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本文报告了对Ce掺杂锰氧化物(La1-xCex)2/3Ca1/3MnO3 (x=0~1.0)系列样品的输运特性和反常磁特性的研究结果.实验表明,Ce掺杂对Tc有明显的抑制作用,整体上电阻率随Ce掺杂含量增加而上升,在外加磁场时表现出极大的磁电阻效应.磁化强度随温度变化的曲线出现了两个转变,高温处对应于Mn离子磁矩的铁磁金属转变,低温处的转变则对应于Ce离子磁矩自旋有序排列的形成.表明Ce掺杂引起样品中铁磁双交换作用和反铁磁超交换作用之间的竞争,Ce离子与Mn离子有很强的相互作用.随Ce掺杂含量的增加,铁磁有序转变温度下降,而反铁磁有序转变温度则向高温处移动,铁磁区域明显减小. 相似文献
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In this paper the measurement result of transmission spectrum of Tb,Tm∶YVO4 is presented. The intensity Ωt, the probabilities of radiating transition, non-radiating transition, the cross-relaxation and the lifetimes of Tm3+ ions at 3H4 and 3F4 levels are calculated. The results show that when the concentration of Tm3+ ions >1.2 at.-%, the fluorescent lifetime of upper level 3H4 is shorter than that of lower level 3F4 due to the interaction between Tm3+ and Tb3+ ions, thus the lasing self-termination at ~1.5 μm can be eliminated. 相似文献
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The Yb3+ laser emitting at a 1.03μm wavelength has been evoking strong interest recently due to its advantages of long fluorescence lifetime, broad absorption band and the fact that it never shows concentration quenching. On the other hand, as a laser of three-level system it has, in general, a relatively high threshold power, which makes it important to seek some suitable host crystals to reduce this. Here, we present a comparison of the lasing performances of Yb3+-doped YAG [Y3Al5O12], FAP [Ca5(PO4)3F] and KYW [KY(WO4)2] crystals, including threshold power and slope efficiency, with those of the Nd:YAG laser based on the threshold formula of three-and four-level systems deduced by the authors. The results show that the Yb3+ laser can output a power larger than the Nd:YAG laser does in the case of comparably higher pumping power, if the length of the lasing rod and the concentration of the active ions satisfy some conditions. The theoretical results are also close to the experimental results reported. 相似文献
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本文推广了有关积分∫a^b f(x)g(x)dx的一个不等式并将其应用于∧-有界变分函数f(x)的Fourier系数的估计。 相似文献
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YU Xin LIU Kangsheng CHEN Pengnian Laboratory of Information Optimization Technologies Ningbo Institute of Technology Zhejiang University Ningbo Zhejiang China 《数学年刊A辑(中文版)》2007,(5)
研究在非自反状态空间和自反控制空间中无限维线性系统x(t)=Ax(t) Bu(t)的精确能控性和精确零能控性.当A生成C-半群时,得到这个系统关于L~p([O,T],U),1
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多元StancU多项式的最优逼近阶及其特征刻画 总被引:4,自引:0,他引:4
对于单纯形上的多元Stancu多项式Mn(f,x)(它是多元Bernstein多项式的广义形式),我们给出其对连续函数的最优逼近阶及其特征刻画,即我们将依据某-K-泛函确定满足‖Mnf-f‖=O(n^-1)的函数类.同时,得到了逼近的正定理和逆定理. 相似文献
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X. H. Sun 《Acta Mathematica Hungarica》1994,65(3):229-236