162 research outputs found
Molecular dynamics study of dislocation nucleation from a crack tip
We have performed a systematic molecular dynamics study of the competition between crack growth and dislocation emission from a crack tip. Two types of boundary conditions are adopted: either planar extension or boundary displacements according to the anisotropic mode-I asymptotic continuum solution. The effects of temperature, loading rate, crystal orientation, sharpness of the crack tip, atomic potential, and system size are investigated. Depending on the crystal orientation, dislocation nucleation can be driven either by the strain or by concerted fluctuations at the crack tip. In the latter case, crystal orientation and temperature have the largest influence on the process of dislocation nucleation
Breaking the silence of the 500-year-old smiling garden of everlasting flowers: The En Tibi book herbarium
We reveal the enigmatic origin of one of the earliest surviving botanical collections. The 16th-century Italian En Tibi herbarium is a large, luxurious book with c. 500 dried plants, made in the Renaissance scholarly circles that developed botany as a distinct discipline. Its Latin inscription, translated as “Here for you a smiling garden of everlasting flowers”, suggests that this herbarium was a gift for a patron of the emerging botanical science. We follow an integrative approach that includes a botanical similarity estimation of the En Tibi with contemporary herbaria (Aldrovandi, Cesalpino, “Cibo”, Merini, Estense) and analysis of the book’s watermark, paper, binding, handwriting, Latin inscription and the morphology and DNA of hairs mounted under specimens. Rejecting the previous origin hypothesis (Ferrara, 1542–1544), we show that the En Tibi was made in Bologna around 1558. We attribute the En Tibi herbarium to Francesco Petrollini, a neglected 16th-century botanist, to whom also belongs, as clarified herein, the controversial “Erbario Cibo” kept in Rome. The En Tibi was probably a work on commission for Petrollini, who provided the plant material for the book. Other people were apparently involved in the compilation and offering of this precious gift to a yet unknown person, possibly the Habsburg Emperor Ferdinand I. The En Tibi herbarium is a Renaissance masterpiece of art and science, representing the quest for truth in herbal medicine and botany. Our multidisciplinary approach can serve as a guideline for deciphering other anonymous herbaria, kept safely “hidden” in treasure rooms of universities, libraries and museums
A Current Induced Transition in atomic-sized contacts of metallic Alloys
We have measured conductance histograms of atomic point contacts made from
the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration
ratio of 1:1. For all alloys these histograms at low bias voltage (below 300
mV) resemble those of the noble metals whereas at high bias (above 300 mV) they
resemble those of the transition metals. We interpret this effect as a change
in the composition of the point contact with bias voltage. We discuss possible
explanations in terms of electromigration and differential diffusion induced by
current heating.Comment: 5 pages, 6 figure
Field of inserted charges during Scanning Electron Microscopy of non-conducting samples
Three different approaches to calculating the electric potential in an inhomogeneous dielectric next to vacuum due to a charge distribution built up by the electron beam are investigated. An analytical solution for the electric potential cannot be found by means of the image charge method or Fourier analysis, both of which do work for a homogenous dielectric with a planar interface to vacuum. A Born approximation gives a good approach to the real electric potential in a homogenous dielectric up to a relative dielectric constant of 5. With this knowledge the electric potential of an inhomogenous dielectric is calculated. Also the electric field is calculated by means of a particle-mesh method. Some inhomogeneous dielectric configurations are calculated and their bound charges are studied. Such a method can yield accurate calculations of the electric potential and can give quantitative insight in the charging process. A capacitor model is described to estimate the potential due to the charge build up. It describes the potential build up in the first microseconds of the charging. Thereafter, it seems that more processes have to be taken into account to describe the potential well. This potential can further be used in a macroscopic approach to the collective motion of the electrons described by the Boltzmann transport equations or a derived density model, which can be a feasible alternative approximation to the more commonly used Monte-Carlo simulation of individual trajectories
Local structure study of In_xGa_(1-x)As semiconductor alloys using High Energy Synchrotron X-ray Diffraction
Nearest and higher neighbor distances as well as bond length distributions
(static and thermal) of the In_xGa_(1-x)As (0<x<1) semiconductor alloys have
been obtained from high real-space resolution atomic pair distribution
functions (PDFs). Using this structural information, we modeled the local
atomic displacements in In_xGa_(1-x)As alloys. From a supercell model based on
the Kirkwood potential, we obtained 3-D As and (In,Ga) ensemble averaged
probability distributions. This clearly shows that As atom displacements are
highly directional and can be represented as a combination of and
displacements. Examination of the Kirkwood model indicates that the standard
deviation (sigma) of the static disorder on the (In,Ga) sublattice is around
60% of the value on the As sublattice and the (In,Ga) atomic displacements are
much more isotropic than those on the As sublattice. The single crystal diffuse
scattering calculated from the Kirkwood model shows that atomic displacements
are most strongly correlated along directions.Comment: 10 pages, 12 figure
Ising Universality in Three Dimensions: A Monte Carlo Study
We investigate three Ising models on the simple cubic lattice by means of
Monte Carlo methods and finite-size scaling. These models are the spin-1/2
Ising model with nearest-neighbor interactions, a spin-1/2 model with
nearest-neighbor and third-neighbor interactions, and a spin-1 model with
nearest-neighbor interactions. The results are in accurate agreement with the
hypothesis of universality. Analysis of the finite-size scaling behavior
reveals corrections beyond those caused by the leading irrelevant scaling
field. We find that the correction-to-scaling amplitudes are strongly dependent
on the introduction of further-neighbor interactions or a third spin state. In
a spin-1 Ising model, these corrections appear to be very small. This is very
helpful for the determination of the universal constants of the Ising model.
The renormalization exponents of the Ising model are determined as y_t = 1.587
(2), y_h = 2.4815 (15) and y_i = -0.82 (6). The universal ratio Q =
^2/ is equal to 0.6233 (4) for periodic systems with cubic symmetry.
The critical point of the nearest-neighbor spin-1/2 model is K_c=0.2216546
(10).Comment: 25 pages, uuencoded compressed PostScript file (to appear in Journal
of Physics A
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