Sums of squares of regular functions on real algebraic varieties

Let be an affine algebraic variety over (or any other real closed field ). We ask when it is true that every positive semidefinite (psd) polynomial function on is a sum of squares (sos). We show that for the answer is always negative if has a real point. Also, if is a smooth non-rational curve all of whose points at infinity are real, the answer is again negative. The same holds if is a smooth surface with only real divisors at infinity. The ``compact' case is harder. We completely settle the case of smooth curves of genus : If such a curve has a complex point at infinity, then every psd function is sos, provided the field is archimedean. If is not archimedean, there are counter-examples of genus .

     

Study of the Interaction of Hemoglobin with Ligands from Resonance Raman Spectra

By the method of resonance Raman spectroscopy the equilibrium between liganded (HbL) and deliganded (Hb) forms of human hemoglobin is studied in destabilization of the solvate protein shell by introducing a cosolvent in the solution. It is shown that the introduction of ethanol leads to a nearly fivefold decrease in the ligand affinity of hemoglobin detected from the increase in the intensity of the 1355 cm^–1 ₄ line. The conformation of the heme and of the hemic pocket remains constant on addition of ethanol to the solution. Owing to the short time spent on measurement, this method can be used for a rapid evaluation of the influence of various preparations on the ligand affinity of hemoglobin.     

Photophysical Properties and Photoreduction of 5,10,15,20-Tetrakis-(4-N-Methylpyridyl)Porphyrin in Formamide

The photophysical characteristics and photochemical conversions of 5,10,15,20-tetrakis-(4-N-methylpyridyl)porphyrin in a highly polar solvent, formamide, are investigated. It has been established that no photochemical reactions occur on excitation in the visible spectral region, while UV excitation ( = 347 nm) causes photoreduction of porphyrin molecules followed by the formation of protonated products. The solvent molecules serve as a reducing agent. Photoreduction occurs in both air-saturated and deoxygenated solutions, but the nature of the products formed is different: chlorin molecules are formed in the air-saturated solutions and the successive photoconversion porphyrinphlorinporphometheneporphyrinogene occurs in the deoxygenated solutions.     

Transfer Matrix Spectrum and Bound States for Lattice Classical Ferromagnetic Spin Systems at High Temperature

We obtain new properties of general d-dimensional lattice ferromagnetic spin systems with nearest neighbor interactions in the high-temperature region (1). Each model is characterized by a single-site a priori spin distribution, taken to be even. We state our results in terms of the parameter =s ⁴–3s ²², where s ^k denotes the kth moment of the a priori distribution. Associated with the model is a lattice quantum field theory which is known to contain particles. We show that for >0, small, there exists a bound state with mass below the two-particle threshold. The existence of the bound state has implications for the decay of correlations, i.e., the 4-point functions decay at a slower rate than twice that of the 2-point function. These results are obtained using a lattice version of the Bethe–Salpeter equation. The existence results generalize to N-component models with rotationally invariant a priori spin distributions.     

γ-LiAlO2上非极性ZnO薄膜制备及其光谱性质研究

利用激光脉冲沉积(PLD)技术在(302)γ-LiAlO₂衬底上成功生长了非极性的a面(1120) ZnO薄膜. 衬底温度为350℃时，薄膜是混合取向(a向和c向)，以c面ZnO为主，且晶粒尺寸分布很宽；提高温度达500℃，薄膜变为单一的(1120)取向，摇摆曲线半高宽0.65°，晶粒尺寸分布趋窄，利用偏振透射谱可以明显看出其面内的各向异性. 衬底温度650℃下制备的样品晶粒继续长大，虽然摇摆曲线半高宽变大，但光致发光谱(PL)带边发射峰半高宽仅为105meV，比在350℃，500℃下制备的样品小1/5. 关键词： 非极性ZnO 2')" href="#">γ-LiAlO₂ PLD 透射谱     

Nd∶KY(WO4)2和Nd∶KG(WO4)2晶体吸收光谱性能分析

通过研究分析钨酸盐晶体Nd∶KY(WO₄)₂和Nd∶KG(WO₄)₂在室温下的吸收光谱，发现这2种晶体具有作为激光晶体的优良特性。根据Judd-Ofelt理论和测试所得的吸收光谱及数据，用VC++编程计算出晶体的谱线强度、振子强度、吸收截面等，拟合得Nd³⁺离子的3个晶场调节参数Ω_λ(λ=2，4，6)的值，并从理论上计算了自发跃迁几率、能级寿命、荧光分支比和积分发射截面。从计算得出的荧光分支比β可以看出，Nd∶KY(WO₄)₂(β_1060nm=0.4380)和Nd∶KG(WO₄)₂(β_1060nm=0.4618)晶体荧光分支比都较大，计算了该晶体的X=Ω₄/Ω₆，并将其X值与其他晶体的X值加以比较，Nd∶KY(WO₄)₂和Nd∶KG(WO₄)₂均易于实现1.06μm激光输出，适合作为LD泵浦的钨酸盐晶体激光器。     

微弱信号源的和波束定向方法与分裂波束定向方法的性能比较

本文讨论水下辐射噪声源的精确定向问题，给出被动声纳和波束定向与分裂波束定向方法的性能比较。指出在一定信噪比下，分裂波束精确测向技术比和波束定向技术具有较高的定向精度。但是，随着信噪比的下降，两者趋于一致。推导了估计定向精度的分析表达式，给出在直线阵和圆弧阵情况下，延时估计和声源入射角偏差之间的换算公式和数值模拟结果。同时给出数字式声纳用以计算入射信号左波束和右波束数据的互谱来实现分裂波束定向的方法。     

InSb纳米晶体的生长和吸收光谱的研究

采用在惰性气体中蒸发的方法获得了沉积在ZnS基片上的InSb纳米晶体,其平均尺寸随惰性气体的压强增加而增大.从实验测量的室温吸收谱上看到,当纳米晶体的平均直径从27.9 nm减小到24.2nm再到21.4 nm时,其吸收边分别向高能方向移动了0.0151 eV和0.0145 eV.用有效质量近似模型计算了半导体纳米晶体的吸收边相对其体材料的移动,将理论计算与实验结果进行了比较.     

Kerr介质中双原子-压缩真空场系统的腔场谱

研究了含Kerr介质腔中耦合双原子间与双模均处于压缩真空态的光场拉曼相互作用过程的腔场谱.通过求解本征方程导出了双模光场处于任意强度时腔场谱的计算公式,给出了数值计算结果.发现在两模初始场均为弱场时,Kerr介质和原子间的耦合作用对谱结构的影响较小;当双模初始场均为强场时,两模主峰的右侧出现了半梳状边峰,随Kerr效应的增强,峰间距逐渐加宽,并向高频方向扩展,在Kerr效应较强时,两模边峰的峰高出现高低相间的现象.     

水分子对碳链的输运性质影响的第一性原理研究

用基于密度泛函理论的非平衡格林函数方法研究了水分子对7个碳原子组成的一维原子链的输运性质的影响.碳原子链放在具有有限截面的Al(100)电极中.研究发现，碳原子链上的水分子的数目和放置的位置的不同将对体系输运性质产生很大的影响.特别是，单个H₂O分子对碳链平衡电导的影响随其摆放位置的不同而出现奇偶振荡，例如，当位于奇数编号的碳原子上时，电导取极大值，当位于偶数编号的碳原子上时，取极小值.将两个H₂O分子置于不同的碳原子正上方时，在不同的位置平衡电导相差很大，在某些特殊的情况下原本受到抑止的第三个本征通道也有较大的贡献.此外，还研究了放置两个水分子时，体系的电流-电压(I-V)特性，随着水分子的数目和放置的位置不同，某些情况可能出现较大幅度的负微分电阻，而在另一些情况下却没有出现. 关键词： 平衡电导 透射谱 负微分电阻