993 research outputs found
Magnetization orientation dependence of the quasiparticle spectrum and hysteresis in ferromagnetic metal nanoparticles
We use a microscopic Slater-Koster tight-binding model with short-range
exchange and atomic spin-orbit interactions that realistically captures generic
features of ferromagnetic metal nanoparticles to address the mesoscopic physics
of magnetocrystalline anisotropy and hysteresis in nanoparticle quasiparticle
excitation spectra. Our analysis is based on qualitative arguments supported by
self-consistent Hartree-Fock calculations for nanoparticles containing up to
260 atoms. Calculations of the total energy as a function of magnetization
direction demonstrate that the magnetic anisotropy per atom fluctuates by
several percents when the number of electrons in the particle changes by one,
even for the largest particles we consider. Contributions of individual
orbitals to the magnetic anisotropy are characterized by a broad distribution
with a mean more than two orders of magnitude smaller than its variance and
with no detectable correlations between anisotropy contribution and
quasiparticle energy. We find that the discrete quasiparticle excitation
spectrum of a nanoparticle displays a complex non-monotonic dependence on an
external magnetic field, with abrupt jumps when the magnetization direction is
reversed by the field, explaining recent spectroscopic studies of magnetic
nanoparticles. Our results suggests the existence of a broad cross-over from a
weak spin-orbit coupling to a strong spin-orbit coupling regime, occurring over
the range from approximately 200- to 1000-atom nanoparticles.Comment: 39 pages, 18 figures, to be published in Physical Review
Thin films of a three-dimensional topological insulator in a strong magnetic field: a microscopic study
The response of thin films of BiSe to a strong perpendicular magnetic
field is investigated by performing magnetic bandstructure calculations for a
realistic multi-band tight-binding model. Several crucial features of Landau
quantization in a realistic three-dimensional topological insulator are
revealed. The Landau level is absent in ultra-thin films, in agreement
with experiment. In films with a crossover thickness of five quintuple layers,
there is a signature of the level, whose overall trend as a function of
magnetic field matches the established low-energy effective-model result.
Importantly, we find a field-dependent splitting and a strong spin-polarization
of the level which can be measured experimentally at reasonable field
strengths. Our calculations show mixing between the surface and bulk Landau
levels which causes the character of levels to evolve with magnetic field.Comment: 5 pages, 4 figure
Elementary Excitations of Ferromagnetic Metal Nanoparticles
We present a theory of the elementary spin excitations in transition metal
ferromagnet nanoparticles which achieves a unified and consistent quantum
description of both collective and quasiparticle physics. The theory starts by
recognizing the essential role played by spin-orbit interactions in determining
the energies of ferromagnetic resonances in the collective excitation spectrum
and the strength of their coupling to low-energy particle-hole excitations. We
argue that a crossover between Landau-damped ferromagnetic resonance and
pure-state collective magnetic excitations occurs as the number of atoms in
typical transition metal ferromagnet nanoparticles drops below approximately
, approximately where the single-particle level spacing, ,
becomes larger than, , where is the
ferromagnetic resonance frequency and is the Gilbert damping
parameter. We illustrate our ideas by studying the properties of semi-realistic
model Hamiltonians, which we solve numerically for nanoparticles containing
several hundred atoms. For small nanoparticles, we find one isolated
ferromagnetic resonance collective mode below the lowest particle-hole
excitation energy, at meV. The spectral weight of
this pure excitation nearly exhausts the transverse dynamical susceptibility
spectral weight. As approaches , the
ferromagnetic collective excitation is more likely to couple strongly with
discrete particle-hole excitations. In this regime the distinction between the
two types of excitations blurs. We discuss the significance of this picture for
the interpretation of recent single-electron tunneling experiments.Comment: 19 pages, 13 figure
Chern number spins of Mn acceptor magnets in GaAs
We determine the effective total spin of local moments formed from
acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern
numbers. We find that when individual Mn atoms are close to the sample surface,
the total spin changes from to , due to quenching of the
acceptor orbital moment. For Mn pairs in bulk, the total depends on the
pair orientation in the GaAs lattice and on the separation between the Mn
atoms. We point out that Berry curvature variation as a function of local
moment orientation can profoundly influence the quantum spin dynamics of these
magnetic entities.Comment: 4 pages, 3 figure
Magnetic interactions of substitutional Mn pairs in GaAs
We employ a kinetic-exchange tight-binding model to calculate the magnetic
interaction and anisotropy energies of a pair of substitutional Mn atoms in
GaAs as a function of their separation distance and direction. We find that the
most energetically stable configuration is usually one in which the spins are
ferromagnetically aligned along the vector connecting the Mn atoms. The
ferromagnetic configuration is characterized by a splitting of the topmost
unoccupied acceptor levels, which is visible in scanning tunneling microscope
studies when the pair is close to the surface and is strongly dependent on pair
orientation. The largest acceptor splittings occur when the Mn pair is oriented
along the symmetry direction, and the smallest when they are oriented
along . We show explicitly that the acceptor splitting is not simply
related to the effective exchange interaction between the Mn local moments. The
exchange interaction constant is instead more directly related to the width of
the distribution of all impurity levels -- occupied and unoccupied. When the Mn
pair is at the (110) GaAs surface, both acceptor splitting and effective
exchange interaction are very small except for the smallest possible Mn
separation.Comment: 25 figure
I sostegni all'ambiente
Diversi sono e saranno i protagonisti nell’ambito del traspor¬to sostenibile (pubblico e privato): la politica e le istituzioni con le con¬seguenti normative, gli utenti-consumatori, le multinazionali dei com¬bustibili, le case costruttrici di autoveicoli e la ricerca che dovrà trovare e proporre soluzioni alternative, dai propulsori dalle emissioni con un impatto ambientale sempre minore, alle materie prime per i carburanti ricavate da fonti rinnovabili. La normativa europea in materia di mobilità sostenibile è piuttosto estesa. Vige un principio di sussidiarietà che delega agli stati membri l’ado-zione di norme in materia di trasporti urbani. Di conseguenza, le diret¬tive si focalizzano principalmente sul miglioramento della qualità dei combustibili, sulla differenziazione delle fonti energetiche usate nel campo dei trasporti, sul miglioramento degli standard emissivi e sulla promozione di buone pratiche. Altro obiettivo da conseguire è la sicurezza e, sebbene l’Unione Europea sia impegnata ad incrementarla relativamente per tutti i modi di traspor¬to, essa continua a rappresentare una delle problematiche più rilevanti
- …