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   204485篇
  免费   3015篇
  国内免费   516篇
化学   107614篇
晶体学   3149篇
力学   7854篇
综合类   3篇
数学   25373篇
物理学   64023篇
  2021年   1485篇
  2020年   1862篇
  2019年   1865篇
  2018年   2385篇
  2017年   2444篇
  2016年   4085篇
  2015年   2678篇
  2014年   3994篇
  2013年   9562篇
  2012年   7242篇
  2011年   8667篇
  2010年   5957篇
  2009年   5783篇
  2008年   8208篇
  2007年   8065篇
  2006年   7524篇
  2005年   6805篇
  2004年   6156篇
  2003年   5449篇
  2002年   5272篇
  2001年   5842篇
  2000年   4399篇
  1999年   3367篇
  1998年   2834篇
  1997年   2739篇
  1996年   2697篇
  1995年   2515篇
  1994年   2521篇
  1993年   2481篇
  1992年   2705篇
  1991年   2543篇
  1990年   2512篇
  1989年   2391篇
  1988年   2408篇
  1987年   2394篇
  1986年   2269篇
  1985年   3024篇
  1984年   3075篇
  1983年   2720篇
  1982年   2842篇
  1981年   2668篇
  1980年   2623篇
  1979年   2753篇
  1978年   2804篇
  1977年   2725篇
  1976年   2716篇
  1975年   2619篇
  1974年   2594篇
  1973年   2781篇
  1972年   1743篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The Nambu-Jona-Lasinio model as a particular model of quark flavour dynamics is bosonized in the low-energy regime using a generalized heat kernel expansion with local mass scale. The resulting effective meson Lagrangian defines a chiral soliton model, the solutions of which show a partial restoration of chiral symmetry inside the soliton.  相似文献   
992.
The first total synthesis of (R)-convolutamydine A has been achieved by the organocatalytic addition of acetone to 4,6-dibromoisatin. The absolute configuration was determined by single crystal X-ray diffraction. DFT studies were used to model the transition states for the aldol reaction and equilibrium geometries of the post-aldol reaction intermediates. The DFT study revealed that the aldol bond forming reaction was considerably endothermic.  相似文献   
993.
The purpose of this paper is two-fold. First, we introduce a new asymptotic expansion in the exponent for the compound binomial approximation of the generalized Poisson binomial distribution. The dependence of its accuracy on the symmetry and shifting of distributions is investigated. Second, for compound binomial and compound Poisson distributions, we present new smoothness estimates, some of which contain explicit constants. Finally, the ideas used in this paper enable us to prove new precise bounds in the compound Poisson approximation. Published in Lietuvos Matematikos Rinkinys, Vol. 46, No. 1, pp. 67–110, January–March, 2006.  相似文献   
994.
995.
In this paper, we re-examine a series of gedanken welcher Weg (WW) experiments introduced by Scully, Englert and Walther that contain the essential ideas underlying the quantum eraser. For this purpose we use the Bohm model which gives a sharp picture of the behaviour of the atoms involved in these experiments. This model supports the thesis that interference disappears in such WW experiments, even though the centre of mass wave function remains coherent throughout the experiment. It also shows exactly what it means to say ‘that the interference can be restored by manipulating the WW detectors long after the atoms have passed’. It does not support Wheeler’s notion that ‘the past is undefined and undefinable without the observation (in the present)’.  相似文献   
996.
The quantum action (dynamical) principle is exploited to investigate the nature and origin of the Faddeev–Popov (FP) factor in gauge theories without recourse to path integrals. Gauge invariant as well as gauge non-invariant interactions are considered to show that the FP factor needs to be modified in more general cases and expressions for these modifications are derived. In particular we show that a gauge invariant theory does not necessarily imply the familiar FP factor for proper quantization. PACS numbers: 11.15.-q; 12.10.-g; 12.15.-y; 12.38.-t  相似文献   
997.
998.
For the methylsilsesquioxane film whose optical birefringence is almost zero, it was recently reported that its vertical thermal expansion coefficient (CTE) was approximately one order of magnitude larger than the lateral CTE. Though the birefringence is not an absolute predictor of anisotropic behavior, the discrepancy in both the CTEs was so remarkable that it was essential to investigate whether the anisotropy was intrinsic property or not. If the effect of Poisson's ratio is considered in the calculation of the vertical CTE and when elastic modulus measured by surface acoustic wave spectroscopy is used in the assessment of the lateral CTE, both the CTEs are coincident with each other. Therefore, it can be concluded that the discrepancy in the CTEs can be attributed to a higher in‐plane polymer chain orientation but it can also arise from the misleadingly assumed modulus and Poisson's ratio. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3109–3120, 2006  相似文献   
999.
Laser Doppler vibrometer (LVD) has been the most favorite instrument for precision dynamics measurement due to its non-contact, high accuracy and high resolution. However, LDV can only give the dynamic data of a particular location on the entire feature. In order to get the whole field data, a laser beam-scanning mechanism has to be implemented. Currently, motor-driven scanning mirror is used to move the measurement probe from one point to another. The mechanical vibrations of the scanning mirror will reduce the measurement accuracy. This paper introduces a novel scanning LDV optical system embodied in an acousto-optic deflector scanning mechanism. It can improve the measurement accuracy since there is no mechanical motion involved. One main advantage of this system is that it generates a laser scanning beam in parallel that is different from the beam scanning in the conventional scanning laser Doppler vibrometer (SLDV). The new system has a board scanning range. The measurement target size ranges from few tens of millimeters down to 10 μm. We have demonstrated the capability of the novel system on scanning measurements of features as big as ultra-precision cutting tool to features as tiny as AFM cantilever. We believe that the novel SLDV will find profound potential applications in the precision engineering field.  相似文献   
1000.
A new method for one-electron propagator calculations of molecular inization potentials is proposed, using a large matrix technique The results of some trial calculations on molecular nitrogen are given.  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