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   587533篇
  免费   22942篇
  国内免费   13973篇
化学   336181篇
晶体学   7983篇
力学   26428篇
综合类   732篇
数学   72048篇
物理学   181076篇
  2022年   5562篇
  2021年   6403篇
  2020年   7395篇
  2019年   7324篇
  2018年   8396篇
  2017年   8145篇
  2016年   11943篇
  2015年   9974篇
  2014年   13008篇
  2013年   28362篇
  2012年   25577篇
  2011年   29437篇
  2010年   19874篇
  2009年   19536篇
  2008年   25191篇
  2007年   24791篇
  2006年   23668篇
  2005年   23180篇
  2004年   20516篇
  2003年   17159篇
  2002年   15731篇
  2001年   16554篇
  2000年   12972篇
  1999年   10660篇
  1998年   8739篇
  1997年   8403篇
  1996年   8538篇
  1995年   7629篇
  1994年   7347篇
  1993年   6865篇
  1992年   7400篇
  1991年   7205篇
  1990年   6675篇
  1989年   6331篇
  1988年   6385篇
  1987年   6074篇
  1986年   5945篇
  1985年   8088篇
  1984年   8170篇
  1983年   6733篇
  1982年   7264篇
  1981年   7024篇
  1980年   6634篇
  1979年   6876篇
  1978年   7032篇
  1977年   7084篇
  1976年   7084篇
  1975年   6746篇
  1974年   6688篇
  1973年   6873篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
941.
The rotational spectra of 34SO2F2 and S18O16OF2 have been measured in their ground vibrational state between 9 and 110 GHz. Accurate rotational constants have been derived. Various experimental structures including the average structure have been determined. The ab initio structure has been calculated at the CCSD(T) level of theory. The different structures are compared and the best equilibrium structure is the ab initio structure: re(SO)=1.401 (3) Å, re(SF)=1.532 (3) Å, ∠e(OSO)=124.91(20)°, ∠e(FSF)=95.53 (20)°.  相似文献   
942.
A low cost, coolerless 980nm diode pumped, gain flattened L band EDFA with fast transient control, high pump efficiency and gain clamping effect was realized by using FBGs as C band seed generators.  相似文献   
943.
Operators on manifolds with corners that have base configurations with geometric singularities can be analysed in the frame of a conormal symbolic structure which is in spirit similar to the one for conical singularities of Kondrat'ev's work. Solvability of elliptic equations and asymptotics of solutions are determined by meromorphic conormal symbols. We study the case when the base has edge singularities which is a natural assumption in a number of applications. There are new phenomena, caused by a specific kind of higher degeneracy of the underlying symbols. We introduce an algebra of meromorphic edge operators that depend on complex parameters and investigate meromorphic inverses in the parameter-dependent elliptic case. Among the examples are resolvents of elliptic differential operators on manifolds with edges.  相似文献   
944.
The title compound, [Cd(SeCN)2(pyCN)2], adopts an extended one‐dimensional chain structure in which the neighboring cadmium atoms are bridged by two selenocyanate ions. The central cadmium atom has a distorted octahedral geometry defined by two isonicotinonitrile and four selenocyanate ions in a 4N2Se fashion. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
945.
The method of nonlinear moments, when used to solve the Boltzmann equation, necessitates the calculation of collision integral matrix elements. The matrix elements are hard to calculate numerically, especially at large indices. The asymptotics of the matrix elements are constructed. In terms of the model of pseudopower particle interaction, a formula free of summation is derived. This makes it possible to find the asymptotic behavior of linear and nonlinear elements when two indices are large. For an arbitrary interaction cross section, asymptotic expansions of linear and nonlinear matrix elements in one index are obtained. For Maxwellian molecules, asymptotic formulas are derived for three large indices.  相似文献   
946.
We argue that the two-dimensional elliptic quantum dot problem with finite barrier cannot be exactly solved, contrary to a recent assertion (van den Broek and Peeters, Physica E 11 (2001) 345. We also prove it explicitly by numerically calculating the correct energy spectrum.  相似文献   
947.
948.
The room temperature structure of perovskite CeAlO3 has been reinvestigated by X-ray powder diffraction. The Rietveld refinement has confirmed the tetragonal symmetry; but revealed a super cell, a=5.32489(6) Å and c=7.58976(10) Å, with the space group I4/mcm. In CeAlO3, the distortion from the ideal cubic perovskite is caused by the cooperative tilting of the AlO6 octahedra around the primitive cubic [001]p-axis.  相似文献   
949.
非完整约束奇异广义力学系统的Poincaré-Cartan积分   总被引:1,自引:0,他引:1       下载免费PDF全文
李爱民  张莹  李子平 《物理学报》2004,53(9):2816-2820
对受高阶微商非完整约束并用奇异Lagrange量描述的广义力学系 统,基于广义Apell-Четаев约束条件,并考虑到系统的内在约束,导出了该非完整 约 束奇异广义力学系统的广义Poincaré-Cartan积分不变量. 并证明了该不变量与非完整约束 奇异广义力学系统的广义正则方程等价.  相似文献   
950.
Binzhong Li  Baida Lü   《Optik》2003,113(12):535-540
Based on the beam coherence-polarization (BCP) matrix, the polarization property of coherent and incoherent Gaussian beam combinations is studied in detail. The general expressions for the degree of polarization P of the resulting beam in case of incoherent and coherent combinations are derived. It is shown that P is dependent on the incoherent or coherent combination, propagation distance, separation, azimuth of the polarization plane and numbers of beamlets in general. The irradiance distribution of the resulting beam for the coherent cases depends on the azimuth of the polarization plane of beamlets. However, for the incoherent case it does not.  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