1,307 research outputs found
Properties and characterization of ALD grown dielectric oxides for MIS structures
We report on an extensive structural and electrical characterization of
under-gate dielectric oxide insulators Al2O3 and HfO2 grown by Atomic Layer
Deposition (ALD). We elaborate the ALD growth window for these oxides, finding
that the 40-100 nm thick layers of both oxides exhibit fine surface flatness
and required amorphous structure. These layers constitute a base for further
metallic gate evaporation to complete the Metal-Insulator-Semiconductor
structure. Our best devices survive energizing up to ~3 MV/cm at 77 K with the
leakage current staying below the state-of-the-art level of 1 nA. At these
conditions the displaced charge corresponds to a change of the sheet carrier
density of 3 \times 1013 cm-2, what promises an effective modulation of the
micromagnetic properties in diluted ferromagnetic semiconductors.Comment: 8 pages, 5 figures, 14 reference
Theory Challenges of the Accelerating Universe
The accelerating expansion of the universe presents an exciting, fundamental
challenge to the standard models of particle physics and cosmology. I highlight
some of the outstanding challenges in both developing theoretical models and
interpreting without bias the observational results from precision cosmology
experiments in the next decade that will return data to help reveal the nature
of the new physics. Examples given focus on distinguishing a new component of
energy from a new law of gravity, and the effect of early dark energy on baryon
acoustic oscillations.Comment: 10 pages, 4 figures; minor changes to match J. Phys. A versio
Lithographic engineering of anisotropies in (Ga,Mn)As
The focus of studies on ferromagnetic semiconductors is moving from material
issues to device functionalities based on novel phenomena often associated with
the anisotropy properties of these materials. This is driving a need for a
method to locally control the anisotropy in order to allow the elaboration of
devices. Here we present a method which provides patterning induced anisotropy
which not only can be applied locally, but also dominates over the intrinsic
material anisotropy at all temperatures
Ellipsoidal universe in the brane world
We study a scenario of the ellipsoidal universe in the brane world cosmology
with a cosmological constant in the bulk . From the five-dimensional Einstein
equations we derive the evolution equations for the eccentricity and the scale
factor of the universe, which are coupled to each other. It is found that if
the anisotropy of our universe is originated from a uniform magnetic field
inside the brane, the eccentricity decays faster in the bulk in comparison with
a four-dimensional ellipsoidal universe. We also investigate the ellipsoidal
universe in the brane-induced gravity and find the evolution equation for the
eccentricity which has a contribution determined by the four- and
five-dimensional Newton's constants. The role of the eccentricity is discussed
in explaining the quadrupole problem of the cosmic microwave background.Comment: 15 pages, 1 figure, Version 3, references added, contents expande
Finding the Center of Mass of a Soft Spring
This article shows how to use calculus to find the center of mass position of
a soft cylindrical helical spring that is suspended vertically. The spring is
non-uniformly stretched by the action of gravity. A general expression for the
vertical position of the center of mass is obtained.Comment: LaTeX, 7 pages, 2 figures. Minor changes to agree with published
versio
Mass Screening in Modified Gravity
Models of modified gravity introduce extra degrees of freedom, which for
consistency with the data, should be suppressed at observable scales. In the
models that share properties of massive gravity such a suppression is due to
nonlinear interactions: An isolated massive astrophysical object creates a halo
of a nonzero curvature around it, shielding its vicinity from the influence of
the extra degrees of freedom. We emphasize that the very same halo leads to a
screening of the gravitational mass of the object, as seen by an observer
beyond the halo. We discuss the case when the screening could be very
significant and may rule out, or render the models observationally interesting.Comment: 16 pages, 4 figures, A contribution to the Proceedings of the
International Workshop on Cosmology and Gravitation, Peyresq 12, June 16-22,
2007, Peyresq, Franc
Modified-Source Gravity and Cosmological Structure Formation
One way to account for the acceleration of the universe is to modify general
relativity, rather than introducing dark energy. Typically, such modifications
introduce new degrees of freedom. It is interesting to consider models with no
new degrees of freedom, but with a modified dependence on the conventional
energy-momentum tensor; the Palatini formulation of theories is one
example. Such theories offer an interesting testing ground for investigations
of cosmological modified gravity. In this paper we study the evolution of
structure in these ``modified-source gravity'' theories. In the linear regime,
density perturbations exhibit scale dependent runaway growth at late times and,
in particular, a mode of a given wavenumber goes nonlinear at a higher redshift
than in the standard CDM model. We discuss the implications of this
behavior and why there are reasons to expect that the growth will be cut off in
the nonlinear regime. Assuming that this holds in a full nonlinear analysis, we
briefly describe how upcoming measurements may probe the differences between
the modified theory and the standard CDM model.Comment: 22 pages, 6 figures, uses iopart styl
Reconstruction of the Scalar-Tensor Lagrangian from a LCDM Background and Noether Symmetry
We consider scalar-tensor theories and reconstruct their potential U(\Phi)
and coupling F(\Phi) by demanding a background LCDM cosmology. In particular we
impose a background cosmic history H(z) provided by the usual flat LCDM
parameterization through the radiation (w_{eff}=1/3), matter (w_{eff}=0) and
deSitter (w_{eff}=-1) eras. The cosmological dynamical system which is
constrained to obey the LCDM cosmic history presents five critical points in
each era, one of which corresponding to the standard General Relativity (GR).
In the cases that differ from GR, the reconstructed coupling and potential are
of the form F(\Phi)\sim \Phi^2 and U(\Phi)\sim F(\Phi)^m where m is a constant.
This class of scalar tensor theories is also theoretically motivated by a
completely independent approach: imposing maximal Noether symmetry on the
scalar-tensor Lagrangian. This approach provides independently: i) the form of
the coupling and the potential as F(\Phi)\sim \Phi^2 and U(\Phi)\sim F(\Phi)^m,
ii) a conserved charge related to the potential and the coupling and iii)
allows the derivation of exact solutions by first integrals of motion.Comment: Added comments, discussion, references. 15 revtex pages, 5 fugure
Electromagnetic form factors of the meson in a light-front constituent quark model
The electromagnetic form factors of the meson are evaluated adopting a
relativistic constituent quark model based on the light-front formalism, and
using a meson wave function with the high-momentum tail generated by the
one-gluon-exchange interaction. The breakdown of the rotational covariance for
the one-body component of the current operator is investigated and the
sensitivity of the ratio of the -meson form factors to the pion (charge)
form factor to the spin-dependent component of the effective
interaction is illustrated.Comment: 8 pages, latex file, 4 figures available as a separate .uu fil
- …