全文获取类型
收费全文 | 54199篇 |
免费 | 3891篇 |
国内免费 | 3993篇 |
专业分类
化学 | 24494篇 |
晶体学 | 232篇 |
力学 | 1637篇 |
综合类 | 1087篇 |
数学 | 20862篇 |
物理学 | 13771篇 |
出版年
2023年 | 464篇 |
2022年 | 1880篇 |
2021年 | 1690篇 |
2020年 | 1046篇 |
2019年 | 1256篇 |
2018年 | 975篇 |
2017年 | 1091篇 |
2016年 | 1320篇 |
2015年 | 1314篇 |
2014年 | 2021篇 |
2013年 | 3355篇 |
2012年 | 2098篇 |
2011年 | 2553篇 |
2010年 | 2332篇 |
2009年 | 3042篇 |
2008年 | 3317篇 |
2007年 | 3475篇 |
2006年 | 2711篇 |
2005年 | 2036篇 |
2004年 | 1763篇 |
2003年 | 1936篇 |
2002年 | 4291篇 |
2001年 | 1833篇 |
2000年 | 1455篇 |
1999年 | 1259篇 |
1998年 | 1200篇 |
1997年 | 937篇 |
1996年 | 868篇 |
1995年 | 641篇 |
1994年 | 656篇 |
1993年 | 636篇 |
1992年 | 590篇 |
1991年 | 420篇 |
1990年 | 362篇 |
1989年 | 297篇 |
1988年 | 318篇 |
1987年 | 227篇 |
1986年 | 242篇 |
1985年 | 362篇 |
1984年 | 277篇 |
1983年 | 173篇 |
1982年 | 336篇 |
1981年 | 499篇 |
1980年 | 446篇 |
1979年 | 500篇 |
1978年 | 400篇 |
1977年 | 296篇 |
1976年 | 247篇 |
1974年 | 112篇 |
1973年 | 190篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
31.
High index of refraction via quantum interference in a three-level system of Er^3+-doped yttrium aluminium garnet crystal
下载免费PDF全文
![点击此处可从《中国物理》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A simple three-level system is proposed to produce high index of refraction with zero absorption in an Er^3+-doped yttrium aluminium garnet (YAG) crystal, which is achieved for a probe field between the excited state 4I13/2 and ground state 4I15/2 by adjusting a strong coherent driving field between the upper excited state 4I11/2 and 4I15/2. It is found that the changes of the frequency of the coherent driving field and the concentration of Er^3+ ions in the YAG crystal can maximize the index of refraction accompanied by vanishing absorption. This result could be useful for the dispersion compensation in fibre communication, laser particle acceleration, high precision magnetometry and so on. 相似文献
32.
33.
本文定义了二阶微分方程的弱 Carathéodory解 ,在不涉及紧型条件的情形下 ,直接用迭代法证明了 Banach空间二阶非线性常微分方程两点边值问题存在唯一解 ,并给出逼近解迭代序列的误差估计 ,对周期边值问题得到类似的结果 相似文献
34.
Recursive algorithms for the computation of standard deviation and average deviation are derived and their applications in data acquisition are discussed. The relative speeds and accuracies of the two algorithms are compared for synthetic data. The performance of recursive estimation under shot and proportional noise limitations is also described. As an example of the utility of these algorithms, absorbance data with constant confidence intervals are collected regardless of incident and transmitted intensities. The desired precision is specified prior to data acquisition and used to control signal-averaging of the data in real time. 相似文献
35.
36.
Shougo Higashi 《Surface science》2006,600(3):591-597
We have studied the individual adsorption of Mn and Bi, and their coadsorption on Cu(0 0 1) by low-energy electron diffraction (LEED). For Mn, we have determined the c(2 × 2) structure formed at 300 K, whose structure had been determined by several methods. We reconfirmed by a tensor LEED analysis that it is a substitutional structure and that a previously reported large corrugation (0.30 Å) between substitutional Mn and remaining surface Cu atoms coincides perfectly with the present value. In the individual adsorption of Bi, we have found a c(4 × 2) structure, which is formed by cooling below ∼250 K a surface prepared by Bi deposition of ∼0.25 ML coverage at 300 K where streaky half-order LEED spots appear. The c(4 × 2) structure has been determined by the tensor LEED analysis at 130 K and it is a substitutional structure. In the coadsorption, we found a c(6 × 4) structure, which has been determined by the tensor LEED analysis. It is very similar to the previously determined structure of the c(6 × 4) formed by coadsorption of Mg and Bi, and embedded MnBi4 clusters are arranged in the top Cu layer instead of MgBi4. Large lateral displacements of Bi atoms in the c(6 × 4)-(Mn + Bi) suggest that the Mn atoms undergo the size-enhancement caused by their large magnetic moment. 相似文献
37.
主成分分析在地区科技竞争力评测中的应用 总被引:16,自引:0,他引:16
近年来对于科技竞争力的研究在国内方兴未艾,其中对于科技竞争力的评测是众多学者研究的重点和热点,也是各级决策者最为关心、最为重要的课题之一。本文根据科技竞争力概念和内涵来确定评测指标体系的构成要素,建立了评测指标体系,并利用主成分分析方法对采集来的数据进行分析,得到最终的评测结果。 相似文献
38.
In this paper we give a realization of some symmetric space G/K as a closed submanifold P of G. We also give several equivalent representations of the submanifold P. Some properties of the set gK∩P are also discussed, where gK is a coset space in G. 相似文献
39.
Efficient measurement of the performance index (the distance of a loading parameter from the voltage collapse point) is one of the key problems in power system operations and planning and such an index indicates the severity of a power system with regard to voltage collapse. There exist many interesting methods and ideas to compute this index. However, some successful methods are not yet mathematically justified while other mathematically sound methods are often proposed directly based on the bifurcation theory and they require the initial stationary state to be too close to the unknown turning point to make the underlying methods practical.This paper first gives a survey of several popular methods for estimating the fold bifurcation point including the continuation methods, bifurcation methods and the test function methods (Seydel's direct solution methods, the tangent vector methods and the reduced Jacobian method) and discuss their relative advantages and problems. Test functions are usually based on scaling of the determinant of the Jacobian matrix and it is generally not clear how to determine the behaviour of such functions. As the underlying nonlinear equations are of a particular type, this allows us to do a new analysis of the determinants of the Jacobian and its submatrices in this paper. Following the analysis, we demonstrate how to construct a class of test functions with a predictable analytical behaviour so that a suitable index can be produced. Finally, examples of two test functions from this class are proposed. For several standard IEEE test systems, promising numerical results have been achieved. 相似文献
40.
Summary In 1980 Dahmen-DeVore-Scherer introduced a modulus of continuity which turns out to reflect invariance properties of compound cubature rules effectively. Accordingly, sharp error bounds are derived, the existence of relevant counterexamples being a consequence of a quantitative resonance principle, established previously. 相似文献