450 research outputs found
Probing the spin polarization in ferromagnets
The emission of correlated electrons from an itinerant ferromagnet following
the impact of a polarized electron beam is analyzed in terms of irreducible
tensorial parameters that can be measured. Under favorable conditions,
specified in this work, these parameters are related to the spin polarization
in the ferromagnet. The formal results are illustrated by numerical studies of
the polarized electron pair emission from a Fe(110) surface and a novel
technique for the investigation of magnetic properties of ferromagnets is
suggested
Many-body scattering theory of electronic systems
This work reviews recent advances in the analytical treatment of the
continuum spectrum of correlated few-body non-relativistic Coulomb systems. The
exactly solvable two-body problem serves as an introduction to the
non-separable three-particle system. For the latter case we discuss the
existence of an approximate separability of the long and the short-range
dynamics which is exposed in an appropriately chosen curvilinear coordinates.
The three-body wave functions of the long-ranged part of the Hamiltonian are
derived and methods are presented to account approximately for the short-ranged
dynamics. Furthermore, we present a generalization of the methods employed for
the derivation of the three-body wave functions to the scattering states of
charged particles.
To deal with thermodynamic properties of finite systems we develop and
discuss a recent Green function methodology designed for the non-perturbative
regime.
In addition, we give a brief account on how thermodynamic properties and
critical phenomena can be exposed in finite interacting systems
Local control of ultrafast dynamics in magnetic nanoparticles
Using the local control theory we derive analytical expressions for magnetic
field pulses that steer the magnetization of a monodomain magnetic nanoparticle
to a predefined state. Finite-temperature full numerical simulations confirm
the analytical results and show that a magnetization switching or freezing is
achievable within few precessional periods and that the scheme is exploitable
for fast thermal switching
Nonlinear magneto-optical response to light carrying orbital angular momentum
We predict a non-thermal magneto-optical effect for magnetic insulators
subject to intense light carrying orbital angular momentum (OAM). Using a
classical approach to second harmonic generation in non-linear media with
specific symmetry properties we predict a significant nonlinear contribution to
the local magnetic field triggered by light with OAM. The resulting magnetic
field originates from the displacement of electrons driven by the electrical
field (with amplitude ) of the spatially inhomogeneous optical pulse,
modeled here as a Laguerre-Gaussian beam carrying OAM. In particular, the
symmetry properties of the irradiated magnet allow for magnetic field responses
which are second-order () and fourth-order () in
electric-field strength and have opposite signs. For sufficiently high laser
intensities, terms dominate and generate magnetic field strengths
which can be as large as several Tesla. Moreover, changing the OAM of the laser
beam is shown to determine the direction of the total light-induced magnetic
field, which is further utilized to study theoretically the non-thermal
magnetization dynamics.Comment: 10 pages, 9 figure
Multipolar, Polarization Shaped High Harmonic Generation by Intense Vector Beams
High harmonic generation (HHG) is a manifestation of the strongly nonlinear
response of matter to intense laser fields and has, as the basis for coherent
XUV sources a variety of applications. Recently, HHG from atoms in a phase and
polarization structured laser was demonstrated and interpreted based on the
transverse electric field component of the driving pulse. Here we point out
that as dictated by Maxwell equations, such fields have a longitudinal
component which in general has a fundamental influence on the charge dynamics.
For instance, its interplay with the transversal field component enables
endowing the emitted radiation locally with circular polarization and a defined
polarity. It is shown that the time-dependent Stokes parameters defining the
polarization state of HHG can be tuned by varying the waist of the driving
field which in turn, changes the ratio between the longitudinal and transverse
electric-field components of the driving laser. In addition, employing a
multipole expansion of the produced harmonics exposes the specific multipolar
character and the relation to the spatial structure of the driving field
polarization states. The scheme proposed here allows a full polarization
control of the emitted harmonics by only one driving laser. A tighter focusing
of the driving pulse renders possible the emission of harmonics with both even
and odd spatial symmetry. The underlying mechanism is due to the fundamental
interplay between the transverse and longitudinal components of the laser's
electromagnetic vector potential. The ratio between those components is
controllable by just focusing the laser spot, pointing to an accessible tool
for polarization and polarity control of the high harmonics.Comment: 7 pages, 5 pictures, submitted to PRL, accepted at PR
Chargeless spin current for switching and coupling of domain walls in magnetic nanowires
The demonstration of the generation and control of a pure spin current
(without net charge flow) by electric fields and/or temperature gradient has
been an essential leap in the quest for low-power consumption electronics. The
key issue of whether and how such a current can be utilized to drive and
control information stored in magnetic domain walls (DWs) is still outstanding
and is addressed here. We demonstrate that pure spin current acts on DWs in a
magnetic stripe with an effective spin-transfer torque resulting in a mutual
DWs separation dynamics and picosecond magnetization reversal. In addition,
long-range ( mm) antiferromagnetic DWs coupling emerges. If one DW is
pinned by geometric constriction, the spin current induces a dynamical spin
orbital interaction that triggers an internal electric field determined by
where
are the effective DWs orientations and is their
spatial separation vector. This leads to charge accumulation or persistent
electric current in the wire. As DWs are routinely realizable and tuneable, the
predicted effects bear genuine potential for power-saving spintronics devices
All-Optical Generation and Tuning of Ultrafast Spin-Hall Current via Optical Vortices
Spin Hall effect, one of the cornerstones in spintronics refers to the
emergence of an imbalance in the spin density transverse to a charge flow in a
sample under voltage bias. This study points to a novel way for an ultrafast
generation and tuning of a unidirectional nonlinear spin Hall current by means
of subpicosecond laser pulses of optical vortices. When interacting with
matter, the optical orbital angular momentum (OAM) carried by the vortex and
quantified by its topological charge is transferred to the charge carriers. The
residual spin-orbital coupling in the sample together with confinement effects
allow exploiting the absorbed optical OAM for spatio-temporally controlling the
spin channels. Both the non-linear spin Hall current and the dynamical spin
Hall angle increase for a higher optical topological charge. The reason is the
transfer of a higher amount of OAM and the enhancement of the effective
spin-orbit interaction strength. No bias voltage is needed. We demonstrate that
the spin Hall current can be all-optically generated in an open circuit
geometry for ring-structured samples. These results follow from a full-fledged
propagation of the spin-dependent quantum dynamics on a time-space grid coupled
to the phononic environment. The findings point to a versatile and controllable
tool for the ultrafast generation of spin accumulations with a variety of
applications such as a source for ultrafast spin transfer torque and charge and
spin current pulse emitter.Comment: 12 pages, 6 figure
Accessing electronic correlations by half-cycle pulses and time-resolved spectroscopy
Ultrashort non-resonant electromagnetic pulses applied to effective
one-electron systems may operate on the electronic state as a position or
momentum translation operator. As derived here, extension to many-body
correlated systems exposes qualitatively new aspects. For instance, to the
lowest order in the electric field intensity the action of the pulse is
expressible in terms of the two-body reduced density matrix enabling thus to
probe various facets of electronic correlations. As an experimental realization
we propose a pump-probe scheme in which after a weak, swift "kick" by the
non-resonant pulse the survival probability for remaining in the initial state
is measured. This probability we correlate to the two-body reduced density
matrix. Since the strength of electronic correlation is bond-length sensitive,
measuring the survival probability may allow for a direct insight into the
bond-dependent two-body correlation in the ground state. As an illustration,
full numerical calculations for two molecular systems are provided and
different measures of electronic correlations are analyzed.Comment: 4 figure
Negative differential magneto-resistance in ferromagnetic wires with domain walls
A domain wall in a ferromagnetic one-dimensional nanowire experiences current
induced motion due to its coupling with the conduction electrons. When the
current is not sufficient to drive the domain wall through the wire, or it is
confined to a perpendicular layer, it nonetheless experiences oscillatory
motion. In turn, this oscillatory motion of the domain wall can couple
resonantly with the electrons in the system affecting the transport properties
further. We investigate the effect of the coupling between these domain wall
modes and the current electrons on the transport properties of the system and
show that such a system demonstrates negative differential magnetoresistance
due to the resonant coupling with the low-lying modes of the domain wall
motion.Comment: 5 pages, 3 figure
Efficient thermal energy harvesting using nanoscale magnetoelectric heterostructures
Thermomechanical cycles with a ferroelectric working substance convert heat
to electrical energy. As shown here, magnetoelectrically coupled
ferroelectric/ferromangtic composites (also called multiferroics) add new
functionalities and allow for an efficient thermal energy harvesting at room
temperature by exploiting the pyroelectric effect. By virtue of the
magnetoelectric coupling, external electric and magnetic fields can steer the
operation of these heat engines. Our theoretical predictions are based on a
combination of Landau-Khalatnikov-Tani approach (with a
Ginzburg-Landau-Devonshire potential) to simulate the ferroelectric dynamics
coupled to the magnetic dynamics. The latter is treated via the
electric-polarization-dependent Landau-Lifshitz-Gilbert equation. Performing an
adapted Olsen cycle we show that a multiferroic working substance is
potentially much more superior to sole ferroelectrics, as far as thermal energy
harvesting using pyroelectric effect is concerned. Our proposal holds promise
not only for low-energy consuming devices but also for cooling technology.Comment: 5 pages, 5 figures, Accepted in Applied Physics Letter
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