Piezoelectricity of ordered (ScxGa1-xN) alloys from first principles

First-principles calculations are performed to compute the e₃₃ piezoelectric coefficients of GaN, ScN and (Sc_xGa_1-xN) alloys exhibiting an alternation of hexagonal GaN, with hexagonal ScN along the c-axis. For Sc compositions larger than 50%, each atom has nearly five nearest neighbors (i.e., the ground state exhibits a phase that is five-fold coordinated). On the other hand, Sc-deficient (Sc, Ga) N alloys adopt a ground-state that is four-fold coordinated). The magnitude of e₃₃ in the Sc-deficient ideally ordered (Sc_0.25Ga_0.75N) is found to be larger than the magnitude of the corresponding e₃₃ coefficients resulting from the compositional weighted average over the hexagonal (h-ScN) and the wurtzite (w-GaN) parent compounds. On the other hand, the e₃₃ coefficients of the Sc-rich ordered (Sc_0.75Ga_0.25N) is found to be negligibly small. In addition, e₃₃ piezoelectric coefficients in ordered (Sc_0.5Ga_0.5N) exhibit quite large magnitudes, due to the nonpolar to polar transition occurs at Sc composition x = 0.5, and thus can bridge the corresponding coefficients of (Ga, In) N and ferroelectric alloys. The microscopic origins for this huge enhancement in the piezoelectric behavior in Sc-low and Sc-intermediate alloys and the role of each atom are revealed and discussed.     

The equivalent potential of water molecules for electronic structure of cysteine

In order to get more reliable electronic structure of protein in aqueous solution, it is necessary to construct a simple, easy-use equivalent potential of water molecules for protein's electronic structure calculation. The first-principles, all-electron, ab initio calculations have been performed to construct the equivalent potential of water molecules for the electronic structure of Cys. The process consists of three steps. First, the electronic structure of the cluster containing Cys and water molecules is calculated. Then, based on the structure, the electronic structure of Cys with the potential of water molecules is calculated using the self-consistent cluster-embedding method. Finally, the electronic structure of Cys with the potential of dipoles is calculated. The dipoles are adjusted so the electronic structure of Cys with the potential of dipoles is close to that of water molecules. The calculations show that the major effect of water molecules on Cys' electronic structure is lowering the occupied electronic states by about 0.032 Ry, and broadening energy gap by 16%. The effect of water molecules on the electronic structure of Cys can be simulated by dipoles potential.     

First principle study of the magnetism of Sr2CoMoO6-δ (δ = 0, 1/2) double perovskites

We investigate the electronic structure of bulk Sr₂CoMoO_6-δ double perovskites using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their magnetic properties within the GGA and GGA+U methods. We discuss the relative stability of ferromagnetic (FM) and antiferromagnetic (AFM) orders (i) without and with taking into account the observed tilting of the oxygen octahedra and (ii) by introducing oxygen vacancies. We show that a very good agreement with experimental results — AFM order for δ= 0 and FM order for δ= 1/2 — is obtained only when the tilting of the oxygen tetrahedra is taking into account and when the GGA+U method is used.     

A new algorithm for accurate evaluation of the generalized Bloch–Gruneisen function and its applications to MgB2 superconductor

The objective of this Letter is to provide an efficient and reliable analytical procedure for the generalized Bloch–Gruneisen function with the Debye temperatures for the wide temperature range using the binomial expansion theorems. As will be seen, the present formulation yields compact closed-form expressions which enable the ready calculation of Bloch–Gruneisen function for integer and noninteger values of parameter m. The proposed procedure guarantees the reliable application of the contribution of electron–photon interaction to the electrical resistivity of metals. Finally, the algorithm is used to simulate the variation of the resistivity and thermal conductivity against temperature sintered polycrystalline MgB₂. Furthermore, the comparison of the method with numerical calculations demonstrates the applicability and accuracy of the method.     

Photoluminescence from electron irradiated ZnO

Photoluminescence of ZnO ceramics irradiated with electrons of 0.8 MeV and 2.6 MeV energy has been investigated. The irradiated samples have been annealed in air at temperatures up to 1100° C. Irradiation causes quenching of the PL green band and as a consequence a peak at 470 nm emerges. Results indicate that part of the quenching can be attributed to damage in the oxygen sublattice.     

Structural and magneto-transport characterization of Co2CrxFe1-xAl Heusler alloy films

We investigate the structure and magneto-transport properties of thin films of the Co₂Cr_0.6Fe_0.4Al full-Heusler compound, which is predicted to be a half-metal by first-principles theoretical calculations. Thin films are deposited by magnetron sputtering at room temperature on various substrates in order to tune the growth from polycrystalline on thermally oxidized Si substrates to highly textured and even epitaxial on MgO(001) substrates, respectively. Our Heusler films are magnetically very soft and ferromagnetic with Curie temperatures up to 630 K. The total magnetic moment is reduced compared to the theoretical bulk value, but still comparable to values reported for films grown at elevated temperature. Polycrystalline Heusler films combined with MgO barriers are incorporated into magnetic tunnel junctions and yield 37% magnetoresistance at room temperature.     

Efficient laser isotope separation of 13C using novel linear multi-pass cavity

¹³C isotope has been separated in the form of enriched product C₂F₄ by selective multi-photon dissociation (MPD) of Freon-22 (CHClF₂) using the 9P(26) laser line of a transversely excited atmospheric CO₂ laser. The non-linearity factor, γ, that determines the dependence of the yield of ¹³C isotope on the fluence (J/cm²) has been determined for various laser rotational lines (9P(20)–9P(26)) and the advantage of a lower γ in the case of 9P(26) is highlighted for macroscopic production of ¹³C isotope. It is also shown that a higher value of the optimum fluence at 9P(26) not only results in a higher enrichment efficiency but in a relatively lower value of γ also. The laser pulse energy is efficiently utilized for selective MPD of Freon-22 by focusing the pulse energy repeatedly with the help of a novel linear multi-pass cavity (LMPC). The novelty of this optical arrangement lies in its ability to maintain the laser fluence around an optimum value for a desired enrichment of ¹³C in the product. This also ensures a higher quantity of enriched product because of the higher reaction volume. The advantage of the LMPC over the conventionally used Herriott multi-pass cell has also been presented. The gain in reaction volume in the present optical cavity having 20 passes with a constant fluence in each pass is as high as 12. Isotope-selective MPD of Freon in a LMPC with constant fluence in each pass showed a distinct advantage in energy utilization to separate ¹³C isotope over the gradually reducing fluence case.This revised version was published online in August 2005 with a corrected cover date.     

4f-electric quadrupolar effects on the lattice constants and magnetism of RCu2Ge2 (R=rare-earths)

The temperature dependence of the lattice constants of the alloys, RCu₂Ge₂ (R=Nd, Tb, Dy, Ho and Tm) shows the existence of a lattice strain due to the alignment of the 4f-electric quadrupole at low temperatures. This strain could be related to the deviation of the Néel temperatures (T _N ) from that predicted by de Gennes scaling, thereby indicating the role of the quadrupole on the magnetism. On the other hand, in PrCu₂Ge₂, for which the enhancement ofT _N is the largest in thisR series, the lattice constants interestingly behave as if the 4f-quadrupole moment is almost quenched and the anomalous magnetism in this case is attributed to some degree of 4f-delocalisation.     

Magnetic Properties of Melted ThMn12-Type Carbides and Their Nitrides

The carbides of NdDy0.2Fe12-yMoyC0.6 （y = 1.5, 2） crystallized in the ThMn12-type structure have been successfully synthesized by arc melting method, followed by a heat treatment. The magnetic properties are strongly enhanced with the addition of carbon. Upon the carbonation the saturation magnetization Ms is increased by about 20emu/g and the Curie temperature Tc is enhanced by 40-70K. The spin reorientation （SR） temperature decreases from 125 K for NdDy0.2Fe10Mo2 to 55 K for NdDy0.2Fe10MO2C0.6 indicating the change of magnetocrystalline anisotropy in the Nd sublattice. It is found that the intrinsic magnetic properties of the carbides can be improved by further nitrogenation, The composite carbon-nitrogen compounds show a Tc - 560K, M8 - 105 emu/g and Ha （anisotropy field） - 86kOe for NdDy0.2Fe10Mo2Co.6Nz and a Tc- 628K, M8 - 119 emu/g and Ha - 115kOe for NdDy0.2Fe10.5Mo1.5C0.6Nz. These magnetic properties are even better than those of simple nitrides, suggesting that these compounds can be considered as a good candidate for permanent magnet applications.     

Quantitative comparison of different preionization schemes of KrF laser discharges

* fluorescence radiation from the ion–ion recombination process in the gas volume during the preionization phase, therefore allowing spatial resolution by partial imaging of the volume. Volume-integrated and temporally resolved measurements are carried out to determine the average of absolute preionization densities by comparison with a theoretical model of the temporal behaviour of the recombination process. Spatially resolved measurements reveal the distribution of the preionization density. The preionization densities determined from spark and sliding/corona discharges schemes are considerably higher (n_e ⁰?10¹² cm^-3) than those obtained from pulsed X-ray preionization (n_e ⁰?10⁷ cm^-3). Received: 3 October 1997/Revised version: 6 January 1998