29,164 research outputs found
On the origin of magnetic anisotropy in two dimensional CrI
The observation of ferromagnetic order in a monolayer of CrI has been
recently reported, with a Curie temperature of 45 Kelvin and off-plane easy
axis. Here we study the origin of magnetic anisotropy, a necessary ingredient
to have magnetic order in two dimensions, combining two levels of modeling,
density functional calculations and spin model Hamiltonians. We find two
different contributions to the magnetic anisotropy of the material, both
favoring off-plane magnetization and contributing to open a gap in the spin
wave spectrum. First, ferromagnetic super-exchange across the 90
degree Cr-I-Cr bonds, are anisotropic, due to the spin orbit interaction of the
ligand I atoms. Second, a much smaller contribution that comes from the single
ion anisotropy of the Cr atom. Our results permit to establish the XXZ
Hamiltonian, with a very small single ion anisotropy, as the adequate spin
model for this system. Using spin wave theory we estimate the Curie temperature
and we highlight the essential role played by the gap that magnetic anisotropy
induces on the magnon spectrum.Comment: 8 pages, 5 figure
Single exciton spectroscopy of semimagnetic quantum dots
A photo-excited II-VI semiconductor nanocrystal doped with a few Mn spins is
considered. The effects of spin-exciton interactions and the resulting
multi-spin correlations on the photoluminescence are calculated by numerical
diagonalization of the Hamiltonian, including exchange interaction between
electrons, holes and Mn spins, as well as spin-orbit interaction. The results
provide a unified description of recent experiments of photoluminesnce of dots
with one and many Mn atoms as well as optically induced ferromagnetism in
semimagnetic nanocrystals.Comment: 5 pages, 3 figure
Semi-classical spectrum of the Homogeneous sine-Gordon theories
The semi-classical spectrum of the Homogeneous sine-Gordon theories
associated with an arbitrary compact simple Lie group G is obtained and shown
to be entirely given by solitons. These theories describe quantum integrable
massive perturbations of Gepner's G-parafermions whose classical
equations-of-motion are non-abelian affine Toda equations. One-soliton
solutions are constructed by embeddings of the SU(2) complex sine-Gordon
soliton in the regular SU(2) subgroups of G. The resulting spectrum exhibits
both stable and unstable particles, which is a peculiar feature shared with the
spectrum of monopoles and dyons in N=2 and N=4 supersymmetric gauge theories.Comment: 28 pages, plain TeX, no figure
Electronic Structure of gated graphene and graphene ribbons
We study the electronic structure of gated graphene sheets. We consider both
infinite graphene and finite width ribbons. The effect of Coulomb interactions
between the electrically injected carriers and the coupling to the external
gate are computed self-consistently in the Hartree approximation. We compute
the average density of extra carriers, , the number of occupied
subbands and the density profiles as a function of the gate potential . We
discuss quantum corrections to the classical capacitance and we calculate the
threshold above which semiconducting armchair ribbons conduct. We find
that the ideal conductance of perfectly transmitting wide ribbons is
proportional to the square root of the gate voltage.Comment: 8 pages, 6 figure
Phase diagram of a polydisperse soft-spheres model for liquids and colloids
The phase diagram of soft spheres with size dispersion has been studied by
means of an optimized Monte Carlo algorithm which allows to equilibrate below
the kinetic glass transition for all sizes distribution. The system
ubiquitously undergoes a first order freezing transition. While for small size
dispersion the frozen phase has a crystalline structure, large density
inhomogeneities appear in the highly disperse systems. Studying the interplay
between the equilibrium phase diagram and the kinetic glass transition, we
argue that the experimentally found terminal polydispersity of colloids is a
purely kinetic phenomenon.Comment: Version to be published in Physical Review Letter
The internationalisation of the Spanish SME sector
As part of a wider research program, we analysed the theoretical framework and the recent developments of the process of internationalisation (transnationalisation) of the small- and medium-sized enterprises in Spain. The paper highlights the main trends and barriers of this internationalisation process. Methodology included document analyses, interviews, and the analyses of statistical databases
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