213 research outputs found
Cotunneling theory of inelastic STM spin spectroscopy
We propose cotunneling as the microscopic mechanism that makes possible
inelastic electron spectroscopy of magnetic atoms in surfaces for a wide range
of systems, including single magnetic adatoms, molecules and molecular stacks.
We describe electronic transport between the scanning tip and the conducting
surface through the magnetic system (MS) with a generalized Anderson model,
without making use of effective spin models. Transport and spin dynamics are
described with an effective cotunneling Hamiltonian in which the correlations
in the magnetic system are calculated exactly and the coupling to the
electrodes is included up to second order in the tip-MS and MS-substrate. In
the adequate limit our approach is equivalent to the phenomenological Kondo
exchange model that successfully describe the experiments . We apply our method
to study in detail inelastic transport in two systems, stacks of Cobalt
Phthalocyanines and a single Mn atom on CuN. Our method accounts both, for
the large contribution of the inelastic spin exchange events to the conductance
and the observed conductance asymmetry.Comment: 12 pages, 6 figure
Exciton condensates in semiconductor quantum wells emit coherent light
We show that a quasi-two dimensional condensate of optically active excitons
emits coherent light even in the absence of population inversion. This allows
an unambiguous and clear experimental detection of the condensed phase. We
prove that, due to the exciton-photon coupling, quantum and thermal
fluctuations do not destroy condensation at finite temperature. Suitable
conditions to achieve condensation are temperatures of a few K for typical
exciton densities, and the use of a pulsed, and preferably circularly
polarized, laser.Comment: 5 pages, no figure
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
Competition between quantum spin tunneling and Kondo effect
Quantum spin tunneling (QST) and Kondo effect are two very different quantum
phenomena that produce the same effect on quantized spins, namely, the
quenching of their magnetization. However, the nature of this quenching is very
different so that QST and Kondo effects compete with each other. Importantly,
both QST and Kondo produce very characteristic features in the spectral
function that can be measured by means of single spin scanning tunneling
spectroscopy that makes it possible to probe the crossover from one regime to
the other. We model this crossover, and the resulting changes in transport,
using a non-perturbative treatment of a generalized Anderson model including
magnetic anisotropy that leads to quantum spin tunneling. We predict that, at
zero magnetic field, integer spins can feature a split-Kondo peak driven by
quantum spin tunneling.Comment: 5 pages, 3 figures; accepted in EPJB; replaced with revised
manuscrip
Emergence of half-metallicity in suspended NiO chains
Contrary to the antiferromagnetic and insulating character of bulk NiO,
one-dimensional chains of this material can become half-metallic due to the
lower coordination of their atoms. Here we present ab initio electronic
structure and quantum transport calculations of ideal infinitely long NiO
chains and of more realistic short ones suspended between Ni electrodes. While
infinite chains are insulating, short suspended chains are half-metallic
minority-spin conductors which display very large magnetoresistance and a
spin-valve behaviour controlled by a single atom.Comment: 5 pages, 4 figures; accepted version; minor changes in introduction
and reference
In-gap impurity states as the hallmark of the Quantum Spin Hall phase
We study the different response to an impurity of the two topologically
different phases shown by a two dimensional insulator with time reversal
symmetry, namely, the Quantum Spin Hall and the normal phase. We consider the
case of graphene as a toy model that features the two phases driven,
respectively, by intrinsic spin-orbit coupling and inversion symmetry breaking.
We find that strictly normalizable in-gap impurity states only occur in the
Quantum Spin Hall phase and carry dissipationless current whose quirality is
determined by the spin and pseudospin of the residing electron. Our results
imply that topological order can be unveiled by local probes of defect states.Comment: 5 pages, 3 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
Spin dynamics of current driven single magnetic adatoms and molecules
A scanning tunneling microscope can probe the inelastic spin excitations of a
single magnetic atom in a surface via spin-flip assisted tunneling in which
transport electrons exchange spin and energy with the atomic spin. If the
inelastic transport time, defined as the average time elapsed between two
inelastic spin flip events, is shorter than the atom spin relaxation time, the
STM current can drive the spin out of equilibrium. Here we model this process
using rate equations and a model Hamiltonian that describes successfully spin
flip assisted tunneling experiments, including a single Mn atom, a Mn dimer and
Fe Phthalocyanine molecules. When the STM current is not spin polarized, the
non-equilibrium spin dynamics of the magnetic atom results in non-monotonic
curves. In the case of spin polarized STM current, the spin orientation
of the magnetic atom can be controlled parallel or anti-parallel to the
magnetic moment of the tip. Thus, spin polarized STM tips can be used both to
probe and to control the magnetic moment of a single atom.Comment: 15 pages, 12 figure
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