64 research outputs found
Kondo resonance of a Co atom exchange coupled to a ferromagnetic tip
The Kondo effect of a Co atom on Cu(100) was investigated with a
low-temperature scanning tunneling microscope using a monoatomically sharp
nickel tip. Upon a tip-Co contact, the differential conductance spectra exhibit
a spin-split asymmetric Kondo resonance. The computed ab initio value of the
exchange coupling is too small to suppress the Kondo effect, but sufficiently
large to produce the splitting observed. A quantitative analysis of the line
shape using the numerical renormalization group technique indicates that the
junction spin polarization is weak.Comment: 5 pages, 4 figure
Structural relaxation effects on interface and transport properties of Fe/MgO(001) tunnel junctions
The interface structure of Fe/MgO(100) magnetic tunnel junctions predicted by
density functional theory (DFT) depends significantly on the choice of exchange
and correlation functional. Bader analysis reveals that structures obtained by
relaxing the cell with the local spin-density approximation (LSDA) display a
different charge transfer than those relaxed with the generalized gradient
approximation (GGA). As a consequence, the electronic transport is found to be
extremely sensitive to the interface structure. In particular, the conductance
for the LSDA-relaxed geometry is about one order of magnitude smaller than that
of the GGA-relaxed one. The high sensitivity of the electronic current to the
details of the interface might explain the discrepancy between the experimental
and calculated values of magnetoresistance.Comment: Submitted to PRL, 5 figure
Electronic structure and magnetism of Mn doped GaN
Mn doped semiconductors are extremely interesting systems due to their novel
magnetic properties suitable for the spintronics applications. It has been
shown recently by both theory and experiment that Mn doped GaN systems have a
very high Curie temperature compared to that of Mn doped GaAs systems. To
understand the electronic and magnetic properties, we have studied Mn doped GaN
system in detail by a first principles plane wave method. We show here the
effect of varying Mn concentration on the electronic and magnetic properties.
For dilute Mn concentration, states of Mn form an impurity band completely
separated from the valence band states of the host GaN. This is in contrast to
the Mn doped GaAs system where Mn states in the gap lie very close to the
valence band edge and hybridizes strongly with the delocalized valence band
states.
To study the effects of electron correlation, LSDA+U calculations have been
performed.
Calculated exchange interaction in (Mn,Ga)N is short ranged in contrary to
that in (Mn,Ga)As where the strength of the ferromagnetic coupling between Mn
spins is not decreased substantially for large Mn-Mn separation. Also, the
exchange interactions are anisotropic in different crystallographic directions
due to the presence or absence of connectivity between Mn atoms through As
bonds.Comment: 6 figures, submitted to Phys. Rev.
Correlated metals and the LDA+U method
While LDA+U method is well established for strongly correlated materials with
well localized orbitals, its application to weakly correlated metals is
questionable. By extending the LDA Stoner approach onto LDA+U, we show that
LDA+U enhances the Stoner factor, while reducing the density of states.
Arguably the most important correlation effects in metals, fluctuation-induced
mass renormalization and suppression of the Stoner factor, are missing from
LDA+U. On the other hand, for {\it moderately} correlated metals LDA+U may be
useful. With this in mind, we derive a new version of LDA+U that is consistent
with the Hohenberg-Kohn theorem and can be formulated as a constrained density
functional theory. We illustrate all of the above on concrete examples,
including the controversial case of magnetism in FeAl.Comment: Substantial changes. In particular, examples of application of the
proposed functional are adde
Kondo screening of the spin and orbital magnetic moments of Fe impurities in Cu
We use x-ray magnetic circular dichroism to evidence the effect of correlations on the local impurity magnetic moment in an archetypal Kondo system, namely, a dilute Cu:Fe alloy. Applying the sum rules on the Fe L2,3 absorption edges, the evolution of the spin and orbital moments across the Kondo temperature are determined separately. The spin moment presents a crossover from a nearly temperature-independent regime below the Kondo temperature to a paramagneticlike regime above. Conversely, the weak orbital moment shows a temperature-independent behavior in the whole temperature range, suggesting different Kondo screening temperature scales for the spin and orbital moments
A self-interaction corrected pseudopotential scheme for magnetic and strongly-correlated systems
Local-spin-density functional calculations may be affected by severe errors
when applied to the study of magnetic and strongly-correlated materials. Some
of these faults can be traced back to the presence of the spurious
self-interaction in the density functional. Since the application of a fully
self-consistent self-interaction correction is highly demanding even for
moderately large systems, we pursue a strategy of approximating the
self-interaction corrected potential with a non-local, pseudopotential-like
projector, first generated within the isolated atom and then updated during the
self-consistent cycle in the crystal. This scheme, whose implementation is
totally uncomplicated and particularly suited for the pseudopotental formalism,
dramatically improves the LSDA results for a variety of compounds with a
minimal increase of computing cost.Comment: 18 pages, 14 figure
Strong-correlation effects in Born effective charges
Large values of Born effective charges are generally considered as reliable
indicators of the genuine tendency of an insulator towards ferroelectric
instability. However, these quantities can be very much influenced by strong
electron correlation and metallic behavior, which are not exclusive properties
of ferroelectric materials. In this paper we compare the Born effective charges
of some prototypical ferroelectrics with those of magnetic, non-ferroelectric
compounds using a novel, self-interaction free methodology that improves on the
local-density approximation description of the electronic properties. We show
that the inclusion of strong-correlation effects systermatically reduces the
size of the Born effective charges and the electron localization lengths.
Furthermore we give an interpretation of the Born effective charges in terms of
band energy structure and orbital occupations which can be used as a guideline
to rationalize their values in the general case.Comment: 10 pages, 4 postscript figure
Implementation of the Projector Augmented Wave LDA+U Method: Application to the Electronic Structure of NiO
The so-called local density approximation plus the multi-orbital mean-field
Hubbard model (LDA+U) has been implemented within the all-electron projector
augmented-wave method (PAW), and then used to compute the insulating
antiferromagnetic ground state of NiO and its optical properties. The
electronic and optical properties have been investigated as a function of the
Coulomb repulsion parameter U. We find that the value obtained from constrained
LDA (U=8 eV) is not the best possible choice, whereas an intermediate value
(U=5 eV) reproduces the experimental magnetic moment and optical properties
satisfactorily. At intermediate U, the nature of the band gap is a mixture of
charge transfer and Mott-Hubbard type, and becomes almost purely of the
charge-transfer type at higher values of U. This is due to the enhancement of
the oxygen 2p states near the top of the valence states with increasing U
value.Comment: 23 pages, 6 figures, submitted to Phys. Rev.
Strain induced exciton fine-structure splitting and shift in bent ZnO microwires
Lattice strain is a useful and economic way to tune the device performance and is commonly present in nanostructures. Here, we investigated for the first time the exciton spectra evolution in bent ZnO microwires along the radial direction via high spatial/energy resolution cathodeluminescence spectroscopy at 5.5â
K. Our experiments show that the exciton peak splits into multi fine peaks towards the compressive part while retains one peak in the tensile part and the emission peak displays a continuous blue-shift from tensile to compressive edges. In combination with first-principles calculations, we show that the observed NBE emission splitting is due to the valence band splitting and the absence of peak splitting in the tensile part maybe due to the highly localized holes in the A band and the carrier density distribution across the microwire. Our studies may pave the way to design nanophotonic and electronic devices using bent ZnO nanowires
The global abundance of tree palms
Aim Palms are an iconic, diverse and often abundant component of tropical ecosystems that provide many ecosystem services. Being monocots, tree palms are evolutionarily, morphologically and physiologically distinct from other trees, and these differences have important consequences for ecosystem services (e.g., carbon sequestration and storage) and in terms of responses to climate change. We quantified global patterns of tree palm relative abundance to help improve understanding of tropical forests and reduce uncertainty about these ecosystems under climate change. Location Tropical and subtropical moist forests. Time period Current. Major taxa studied Palms (Arecaceae). Methods We assembled a pantropical dataset of 2,548 forest plots (covering 1,191 ha) and quantified tree palm (i.e., â„10 cm diameter at breast height) abundance relative to coâoccurring nonâpalm trees. We compared the relative abundance of tree palms across biogeographical realms and tested for associations with palaeoclimate stability, current climate, edaphic conditions and metrics of forest structure. Results On average, the relative abundance of tree palms was more than five times larger between Neotropical locations and other biogeographical realms. Tree palms were absent in most locations outside the Neotropics but present in >80% of Neotropical locations. The relative abundance of tree palms was more strongly associated with local conditions (e.g., higher mean annual precipitation, lower soil fertility, shallower water table and lower plot mean wood density) than metrics of longâterm climate stability. Lifeâform diversity also influenced the patterns; palm assemblages outside the Neotropics comprise many nonâtree (e.g., climbing) palms. Finally, we show that tree palms can influence estimates of aboveâground biomass, but the magnitude and direction of the effect require additional work. Conclusions Tree palms are not only quintessentially tropical, but they are also overwhelmingly Neotropical. Future work to understand the contributions of tree palms to biomass estimates and carbon cycling will be particularly crucial in Neotropical forests
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