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The ability to artificially grow different configurations of semiconductor alloys--random structures, spontaneously ordered and layered superlattices--raises the issue of how different alloy configurations may lead to new and different alloy physical properties. We address this question in the context of nitrogen impurities in GaP, which form deep levels in the gap whose energy and optical absorption sensitively depend on configuration. We use the "inverse band structure" approach in which we first specify a desired target physical property (such as the deepest nitrogen level, or lowest strain configuration), and then we search, via genetic algorithm, for the alloy atomic configurations that have this property. We discover the essential structural motifs leading to such target properties. This strategy opens the way to efficient alloy design. 相似文献
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A systematic method to unravel a large class of single-bonded (SB) polymeric phases of nitrogen under high pressure is presented. The approach is based on the combinatorial generation of different Peierls-like distortions of a given reference structure that maintain the threefold connectivity of SB nitrogen, followed by first-principles calculations. Using an eight atom simple cubic reference structure, the approach not only recovers all four SB nitrogen phases reported to date, but eight new metastable structures (confirmed by phonon density of states calculations) are found. Basic properties of the structures are computed and the trends analyzed. Extensions to the method are straightforward and should lead to the discovery of more phases of polynitrogen. 相似文献
3.
Intrinsic bistability and hysteresis have been observed in the photoluminescence of bulk glass doped with Yb3+ and Tm3+ impurities at room temperature. Multiple instabilities also occur in channel waveguide lasers in this material. Consequently, local field effects exhibit more degrees of freedom than expected and have important implications for compact optical amplifiers and oscillators. 相似文献
4.
G Fuchs K-H Müller J Freudenberger K Nenkov S-L Drechsler SV Shulga D Lipp A Gladun T Cichorek P Gegenwart 《Pramana》2002,58(5-6):791-797
The effect of substitutional disorder on the superconducting properties of YNi2B2C was studied by partially replacing yttrium and nickel by Lu and Pt, respectively. For the two series of (Y, Lu)Ni2B2C and Y(Ni, Pt)2B2C compounds, the upper critical field H
c2(T) and the specific heat c
p(T, H) in the superconducting mixed state have been investigated. Disorder is found to reduce several relevant quantities such
as T
c, the upper critical field H
c2(0) at T=0 and a characteristic positive curvature of H
c2(T) observed for these compounds near T
c. The H
c2(T) data point to the clean limit for (Y, Lu) substitutions and to a transition to the quasi-dirty limit for (Ni, Pt) substitutions.
The electronic specific heat contribution γ(H) exhibits significant deviations from the usual linear γ(H) law. These deviations reduce with growing substitutional disorder but remain even in the quasidirty limit which is reached
in the Y(Ni1−x
, Pt
x
)2B2C samples for x=0.1. 相似文献
5.
Non-resonant microwave absorption (NRMA) studies of superconducting MgB2 and a sample containing ∼10% by weight of MgO in MgB2 are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB2 sample. A linear temperature dependence of the lower critical field H
c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB2 + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links.
The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB2 sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong
pinning of flux lines. This hysteresis saturates a few degrees below T
c while in the MgB2 + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning. 相似文献
6.
Abou-Rachid H Song Y Hu A Dudiy S Zybin SV Goddard WA 《The journal of physical chemistry. A》2008,112(46):11914-11920
We present density functional theory level predictions and analysis of the basic properties of newly synthesized high-nitrogen compounds together with 3,6-bis(2H-tetrazol-5-yl)-1,2,4,5-tetrazine (BTT) and 3,3'-azobis(6-amino-1,2,4,5-tetrazine) (DAAT), for which experimental data are available. The newly synthesized high-nitrogen compounds are based on tricycle fused 1,2,4-triazine and 1,2,4,5-tetrazine heterocycles. In this work, the molecules BTT and DAAT have been studied in order to validate the theoretical approach and to facilitate further progress developments for the molecules of interest. Molecular structural properties are clarified, and IR spectra predictions are provided to help detection of those compounds in the experiment. The energy content of the molecules in the gas phase is evaluated by calculating standard enthalpies of formation, by using a special selection of isodesmic reaction paths. We also include estimates of the condensed-phase heats of formation and heats of sublimation in the framework of the Politzer approach. The obtained properties are consistent with those new high-nitrogen compounds being a promising set of advanced energetic materials. 相似文献
7.
Franceschetti A Dudiy SV Barabash SV Zunger A Xu J van Schilfgaarde M 《Physical review letters》2006,97(4):047202
The transition temperature TC of multicomponent systems--ferromagnetic, superconducting, or ferroelectric--depends strongly on the atomic arrangement, but an exhaustive search of all configurations for those that optimize TC is difficult, due to the astronomically large number of possibilities. Here we address this problem by parametrizing the TC of a set of approximately 50 input configurations, calculated from first principles, in terms of configuration variables ("cluster expansion"). Once established, this expansion allows us to search almost effortlessly the transition temperature of arbitrary configurations. We apply this approach to search for the configuration of Mn dopants in GaAs having the highest ferromagnetic Curie temperature. Our general approach of cluster expanding physical properties opens the way to design based on exploring a large space of configurations. 相似文献
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