333 research outputs found
Transient lateral photovoltaic effect in patterned metal-oxide-semiconductor films
The time dependent transient lateral photovoltaic effect has been studied
with us time resolution and with chopping frequencies in the kHz range, in
lithographically patterned 21 nm thick, 5, 10 and 20 um wide and 1500 um long
Co lines grown over naturally passivated p-type Si (100). We have observed a
nearly linear dependence of the transitorial response with the laser spot
position. A transitorial response with a sign change in the laser-off stage has
been corroborated by numerical simulations. A qualitative explanation suggests
a modification of the drift-diffusion model by including the in uence of a
local inductance. Our findings indicate that the microstructuring of position
sensitive detectors could improve their space-time resolution.Comment: 4 pages, 4 figure
Controlling shot noise in double-barrier magnetic tunnel junctions
We demonstrate that shot noise in Fe/MgO/Fe/MgO/Fe double-barrier magnetic
tunnel junctions is determined by the relative magnetic configuration of the
junction and also by the asymmetry of the barriers. The proposed theoretical
model, based on sequential tunneling through the system and including spin
relaxation, successfully accounts for the experimental observations for bias
voltages below 0.5V, where the influence of quantum well states is negligible.
A weak enhancement of conductance and shot noise, observed at some voltages
(especially above 0.5V), indicates the formation of quantum well states in the
middle magnetic layer. The observed results open up new perspectives for a
reliable magnetic control of the most fundamental noise in spintronic
structures.Comment: 8 pages, 4 figure
Heterotic compactifications and nearly-Kahler manifolds
We propose that under certain conditions heterotic string compactifications
on half-flat and nearly-Kahler manifolds are equivalent. Based on this
correspondence we argue that the moduli space of the nearly-Kahler manifolds
under discussion consists only of the Kahler deformations moduli space and
there is no correspondent for the complex structure deformations.Comment: 5 pages, references added, typos correcte
D6-branes and torsion
The D6-brane spectrum of type IIA vacua based on twisted tori and RR
background fluxes is analyzed. In particular, we compute the torsion factors of
the (co)homology groups H_n and describe the effect that they have on D6-brane
physics. For instance, the fact that H_3 contains Z_N subgroups explains why RR
tadpole conditions are affected by geometric fluxes. In addition, the presence
of torsional (co)homology shows why some D6-brane moduli are lifted, and it
suggests how the D-brane discretum appears in type IIA flux compactifications.
Finally, we give a clear, geometrical understanding of the Freed-Witten anomaly
in the present type IIA setup, and discuss its consequences for the
construction of semi-realistic flux vacua.Comment: 35 pages, 1 figure. One reference adde
M-Branes on k-center Instantons
We present analytic solutions for membrane metric function based on
transverse -center instanton geometries. The membrane metric functions
depend on more than two transverse coordinates and the solutions provide
realizations of fully localized type IIA D2/D6 and NS5/D6 brane intersections.
All solutions have partial preserved supersymmetries.Comment: 22 pages, 5 figure
Magnetization reversal in sub-100nm magnetic tunnel junctions with ultrathin MgO barrier biased along hard axis
We report on room temperature magnetoresistance and low frequency noise in
sub-100nm elliptic CoFeB/MgO/CoFeB magnetic tunnel junctions with ultrathin
(0.9nm) barriers. For magnetic fields applied along the hard axis, we observe
current induced magnetization switching between the antiparallel and parallel
alignments at DC current densities as low as 4*106A/cm2. We attribute the low
value of the critical current to the influence of localized reductions in the
tunnel barrier, which affects the current distribution. The analysis of random
telegraph noise, which appears in the field interval near a magnetization
switch, provides an estimate to the dimension of the pseudo pinholes that
trigger the magnetization switching via local spin torque. Micromagnetic
simulations qualitatively and quantitatively reproduce the main experimental
observations
An Alternative Interpretation for the Moduli Fields of the Cosmology Associated to Type IIB Supergravity with Fluxes
We start with a particular cosmological model derived from type IIB
supergravity theory with fluxes, where usually the dilaton is interpreted as a
Quintessence field. Instead of that, in this letter we interpret the dilaton as
the dark matter of the universe. With this alternative interpretation we find
that in this supergravity model gives a similar evolution and structure
formation of the universe compared with the CDM model in the linear
regime of fluctuations of the structure formation. Some free parameters of the
theory are fixed using the present cosmological observations. In the non-linear
regimen there are some differences between the type IIB supergravity theory
with the traditional CDM paradigm. The supergravity theory predicts the
formation of galaxies earlier than the CDM and there is no density cusp in the
center of galaxies. These differences can distinguish both models and can give
a distinctive feature to the phenomenology of the cosmology coming from
superstring theory with fluxes.Comment: 7 pages, 5 figures, references added, minor modifications, typos
corrected. Version accepted for publication in IJMP
Branes on Generalized Calibrated Submanifolds
We extend previous results on generalized calibrations to describe
supersymmetric branes in supergravity backgrounds with diverse fields turned
on, and provide several new classes of examples. As an important application,
we show that supersymmetric D-branes in compactifications with field strength
fluxes, and on SU(3)-structure spaces, wrap generalized calibrated
submanifolds, defined by simple conditions in terms of the underlying globally
defined, but non-closed, 2- and 3-forms. We provide examples where the
geometric moduli of D-branes (for instance D7-branes in 3-form flux
configurations) are lifted by the generalized calibration condition. In
addition, we describe supersymmetric D6-branes on generalized calibrated
3-submanifolds of half-flat manifolds, which provide the mirror of B-type
D-branes in IIB CY compactifications with 3-form fluxes. Supersymmetric sets of
such D-branes carrying no homology charges are mirror to supersymmetric sets of
D-branes which are homologically non-trivial, but trivial in K-theory. As an
additional application, we describe models with chiral gauge sectors, realized
in terms of generalized calibrated brane box configurations of NS- and
D5-branes, which are supersymmetric but carry no charges, so that no
orientifold planes are required in the compactification.Comment: 40 pages, 3 figures, references adde
Fluxes, moduli fixing and MSSM-like vacua in a simple IIA orientifold
We study the effects of adding RR, NS and metric fluxes on a T^6/(\Omega
(-1)^{F_L} I_3) Type IIA orientifold. By using the effective flux-induced
superpotential we obtain Minkowski or AdS vacua with broken or unbroken
supersymmetry. In the Minkowski case some combinations of real moduli remain
undetermined, whereas all can be stabilized in the AdS solutions. Many flux
parameters are available which are unconstrained by RR tadpole cancellation
conditions allowing to locate the minima at large volume and small dilaton. We
also find that in AdS supersymmetric vacua with metric fluxes, the overall flux
contribution to RR tadpoles can vanish or have opposite sign to that of
D6-branes, allowing for new model-building possibilities. In particular, we
construct the first N=1 supersymmetric intersecting D6-brane models with
MSSM-like spectrum and with all closed string moduli stabilized. Some
axion-like fields remain undetermined but they are precisely required to give
St\"uckelberg masses to (potentially anomalous) U(1) brane fields. We show that
the cancellation of the Freed-Witten anomaly guarantees that the axions with
flux-induced masses are orthogonal to those giving masses to the U(1)'s.
Cancellation of such anomalies also guarantees that the D6-branes in our N=1
supersymmetric AdS vacua are calibrated so that they are forced to preserve one
unbroken supersymmetry.Comment: 61 pages, Latex, v2: added references, v3: minor correction
Intersecting D-Branes on Shift Z2 x Z2 Orientifolds
We investigate Z2 x Z2 orientifolds with group actions involving shifts. A
complete classification of possible geometries is presented where also previous
work by other authors is included in a unified framework from an intersecting
D-brane perspective. In particular, we show that the additional shifts not only
determine the topology of the orbifold but also independently the presence of
orientifold planes. In the second part, we work out in detail a basis of
homological three cycles on shift Z2 x Z2 orientifolds and construct all
possible fractional D-branes including rigid ones. A Pati-Salam type model with
no open-string moduli in the visible sector is presented.Comment: 36 pages, 4 figures, refs. adde
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