807 research outputs found
Exchange interactions in transition metal oxides: The role of oxygen spin polarization
Magnetism of transition metal (TM) oxides is usually described in terms of
the Heisenberg model, with orientation-independent interactions between the
spins. However, the applicability of such a model is not fully justified for TM
oxides because spin polarization of oxygen is usually ignored. In the
conventional model based on the Anderson principle, oxygen effects are
considered as a property of the TM ion and only TM interactions are relevant.
Here, we perform a systematic comparison between two approaches for spin
polarization on oxygen in typical TM oxides. To this end, we calculate the
exchange interactions in NiO, MnO, and hematite (Fe2O3) for different magnetic
configurations using the magnetic force theorem. We consider the full spin
Hamiltonian including oxygen sites, and also derive an effective model where
the spin polarization on oxygen renormalizes the exchange interactions between
TM sites. Surprisingly, the exchange interactions in NiO depend on the magnetic
state if spin polarization on oxygen is neglected, resulting in non-Heisenberg
behavior. In contrast, the inclusion of spin polarization in NiO makes the
Heisenberg model more applicable. Just the opposite, MnO behaves as a
Heisenberg magnet when oxygen spin polarization is neglected, but shows strong
non-Heisenberg effects when spin polarization on oxygen is included. In
hematite, both models result in non-Heisenberg behavior. General applicability
of the magnetic force theorem as well as the Heisenberg model to TM oxides is
discussed.Comment: 19 pages, 2 figure
Geometric, electronic and magnetic structure of FeO clusters
Correlation between geometry, electronic structure and magnetism of solids is
both intriguing and elusive. This is particularly strongly manifested in small
clusters, where a vast number of unusual structures appear. Here, we employ
density functional theory in combination with a genetic search algorithm,
GGA and a hybrid functional to determine the structure of gas phase
FeO clusters. For FeO cation clusters we also
calculate the corresponding vibration spectra and compare them with
experiments. We successfully identify FeO, FeO,
FeO, FeO and propose structures for
FeO. Within the triangular geometric structure of
FeO a non-collinear, ferrimagnetic and ferromagnetic state are
comparable in energy. FeO and FeO are
ferrimagnetic with a residual magnetic moment of 1~\muB{} due to ionization.
FeO is ferrimagnetic due to the odd number of Fe atoms. We
compare the electronic structure with bulk magnetite and find
FeO, FeO, FeO to be mixed
valence clusters. In contrast, in FeO and FeO
all Fe are found to be trivalent.Comment: 14 pages, 21 figure
Modeling Klein tunneling and caustics of electron waves in graphene
We employ the tight-binding propagation method to study Klein tunneling and
quantum interference in large graphene systems. With this efficient numerical
scheme, we model the propagation of a wave packet through a potential barrier
and determine the tunneling probability for different incidence angles. We
consider both sharp and smooth potential barriers in n-p-n and n-n' junctions
and find good agreement with analytical and semiclassical predictions. When we
go outside the Dirac regime, we observe that sharp n-p junctions no longer show
Klein tunneling because of intervalley scattering. However, this effect can be
suppressed by considering a smooth potential. Klein tunneling holds for
potentials changing on the scale much larger than the interatomic distance.
When the energies of both the electrons and holes are above the Van Hove
singularity, we observe total reflection for both sharp and smooth potential
barriers. Furthermore, we consider caustic formation by a two-dimensional
Gaussian potential. For sufficiently broad potentials we find a good agreement
between the simulated wave density and the classical electron trajectories.Comment: 14 pages, 12 figure
Magnetic properties of Co doped Nb clusters
From magnetic deflection experiments on isolated Co doped Nb clusters we made
the interesting observation of some clusters being magnetic, while others
appear to be non-magnetic. There are in principle two explanations for this
behavior. Either the local moment at the Co site is completely quenched or it
is screened by the delocalized electrons of the cluster, i.e. the Kondo effect.
In order to reveal the physical origin, we conducted a combined theoretical and
experimental investigation. First, we established the ground state geometry of
the clusters by comparing the experimental vibrational spectra with those
obtained from a density functional theory study. Then, we performed an analyses
based on the Anderson impurity model. It appears that the non-magnetic clusters
are due to a complete quenching of the local Co moment and not due to the Kondo
effect. In addition, the magnetic behavior of the clusters can be understood
from an inspection of their electronic structure. Here magnetism is favored
when the effective hybridization around the chemical potential is small, while
the absence of magnetism is signalled by a large effective hybridization around
the chemical potential.Comment: 14 pages, 8 figure
Tracking with prescribed transient performance for hysteretic systems
Tracking of reference signals (assumed bounded with essentially bounded derivative) is considered for a class of single-input, single-output, nonlinear systems, described by a functional differential equation with a hysteresis nonlinearity in the input channel. The first control objective is tracking, by the output, with prescribed accuracy: determine a feedback strategy which ensures that, for every reference signal and every system of the underlying class, the tracking error ultimately satisfies the prescribed accuracy requirements. The second objective is guaranteed output transient performance: the graph of the tracking error should be contained in a prescribed set (performance funnel). Under a weak sector boundedness assumption on the hysteresis operator, both objectives are achieved by a memoryless feedback which is universal for the underlying class of systems
Standardization of surface electromyography utilized to evaluate patients with dysphagia
<p>Abstract</p> <p>Backgorund</p> <p>Patients suspected of having swallowing disorders, could highly benefit from simple diagnostic screening before being referred to specialist evaluations. We introduce surface electromyography (sEMG) to carry out rapid assessment of such patients and propose suggestions for standardizing sEMGs in order to identify abnormal deglutition.</p> <p>Methods</p> <p>Specifics steps for establishing standards for applying the technique for screening purposes (e.g., evaluation of specific muscles), the requirements for diagnostic sEMG equipment, the sEMG technique itself, and defining the tests suitable for assessing deglutition (e.g., saliva, normal, and excessive swallows and uninterrupted drinking of water) are presented in detail. A previously described normative database for single swallowing and drinking and standard approach to analysis was compared to data on the duration and electric activity of muscles involved in deglutition and with sEMG recordings in order to estimate stages of a swallow.</p> <p>Conclusion</p> <p>SEMG of swallowing is a simple and reliable method for screening and preliminary differentiation among dysphagia and odynophagia of various origins. This noninvasive radiation-free examination has a low level of discomfort, and is simple, timesaving and inexpensive to perform. With standardization of the technique and an established normative database, sEMG can serve as a reliable screening method for optimal patient management.</p
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