13,685 research outputs found
Electron Transport through Disordered Domain Walls: Coherent and Incoherent Regimes
We study electron transport through a domain wall in a ferromagnetic nanowire
subject to spin-dependent scattering. A scattering matrix formalism is
developed to address both coherent and incoherent transport properties. The
coherent case corresponds to elastic scattering by static defects, which is
dominant at low temperatures, while the incoherent case provides a
phenomenological description of the inelastic scattering present in real
physical systems at room temperature. It is found that disorder scattering
increases the amount of spin-mixing of transmitted electrons, reducing the
adiabaticity. This leads, in the incoherent case, to a reduction of conductance
through the domain wall as compared to a uniformly magnetized region which is
similar to the giant magnetoresistance effect. In the coherent case, a
reduction of weak localization, together with a suppression of spin-reversing
scattering amplitudes, leads to an enhancement of conductance due to the domain
wall in the regime of strong disorder. The total effect of a domain wall on the
conductance of a nanowire is studied by incorporating the disordered regions on
either side of the wall. It is found that spin-dependent scattering in these
regions increases the domain wall magnetoconductance as compared to the effect
found by considering only the scattering inside the wall. This increase is most
dramatic in the narrow wall limit, but remains significant for wide walls.Comment: 23 pages, 12 figure
Radical pair intersystem crossing: Quantum dynamics or incoherent kinetics?
Magnetic field effects on radical pair reactions arise due to the interplay
of coherent electron spin dynamics and spin relaxation effects, a rigorous
treatment of which requires the solution of the Liouville-von Neumann equation.
However, it is often found that simple incoherent kinetic models of the radical
pair singlet-triplet intersystem crossing provide an acceptable description of
experimental measurements. In this paper we outline the theoretical basis for
this incoherent kinetic description, elucidating its connection to exact
quantum mechanics. We show in particular how the finite lifetime of the radical
pair spin states, as well as any additional spin-state dephasing, leads to
incoherent intersystem crossing. We arrive at simple expressions for the
radical pair spin state interconversion rates to which the functional form
proposed recently by Steiner et al. [J. Phys. Chem. C 122, 11701 (2018)] can be
regarded as an approximation. We also test the kinetic master equation against
exact quantum dynamical simulations for a model radical pair and for a series
of molecular
wires
Neutrino production coherence and oscillation experiments
Neutrino oscillations are only observable when the neutrino production,
propagation and detection coherence conditions are satisfied. In this paper we
consider in detail neutrino production coherence, taking \pi\to \mu \nu \ decay
as an example. We compare the oscillation probabilities obtained in two
different ways: (1) coherent summation of the amplitudes of neutrino production
at different points along the trajectory of the parent pion; (2) averaging of
the standard oscillation probability over the neutrino production coordinate in
the source. We demonstrate that the results of these two different approaches
exactly coincide, provided that the parent pion is considered as pointlike and
the detection process is perfectly localized. In this case the standard
averaging of the oscillation probability over the finite spatial extensions of
the neutrino source (and detector) properly takes possible decoherence effects
into account. We analyze the reason for this equivalence of the two approaches
and demonstrate that for pion wave packets of finite width \sigma_{x\pi} the
equivalence is broken. The leading order correction to the oscillation
probability due to \sigma_{x\pi}\ne 0 is shown to be \sim
[v_g/(v_g-v_\pi)]\sigma_{x\pi}/l_{osc}, where v_g and v_\pi \ are the group
velocities of the neutrino and pion wave packets, and l_{osc} is the neutrino
oscillation length.Comment: LaTeX, 40 pages, 4 figures. v2: minor typos correcte
Clustering in Highest Energy Cosmic Rays: Physics or Statistics?
Directional clustering can be expected in cosmic ray observations due to
purely statistical fluctuations for sources distributed randomly in the sky. We
develop an analytic approach to estimate the probability of random cluster
configurations, and use these results to study the strong potential of the
HiRes, Auger, Telescope Array and EUSO/OWL/AirWatch facilities for deciding
whether any observed clustering is most likely due to non-random sources.Comment: 19 pages, LaTeX, 3 figure
Decays of supernova neutrinos
Supernova neutrinos could be well-suited for probing neutrino decay, since
decay may be observed even for very small decay rates or coupling constants. We
will introduce an effective operator framework for the combined description of
neutrino decay and neutrino oscillations for supernova neutrinos, which can
especially take into account two properties: One is the radially symmetric
neutrino flux, allowing a decay product to be re-directed towards the observer
even if the parent neutrino had a different original direction of propagation.
The other is decoherence because of the long baselines for coherently produced
neutrinos. We will demonstrate how to use this effective theory to calculate
the time-dependent fluxes at the detector. In addition, we will show the
implications of a Majoron-like decay model. As a result, we will demonstrate
that for certain parameter values one may observe some effects which could also
mimic signals similar to the ones expected from supernova models, making it in
general harder to separate neutrino and supernova properties.Comment: 33 pages, 10 figures, Elsevier LaTeX. Final version to be published
in Nuclear Physics
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