1,816 research outputs found
Antiferromagnetic order in (Ga,Mn)N nanocrystals: A density functional theory study
We investigate the electronic and magnetic properties of (Ga,Mn)N
nanocrystals using the density functional theory. We study both wurtzite and
zinc-blende structures doped with one or two substitutional Mn impurities. For
a single Mn dopant placed close to surface, the behavior of the empty
Mn-induced state, hereafter referred to as "Mn hole", is different from bulk
(Ga,Mn)N. The energy level corresponding to this off-center Mn hole lies within
the nanocrystal gap near the conduction edge. For two Mn dopants, the most
stable magnetic configuration is antiferromagnetic, and this was unexpected
since (Ga,Mn)N bulk shows ferromagnetism in the ground state. The surprising
antiferromagnetic alignment of two Mn spins is ascribed also to the holes
linked to the Mn impurities located close to surface. Unlike Mn holes in
(Ga,Mn)N bulk, these Mn holes in confined (Ga,Mn)N nanostructures do not
contribute to the ferromagnetic alignment of the two Mn spins
Optical spin control in nanocrystalline magnetic nanoswitches
We investigate the optical properties of (Cd,Mn)Te quantum dots (QDs) by
looking at the excitons as a function of the Mn impurities positions and their
magnetic alignments. When doped with two Mn impurities, the Mn spins, aligned
initially antiparallel in the ground state, have lower energy in the parallel
configuration for the optically active spin-up exciton. Hence, the
photoexcitation of the QD ground state with antiparallel Mn spins induces one
of them to flip and they align parallel. This suggests that (Cd,Mn)Te QDs are
suitable for spin-based operations handled by light
First-principles calculations of the magnetic properties of (Cd,Mn)Te nanocrystals
We investigate the electronic and magnetic properties of Mn-doped CdTe
nanocrystals (NCs) with 2 nm in diameter which can be experimentally
synthesized with Mn atoms inside. Using the density-functional theory, we
consider two doping cases: NCs containing one or two Mn impurities. Although
the Mn d peaks carry five up electrons in the dot, the local magnetic moment on
the Mn site is 4.65 mu_B. It is smaller than 5 mu_B because of the sp-d
hybridization between the localized 3d electrons of the Mn atoms and the s- and
p-type valence states of the host compound. The sp-d hybridization induces
small magnetic moments on the Mnnearest- neighbor Te sites, antiparallel to the
Mn moment affecting the p-type valence states of the undoped dot, as usual for
a kinetic-mediated exchange magnetic coupling. Furthermore, we calculate the
parameters standing for the sp-d exchange interactions. Conduction N0\alpha and
valence N0\beta are close to the experimental bulk values when the Mn
impurities occupy bulklike NCs' central positions, and they tend to zero close
to the surface. This behavior is further explained by an analysis of
valence-band-edge states showing that symmetry breaking splits the states and
in consequence reduces the exchange. For two Mn atoms in several positions, the
valence edge states show a further departure from an interpretation based in a
perturbative treatment. We also calculate the d-d exchange interactions |Jdd|
between Mn spins. The largest |Jdd| value is also for Mn atoms on bulklike
central sites; in comparison with the experimental d-d exchange constant in
bulk Cd0.95Mn0.05Te, it is four times smaller
The Kondo-Hubbard model at half-filling
We have analyzed the antiferromagnetic (J>0) Kondo-Hubbard lattice with the
band at half-filling by means of a perturbative approach in the strong coupling
limit, the small parameter is an arbitrary tight-binding band. The results are
valid for any band shape and any dimension. We have obtained the energies of
elementary charge and spin excitations as well as the magnetic correlations in
order to elucidate the magnetic and charge behavior of the Kondo lattice at
half-filling. Finally, we have briefly analyzed the ferromagnetic case (J<0),
which is shown to be equivalent to an effective antiferromagnetic Heisenberg
model.Comment: 4 pages, Proceedings of SCES98/Pari
Electronic structure and optical properties of ZnS/CdS nanoheterostructures
The electronic and optical properties of spherical nanoheterostructures are
studied within the semi-empirical tight-binding model including
the spin-orbit interaction. We use a symmetry-based approach previously applied
to CdSe and CdTe quantum dots. The complete one-particle spectrum is obtained
by using group-theoretical methods. The excitonic eigenstates are then deduced
in the configuration-interaction approach by fully taking into account the
Coulomb direct and exchange interactions. Here we focus on ZnS/CdS, ZnS/CdS/ZnS
and CdS/ZnS nanocrystals with particular emphasis on recently reported
experimental data. The degree of carrier localization in the CdS well layer is
analyzed as a function of its thickness. We compute the excitonic fine
structure, i.e., the relative intensities of low-energy optical transitions.
The calculated values of the absorption gap show a good agreement with the
experimental ones. Enhanced resonant photoluminescence Stokes shifts are
predicted.Comment: 6 pages, 4 Figures, revtex
Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals
The photoluminescence spectra of spherical CdTe nanocrystals with zincblende
structure are studied by size-selective spectroscopic techniques. We observe a
resonant Stokes shift of 15 meV when the excitation laser energy is tuned to
the red side of the absorption band at 2.236 eV. The experimental data are
analyzed within a symmetry-based tight-binding theory of the exciton spectrum,
which is first shown to account for the size dependence of the fundamental gap
reported previously in the literature. The theoretical Stokes shift presented
as a function of the gap shows a good agreement with the experimental data,
indicating that the measured Stokes shift indeed arises from the electron-hole
exchange interaction.Comment: 8 pages, 4 figures, LaTe
Content in metallic ions of wines from the Madeira and Azores archipelagos
This study determines for the first time Na, K, Ca, Mg, Fe, Cu, Zn, Mn, Sr, Li and Rb contents in wines from the archipelagos of Madeira and Azores (Portugal). The greater part of the mean content for the different parameters fell within the ranges described in the literature, except for sodium whose higher content may be due to the effect of marine spray. ANOVA was used to establish the metals with significant differences in mean content between the wines from both archipelagos, between table and liquor wines of Madeira, and between wines of Pico and Terceira Islands from the Azores archipelago. Principal component analysis shows differences in the wines according to the wine-making process and/or the equipment employed. Stepwise linear discriminant analysis achieves a good classification and validation of wines according to the archipelago of origin, and the island in the case of Azores wines
Análisis sobre la influencia de la densidad en la termografía de infrarrojos y el alcance de esta técnica en la detección de defectos internos en la madera
This paper shows the results of a laboratory phase for the determination of the 1limits of infrared thermography in detecting internal defects in wood and, furthermore, it analyses how the density of this material can influence the surface temperature displayed in a thermogram. To this end, experimental work is carried out whereby a series of work and environmental parameters are monitored (such as environmental temperature, relative humidity, distance to target), and the process is systematized by using pieces of different wood species (different densities) and by devising a set of samples for which various possible cases are analysed in terms of size and internal position of the damage. Similarly, using these samples, a study has been conducted on the effect on the thermal image produced by an increase of humidity inside the defect phenomena normally associated with wood decay.En el presente artículo se muestran los resultados obtenidos en una fase de laboratorio al tratar de determinar el alcance de la termografía de infrarrojos en la detección de defectos internos en la madera y, por otro lado, cómo interviene la densidad de este material sobre la temperatura superficial mostrada en un termograma. Para ello se ha desarrollado un trabajo experimental donde se han controlado una serie de parámetros ambientales y de trabajo (temperatura ambiente, humedad relativa, distancia al objetivo…), y se ha sistematizado el proceso utilizando piezas de distintas especies de madera (distintas densidades) y diseñando una serie de probetas donde se han analizado varias casuísticas posibles en cuanto a tamaño y posición interna de los daños. Análogamente, sobre estas probetas se ha estudiado el efecto que produce sobre la imagen termográfica un incremento de humedad interna en el defecto asociada normalmente a los fenómenos de pudrición de la madera
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