3,250 research outputs found
Reversal and Termination of Current-Induced Domain Wall Motion via Magnonic Spin-Transfer Torque
We investigate the domain wall dynamics of a ferromagnetic wire under the
combined influence of a spin-polarized current and magnonic spin-transfer
torque generated by an external field, taking also into account Rashba
spin-orbit coupling interactions. It is demonstrated that current-induced
motion of the domain wall may be completely reversed in an oscillatory fashion
by applying a magnonic spin-transfer torque as long as the spin-wave velocity
is sufficiently high. Moreover, we show that the motion of the domain wall may
be fully terminated by means of the generation of spin-waves, suggesting the
possibility to pin the domain-walls to predetermined locations. We also discuss
how strong spin-orbit interactions modify these results.Comment: Accepted for publication in Phys. Rev.
Strongly spin-polarized current generated in Zeeman-split unconventional superconductors
We consider a thin-film normal metal/superconductor junction in the presence
of an externally applied in-plane magnetic field for several symmetries of the
superconducting order parameter. For p-wave superconductors, a strongly
spin-polarized current emerges due to an interplay between the nodal structure
of the superconducting order parameter, the existence or non-existence of
zero-energy surface states, and the Zeeman-splitting of the bands which form
superconductivity. Thus, the polarization depends strongly on the orbital
symmetry of the superconducting state. Our findings suggest a mechanism for
obtaining fully spin-polarized currents crucially involving zero-energy surface
states, not present in s-wave superconductors.Comment: 5 pages, 5 figures. Accepted for publication in Phys. Rev.
Chemical species spatial distribution and relationship to elevation and snow accumulation rate over the Greenland Ice Sheet
Major chemical species (Cl−, NO−3, SO2−4, Na+, K+, Mg2+, Ca2+) from 24 snowpits (sampled at a resolution of 3 cm, total 2995 samples) collected from northern, central, and southern Greenland were used for this investigation. The annual and seasonal (winter and summer) concentration of each chemical species was calculated and used to study the spatial distribution of chemical species over the central portion of the Greenland Ice Sheet. A two‐sided t‐distribution test (α = 0.05) suggests that concentrations of major chemical species in snow do not vary significantly over this portion of central Greenland. The relationship between chemical concentration and snow accumulation rate was investigated using annual data from two groups of snowpits: those from coastal sites (northern and southern Greenland); and those from high‐altitude inland sites (central Greenland). The snowpit data from a single group, when examined independently of the other group, show that chemical concentrations do not vary with snow accumulation rate. However, when data from the two groups are integrated into a single data set, pseudorelationships appear, with NO−3 concentration decreasing and Na+, K+, Mg2+, and Cl− increasing as snow accumulation rate increases. Therefore we suggest that it is improper to study the relationship between chemical concentration and snow accumulation rate by using data collected from different geographic sites. The relationship between elevation and chemical concentration was investigated using the same suite of annual data sets. We find that Cl−, Na+, and Mg2+ concentrations decrease, while NO−3 concentration increases, with increasing elevation on the Greenland Ice Sheet
Galaxies as Fluctuations in the Ionizing Background Radiation at Low Redshift
Some Lyman continuum photons are likely to escape from most galaxies, and
these can play an important role in ionizing gas around and between galaxies,
including gas that gives rise to Lyman alpha absorption. Thus the gas
surrounding galaxies and in the intergalactic medium will be exposed to varying
amounts of ionizing radiation depending upon the distances, orientations, and
luminosities of any nearby galaxies. The ionizing background can be
recalculated at any point within a simulation by adding the flux from the
galaxies to a uniform quasar contribution. Normal galaxies are found to almost
always make some contribution to the ionizing background radiation at redshift
zero, as seen by absorbers and at random points in space. Assuming that about 2
percent of ionizing photons escape from a galaxy like the Milky Way, we find
that normal galaxies make a contribution of at least 30 to 40 percent of the
assumed quasar background. Lyman alpha absorbers with a wide range of neutral
column densities are found to be exposed to a wide range of ionization rates,
although the distribution of photoionization rates for absorbers is found to be
strongly peaked. On average, less highly ionized absorbers are found to arise
farther from luminous galaxies, while local fluctuations in the ionization rate
are seen around galaxies having a wide range of properties.Comment: 10 pages, 8 figures, references added, clarified explanation of first
two equation
Identifying the odd-frequency superconducting state by a field-induced Josephson effect
Superconducting order parameters that are odd under exchange of
time-coordinates of the electrons constituting a Cooper-pair, are potentially
of great importance both conceptually and technologically. Recent experiments
report that such an odd-frequency superconducting {\it bulk} state may be
realized in certain heavy-fermion compounds. While the Josephson current
normally only flows between superconductors with the same symmetries with
respect to frequency, we demonstrate that an exchange field may induce a
current between diffusive even- and odd-frequency superconductors. This
suggests a way to identify the possible existence of bulk odd-frequency
superconductors.Comment: 10 pages, 7 figures. To appear in Physical Review
Josephson effect in thin-film superconductor/insulator/superconductor junctions with misaligned in-plane magnetic fields
We study a tunnel junction consisting of two thin-film s-wave superconductors
separated by a thin, insulating barrier in the presence of misaligned in-plane
exchange fields. We find an interesting interplay between the superconducting
phase difference and the relative orientation of the exchange fields,
manifested in the Josephson current across the junction. Specifically, this may
be written , where
I_0 and I_m are constants, and is the relative orientation of the
exchange fields while is the superconducting phase difference.
Similar results have recently been obtained in other S/I/S junctions coexisting
with helimagnetic or ferromagnetic order. We calculate the superconducting
order parameter self-consistently, and investigate quantitatively the effect
which the misaligned exchange fields constitute on the Josephson current, to
see if I_m may have an appreciable effect on the Josephson current. It is found
that I_0 and I_m become comparable in magnitude at sufficiently low
temperatures and fields close to the critical value, in agreement with previous
work. From our analytical results, it then follows that the Josephson current
in the present system may be controlled in a well-defined manner by a rotation
of the exchange fields on both sides of the junction. We discuss a possible
experimental realization of this proposition.Comment: 8 pages, 8 figures. Accepted for publication in Phys. Rev.
Global perspective of nitrate flux in ice cores
The relationships between the concentration and the flux of chemical species (Cl-, NO3 - , SO42-, Na +, K + , NH4 + , Mg 2+ , Ca 2+) versus snow accumulation rate were examined at GISP2 and 20D in Greenland, Mount Logan from the St. Elias Range, Yukon Territory, Canada, and Sentik Glacier from the northwest end of the Zanskar Range in the Indian Himalayas. At all sites, only nitrate flux is significantly (a = 0.05) related to snow accumulation rate. Of all the chemical series, only nitrate concentration data are normally distributed. Therefore we suggest that nitrate concentration in snow is affected by postdepositionaJ exchange with the atmosphere over a broad range of environmental conditions. The persistent summer maxima in nitrate observed in Greenland snow over the entire range of record studied (the last 800 years) may be mainly due to NO• released from peroxyacetyl nitrate by thermal decomposition in the presence of higher OH concentrations in summer. The late winter/early spring nitrate peak observed in modern Greenland snow may be related to the buildup of anthropogenically derived N Oy in the Arctic troposphere during the long polar winter
Theory of superconducting and magnetic proximity effect in SF structures with inhomogeneous magnetization textures and spin-active interfaces
We present a study of the proximity effect and the inverse proximity effect
in a superconductorferromagnet bilayer, taking into account several
important factors which mostly have been ignored in the literature so far.
These include spin-dependent interfacial phase shifts (spin-DIPS) and
inhomogeneous textures of the magnetization in the ferromagnetic layer, both of
which are expected to be present in real experimental samples. Our approach is
numerical, allowing us to access the full proximity effect regime. In Part I of
this work, we study the superconducting proximity effect and the resulting
local density of states in an inhomogeneous ferromagnet with a non-trivial
magnetic texture.
Our two main results in Part I are a study of how Bloch and N\'eel domain
walls affect the proximity-induced superconducting correlations and a study of
the superconducting proximity effect in a conical ferromagnet. The latter topic
should be relevant for the ferromagnet Ho, which was recently used in an
experiment to demonstrate the possibility to generate and sustain long-range
triplet superconducting correlations. In Part II of this work, we investigate
the inverse proximity effect with emphasis on the induced magnetization in the
superconducting region as a result of the "leakage" from the ferromagnetic
region. It is shown that the presence of spin-DIPS modify conclusions obtained
previously in the literature with regard to the induced magnetization in the
superconducting region. In particular, we find that the spin-DIPS can trigger
an anti-screening effect of the magnetization, leading to an induced
magnetization in the superconducting region with \textit{the same sign} as in
the proximity ferromagnet.Comment: 16 pages, 18 figures. Accepted for publication in Phys. Rev.
Tunneling currents in ferromagnetic systems with multiple broken symmetries
SHORTENED ABSTRACT: A system exhibiting multiple simultaneously broken
symmetries offers the opportunity to influence physical phenomena such as
tunneling currents by means of external control parameters. In this paper, we
consider the broken SU(2) (internal spin) symmetry of ferromagnetic systems
coexisting with \textit{i)} the broken U(1) symmetry of superconductors and
\textit{ii)} the broken spatial inversion symmetry induced by a Rashba term in
a spin-orbit coupling Hamiltonian. In order to study the effect of these broken
symmetries, we consider tunneling currents that arise in two different systems;
tunneling junctions consisting of non-unitary spin-triplet ferromagnetic
superconductors and junctions consisting of ferromagnets with spin-orbit
coupling.Comment: Accepted for publication in Phys. Rev.
Quinstant Dark Energy Predictions for Structure Formation
We explore the predictions of a class of dark energy models, quinstant dark
energy, concerning the structure formation in the Universe, both in the linear
and non-linear regimes. Quinstant dark energy is considered to be formed by
quintessence and a negative cosmological constant. We conclude that these
models give good predictions for structure formation in the linear regime, but
fail to do so in the non-linear one, for redshifts larger than one.Comment: 9 pages, 14 figures, "Accepted for publication in Astrophysics &
Space Science
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