首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   530558篇
  免费   4217篇
  国内免费   1304篇
化学   270410篇
晶体学   7653篇
力学   26063篇
综合类   18篇
数学   68107篇
物理学   163828篇
  2021年   5148篇
  2020年   5553篇
  2019年   6289篇
  2018年   8487篇
  2017年   8618篇
  2016年   11719篇
  2015年   6234篇
  2014年   10505篇
  2013年   23647篇
  2012年   18440篇
  2011年   21907篇
  2010年   16506篇
  2009年   16347篇
  2008年   20748篇
  2007年   20582篇
  2006年   18683篇
  2005年   16684篇
  2004年   15524篇
  2003年   14015篇
  2002年   13893篇
  2001年   14754篇
  2000年   11358篇
  1999年   8770篇
  1998年   7606篇
  1997年   7478篇
  1996年   6947篇
  1995年   6265篇
  1994年   6273篇
  1993年   6026篇
  1992年   6387篇
  1991年   6872篇
  1990年   6583篇
  1989年   6488篇
  1988年   6325篇
  1987年   6135篇
  1986年   5875篇
  1985年   7415篇
  1984年   7777篇
  1983年   6540篇
  1982年   6824篇
  1981年   6327篇
  1980年   6008篇
  1979年   6510篇
  1978年   6747篇
  1977年   6630篇
  1976年   6588篇
  1975年   6306篇
  1974年   6131篇
  1973年   6425篇
  1972年   4707篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
952.
Adaptation of populations takes place with the occurrence and subsequent fixation of mutations that confer some selective advantage to the individuals which acquire it. For this reason, the study of the process of fixation of advantageous mutations has a long history in the population genetics literature. Particularly, the previous investigations aimed to find out the main evolutionary forces affecting the strength of natural selection in the populations. In the current work, we investigate the dynamics of fixation of beneficial mutations in a subdivided population. The subpopulations (demes) can exchange migrants among their neighbors, in a migration network which is assumed to have either a random graph or a scale-free topology. We have observed that the migration rate drastically affects the dynamics of mutation fixation, despite of the fact that the probability of fixation is invariant on the migration rate, accordingly to Maruyama's conjecture. In addition, we have noticed a topological dependence of the adaptive evolution of the population when clonal interference becomes effective.  相似文献   
953.
In experiment PAX, proposed for the new accelerator complex at GSI, investigations in the field of high-energy spin physics with the use of a polarized antiproton beam are planned. In this paper the possibilities for triggering are considered taking into account the PAX detector properties and a probable configuration of the trigger system is discussed for the first time. The text was submitted by the authors in English.  相似文献   
954.
We present a framework for efficiently performing Monte Carlo wave-function simulations in cavity QED with moving particles. It relies heavily on the object-oriented programming paradigm as realised in C++, and is extensible and applicable for simulating open interacting qua ntum dynamics in general. The user is provided with a number of “elements”, e.g. pumped moving particles, pumped lossy cavity modes, and various interactions to compose complex interacting systems, which contain several particles moving in electromagnetic fields of various configurations, and perform wave-function simulations on such systems. A number of tools are provided to facilitate the implementation of new elements.  相似文献   
955.
An explicitly solvable analog of the Kirchhoff flow for the case of a semipenetrable obstacle is considered. Its application to estimating the efficiency of free flow turbines is discussed.  相似文献   
956.
A review of the experimental and theoretical studies of the crown ether complexes with polar molecules in their crystals, solutions, and in a gas phase is given. The type of the molecular bonds in the complexes, their stoichiometry, and the change in the macrocycle conformation during complex formation are considered, as well as the effect of the macrocycle structure and the nature of the medium on the efficiency of the molecular bonding. New data are given on the enthalpies of transfer of the crown ethers from tetrachloromethane into solvents capable of forming hydrogen bonds. The enthalpies of specific interactions of macrocycles with the molecules of the solvents in the medium of the same solvents are characterized. The conformations of the crown ethers in the media under study are discussed.  相似文献   
957.
958.
959.
Thomas  M. F.  Case  G. S.  Bland  J.  Herring  A. D. F.  Stirling  W. G.  Tixier  S.  Boni  P.  Ward  R. C. C.  Wells  M. R.  Langridge  S. 《Hyperfine Interactions》2002,141(1-4):471-476
Hyperfine Interactions - Multilayers of Ce/Fe and U/Fe were fabricated by sputtering and studied by X-ray diffraction and reflectivity, Mössbauer spectroscopy and polarised neutron...  相似文献   
960.
Experimental research on the improvement of the sensitivity and time resolution of pulsed magnetic resonance spectrometers is discussed. It is shown that application of a time-variable feedback of a signal to the input of the receiver amplifier can decouple, the “fixed” relationship between the quality factorQ and the ringdown time of the resonance system. Experiments were performed with low-frequency, radio-frequency and microwave pulse-type magnetic resonance receivers. Modifications of an S/C-band electron spin echo modulation spectrometer carried out to check the “time-variable feedback” performance are described. It is demonstrated that the application of a time-variable feedback can significantly reduce the ringdown time and improve the recovery properties of the magnetic resonance receiver system. It is also demonstrated that the time-variable feedback can improve the overall receiver sensitivity due to the fact that the working bandwidth of the resonance system can be optimized separately for the transmitting and the receiving mode. Signal values could be increased at least three times and the signal-to-noise ratio about 1.5–2 times. The largest improvement is achieved with the initially overcoupled resonator. Experimental spectra of test samples for different settings of the time-variable feedback are shown.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号