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61.
Goennenwein ST Wassner TA Huebl H Brandt MS Philipp JB Opel M Gross R Koeder A Schoch W Waag A 《Physical review letters》2004,92(22):227202
We show that upon exposure to a remote dc hydrogen plasma, the magnetic and electronic properties of the dilute magnetic semiconductor Ga1-xMnxAs change qualitatively. While the as-grown Ga1-xMnxAs thin films are ferromagnetic at temperatures T less, similar 70 K, the samples are found to be paramagnetic after the hydrogenation, with a Brillouin-type magnetization curve even at T=2 K. Comparing magnetization and electronic transport measurements, we conclude that the density of free holes p is significantly reduced by the plasma process, while the density of Mn magnetic moments does not change. 相似文献
62.
Boron trifluoride and BBr(3) mediated [2,3]-sigmatropic rearrangements of allylic alpha-amino amides have been developed affording secondary amines in good yields. (E)-Crotyl and (E)-cinnamyl alpha-amino amides 2b and 2c exhibit excellent syn-diastereoselectivity upon rearrangement with either Lewis acid. The allylic amine 2a forms upon treatment with BF(3) or BBr(3) a five-membered heterocylic complex in which a single halide anion has been displaced by the carbonyl oxygen atom. The structures of the Lewis acid-amine complexes were elucidated using NMR spectroscopy. A plausible reaction mechanism, based on DFT calculations, is presented. Thus, BF(3)- or BBr(3)-complexed allylic amines 2 are shown to preferentially proceed, after deprotonation, via an endo transition state. 相似文献
63.
The reactions of [Rh2(kappa2-acac)2(mu-CPh2)2(mu-PR3)] (PR3= PMe34, PMe2Ph 7, PEt38) with an equimolar amount of Me3SiX (X = Cl, Br, I) afforded the unsymmetrical complexes [Rh2X(kappa2-acac)(mu-CPh2)2(mu-PR3)]5, 9-12, which contain the phosphine in a semi-bridging coordination mode. From 4 and excess Me3SiCl, the tetranuclear complex [[Rh2Cl(mu-Cl)(mu-CPh2)2(mu-PMe3)]2]6 was obtained. In contrast, the reaction of 4 with an excess of Me3SiX (X = Br, I) yielded the dinuclear complexes [Rh2X2(mu-CPh2)2(mu-PMe3)]13, 14 in which, as shown by the X-ray crystal structure analysis of 14, the bridging phosphine is coordinated in a truly symmetrical bonding mode. While related compounds with PEt3 and PMe2Ph as bridging ligands were prepared on a similar route, the complex [Rh2Cl2(mu-CPh2)2(mu-PiPr3)]19 was obtained from the mixed-valence species [(PiPr3)Rh(mu-CPh2)2Rh(kappa2-acac)2]17 and HCl. The reaction of [Rh2(kappa2-acac)2(mu-CPh2)2(mu-SbiPr3)]3 with AsMe3 gave the related Rh(mu-AsMe3)Rh compound 21. With Me3SiCl, the acac ligands of 21 can be replaced stepwise by chloride to give [Rh2Cl(kappa2-acac)(mu-CPh2)2(mu-AsMe3)]23 and [[Rh2Cl(mu-Cl)(mu-CPh2)2(mu-AsMe3)]2]24, the latter being isomorphous to the phosphine-bridged dimer 6. 相似文献
64.
Abazov VM Abbott B Abdesselam A Abolins M Abramov V Acharya BS Adams DL Adams M Ahmed SN Alexeev GD Alves GA Amos N Anderson EW Baarmand MM Babintsev VV Babukhadia L Bacon TC Baden A Baldin B Balm PW Banerjee S Barberis E Baringer P Barreto J Bartlett JF Bassler U Bauer D Bean A Begel M Belyaev A Beri SB Bernardi G Bertram I Besson A Beuselinck R Bezzubov VA Bhat PC Bhatnagar V Bhattacharjee M Blazey G Blessing S Boehnlein A Bojko NI Borcherding F Bos K Brandt A Breedon R Briskin G Brock R 《Physical review letters》2001,87(6):061802
We present results of searches for technirho (rho(T)), techniomega (omega(T)), and Z' particles, using the decay channels rho(T),omega(T),Z'-->e(+)e(-). The search is based on 124.8 pb(-1) of data collected by the D0 detector at the Fermilab Tevatron during 1992-1996. In the absence of a signal, we set 95% C.L. upper limits on the cross sections for the processes pp-->rho(T),omega(T),Z'-->e(+)e(-) as a function of the mass of the decaying particle. For certain model parameters, we exclude the existence of degenerate rho(T) and omega(T) states with masses below about 200 GeV. We exclude a Z' with mass below 670 GeV, assuming that it has the same couplings to fermions as the Z boson. 相似文献
65.
Spin-dependent changes in the noise power of undoped amorphous hydrogenated silicon ( a-Si:H) are observed under electron spin resonance conditions. The noise-detected magnetic resonance (NDMR) signal has the g value of holes in the valence band tail of a-Si:H. Both the sign of the NDMR signal and the frequency dependence of its intensity can be quantitatively accounted for by a resonant reduction of the generation-recombination noise time constant tau. This identifies hopping in the valence-band tail as the dominant spin-dependent step governing noise in this material. 相似文献
66.
H. von Wenckstern M. Brandt H. Schmidt G. Biehne R. Pickenhain H. Hochmuth M. Lorenz M. Grundmann 《Applied Physics A: Materials Science & Processing》2007,88(1):135-139
We have investigated donor-like defects in ZnO substrate material grown by three different methods, and in epitaxial ZnO thin
films grown on sapphire by pulsed laser deposition. Temperature dependent Hall effect measurements yield information about
dominant donors. The thermal activation energies lie in a wide range from about 20 meV to about 370 meV. Deep level transient
spectroscopy is used to obtain parameters of deep donor-like defects. For that, a high-speed diode contact configuration was
laid out for the epitaxial thin films in order to determine the defect parameters with high precision. The identified levels
are E1, E3 and E4, though the level E4 is observed only in single crystals grown by seeded chemical vapor transport.
PACS 72.10.Fk; 72.80.Ey; 73.50.-h 相似文献
67.
A simple method for nano‐scale texturing of silicon surfaces based on local metal‐catalyzed wet chemical etching, which results in an almost complete suppression of reflectivity in a broad spectral range, has been successfully applied to produce black multi‐crystalline silicon solar cells. The performance of the cells is compared to that of reference cells without surface nano‐texturing. A considerable increase of the short circuit current (by 36–42% with respect to the reference cells) without deterioration of other performance parameters is observed under natural sun illumination. Means of further optimization of such black solar cells are discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
68.
Computing the minimal covering set 总被引:1,自引:0,他引:1
We present the first polynomial-time algorithm for computing the minimal covering set of a (weak) tournament. The algorithm draws upon a linear programming formulation of a subset of the minimal covering set known as the essential set. On the other hand, we show that no efficient algorithm exists for two variants of the minimal covering set–the minimal upward covering set and the minimal downward covering set–unless P equals NP. Finally, we observe a strong relationship between von Neumann–Morgenstern stable sets and upward covering on the one hand, and the Banks set and downward covering on the other. 相似文献
69.
70.
Rüdiger Brandt Hanskarl Müller-Buschbaum 《Monatshefte für Chemie / Chemical Monthly》1986,117(11):1239-1244
Solid state reaction of BaCO3, FeC2O4·2H2O and Nb2O5 gave single crystals of Ba6FeNb9O30. The crystal structure was solved by X-ray investigations (a=12.597,c=3.990Å, space group P 4 bm-C
4v
2
,Z=1). Ba6FeNb9O30 crystallyzes in the tetragonal bronze type with a statistical distribution of Fe3+ and Nb5+ in the octahedral framework. The anisotropic temperature factors of barium are discussed with respect to the oxygen coordination. 相似文献