7,737 research outputs found
Stretching dependence of the vibration modes of a single-molecule Pt-H2-Pt bridge
A conducting bridge of a single hydrogen molecule between Pt electrodes is
formed in a break junction experiment. It has a conductance near the quantum
unit, G_0 = 2e^2/h, carried by a single channel. Using point contact
spectroscopy three vibration modes are observed and their variation upon
stretching and isotope substitution is obtained. The interpretation of the
experiment in terms of a Pt-H_2-Pt bridge is verified by Density Functional
Theory calculations for the stability, vibrational modes, and conductance of
the structure.Comment: 5 pages, 4 figure
Inductance mode characteristics of a ceramic YBa2Cu3O7-x radio-frequency superconducting quantum interference device at 77 K
Interacting classical dimers on the square lattice
We study a model of close-packed dimers on the square lattice with a nearest
neighbor interaction between parallel dimers. This model corresponds to the
classical limit of quantum dimer models [D.S. Rokhsar and S.A. Kivelson, Phys.
Rev. Lett.{\bf 61}, 2376 (1988)]. By means of Monte Carlo and Transfer Matrix
calculations, we show that this system undergoes a Kosterlitz-Thouless
transition separating a low temperature ordered phase where dimers are aligned
in columns from a high temperature critical phase with continuously varying
exponents. This is understood by constructing the corresponding Coulomb gas,
whose coupling constant is computed numerically. We also discuss doped models
and implications on the finite-temperature phase diagram of quantum dimer
models.Comment: 4 pages, 4 figures; v2 : Added results on doped models; published
versio
Dislocation Kinks in Copper: Widths, Barriers, Effective Masses, and Quantum Tunneling
We calculate the widths, migration barriers, effective masses, and quantum
tunneling rates of kinks and jogs in extended screw dislocations in copper,
using an effective medium theory interatomic potential. The energy barriers and
effective masses for moving a unit jog one lattice constant are close to
typical atomic energies and masses: tunneling will be rare. The energy barriers
and effective masses for the motion of kinks are unexpectedly small due to the
spreading of the kinks over a large number of atoms. The effective masses of
the kinks are so small that quantum fluctuations will be important. We discuss
implications for quantum creep, kink--based tunneling centers, and Kondo
resonances
Simulations of energetic beam deposition: from picoseconds to seconds
We present a new method for simulating crystal growth by energetic beam
deposition. The method combines a Kinetic Monte-Carlo simulation for the
thermal surface diffusion with a small scale molecular dynamics simulation of
every single deposition event. We have implemented the method using the
effective medium theory as a model potential for the atomic interactions, and
present simulations for Ag/Ag(111) and Pt/Pt(111) for incoming energies up to
35 eV. The method is capable of following the growth of several monolayers at
realistic growth rates of 1 monolayer per second, correctly accounting for both
energy-induced atomic mobility and thermal surface diffusion. We find that the
energy influences island and step densities and can induce layer-by-layer
growth. We find an optimal energy for layer-by-layer growth (25 eV for Ag),
which correlates with where the net impact-induced downward interlayer
transport is at a maximum. A high step density is needed for energy induced
layer-by-layer growth, hence the effect dies away at increased temperatures,
where thermal surface diffusion reduces the step density. As part of the
development of the method, we present molecular dynamics simulations of single
atom-surface collisions on flat parts of the surface and near straight steps,
we identify microscopic mechanisms by which the energy influences the growth,
and we discuss the nature of the energy-induced atomic mobility
Mechanical properties and formation mechanisms of a wire of single gold atoms
A scanning tunneling microscope (STM) supplemented with a force sensor is
used to study the mechanical properties of a novel metallic nanostructure: a
freely suspended chain of single gold atoms. We find that the bond strength of
the nanowire is about twice that of a bulk metallic bond. We perform ab initio
calculations of the force at chain fracture and compare quantitatively with
experimental measurements. The observed mechanical failure and nanoelastic
processes involved during atomic wire fabrication are investigated using
molecular dynamics (MD) simulations, and we find that the total effective
stiffness of the nanostructure is strongly affected by the detailed local
atomic arrangement at the chain bases.Comment: To be published in Phys. Rev. Lett. 4 pages with 3 figure
Continuous melting of compact polymers
The competition between chain entropy and bending rigidity in compact
polymers can be addressed within a lattice model introduced by P.J. Flory in
1956. It exhibits a transition between an entropy dominated disordered phase
and an energetically favored crystalline phase. The nature of this
order-disorder transition has been debated ever since the introduction of the
model. Here we present exact results for the Flory model in two dimensions
relevant for polymers on surfaces, such as DNA adsorbed on a lipid bilayer. We
predict a continuous melting transition, and compute exact values of critical
exponents at the transition point.Comment: 5 pages, 1 figur
Apoplexia em tumor hipofisário
Pituitary tumor apoplexy is a medical emergency due to acute infarction or hemorrhage in the pituitary gland. In this review, the authors discuss the sellar anatomy, the pituitary gland and adenomas' vascularization and the general aspects of the syndrome such as its ethiopatogenesis, predisposing factors, clinical features, treatment and prognosis.A apoplexia em tumor hipofisário é uma emergência médica decorrente do infarto agudo ou hemorrágico na glândula hipófise. Nesta revisão os autores discutem a anatomia da região selar, a vascularização da hipófise e adenomas hipofisários, e demais aspectos da sÃndrome como etiopatogenia, fatores predisponentes, quadro clÃnico, tratamento e prognóstico
Signatures of polaronic excitations in quasi-one-dimensional LaTiO
The optical properties of quasi-one-dimensional metallic LaTiO are
studied for the polarization along the and axes. With decreasing
temperature modes appear along both directions suggestive for a phase
transition. The broadness of these modes along the conducting axis might be due
to the coupling of the phonons to low-energy electronic excitations across an
energy gap. We observe a pronounced midinfrared band with a temperature
dependence consistent with (interacting) polaron models. The polaronic picture
is corroborated by the presence of strong electron-phonon coupling and the
temperature dependence of the dc conductivity.Comment: 5 pages, 5 figure
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