2,482 research outputs found
Towards absolute calibration of optical tweezers
Aiming at absolute force calibration of optical tweezers, following a
critical review of proposed theoretical models, we present and test the results
of MDSA (Mie-Debye-Spherical Aberration) theory, an extension of a previous
(MD) model, taking account of spherical aberration at the glass/water
interface. This first-principles theory is formulated entirely in terms of
experimentally accessible parameters (none adjustable). Careful experimental
tests of the MDSA theory, undertaken at two laboratories, with very different
setups, are described. A detailed description is given of the procedures
employed to measure laser beam waist, local beam power at the transparent
microspheres trapped by the tweezers, microsphere radius and the trap
transverse stiffness, as a function of radius and height in the (inverted
microscope) sample chamber. We find generally very good agreement with MDSA
theory predictions, for a wide size range, from the Rayleigh domain to large
radii, including the values most often employed in practice, and at different
chamber heights, both with objective overfilling and underfilling. The results
asymptotically approach geometrical optics in the mean over size intervals, as
they should, and this already happens for size parameters not much larger than
unity. MDSA predictions for the trapping threshold, position of stiffness peak,
stiffness variation with height, multiple equilibrium points and `hopping'
effects among them are verified. Remaining discrepancies are ascribed to focus
degradation, possibly arising from objective aberrations in the infrared, not
yet included in MDSA theory.Comment: 15 pages, 20 figure
3D-QSARpy: Combining variable selection strategies and machine learning techniques to build QSAR models
Quantitative Structure-Activity Relationship (QSAR) is a computer-aided technology in the field of medicinal chemistry that seeks to clarify the relationships between molecular structures and their biological activities. Such technologies allow for the acceleration of the development of new compounds by reducing the costs of drug design. This work presents 3D-QSARpy, a flexible, user-friendly and robust tool, freely available without registration, to support the generation of QSAR 3D models in an automated way. The user only needs to provide aligned molecular structures and the respective dependent variable. The current version was developed using Python with packages such as scikit-learn and includes various techniques of machine learning for regression. The diverse techniques employed by the tool is a differential compared to known methodologies, such as CoMFA and CoMSIA, because it expands the search space of possible solutions, and in this way increases the chances of obtaining relevant models. Additionally, approaches for select variables (dimension reduction) were implemented in the tool. To evaluate its potentials, experiments were carried out to compare results obtained from the proposed 3D-QSARpy tool with the results from already published works. The results demonstrated that 3D-QSARpy is extremely useful in the field due to its expressive results
Atributos químicos das fases sólida e aquosa do solo de sítios de restinga sob diferentes coberturas vegetais no estado de Sergipe.
bitstream/CPATC/19773/1/f_11_2007.pd
The Little-Hopfield model on a Random Graph
We study the Hopfield model on a random graph in scaling regimes where the
average number of connections per neuron is a finite number and where the spin
dynamics is governed by a synchronous execution of the microscopic update rule
(Little-Hopfield model).We solve this model within replica symmetry and by
using bifurcation analysis we prove that the spin-glass/paramagnetic and the
retrieval/paramagnetictransition lines of our phase diagram are identical to
those of sequential dynamics.The first-order retrieval/spin-glass transition
line follows by direct evaluation of our observables using population dynamics.
Within the accuracy of numerical precision and for sufficiently small values of
the connectivity parameter we find that this line coincides with the
corresponding sequential one. Comparison with simulation experiments shows
excellent agreement.Comment: 14 pages, 4 figure
Replicated Transfer Matrix Analysis of Ising Spin Models on `Small World' Lattices
We calculate equilibrium solutions for Ising spin models on `small world'
lattices, which are constructed by super-imposing random and sparse Poissonian
graphs with finite average connectivity c onto a one-dimensional ring. The
nearest neighbour bonds along the ring are ferromagnetic, whereas those
corresponding to the Poisonnian graph are allowed to be random. Our models thus
generally contain quenched connectivity and bond disorder. Within the replica
formalism, calculating the disorder-averaged free energy requires the
diagonalization of replicated transfer matrices. In addition to developing the
general replica symmetric theory, we derive phase diagrams and calculate
effective field distributions for two specific cases: that of uniform sparse
long-range bonds (i.e. `small world' magnets), and that of (+J/-J) random
sparse long-range bonds (i.e. `small world' spin-glasses).Comment: 22 pages, LaTeX, IOP macros, eps figure
Twisted bilayer graphene: low-energy physics, electronic and optical properties
Van der Waals (vdW) heterostructures ---formed by stacking or growing
two-dimensional (2D) crystals on top of each other--- have emerged as a new
promising route to tailor and engineer the properties of 2D materials. Twisted
bilayer graphene (tBLG), a simple vdW structure where the interference between
two misaligned graphene lattices leads to the formation of a moir\'e pattern,
is a test bed to study the effects of the interaction and misalignment between
layers, key players for determining the electronic properties of these
stackings. In this chapter, we present in a pedagogical way the general theory
used to describe lattice mismatched and misaligned vdW structures. We apply it
to the study of tBLG in the limit of small rotations and see how the coupling
between the two layers leads both to an angle dependent renormalization of
graphene's Fermi velocity and appearance of low-energy van Hove singularities.
The optical response of this system is then addressed by computing the optical
conductivity and the dispersion relation of tBLG surface plasmon-polaritons
How to determine an effective potential for a variable cosmological term
It is shown that if a variable cosmological term in the present Universe is
described by a scalar field with minimal coupling to gravity and with some
phenomenological self-interaction potential , then this potential
can be unambiguously determined from the following observational data: either
from the behaviour of density perturbations in dustlike matter component as a
function of redshift (given the Hubble constant additionally), or from the
luminosity distance as a function of redshift (given the present density of
dustlike matter in terms of the critical one).Comment: Latex, 7 pages, JETP Lett., in press, 199
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