619 research outputs found
Towards a statistical theory of solid dry friction
Wearless dry friction of an elastic block of weight N, driven by an external
force F over a rigid substrate, is investigated. The slider and substrate
surfaces are both microscopically rough, interacting via a repulsive potential
that depends on the local overlap. The model reproduces Amontons's laws which
state that the friction force is proportional to the normal loading force N and
independent of the nominal surface area. In this model, the dynamic friction
force decays for large velocities and approaches a finite static friction for
small velocities if the surface profiles are self-affine on small length
scales.Comment: Latex, 10 pages. Jounal reference adde
On the efficient numerical solution of lattice systems with low-order couplings
We apply the Quasi Monte Carlo (QMC) and recursive numerical integration
methods to evaluate the Euclidean, discretized time path-integral for the
quantum mechanical anharmonic oscillator and a topological quantum mechanical
rotor model. For the anharmonic oscillator both methods outperform standard
Markov Chain Monte Carlo methods and show a significantly improved error
scaling. For the quantum mechanical rotor we could, however, not find a
successful way employing QMC. On the other hand, the recursive numerical
integration method works extremely well for this model and shows an at least
exponentially fast error scaling
Contact-induced charge contributions to non-local spin transport measurements in Co/MgO/graphene devices
Recently, it has been shown that oxide barriers in graphene-based non-local
spin-valve structures can be the bottleneck for spin transport. The barriers
may cause spin dephasing during or right after electrical spin injection which
limit spin transport parameters such as the spin lifetime of the whole device.
An important task is to evaluate the quality of the oxide barriers of both spin
injection and detection contacts in a fabricated device. To address this issue,
we discuss the influence of spatially inhomogeneous oxide barriers and
especially conducting pinholes within the barrier on the background signal in
non-local measurements of graphene/MgO/Co spin-valve devices. By both
simulations and reference measurements on devices with non-ferromagnetic
electrodes, we demonstrate that the background signal can be caused by
inhomogeneous current flow through the oxide barriers. As a main result, we
demonstrate the existence of charge accumulation next to the actual spin
accumulation signal in non-local voltage measurements, which can be explained
by a redistribution of charge carriers by a perpendicular magnetic field
similar to the classical Hall effect. Furthermore, we present systematic
studies on the phase of the low frequency non-local ac voltage signal which is
measured in non-local spin measurements when applying ac lock-in techniques.
This phase has so far widely been neglected in the analysis of non-local spin
transport. We demonstrate that this phase is another hallmark of the
homogeneity of the MgO spin injection and detection barriers. We link backgate
dependent changes of the phase to the interplay between the capacitance of the
oxide barrier to the quantum capacitance of graphene.Comment: 19 pages, 7 figure
Absence of non-trivial asymptotic scaling in the Kashchiev model of polynuclear growth
In this brief comment we show that, contrary to previous claims [Bartelt M C
and Evans J W 1993 {\it J.\ Phys.\ A} 2743], the asymptotic
behaviour of the Kashchiev model of polynuclear growth is trivial in all
spatial dimensions, and therefore lies outside the Kardar-Parisi-Zhang
universality class.Comment: 3 pages, 4 postscript figures, uses eps
Pion form factor in large QCD
The electromagnetic form factor of the pion is obtained using a particular
realization of QCD in the large limit, which sums up the infinite number
of zero-width resonances to yield an Euler's Beta function of the Veneziano
type. This form factor agrees with space-like data much better than single
rho-meson dominance. A simple unitarization ansatz is illustrated, and the
resulting vector spectral function in the time-like region is shown to be in
reasonable agreement with the ALEPH data from threshold up to about 1.3
.Comment: Plain Latex, 9 pages, 2 figure
Nanosecond spin lifetimes in single- and few-layer graphene-hBN heterostructures at room temperature
We present a new fabrication method of graphene spin-valve devices which
yields enhanced spin and charge transport properties by improving both the
electrode-to-graphene and graphene-to-substrate interface. First, we prepare
Co/MgO spin injection electrodes onto Si/SiO. Thereafter, we
mechanically transfer a graphene-hBN heterostructure onto the prepatterned
electrodes. We show that room temperature spin transport in single-, bi- and
trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion
lengths reaching 10m combined with carrier mobilities exceeding 20,000
cm/Vs.Comment: 15 pages, 5 figure
Point island models for nucleation and growth of supported nanoclusters during surface deposition
Point island models (PIMs) are presented for the formation of supported nanoclusters (or islands) during deposition on flat crystalline substrates at lower submonolayer coverages. These models treat islands as occupying a single adsorption site, although carrying a label to track their size (i.e., they suppress island structure). However, they are particularly effective in describing the island size and spatial distributions. In fact, these PIMs provide fundamental insight into the key features for homogeneous nucleation and growth processes on surfaces. PIMs are also versatile being readily adapted to treat both diffusion-limited and attachment-limited growth and also a variety of other nucleation processes with modified mechanisms. Their behavior is readily and precisely assessed by kinetic Monte Carlo simulation
Light-Front Model of Transition Form-Factors in Heavy Meson Decay
Electroweak transition form factors of heavy meson decays are important
ingredients in the extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix
elements from experimental data. In this work, within a light-front framework,
we calculate electroweak transition form factor for the semileptonic decay of
mesons into a pion or a kaon. The model results underestimate in both cases
the new data of CLEO for the larger momentum transfers accessible in the
experiment. We discuss possible reasons for that in order to improve the model.Comment: Paper with 5 pages and 2 eps figures. To appear to Nuclear Physics B.
Talk at Light Cone 2009: Relativistic Hadronic and Particle Physics (LC
2009), Sao Jose dos Campos, S.P, Brazil, 8-13 Jul 2009
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