579 research outputs found
Measuring the Fourth Generation b --> s Quadrangle at the LHC
We show that simultaneous precision measurement of the CP-violating phase in
time-dependent Bs --> J/psi phi study and the Bs --> mu+ mu- rate, together
with measuring m_t' by direct search at the LHC, would determine
V_{t's}^*V_{t'b} and therefore the b --> s quadrangle in the four-generation
standard model. The forward-backward asymmetry in B --> K* l+ l- provides
further discrimination.Comment: 6 pages, 7 figures, revised based on LHC results released in this
summer, to appear in PR
First-order restoration of SU(Nf) x SU(Nf) chiral symmetry with large Nf and Electroweak phase transition
It has been argued by Pisarski and Wilczek that finite temperature
restoration of the chiral symmetry SU(Nf) x SU(Nf) is first-order for Nf >=3.
This type of chiral symmetry with a large Nf may appear in the Higgs sector if
one considers models such as walking technicolor theories. We examine the
first-order restoration of the chiral symmetry from the point of view of the
electroweak phase transition. The strength of the transition is estimated in
SU(2) x U(1) gauged linear sigma model by means of the finite temperature
effective potential at one-loop with the ring improvement. Even if the mass of
the neutral scalar boson corresponding to the Higgs boson is larger than 114
GeV, the first-order transition can be strong enough for the electroweak
baryogenesis, as long as the extra massive scalar bosons (required for the
linear realization) are kept heavier than the neutral scalar boson. Explicit
symmetry breaking terms reduce the strength of the first-order transition, but
the transition can remain strongly first-order even when the masses of pseudo
Nambu-Goldstone bosons become as large as the current lower bound of direct
search experiments.Comment: 18 pages, 18 figures, minor corrections, references adde
Strong spin relaxation length dependence on electric field gradients
We discuss the influence of electrical effects on spin transport, and in
particular the propagation and relaxation of spin polarized electrons in the
presence of inhomogeneous electric fields. We show that the spin relaxation
length strongly depends on electric field gradients, and that significant
suppression of electron spin polarization can occur as a result thereof. A
discussion in terms of a drift-diffusion picture, and self-consistent numerical
calculations based on a Boltzmann-Poisson approach shows that the spin
relaxation length in fact can be of the order of the charge screening length.Comment: 4 pages, 3 figures, to be presented at PASPSI
Incommensurate spin correlations induced by magnetic Fe ions substituted into overdoped Bi1.75Pb0.35Sr1.90CuO6+z
Spin correlations in the overdoped region of Bi1.75Pb0.35Sr1.90CuO6+z have
been explored with Fe-doped single crystals characterized by neutron
scattering, muon-spin-rotation (muSR) spectroscopy, and magnetic susceptibility
measurements. Static incommensurate spin correlations induced by the Fe spins
are revealed by elastic neutron scattering. The resultant incommensurability
delta is unexpectedly large (~0.2 r.l.u.), as compared with delta ~ 1/8 in
overdoped superconductor La2-xSrxCuO4. Intriguingly, the large delta in this
overdoped region is close to the hole concentration p. This result is
reminiscent of the delta ~ p trend observed in underdoped La2-xSrxCuO4;
however, it is inconsistent with the saturation of delta in the latter compound
in the overdoped regime. While our findings in Fe-doped
Bi1.75Pb0.35Sr1.90CuO6+z support the commonality of incommensurate spin
correlations in high-Tc cuprate superconductors, they also suggest that the
magnetic response might be dominated by a distinct mechanism in the overdoped
region.Comment: 4 pages, 5 figures. Revision in introduction, discussion, and
conclusion
Direct determination of spin orbit interaction coefficients and realization of the persistent spin helix symmetry
The spin orbit interaction plays a crucial role in diverse fields of
condensed matter, including the investigation of Majorana fermions, topological
insulators, quantum information and spintronics. In III V zinc blende
semiconductor heterostructures, two types of spin orbit interaction, Rashba and
Dresselhaus act on the electron spin as effective magnetic fields with
different directions. They are characterized by coefficients alpha and beta,
respectively. When alpha is equal to beta, the so called persistent spin helix
symmetry is realized. In this condition, invariance with respect to spin
rotations is achieved even in the presence of the spin orbit interaction,
implying strongly enhanced spin lifetimes for spatially periodic spin modes.
Existing methods to evaluate alpha/beta require fitting analyses that often
include ambiguity in the parameters used. Here, we experimentally demonstrate a
simple and fitting parameter free technique to determine alpha/beta and to
deduce the absolute values of alpha and beta. The method is based on the
detection of the effective magnetic field direction and the strength induced by
the two spin orbit interactions. Moreover, we observe the persistent spin helix
symmetry by gate tuning.Comment: 34 pages with 7 figures including supplementary information. appears
in Nature Nanotechnology (2014) Published online 13 July 201
Spin-polarized current amplification and spin injection in magnetic bipolar transistors
The magnetic bipolar transistor (MBT) is a bipolar junction transistor with
an equilibrium and nonequilibrium spin (magnetization) in the emitter, base, or
collector. The low-injection theory of spin-polarized transport through MBTs
and of a more general case of an array of magnetic {\it p-n} junctions is
developed and illustrated on several important cases. Two main physical
phenomena are discussed: electrical spin injection and spin control of current
amplification (magnetoamplification). It is shown that a source spin can be
injected from the emitter to the collector. If the base of an MBT has an
equilibrium magnetization, the spin can be injected from the base to the
collector by intrinsic spin injection. The resulting spin accumulation in the
collector is proportional to , where is the proton
charge, is the bias in the emitter-base junction, and is the
thermal energy. To control the electrical current through MBTs both the
equilibrium and the nonequilibrium spin can be employed. The equilibrium spin
controls the magnitude of the equilibrium electron and hole densities, thereby
controlling the currents. Increasing the equilibrium spin polarization of the
base (emitter) increases (decreases) the current amplification. If there is a
nonequilibrium spin in the emitter, and the base or the emitter has an
equilibrium spin, a spin-valve effect can lead to a giant magnetoamplification
effect, where the current amplifications for the parallel and antiparallel
orientations of the the equilibrium and nonequilibrium spins differ
significantly. The theory is elucidated using qualitative analyses and is
illustrated on an MBT example with generic materials parameters.Comment: 14 PRB-style pages, 10 figure
Theory of spin-polarized bipolar transport in magnetic p-n junctions
The interplay between spin and charge transport in electrically and
magnetically inhomogeneous semiconductor systems is investigated theoretically.
In particular, the theory of spin-polarized bipolar transport in magnetic p-n
junctions is formulated, generalizing the classic Shockley model. The theory
assumes that in the depletion layer the nonequilibrium chemical potentials of
spin up and spin down carriers are constant and carrier recombination and spin
relaxation are inhibited. Under the general conditions of an applied bias and
externally injected (source) spin, the model formulates analytically carrier
and spin transport in magnetic p-n junctions at low bias. The evaluation of the
carrier and spin densities at the depletion layer establishes the necessary
boundary conditions for solving the diffusive transport equations in the bulk
regions separately, thus greatly simplifying the problem. The carrier and spin
density and current profiles in the bulk regions are calculated and the I-V
characteristics of the junction are obtained. It is demonstrated that spin
injection through the depletion layer of a magnetic p-n junction is not
possible unless nonequilibrium spin accumulates in the bulk regions--either by
external spin injection or by the application of a large bias. Implications of
the theory for majority spin injection across the depletion layer, minority
spin pumping and spin amplification, giant magnetoresistance, spin-voltaic
effect, biasing electrode spin injection, and magnetic drift in the bulk
regions are discussed in details, and illustrated using the example of a GaAs
based magnetic p-n junction.Comment: 36 pages, 11 figures, 2 table
Thymulin inhibits monocrotaline-induced pulmonary hypertension modulating interleukin-6 expression and suppressing p38 pathway
The pathogenesis of pulmonary hypertension (PH) includes an inflammatory response. Thymulin, a zinc-dependent thymic hormone, has important immunobiological effects by inhibiting various proinflammatory cytokines and chemokines. We investigated morphological and hemodynamic effects of thymulin administration in a rat model of monocrotaline (MCT)-induced PH, as well as the pattern of proinflammatory cytokine gene expression and the intracellular pathways involved. Adult Wistar rats received an injection of MCT (60 mg/kg, sc) or an equal volume of saline. One day after, the animals randomly received during 3 wk an injection of saline, vehicle (zinc plus carboxymethyl cellulose), or thymulin (100 ng/kg, sc, daily). At d 23-25, the animals were anesthetized for hemodynamic recordings, whereas heart and lungs were collected for morphometric and molecular analysis. Thymulin prevented morphological, hemodynamic, and inflammatory cardiopulmonary profile characteristic of MCT-induced PH, whereas part of these effects were also observed in MCT-treated animals injected with the thymulin's vehicle containing zinc. The pulmonary thymulin effect was likely mediated through suppression of p38 pathway.Portuguese Foundation for Science and Technology (No. POCI/SAVFCF/60803/2004; POCTI/SAV-MMO/61547/2004 and PTDC/SAV-FCF/65793/2006) through Cardiovascular R&D Unit (FCT No. 51/94). R.S.M. was supported by Fundação para a Ciência e a Tecnologia (reference SFRH/BPD/15408/2005
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
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