Chiral magnetic order at surfaces driven by inversion asymmetry

Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed single-handedness of biomolecules) and in particle physics (in the charge-parity violation of electroweak interactions). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii-Moriya interaction, which arises from spin-orbit scattering of electrons in an inversion-asymmetric crystal field. This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur. Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten (110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii-Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure, causing a variety of excitations or manipulations of the magnetization and giving rise to microwave emission, magnetization switching, or magnetic motors.     

耐水脱硫石膏、粉煤灰胶结材的研究

本文较深入地研究了脱硫石膏粉煤灰化学成分、杂质、颗粒特征和激发剂等对脱硫石膏、粉煤灰胶结材性能的影响 研究表明 ,激发剂是影响脱硫石膏、粉煤灰胶结材性能的主要因素 这为脱硫石膏、粉煤灰资源化利用打下良好理论基础 经实际应用表明 ,研究结果与实际情况能很好相符     

Brain potential and functional MRI evidence for how to handle two languages with one brain

Bilingual individuals need effective mechanisms to prevent interference from one language while processing material in the other. Here we show, using event-related brain potentials and functional magnetic resonance imaging (fMRI), that words from the non-target language are rejected at an early stage before semantic analysis in bilinguals. Bilingual Spanish/Catalan and monolingual Spanish subjects were instructed to press a button when presented with words in one language, while ignoring words in the other language and pseudowords. The brain potentials of bilingual subjects in response to words of the non-target language were not sensitive to word frequency, indicating that the meaning of non-target words was not accessed in bilinguals. The fMRI activation patterns of bilinguals included a number of areas previously implicated in phonological and pseudoword processing, suggesting that bilinguals use an indirect phonological access route to the lexicon of the target language to avoid interference.     

正弦稳态扫频信号的产生与输出

介绍用计算机软件产生正弦稳态扫频信号的编程方法。通过对Ｄ／Ａ变换与Ａ／Ｄ采集的同步输出可以获得不同频率量程和设定电压的扫频输出     

一类高阶非线性系统的零解稳定性

由Барбащин公式得到的四阶常系数线性系统的Ляпунов函数出发，通过类比法构造了一类四阶非线性系统的Ляпунов函数，并由此得到了这些系统零解的全局渐近稳定性的充分条件     

Refined crystal structure of beta-lactamase from Citrobacter freundii indicates a mechanism for beta-lactam hydrolysis

Beta-Lactamases (EC 3.5.2.6, 'penicillinases') are a family of enzymes that protect bacteria against the lethal effects of cell-wall synthesis of penicillins, cephalosporins and related antibiotic agents, by hydrolysing the beta-lactam antibiotics to biologically inactive compounds. Their production can, therefore, greatly contribute to the clinical problem of antibiotic resistance. Three classes of beta-lactamases--A, B and C--have been identified on the basis of their amino-acid sequence; class B beta-lactamases are metalloenzymes, and are clearly distinct from members of class A and C beta-lactamases, which both contain an active-site serine residue involved in the formation of an acyl enzyme with beta-lactam substrates during catalysis. It has been predicted that class C beta-lactamases share common structural features with D,D-carboxypeptidases and class A beta-lactamases, and further, suggested that class A and class C beta-lactamases have the same evolutionary origin as other beta-lactam target enzymes. We report here the refined three-dimensional structure of the class C beta-lactamase from Citrobacter freundii at 2.0-A resolution and confirm the predicted structural similarity. The refined structure of the acyl-enzyme formed with the monobactam inhibitor aztreonam at 2.5-A resolution defines the enzyme's active site and, along with molecular modelling, indicates a mechanism for beta-lactam hydrolysis. This leads to the hypothesis that Tyr 150 functions as a general base during catalysis.     

Genome-wide association analysis identifies susceptibility loci for migraine without aura

Migraine without aura is the most common form of migraine, characterized by recurrent disabling headache and associated autonomic symptoms. To identify common genetic variants associated with this migraine type, we analyzed genome-wide association data of 2,326 clinic-based German and Dutch individuals with migraine without aura and 4,580 population-matched controls. We selected SNPs from 12 loci with 2 or more SNPs associated with P values of <1 × 10(-5) for replication testing in 2,508 individuals with migraine without aura and 2,652 controls. SNPs at two of these loci showed convincing replication: at 1q22 (in MEF2D; replication P = 4.9 × 10(-4); combined P = 7.06 × 10(-11)) and at 3p24 (near TGFBR2; replication P = 1.0 × 10(-4); combined P = 1.17 × 10(-9)). In addition, SNPs at the PHACTR1 and ASTN2 loci showed suggestive evidence of replication (P = 0.01; combined P = 3.20 × 10(-8) and P = 0.02; combined P = 3.86 × 10(-8), respectively). We also replicated associations at two previously reported migraine loci in or near TRPM8 and LRP1. This study identifies the first susceptibility loci for migraine without aura, thereby expanding our knowledge of this debilitating neurological disorder.