379 research outputs found
An ab initio approach to anisotropic alloying into the Si(001) surface
Employing density functional theory calculations we explore initial stage of
competitive alloying of co-deposited silver and indium atoms into a silicon
surface. Particularly, we identify respective adsorption positions and
activation barriers governing their diffusion on the dimer-reconstructed
silicon surface. Further, we develop a growth model that properly describes
diffusion mechanisms and silicon morphology with the account of silicon
dimerization and the presence of C-type defects. Based on the surface kinetic
Monte Carlo simulations we examine dynamics of bimetallic adsorption and
elaborate on the temperature effects on the submonolayer growth of Ag-In alloy.
A close inspection of adatom migration clearly indicates effective nucleation
of Ag and In atoms, followed by the formation of orthogonal atomic chains. We
show that the epitaxial bimetal growth might potentially lead to exotic
ordering of adatoms in the form of anisotropic two-dimensional lattices via
orthogonal oriented single-metal rows. We argue that this scenario becomes
favorable provided above room temperature, while our numerical results are
shown to be in agreement with experimental findings.Comment: 8 pages, 5 figure
Distributed Environment for Efficient Virtual Machine Image Management in Federated Cloud Architectures
The use of Virtual Machines (VM) in Cloud computing provides various benefits in the overall software engineering lifecycle. These include efficient elasticity mechanisms resulting in higher resource utilization and lower operational costs. VM as software artifacts are created using provider-specific templates, called VM images (VMI), and are stored in proprietary or public repositories for further use. However, some technology specific choices can limit the interoperability among various Cloud providers and bundle the VMIs with nonessential or redundant software packages, leading to increased storage size, prolonged VMI delivery, stagnant VMI instantiation and ultimately vendor lock-in. To address these challenges, we present a set of novel functionalities and design approaches for efficient operation of distributed VMI repositories, specifically tailored for enabling: (i) simplified creation of lightweight and size optimized VMIs tuned for specific application requirements; (ii) multi-objective VMI repository optimization; and (iii) efficient reasoning mechanism to help optimizing complex VMI operations. The evaluation results confirm that the presented approaches can enable VMI size reduction by up to 55%, while trimming the image creation time by 66%. Furthermore, the repository optimization algorithms, can reduce the VMI delivery time by up to 51% and cut down the storage expenses by 3%. Moreover, by implementing replication strategies, the optimization algorithms can increase the system reliability by 74%
Mott Transition of MnO under Pressure: Comparison of Correlated Band Theories
The electronic structure, magnetic moment, and volume collapse of MnO under
pressure are obtained from four different correlated band theory methods; local
density approximation + Hubbard U (LDA+U), pseudopotential self-interaction
correction (pseudo-SIC), the hybrid functional (combined local exchange plus
Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each
method treats correlation among the five Mn 3d orbitals (per spin), including
their hybridization with three O orbitals in the valence bands and their
changes with pressure. The focus is on comparison of the methods for rocksalt
MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa
range). Each method predicts a first-order volume collapse, but with variation
in the predicted volume and critical pressure. Accompanying the volume collapse
is a moment collapse, which for all methods is from high-spin to low-spin (5/2
to 1/2), not to nonmagnetic as the simplest scenario would have. The specific
manner in which the transition occurs varies considerably among the methods:
pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives
insulator-to-insulator and the hybrid method gives an insulator-to-semimetal
transition. Projected densities of states above and below the transition are
presented for each of the methods and used to analyze the character of each
transition. In some cases the rhombohedral symmetry of the
antiferromagnetically ordered phase clearly influences the character of the
transition.Comment: 14 pages, 9 figures. A 7 institute collaboration, Updated versio
Giant natural optical rotation from chiral electromagnons in a collinear antiferromagnet
In NiTeO with a chiral crystal structure, we report on a giant
natural optical rotation of the lowest-energy magnon. This polarization
rotation, as large as 140 deg/mm, corresponds to a path difference between
right and left circular polarizations that is comparable to the sample
thickness. Natural optical rotation, being a measure of structural chirality,
is highly unusual for long-wavelength magnons. The collinear antiferromagnetic
order of NiTeO makes this giant effect even more peculiar: Chirality of
the crystal structure does not affect the magnetic ground state but is strongly
manifested in the lowest excited state. We show that the dynamic
magnetoelectric effect, turning this magnon to a magnetic- and electric-dipole
active hybrid mode, generates the giant natural optical rotation. In finite
magnetic fields, it also leads to a strong optical magnetochiral effect.Comment: 9 pages, 4 figure
Concave Plasmonic Particles: Broad-Band Geometrical Tunability in the Near Infra-Red
Optical resonances spanning the Near and Short Infra-Red spectral regime were
exhibited experimentally by arrays of plasmonic nano-particles with concave
cross-section. The concavity of the particle was shown to be the key ingredient
for enabling the broad band tunability of the resonance frequency, even for
particles with dimensional aspect ratios of order unity. The atypical
flexibility of setting the resonance wavelength is shown to stem from a unique
interplay of local geometry with surface charge distributions
symmetric non-selfadjoint operators, diagonalizable and non-diagonalizable, with real discrete spectrum
Consider in , , the operator family . \ds
H_0= a^\ast_1a_1+... +a^\ast_da_d+d/2 is the quantum harmonic oscillator with
rational frequencies, a symmetric bounded potential, and a real
coupling constant. We show that if , being an explicitly
determined constant, the spectrum of is real and discrete. Moreover we
show that the operator \ds H(g)=a^\ast_1 a_1+a^\ast_2a_2+ig a^\ast_2a_1 has
real discrete spectrum but is not diagonalizable.Comment: 20 page
Patterned Irradiation of YBa_2Cu_3O_(7-x) Thin Films
We present a new experiment on YBa_2Cu_3O_{7-x} (YBCO) thin films using
spatially resolved heavy ion irradiation. Structures consisting of a periodic
array of strong and weak pinning channels were created with the help of metal
masks. The channels formed an angle of +/-45 Deg with respect to the symmetry
axis of the photolithographically patterned structures. Investigations of the
anisotropic transport properties of these structures were performed. We found
striking resemblance to guided vortex motion as it was observed in YBCO single
crystals containing an array of unidirected twin boundaries. The use of two
additional test bridges allowed to determine in parallel the resistivities of
the irradiated and unirradiated parts as well as the respective current-voltage
characteristics. These measurements provided the input parameters for a
numerical simulation of the potential distribution of the Hall patterning. In
contrast to the unidirected twin boundaries in our experiment both strong and
weak pinning regions are spatially extended. The interfaces between
unirradiated and irradiated regions therefore form a Bose-glass contact. The
experimentally observed magnetic field dependence of the transverse voltage
vanishes faster than expected from the numerical simulation and we interpret
this as a hydrodynamical interaction between a Bose-glass phase and a vortex
liquid.Comment: 7 pages, 8 Eps figures included. Submitted to PR
Effect of electron irradiation on vortex dynamics in YBa_2Cu_3O_{7-x} single crystals
We report on drastic change of vortex dynamics with increase of quenched
disorder: for rather weak disorder we found a single vortex creep regime, which
we attribute to a Bragg-glass phase, while for enhanced disorder we found an
increase of both the depinning current and activation energy with magnetic
field, which we attribute to entangled vortex phase. We also found that
introduction of additional defects always increases the depinning current, but
it increases activation energy only for elastic vortex creep, while it
decreases activation energy for plastic vortex creep.Comment: 4 pages, 3 figures, submited to Phys. Rev.
Norm estimates of complex symmetric operators applied to quantum systems
This paper communicates recent results in theory of complex symmetric
operators and shows, through two non-trivial examples, their potential
usefulness in the study of Schr\"odinger operators. In particular, we propose a
formula for computing the norm of a compact complex symmetric operator. This
observation is applied to two concrete problems related to quantum mechanical
systems. First, we give sharp estimates on the exponential decay of the
resolvent and the single-particle density matrix for Schr\"odinger operators
with spectral gaps. Second, we provide new ways of evaluating the resolvent
norm for Schr\"odinger operators appearing in the complex scaling theory of
resonances
Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle
Optical nanoantennas are a novel tool to investigate previously unattainable
dimensions in the nanocosmos. Just like their radio-frequency equivalents,
nanoantennas enhance the light-matter interaction in their feed gap. Antenna
enhancement of small signals promises to open a new regime in linear and
nonlinear spectroscopy on the nanoscale. Without antennas especially the
nonlinear spectroscopy of single nanoobjects is very demanding. Here, we
present for the first time antenna-enhanced ultrafast nonlinear optical
spectroscopy. In particular, we utilize the antenna to determine the nonlinear
transient absorption signal of a single gold nanoparticle caused by mechanical
breathing oscillations. We increase the signal amplitude by an order of
magnitude which is in good agreement with our analytical and numerical models.
Our method will find applications in linear and nonlinear spectroscopy of
nanoobjects, ranging from single protein binding events via nonlinear tensor
elements to the limits of continuum mechanics
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