133 research outputs found
Electronic and magnetic properties of substitutional Mn clusters in (Ga,Mn)As
The magnetization and hole distribution of Mn clusters in (Ga,Mn)As are
investigated by all-electron total energy calculations using the projector
augmented wave method within the density-functional formalism. It is found that
the energetically most favorable clusters consist of Mn atoms surrounding one
center As atom. As the Mn cluster grows the hole band at the Fermi level splits
increasingly and the hole distribution gets increasingly localized at the
center As atom. The hole distribution at large distances from the cluster does
not depend significantly on the cluster size. As a consequence, the spin-flip
energy differences of distant clusters are essentially independent of the
cluster size. The Curie temperature is estimated directly from these
spin-flip energies in the mean field approximation. When clusters are present
estimated values are around 250 K independent of Mn concentration whereas
for a uniform Mn distribution we estimate a of about 600 K.Comment: 7 pages, 5 figures, 2 tables; Revised manuscript 26. May 200
Intrinsic hole localization mechanism in magnetic semiconductors
The interplay between clustering and exchange coupling in magnetic
semiconductors for the prototype (Ga_{1-x},Mn_x)As with manganese
concentrations x of 1/16 and 1/32 in the interesting experimental range is
investigated. For x ~ 6 %, when all possible arrangements of two atoms within a
large supercell are considered, the clustering of Mn atoms at nearest-neighbour
Ga sites is energetically preferred. As shown by spin density analysis, this
minimum energy configuration localizes further one hole and reduces the
effective charge carrier concentration. Also the exchange coupling constant
increases to a value corresponding to lower Mn concentrations with decreasing
inter Mn distance.Comment: Accepted for publication in Journal of Physics: Condensed Matte
Spontaneous magnetization of aluminum nanowires deposited on the NaCl(100) surface
We investigate electronic structures of Al quantum wires, both unsupported
and supported on the (100) NaCl surface, using the density-functional theory.
We confirm that unsupported nanowires, constrained to be linear, show
magnetization when elongated beyond the equilibrium length. Allowing ions to
relax, the wires deform to zig-zag structures with lower magnetization but no
dimerization occurs. When an Al wire is deposited on the NaCl surface, a
zig-zag geometry emerges again. The magnetization changes moderately from that
for the corresponding unsupported wire. We analyse the findings using electron
band structures and simple model wires.Comment: submitted to PHys. Rev.
Identification and design principles of low hole effective mass p-type transparent conducting oxides
The development of high-performance transparent conducting oxides is critical to many technologies from transparent electronics to solar cells. Whereas n-type transparent conducting oxides are present in many devices, their p-type counterparts are not largely commercialized, as they exhibit much lower carrier mobilities due to the large hole effective masses of most oxides. Here we conduct a high-throughput computational search on thousands of binary and ternary oxides and identify several highly promising compounds displaying exceptionally low hole effective masses (up to an order of magnitude lower than state-of-the-art p-type transparent conducting oxides), as well as wide band gaps. In addition to the discovery of specific compounds, the chemical rationalization of our findings opens new directions, beyond current Cu-based chemistries, for the design and development of future p-type transparent conducting oxides.United States. Office of Naval Research (Award N00014-11-1-0212
Preparation and Instability of Nanocrystalline Cuprous Nitride
Low-dimensional cuprous nitride (Cu3N) was synthesized by nitridation (ammonolysis) of cuprous oxide (Cu2O) nanocrystals using either ammonia (NH3) or urea (H2NCONH2) as the nitrogen source. The resulting nanocrystalline Cu3N spontaneously decomposes to nanocrystalline CuO in the presence of both water and oxygen from air at room temperature. Ammonia was produced in 60% chemical yield during Cu3N decomposition, as measured using the colorimetric indophenol method. Because Cu3N decomposition requires H2O and produces substoichiometric amounts of NH3\u3e, we conclude that this reaction proceeds through a complex stoichiometry that involves the concomitant release of both N2 and NH3. This is a thermodynamically unfavorable outcome, strongly indicating that H2O (and thus NH3 production) facilitate the kinetics of the reaction by lowering the energy barrier for Cu3N decomposition. The three different Cu2O, Cu3N, and CuO nanocrystalline phases were characterized by a combination of optical absorption, powder X-ray diffraction, transmission electron microscopy, and electronic density of states obtained from electronic structure calculations on the bulk solids. The relative ease of interconversion between these interesting and inexpensive materials bears possible implications for catalytic and optoelectronic applications
A computational study of the electronic properties, ionic conduction, and thermal expansion of Sm1−xAxCoO3 and Sm1−xAxCoO3−x/2 (A = Ba2+, Ca2+, Sr2+, and x = 0.25, 0.5) as intermediate temperature SOFC cathodes
The substitutional doping of Ca2+, Sr2+, and Ba2+ on the Sm-site in the cubic perovskite SmCoO3 is reported to improve both electronic and ionic conductivities for applications as solid oxide fuel cell (SOFC) cathodes. Hence, in this study we have used density functional theory (DFT) calculations to investigate dopant configurations at two different dopant concentrations: 25 and 50%. To preserve the electroneutrality of the system, we have studied two different charge compensation mechanisms: the creation of oxygen vacancies, and electronic holes. After examining the electronic structure, charge density difference, and oxygen vacancy formation energies, we concluded that oxygen vacancy charge compensation is the preferred mechanism to maintain the electroneutrality of the system. Furthermore, we found that the improvement of the electronic conduction is not a direct consequence of the appearance of electron holes, but a result of the distortion of the material, more specifically, the distortion of the Co–O bonds. Finally, molecular dynamics were employed to model ionic conduction and thermal expansion coefficients. It was found that all dopants at both concentrations showed high ionic conduction comparable to experimental results
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