全文获取类型
收费全文 | 76784篇 |
免费 | 10617篇 |
国内免费 | 6688篇 |
专业分类
化学 | 53473篇 |
晶体学 | 1104篇 |
力学 | 3930篇 |
综合类 | 881篇 |
数学 | 8256篇 |
物理学 | 26445篇 |
出版年
2024年 | 126篇 |
2023年 | 731篇 |
2022年 | 2244篇 |
2021年 | 2858篇 |
2020年 | 2831篇 |
2019年 | 2423篇 |
2018年 | 2099篇 |
2017年 | 2310篇 |
2016年 | 3191篇 |
2015年 | 3147篇 |
2014年 | 3819篇 |
2013年 | 6011篇 |
2012年 | 4194篇 |
2011年 | 4568篇 |
2010年 | 3905篇 |
2009年 | 4457篇 |
2008年 | 4552篇 |
2007年 | 4821篇 |
2006年 | 4033篇 |
2005年 | 3042篇 |
2004年 | 2710篇 |
2003年 | 2621篇 |
2002年 | 4701篇 |
2001年 | 2284篇 |
2000年 | 1784篇 |
1999年 | 1434篇 |
1998年 | 1364篇 |
1997年 | 1025篇 |
1996年 | 1030篇 |
1995年 | 882篇 |
1994年 | 869篇 |
1993年 | 815篇 |
1992年 | 755篇 |
1991年 | 510篇 |
1990年 | 462篇 |
1989年 | 380篇 |
1988年 | 379篇 |
1987年 | 284篇 |
1986年 | 302篇 |
1985年 | 382篇 |
1984年 | 310篇 |
1983年 | 186篇 |
1982年 | 340篇 |
1981年 | 504篇 |
1980年 | 447篇 |
1979年 | 488篇 |
1978年 | 390篇 |
1977年 | 287篇 |
1976年 | 240篇 |
1973年 | 199篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
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
62.
Wolfram Bremser Roland Becker Heinrich Kipphardt Petra Lehnik-Habrink Ulrich Panne Antje Töpfer 《Accreditation and quality assurance》2006,11(10):489-495
ISO Guide 35 deals with RM stability issues and scrutinizes the evaluation of stability testing results under the assumption that either there is no trend at all (a rather rare situation), or any observed deterministic change is insignificant and thus can be neglected. However, market demands for reliable reference materials are obviously not limited to stable or at least seemingly stable materials. In many analytical applications, analytes and measurands under consideration are known, or at least suspected, to be unstable on time scales that may vary widely from measurand to measurand. The Federal Institute for Materials Research and Testing (BAM) has developed (and successfully uses) an integrated approach in its certification practice. The approach is based on an initial stability study and subsequent post-certification monitoring. Data evaluation is model-based and takes advantage of all information collected in the stability testing scheme(s). It thus allows one to deal with any kind of instability observed, to assess limiting time intervals at any stress condition in the range tested, to estimate a final expiry date for materials with detected instabilities or the maximum admissible re-testing interval for seemingly stable materials, and to assess maximum admissible stress loads during delivery of the material to the customer. The article describes (and exemplifies) typical study layout, the model selection, and the integrated data assessment. 相似文献
63.
64.
超强脉冲激光在低密度等离子体中的相对论自导引效应 总被引:1,自引:1,他引:0
分析了相对论条件下激光超短脉冲在等离子体中的传输特性 ,在傍轴近似和慢变振幅近似条件下 ,推导了折射率、电子密度、静电场以及电子空腔尺度的表达式。当激光功率超过产生自导引阈值功率时 ,激光束斑沿着传输光轴方向振荡。在有质动力产生的压力非常强时 ,聚焦光束中央部分的电子被全部排开形成电子空腔。给出了电子空腔的尺寸以及在出现电子空腔时的处理方法。在超过形成电子空腔的阈值功率 (Pc≈ 2 .5TW )时 ,空腔的尺度几乎与激光功率无关 ,这意味着电子空腔阻止了激光脉冲的进一步聚焦。 相似文献
65.
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
66.
用密耦方法计算了非对称同位素替代分子与氦原子碰撞(He-HD,HT,DT)转动激发,当入射能量E=0.3 eV时,得到了上述碰撞体系的微分散射截面或角分布.基于上述理论计算,讨论了原子与双原子分子碰撞的同位素效应. 相似文献
67.
We report studies on the structure of the metallic core and the alkyl cap layer in monolayer protected gold nanoparticles having sizes down to 1.6 nm. These particles are obtained by laser ablating gold targets in alkane-thiol solutions at different concentrations. The electronic structure of gold core and the vibrational properties of the capping hydrocarbon chains reveal effects connected with the nanosized nature of the particles. 相似文献
68.
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
69.
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
70.
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