172 research outputs found
Lifetimes of Magnons in Two-Dimensional Diluted Ferromagnetic Systems
Spin dynamics in low dimensional magnetic systems has been of fundamental
importance for a long time and has currently received an impetus owing to the
emerging field of nanoelectronics. Knowledge of the spin wave lifetimes, in
particular, can be favorable for future potential applications. We investigate
the low-temperature spin wave excitations in two-dimensional disordered
ferromagnetic systems, with a particular focus on the long wavelength magnon
lifetimes. A semi-analytical Green's functions based approach is used to
determine the dynamical spectral functions, for different magnetic impurity
concentrations, from which the intrinsic linewidth is extracted. We obtain an
unambiguous scaling of the magnon linewidth which is ascribed to the
disorder induced damping of the spin waves, thereby settling a longstanding
unresolved issue on the wave-vector dependence. Our findings are also in good
agreement with previous theoretical studies on Heisenberg ferromagnets.
Additionally, we demonstrate the futility of using the low moments associated
with the spectral densities to evaluate the magnon dispersions and lifetimes.Comment: 9 pages, 9 figures, improved discussion, references added, and
revised to match the published versio
In-plane magnetoelectric response in bilayer graphene
A graphene bilayer shows an unusual magnetoelectric response whose magnitude
is controlled by the valley-isospin density, making it possible to link
magnetoelectric behavior to valleytronics. Complementary to previous studies,
we consider the effect of static homogeneous electric and magnetic fields that
are oriented parallel to the bilayer's plane. Starting from a tight-binding
description and using quasi-degenerate perturbation theory, the low-energy
Hamiltonian is derived including all relevant magnetoelectric terms whose
prefactors are expressed in terms of tight-binding parameters. We confirm the
existence of an expected axion-type pseudoscalar term, which turns out to have
the same sign and about twice the magnitude of the previously obtained
out-of-plane counterpart. Additionally, small anisotropic corrections to the
magnetoelectric tensor are found that are fundamentally related to the skew
interlayer hopping parameter . We discuss possible ways to identify
magnetoelectric effects by distinctive features in the optical conductivity.Comment: 14 pages, 7 figure
Spontaneous magnetization in presence of nanoscale inhomogeneities in diluted magnetic systems
The presence of nanoscale inhomogeneities has been experimentally evidenced
in several diluted magnetic systems, which in turn often leads to interesting
physical phenomena. However, a proper theoretical understanding of the
underlying physics is lacking in most of the cases. Here we present a detailed
and comprehensive theoretical study of the effects of nanoscale inhomogeneities
on the temperature dependent spontaneous magnetization in diluted magnetic
systems, which is found to exhibit an unusual and unconventional behavior. The
effects of impurity clustering on the magnetization response have hardly been
studied until now. We show that nanosized clusters of magnetic impurities can
lead to drastic effects on the magnetization compared to that of homogeneously
diluted compounds. The anomalous nature of the magnetization curves strongly
depends on the relative concentration of the inhomogeneities as well as the
effective range of the exchange interactions. In addition we also provide a
systematic discussion of the nature of the distributions of the local
magnetization.Comment: 18 pages, 9 figures, 4 new references added and Text modified to
match the published versio
Weak (anti)localization in tubular semiconductor nanowires with spin-orbit coupling
We compute analytically the weak (anti)localization correction to the Drude
conductivity for electrons in tubular semiconductor systems of zinc blende
type. We include linear Rashba and Dresselhaus spin-orbit coupling (SOC) and
compare wires of standard growth directions ,
, and . The motion on the
quasi-two-dimensional surface is considered diffusive in both directions:
transversal as well as along the cylinder axis. It is shown that Dresselhaus
and Rashba SOC similarly affect the spin relaxation rates. For the
growth direction, the long-lived spin states are of helical
nature. We detect a crossover from weak localization to weak anti-localization
depending on spin-orbit coupling strength as well as dephasing and scattering
rate. The theory is fitted to experimental data of an undoped
InAs nanowire device which exhibits a top-gate-controlled
crossover from positive to negative magnetoconductivity. Thereby, we extract
transport parameters where we quantify the distinct types of SOC individually.Comment: 17 pages, 9 figure
Topological transitions in two-dimensional Floquet superconductors
We demonstrate the occurrence of a topological phase transition induced by an effective magnetic field in a two-dimensional electron gas with spin-orbit coupling and in proximity to an s-wave superconductor. The effective, perpendicular magnetic field is generated by an in plane, off-resonant ac- magnetic field or by circularly polarized light. The conditions for entering the topological phase do not rely on fine parameter tuning: For fixed frequency, one requires a minimal amplitude of the effective field which can be evaluated analytically. In this phase, chiral edge states generally emerge for a system in stripe geometry unless the Rashba and Dresselhaus coupling have the same magnitude. In this special case, for magnetic field driving the edge states become Majorana flat bands, due to the presence of a chiral symmetry; the light irradiated system is a trivial superconductor
Conserved spin quantity in strained hole systems with Rashba and Dresselhaus spin-orbit coupling
We derive an effective Hamiltonian for a (001)-confined quasi-two-dimensional hole gas in a strained zinc-blende semiconductor heterostructure including both Rashba and Dresselhaus spin-orbit coupling. In the presence of uniaxial strain along the axes, we find a conserved spin quantity in the vicinity of the Fermi contours in the lowest valence subband. In contrast to previous works, this quantity meets realistic requirements for the Luttinger parameters. For more restrictive conditions, we even find a conserved spin quantity for vanishing strain, restricted to the vicinity of the Fermi surface
Signatures of spin-preserving symmetries in two-dimensional hole gases
We investigate ramifications of the persistent spin helix symmetry in
two-dimensional hole gases in the conductance of disordered mesoscopic systems.
To this end we extend previous models by going beyond the axial approximation
for III-V semiconductors. For heavy-hole subbands we identify an exact
spin-preserving symmetry analogous to the electronic case by analyzing the
crossover from weak antilocalization to weak localization and spin transmission
as a function of extrinsic spin-orbit interaction strength.Comment: 7 pages, 3 figures; reference adde
Spin relaxation in wurtzite nanowires
We theoretically investigate the D'yakonov-Perel' spin-relaxation properties in diffusive wurtzite semiconductor nanowires and their impact on the quantum correction to the conductivity. Although the lifetime of the long-lived spin states is limited by the dominant k-linear spin-orbit contributions in the bulk, these terms show almost no effect in the finite-size nanowires. Here, the spin lifetime is essentially determined by the small k-cubic spin-orbit terms and nearly independent of the wire radius. At the same time, these states possess in general a complex helical structure in real space that is modulated by the spin-precession length induced by the k-linear terms. For this reason, the experimentally detected spin relaxation largely depends on the ratio between the nanowire radius and the spin-precession length as well as the type of measurement. In particular, it is shown that while a variation of the radius hardly affects the magnetoconductance correction, which is governed by the long-lived spin states, the change in the spin lifetime observed in optical experiments can be dramatic. We compare our results with recent experimental studies on wurtzite InAs nanowires
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