7,184 research outputs found
Ferromagnetic Film on a Superconducting Substrate
We study the equilibrium domain structure and magnetic flux around a
ferromagnetic (FM) film with perpendicular magnetization M_0 on a
superconducting (SC) substrate. At 4{\pi}M_0<H_{c1} the SC is in the Meissner
state and the equilibrium domain width in the film, l, scales as
(l/4{\pi}{\lambda}_{L}) = (l_{N}/4{\pi}{\lambda}_{L})^{2/3} with the domain
width on a normal (non-superconducting) substrate, l_{N}/4\pi\lambda_L >> 1.
Here \lambda_L is the London penetration length. For 4{\pi}M_0 > H_{c1} and
l_{N} in excess of about 35 {\lambda}_{L}, the domains are connected by SC
vortices. We argue that pinning of vortices by magnetic domains in FM/SC
multilayers can provide high critical currents.Comment: 4 pages, 2 figures, submitted to PR
Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics
We revisit the color-diffusion algorithm [P. C. Aeberhard et al., Phys. Rev.
Lett. 108, 095901 (2012)] in nonequilibrium ab initio molecular dynamics
(NE-AIMD), and propose a simple efficient approach for the estimation of
monovacancy jump rates in crystalline solids at temperatures well below
melting. Color-diffusion applied to monovacancy migration entails that one
lattice atom (colored-atom) is accelerated toward the neighboring defect-site
by an external constant force F. Considering bcc molybdenum between 1000 and
2800 K as a model system, NE-AIMD results show that the colored-atom jump rate
k_{NE} increases exponentially with the force intensity F, up to F values far
beyond the linear-fitting regime employed previously. Using a simple model, we
derive an analytical expression which reproduces the observed k_{NE}(F)
dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in
excellent agreement with those of unconstrained dynamics. The gain in
computational efficiency achieved with our approach increases rapidly with
decreasing temperatures, and reaches a factor of four orders of magnitude at
the lowest temperature considered in the present study
Temperature dependent effective potential method for accurate free energy calculations of solids
We have developed a thorough and accurate method of determining anharmonic
free energies, the temperature dependent effective potential technique (TDEP).
It is based on \emph{ab initio} molecular dynamics followed by a mapping onto a
model Hamiltonian that describes the lattice dynamics. The formalism and the
numerical aspects of the technique are described in details. A number of
practical examples are given, and results are presented, which confirm the
usefulness of TDEP within \emph{ab initio} and classical molecular dynamics
frameworks. In particular, we examine from first-principles the behavior of
force constants upon the dynamical stabilization of body centered phase of Zr,
and show that they become more localized. We also calculate phase diagram for
He modeled with the Aziz \emph{et al.} potential and obtain results which
are in favorable agreement both with respect to experiment and established
techniques
From electronic structure to catalytic activity: A single descriptor for adsorption and reactivity on transition-metal carbides
Adsorption and catalytic properties of the polar (111) surface of
transition-metal carbides (TMC's) are investigated by density-functional
theory. Atomic and molecular adsorption are rationalized with the
concerted-coupling model, in which two types of TMC surface resonances (SR's)
play key roles. The transition-metal derived SR is found to be a single
measurable descriptor for the adsorption processes, implying that the
Br{\o}nsted-Evans-Polanyi relation and scaling relations apply. This gives a
picture with implications for ligand and vacancy effects and which has a
potential for a broad screening procedure for heterogeneous catalysts.Comment: 5 pages, 3 figure
Lattice dynamics of anharmonic solids from first principles
An accurate and easily extendable method to deal with lattice dynamics of
solids is offered. It is based on first-principles molecular dynamics
simulations and provides a consistent way to extract the best possible harmonic
- or higher order - potential energy surface at finite temperatures. It is
designed to work even for strongly anharmonic systems where the traditional
quasiharmonic approximation fails. The accuracy and convergence of the method
are controlled in a straightforward way. Excellent agreement of the calculated
phonon dispersion relations at finite temperature with experimental results for
bcc Li and bcc Zr is demonstrated
Fred and Steve Hellman and Roger Redoing Interview
Transcript of an oral history interview with Fred and Steve Hellman and Roger Redoing by Debbie Perry and Delores Roe on their experiences during the Vietnam War in May of 1998
Dissociation of O2 at Al(111): The Role of Spin Selection Rules
A most basic and puzzling enigma in surface science is the description of the
dissociative adsorption of O2 at the (111) surface of Al. Already for the
sticking curve alone, the disagreement between experiment and results of
state-of-the-art first-principles calculations can hardly be more dramatic. In
this paper we show that this is caused by hitherto unaccounted spin selection
rules, which give rise to a highly non-adiabatic behavior in the O2/Al(111)
interaction. We also discuss problems caused by the insufficient accuracy of
present-day exchange-correlation functionals.Comment: 4 pages including 3 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
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