for the transformed quark fields, we obtain effective
contact interactions of the NJL type. The parameters of the model that maps a
system of non-interacting transformed fields into quarks interacting via NJL
contact terms are discussed.
|
Magnetospectroscopy of epitaxial few-layer graphene | The inter-Landau level transitions observed in far-infrared transmission
experiments on few-layer graphene samples show a behaviour characteristic of
the linear dispersion expected in graphene. This behaviour persists in
relatively thick samples, and is qualitatively different from that of thin
samples of bulk graphite.
|
Dust Formation and Survival in Supernova Ejecta | The presence of dust at high redshift requires efficient condensation of
grains in SN ejecta, in accordance with current theoretical models. Yet,
observations of the few well studied SNe and SN remnants imply condensation
efficiencies which are about two orders of magnitude smaller. Motivated by this
tension, we have (i) revisited the model of Todini & Ferrara (2001) for dust
formation in the ejecta of core collapse SNe and (ii) followed, for the first
time, the evolution of newly condensed grains from the time of formation to
their survival - through the passage of the reverse shock - in the SN remnant.
We find that 0.1 - 0.6 M_sun of dust form in the ejecta of 12 - 40 M_sun
stellar progenitors. Depending on the density of the surrounding ISM, between
2-20% of the initial dust mass survives the passage of the reverse shock, on
time-scales of about 4-8 x 10^4 yr from the stellar explosion. Sputtering by
the hot gas induces a shift of the dust size distribution towards smaller
grains. The resulting dust extinction curve shows a good agreement with that
derived by observations of a reddened QSO at z =6.2. Stochastic heating of
small grains leads to a wide distribution of dust temperatures. This supports
the idea that large amounts (~ 0.1 M_sun) of cold dust (T ~ 40K) can be present
in SN remnants, without being in conflict with the observed IR emission.
|
Preferential interaction coefficient for nucleic acids and other
cylindrical poly-ions | The thermodynamics of nucleic acid processes is heavily affected by the
electric double-layer of micro-ions around the polyions. We focus here on the
Coulombic contribution to the salt-polyelectrolyte preferential interaction
(Donnan) coefficient and we report extremely accurate analytical expressions
valid in the range of low salt concentration (when polyion radius is smaller
than the Debye length). The analysis is performed at Poisson-Boltzmann level,
in cylindrical geometry, with emphasis on highly charged poly-ions (beyond
``counter-ion condensation''). The results hold for any electrolyte of the form
$z_-$:$z_+$. We also obtain a remarkably accurate expression for the electric
potential in the vicinity of the poly-ion.
|
A new approach to mutual information | A new expression as a certain asymptotic limit via "discrete micro-states" of
permutations is provided to the mutual information of both continuous and
discrete random variables.
|
Analysis of the real estate market in Las Vegas: Bubble, seasonal
patterns, and prediction of the CSW indexes | We analyze 27 house price indexes of Las Vegas from Jun. 1983 to Mar. 2005,
corresponding to 27 different zip codes. These analyses confirm the existence
of a real-estate bubble, defined as a price acceleration faster than
exponential, which is found however to be confined to a rather limited time
interval in the recent past from approximately 2003 to mid-2004 and has
progressively transformed into a more normal growth rate comparable to
pre-bubble levels in 2005. There has been no bubble till 2002 except for a
medium-sized surge in 1990. In addition, we have identified a strong yearly
periodicity which provides a good potential for fine-tuned prediction from
month to month. A monthly monitoring using a model that we have developed could
confirm, by testing the intra-year structure, if indeed the market has returned
to ``normal'' or if more turbulence is expected ahead. We predict the evolution
of the indexes one year ahead, which is validated with new data up to Sep.
2006. The present analysis demonstrates the existence of very significant
variations at the local scale, in the sense that the bubble in Las Vegas seems
to have preceded the more global USA bubble and has ended approximately two
years earlier (mid 2004 for Las Vegas compared with mid-2006 for the whole of
the USA).
|
A Low Complexity Algorithm and Architecture for Systematic Encoding of
Hermitian Codes | We present an algorithm for systematic encoding of Hermitian codes. For a
Hermitian code defined over GF(q^2), the proposed algorithm achieves a run time
complexity of O(q^2) and is suitable for VLSI implementation. The encoder
architecture uses as main blocks q varying-rate Reed-Solomon encoders and
achieves a space complexity of O(q^2) in terms of finite field multipliers and
memory elements.
|
Quantum criticality and disorder in the antiferromagnetic critical point
of NiS$_{2}$ pyrite | A quantum critical point (QCP) between the antiferromagnetic and the
paramagnetic phases was realized by applying a hydrostatic pressure of ~ 7 GPa
on single crystals of NiS_{2} pyrite with a low residual resistivity, rho_{0},
of 0.5 mu-Omega-cm. We found that the critical behavior of the resistivity,
rho, in this clean system contrasts sharply with those observed in its
disordered analogue, NiS_{2-x}Se_{x} solid-solution, demonstrating the
unexpectedly drastic effect of disorder on the quantum criticality. Over a
whole paramagnetic region investigated up to P = 9 GPa, a crossover
temperature, defined as the onset of T^{2} dependence of rho, an indication of
Fermi liquid, was suppressed to a substantially low temperature T sim 2 K and,
instead, a non Fermi liquid behavior of rho, T^{3/2}-dependence, robustly
showed up.
|
Spin coherence of holes in GaAs/AlGaAs quantum wells | The carrier spin coherence in a p-doped GaAs/(Al,Ga)As quantum well with a
diluted hole gas has been studied by picosecond pump-probe Kerr rotation with
an in-plane magnetic field. For resonant optical excitation of the positively
charged exciton the spin precession shows two types of oscillations. Fast
oscillating electron spin beats decay with the radiative lifetime of the
charged exciton of 50 ps. Long lived spin coherence of the holes with dephasing
times up to 650 ps. The spin dephasing time as well as the in-plane hole g
factor show strong temperature dependence, underlining the importance of hole
localization at cryogenic temperatures.
|
Local-field effects in radiatively broadened magneto-dielectric media:
negative refraction and absorption reduction | We give a microscopic derivation of the Clausius-Mossotti relations for a
homogeneous and isotropic magneto-dielectric medium consisting of radiatively
broadened atomic oscillators. To this end the diagram series of electromagnetic
propagators is calculated exactly for an infinite bi-cubic lattice of
dielectric and magnetic dipoles for a lattice constant small compared to the
resonance wavelength $\lambda$. Modifications of transition frequencies and
linewidth of the elementary oscillators are taken into account in a
selfconsistent way by a proper incorporation of the singular self-interaction
terms. We show that in radiatively broadened media sufficiently close to the
free-space resonance the real part of the index of refraction approaches the
value -2 in the limit of $\rho \lambda^3 \gg 1$, where $\rho$ is the number
density of scatterers. Since at the same time the imaginary part vanishes as
$1/\rho$ local field effects can have important consequences for realizing
low-loss negative index materials.
|
Search for a fourth generation b'-quark at LEP-II at sqrt{s}=196-209 GeV | A search for the pair production of fourth generation b'-quarks was performed
using data taken by the DELPHI detector at LEP-II. The analysed data were
collected at centre-of-mass energies ranging from 196 to 209 GeV, corresponding
to an integrated luminosity of 420 pb^{-1}. No evidence for a signal was found.
Upper limits on BR(b' -> bZ) and BR(b' -> cW) were obtained for b' masses
ranging from 96 to 103 GeV/c^2. These limits, together with the theoretical
branching ratios predicted by a sequential four generations model, were used to
constrain the value of R_{CKM}=|V_{cb'}/V_{tb'}V_{tb}|, where V_{cb'}, V_{tb'}
and V_{tb} are elements of the extended CKM matrix.
|
About curvature, conformal metrics and warped products | We consider the curvature of a family of warped products of two
pseduo-Riemannian manifolds $(B,g_B)$ and $(F,g_F)$ furnished with metrics of
the form $c^{2}g_B \oplus w^2 g_F$ and, in particular, of the type $w^{2
\mu}g_B \oplus w^2 g_F$, where $c, w \colon B \to (0,\infty)$ are smooth
functions and $\mu$ is a real parameter. We obtain suitable expressions for the
Ricci tensor and scalar curvature of such products that allow us to establish
results about the existence of Einstein or constant scalar curvature structures
in these categories. If $(B,g_B)$ is Riemannian, the latter question involves
nonlinear elliptic partial differential equations with concave-convex
nonlinearities and singular partial differential equations of the
Lichnerowicz-York type among others.
|
The local structure of conformally symmetric manifolds | This is a final step in a local classification of pseudo-Riemannian manifolds
with parallel Weyl tensor that are not conformally flat or locally symmetric.
|
Investigation of Colour Reconnection in WW Events with the DELPHI
detector at LEP-2 | In the reaction e+e- -> WW -> (q_1 qbar_2)(q_3 qbar_4) the usual
hadronization models treat the colour singlets q_1 qbar_2 and q_3 qbar_4 coming
from two W bosons independently. However, since the final state partons may
coexist in space and time, cross-talk between the two evolving hadronic systems
may be possible during fragmentation through soft gluon exchange. This effect
is known as Colour Reconnection. In this article the results of the
investigation of Colour Reconnection effects in fully hadronic decays of W
pairs in DELPHI at LEP are presented. Two complementary analyses were
performed, studying the particle flow between jets and W mass estimators, with
negligible correlation between them, and the results were combined and compared
to models. In the framework of the SK-I model, the value for its kappa
parameter most compatible with the data was found to be: kappa_{SK-I} =
2.2^{+2.5}_{-1.3} corresponding to the probability of reconnection P_{reco} to
be in the range 0.31 < P_{reco} < 0.68 at 68% confidence level with its best
value at 0.52.
|
Evolutionary Neural Gas (ENG): A Model of Self Organizing Network from
Input Categorization | Despite their claimed biological plausibility, most self organizing networks
have strict topological constraints and consequently they cannot take into
account a wide range of external stimuli. Furthermore their evolution is
conditioned by deterministic laws which often are not correlated with the
structural parameters and the global status of the network, as it should happen
in a real biological system. In nature the environmental inputs are noise
affected and fuzzy. Which thing sets the problem to investigate the possibility
of emergent behaviour in a not strictly constrained net and subjected to
different inputs. It is here presented a new model of Evolutionary Neural Gas
(ENG) with any topological constraints, trained by probabilistic laws depending
on the local distortion errors and the network dimension. The network is
considered as a population of nodes that coexist in an ecosystem sharing local
and global resources. Those particular features allow the network to quickly
adapt to the environment, according to its dimensions. The ENG model analysis
shows that the net evolves as a scale-free graph, and justifies in a deeply
physical sense- the term gas here used.
|
X-ray Dichroism and the Pseudogap Phase of Cuprates | A recent polarized x-ray absorption experiment on the high temperature
cuprate superconductor Bi2Sr2CaCu2O8 indicates the presence of broken parity
symmetry below the temperature, T*, where a pseudogap appears in photoemission.
We critically analyze the x-ray data, and conclude that a parity-breaking
signal of the kind suggested is unlikely based on the crystal structures
reported in the literature. Possible other origins of the observed dichroism
signal are discussed. We propose x-ray scattering experiments that can be done
in order to determine whether such alternative interpretations are valid or
not.
|
Solvability of linear equations within weak mixing sets | We introduce a new class of "random" subsets of natural numbers, WM sets.
This class contains normal sets (sets whose characteristic function is a normal
binary sequence). We establish necessary and sufficient conditions for
solvability of systems of linear equations within every WM set and within every
normal set. We also show that partition-regular system of linear equations with
integer coefficients is solvable in any WM set.
|