262 research outputs found
Spin-dependent Fano resonance induced by conducting chiral helimagnet contained in a quasi-one-dimensional electron waveguide
Fano resonance appears for conduction through an electron waveguide
containing donor impurities. In this work, we consider the thin-film conducting
chiral helimagnet (CCH) as the donor impurity in a one-dimensional waveguide
model. Due to the spin spiral coupling, interference between the direct and
intersubband transmission channels gives rise to spin-dependent Fano resonance
effect. The spin-dependent Fano resonance is sensitively dependent on the
helicity of the spiral. By tuning the CCH potential well depth and the incident
energy, this provides a potential way to detect the spin structure in the CCH.Comment: 14 pages, 6 figure
Spin-dependent diffraction at ferromagnetic/spin spiral interface
Spin-dependent transport is investigated in ballistic regime through the
interface between a ferromagnet and a spin spiral. We show that spin-dependent
interferences lead to a new type of diffraction called "spin-diffraction". It
is shown that this spin-diffraction leads to local spin and electrical currents
along the interface. This study also shows that in highly non homogeneous
magnetic configuration (non adiabatic limit), the contribution of the
diffracted electrons is crucial to describe spin transport in such structures
Crossover from Diffusive to Superfluid Transport in Frustrated Magnets
We investigate the spin transport across the magnetic phase diagram of a
frustrated antiferromagnetic insulator and uncover a drastic modification of
the transport regime from spin diffusion to spin superfluidity. Adopting a
triangular lattice accounting for both nearest neighbor and next-nearest
neighbor exchange interactions with easy-plane anisotropy, we perform atomistic
spin simulations on a two-terminal configuration across the full magnetic phase
diagram. We found that as long as the ground state magnetic moments remain
in-plane, irrespective of whether the magnetic configuration is ferromagnetic,
collinear or non-collinear antiferromagnetic, the system exhibits spin
superfluid behavior with a device output that is independent on the value of
the exchange interactions. When the magnetic frustration is large enough to
compete with the easy-plane anisotropy and cant the magnetic moments out of the
plane, the spin transport progressively evolves towards the diffusive regime.
The robustness of spin superfluidity close to magnetic phase boundaries is
investigated and we uncover the possibility for {\em proximate} spin
superfluidity close to the ferromagnetic transition.Comment: 9 pages, 7 figure
Theory of laser-induced demagnetization at high temperatures
Laser-induced demagnetization is theoretically studied by explicitly taking
into account interactions among electrons, spins and lattice. Assuming that the
demagnetization processes take place during the thermalization of the
sub-systems, the temperature dynamics is given by the energy transfer between
the thermalized interacting baths. These energy transfers are accounted for
explicitly through electron-magnons and electron-phonons interaction, which
govern the demagnetization time scale. By properly treating the spin system in
a self-consistent random phase approximation, we derive magnetization dynamic
equations for a broad range of temperature. The dependence of demagnetization
on the temperature and pumping laser intensity is calculated in detail. In
particular, we show several salient features for understanding magnetization
dynamics near the Curie temperature. While the critical slowdown in dynamics
occurs, we find that an external magnetic field can restore the fast dynamics.
We discuss the implication of the fast dynamics in the application of heat
assisted magnetic recording.Comment: 11 Pages, 7 Figure
Description of current-driven torques in magnetic tunnel junctions
A free electron description of spin-dependent tranport in magnetic tunnel
junctions with non collinear magnetizations is presented. We investigate the
origin of transverse spin density in tunnelling transport and the quantum
interferences which give rise to oscillatory torques on the local
magnetization. Spin transfer torque is also analyzed and an important bias
asymmetry is found as well as a damped oscillatory behaviour. Furthermore, we
investigate the influence of the s-d exchange coupling on torque in particular
in the case of half-metallic MTJ in which the spin transfer torque is due to
interfacial spin-dependent reflections
Entropy-calibrated stellar modeling: Testing and improving the use of prescriptions for entropy of adiabatic convection
The modeling of convection is a long standing problem in stellar physics.
Up-to-now, all ad hoc models rely on a free parameter alpha (among others)
which has no real physical justification and is therefore poorly constrained.
However, a link exists between this free parameter and the entropy of the
stellar adiabat. Prescriptions, derived from 3D stellar atmospheric models, are
available that provide entropy as a function of stellar atmospheric parameters
(effective temperature, surface gravity, chemical composition). This can
provide constraints on alpha through the development of entropy-calibrated
models. Several questions arise as these models are increasingly used. Which
prescription should be used? How do uncertainties impact entropy-calibrated
models? We aim to study the three existing prescriptions and determine which
one should be used, and how. We implemented the entropy-calibration method into
the stellar evolution code Cesam2k20 and performed comparisons with the Sun and
the alpha Cen system. In addition, we used data from the CIFIST grid of 3D
atmosphere models to evaluate the accuracy of the prescriptions. Of the three
entropy prescriptions available, we determine which one best reproduces the
entropies of the 3D models. We also demonstrate that the entropy obtained from
this prescription should be corrected for the evolving chemical composition and
for an entropy offset different between various EoS tables, following a precise
procedure, otherwise classical parameters obtained from the models will be
strongly biased. Finally, we also provide table with entropy of the adiabat of
the CIFIST grid, as well as fits of these entropies. We performed a precise
examination of entropy-calibrated modelling, and gave recommendations on which
adiabatic entropy prescription to use, how to correct it and to implement the
method into a stellar evolution code.Comment: 18 pages, 11 figures. Submitted to Astronomy and Astrophysics,
recommended for publication with minor revision
Experimental observation of the optical spin-orbit torque
Spin polarized carriers electrically injected into a magnet from an external
polarizer can exert a spin transfer torque (STT) on the magnetization. The phe-
nomenon belongs to the area of spintronics research focusing on manipulating
magnetic moments by electric fields and is the basis of the emerging
technologies for scalable magnetoresistive random access memories. In our
previous work we have reported experimental observation of the optical
counterpart of STT in which a circularly polarized pump laser pulse acts as the
external polarizer, allowing to study and utilize the phenomenon on several
orders of magnitude shorter timescales than in the electric current induced
STT. Recently it has been theoretically proposed and experimentally
demonstrated that in the absence of an external polarizer, carriers in a magnet
under applied electric field can develop a non-equilibrium spin polarization
due to the relativistic spin-orbit coupling, resulting in a current induced
spin-orbit torque (SOT) acting on the magnetization. In this paper we report
the observation of the optical counterpart of SOT. At picosecond time-scales,
we detect excitations of magnetization of a ferromagnetic semiconductor
(Ga,Mn)As which are independent of the polarization of the pump laser pulses
and are induced by non-equilibrium spin-orbit coupled photo-holes.Comment: 4 figure, supplementary information. arXiv admin note: text overlap
with arXiv:1101.104
Spitzer's Identity and the Algebraic Birkhoff Decomposition in pQFT
In this article we continue to explore the notion of Rota-Baxter algebras in
the context of the Hopf algebraic approach to renormalization theory in
perturbative quantum field theory. We show in very simple algebraic terms that
the solutions of the recursively defined formulae for the Birkhoff
factorization of regularized Hopf algebra characters, i.e. Feynman rules,
naturally give a non-commutative generalization of the well-known Spitzer's
identity. The underlying abstract algebraic structure is analyzed in terms of
complete filtered Rota-Baxter algebras.Comment: 19 pages, 2 figure
Backward error analysis and the substitution law for Lie group integrators
Butcher series are combinatorial devices used in the study of numerical
methods for differential equations evolving on vector spaces. More precisely,
they are formal series developments of differential operators indexed over
rooted trees, and can be used to represent a large class of numerical methods.
The theory of backward error analysis for differential equations has a
particularly nice description when applied to methods represented by Butcher
series. For the study of differential equations evolving on more general
manifolds, a generalization of Butcher series has been introduced, called
Lie--Butcher series. This paper presents the theory of backward error analysis
for methods based on Lie--Butcher series.Comment: Minor corrections and additions. Final versio
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