58,115 research outputs found
Complex evolution of the electronic structure from polycrystalline to monocrystalline graphene: generation of a new Dirac point
First principles calculations, employed to address the properties of
polycrystalline graphene, indicate that the electronic structure of tilt grain
boundaries in this system displays a rather complex evolution towards graphene
bulk, as the tilt angle decreases, with the generation of a new Dirac point at
the Fermi level, and an anisotropic Dirac cone of low energy excitations.
Moreover, the usual Dirac point at the {\bf K} point falls below the Fermi
level, and rises towards it as the tilt angle decreases. Further, our
calculations indicate that the grain-boundary formation energy behaves
non-monotonically with the tilt angle, due to a change in the the spatial
distribution and relative contributions of the bond-stretching and bond-bending
deformations associated with the formation of the defect.Comment: 4 pages (+ a few references on 5th page). Contains text (.tex) file +
4 figures + pdf fil
Magnetization reversals in a disk-shaped small magnet with an interface
We consider a nanodisk possessing two coupled materials with different
ferromagnetic exchange constant. The common border line of the two media passes
at the disk center dividing the system exactly in two similar half-disks. The
vortex core motion crossing the interface is investigated with a simple
description based on a two-dimensional model which mimics a very thin real
material with such a line defect. The main result of this study is that,
depending on the magnetic coupling which connects the media, the vortex core
can be dramatically and repeatedly flipped from up to down and vice versa by
the interface. This phenomenon produces burst-like emission of spin waves each
time the switching process takes place.Comment: 11 pages, 10 figure
Oxygen adsorption effect on magnetic properties of graphite
Both experimental and theoretical studies of the magnetic properties of
micrographite and nanographite indicate a crucial role of the partial oxidation
of graphitic zigzag edges in ferromagnetism. In contrast to total and partial
hydrogenation, the oxidation of half of the carbon atoms on the graphite edges
transforms the antiferromagnetic exchange interaction between graphite planes
and over graphite ribbons to the ferromagnetic interaction. The stability of
the ferromagnetism is discussed.Comment: 14 pages, 6 figure
Magnetic monopole and string excitations in a two-dimensional spin ice
We study the magnetic excitations of a square lattice spin-ice recently
produced in an artificial form, as an array of nanoscale magnets. Our analysis,
based upon the dipolar interaction between the nanomagnetic islands, correctly
reproduces the ground-state observed experimentally. In addition, we find
magnetic monopole-like excitations effectively interacting by means of the
usual Coulombic plus a linear confining potential, the latter being related to
a string-like excitation binding the monopoles pairs, what indicates that the
fractionalization of magnetic dipoles may not be so easy in two dimensions.
These findings contrast this material with the three-dimensional analogue,
where such monopoles experience only the Coulombic interaction. We discuss,
however, two entropic effects that affect the monopole interactions: firstly,
the string configurational entropy may loose the string tension and then, free
magnetic monopoles should also be found in lower dimensional spin ices;
secondly, in contrast to the string configurational entropy, an entropically
driven Coulomb force, which increases with temperature, has the opposite effect
of confining the magnetic defects.Comment: 8 pages. Accepted by Journal of Applied Physics (2009
A novel and precise time domain description of MOSFET low frequency noise due to random telegraph signals
Nowadays, random telegraph signals play an important role in integrated
circuit performance variability, leading for instance to failures in memory
circuits. This problem is related to the successive captures and emissions of
electrons at the many traps stochastically distributed at the silicon-oxide
(Si-SiO2) interface of MOS transistors. In this paper we propose a novel
analytical and numerical approach to statistically describe the fluctuations of
current due to random telegraph signal in time domain. Our results include two
distinct situations: when the density of interface trap density is uniform in
energy, and when it is an u-shape curve as prescribed in literature, here
described as simple quadratic function. We establish formulas for relative
error as function of the parameters related to capture and emission
probabilities. For a complete analysis experimental u-shape curves are used and
compared with the theoretical aproach
Non-white frequency noise in spin torque oscillators and its effect on spectral linewidth
We measure the power spectral density of frequency fluctuations in
nanocontact spin torque oscillators over time scales up to 50 ms. We use a
mixer to convert oscillator signals ranging from 10 GHz to 40 GHz into a band
near 70 MHz before digitizing the time domain waveform. We analyze the waveform
using both zero crossing time stamps and a sliding Fourier transform, discuss
the different limitations and advantages of these two methods, and combine them
to obtain a frequency noise spectrum spanning more than five decades of Fourier
frequency . For devices having a free layer consisting of either a single
NiFe layer or a Co/Ni multilayer we find a
frequency noise spectrum that is white at large and varies as \emph{}
at small . The crossover frequency ranges from \approx\unit[10^{4}]{Hz} to
\approx\unit[10^{6}]{Hz} and the component is stronger in the
multilayer devices. Through actual and simulated spectrum analyzer
measurements, we show that frequency noise causes both broadening and a
change in shape of the oscillator's spectral line as measurement time
increases. Our results indicate that the long term stability of spin torque
oscillators cannot be accurately predicted from models based on thermal (white)
noise sources
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