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   24977篇
  免费   544篇
  国内免费   99篇
化学   13939篇
晶体学   150篇
力学   770篇
综合类   9篇
数学   3488篇
物理学   7264篇
  2022年   171篇
  2021年   394篇
  2020年   363篇
  2019年   316篇
  2018年   407篇
  2017年   369篇
  2016年   634篇
  2015年   526篇
  2014年   612篇
  2013年   979篇
  2012年   1266篇
  2011年   1450篇
  2010年   814篇
  2009年   771篇
  2008年   1205篇
  2007年   1197篇
  2006年   1078篇
  2005年   1501篇
  2004年   1548篇
  2003年   1076篇
  2002年   724篇
  2001年   603篇
  2000年   563篇
  1999年   345篇
  1998年   292篇
  1997年   290篇
  1996年   358篇
  1995年   319篇
  1994年   307篇
  1993年   329篇
  1992年   320篇
  1991年   269篇
  1990年   215篇
  1989年   205篇
  1988年   181篇
  1987年   177篇
  1986年   174篇
  1985年   227篇
  1984年   214篇
  1983年   185篇
  1982年   192篇
  1981年   184篇
  1980年   169篇
  1979年   151篇
  1978年   175篇
  1977年   140篇
  1976年   134篇
  1975年   112篇
  1974年   131篇
  1973年   121篇
排序方式: 共有10000条查询结果,搜索用时 535 毫秒
991.
A multineuron interaction model (RS model) with an energy function given by the product of the squared distances in phase space between the state of the net and the stored patterns is studied in detail within a mean-field approach. Two limits are considered: when the patterns and antipatterns are stored (as in the Hopfield model), PAS case, and when only the patterns are taken into account, OPS case. TheT=0 solutions for the proper memories are exactly obtained for all finite values of, as a consequence of the energy function: whenever one of the overlaps is exactly one the corresponding equations decouple and no configuration average is required. Special interest is focused on the OPS situation, which presents a peculiar phase space topology. On the other hand, the PAS configuration recovers the Hopfield model in the appropriate limit, while keeping associative memory abilities far beyond the critical values of other models when the full Hamiltonian is considered.  相似文献   
992.
993.
994.
995.
Blocking sets in symmetric BIBD's with 2 are defined and bounds for their sizes are found. Thus, some results known in projective planes are generalized to symmetric BIBD's.  相似文献   
996.
We show that particle-antiparticle exchange and covariant motion reversal are two physically different aspects of the same mathematical transformation, either in the prequantal relativistic equation of motion of a charged point particle, in the general scheme of second quantization, or in the spinning wave equations of Dirac and of Petiau-Duffin-Kemmer. While, classically, charge reversal and rest mass reversal are equivalent operations, in the wave mechanical case mass reversal must be supplemented by exchange of the two adjoint equations, implying .Denoting by M the rest mass reversal, P the parity reversal, T the Racah time reversal, and Z the exchange, the connection with the usual scheme of charge conjugation, parity reversal, and Wigner motion reversal, is with, of course,  相似文献   
997.
The cell constants of four new monoclinic compounds BaR4X5O17 (R = Y, Gd; X = Si, Ge) are given. The luminescence of various RE activators in the silicates is reported. Pr3+-activated BaY4Si5O17 shows efficient ultraviolet 5d → 4f emission and weak 4f → 4f emission (mainly red luminescence from the 1D2 level). The 5d → 4f emission is ascribed to Pr3+ on Y sites, the 4f → 4f emission to Pr3+ on Ba sites. Energy transfer from Pr3+ to Gd3+ has been observed. Gd3+ plays an intermediate role in the energy transfer from Pr3+ to Sm3+ and to Dy3+ in BaGd4Si5O17. Upon activation with Tb3+ the silicates show characteristic green Tb3+ luminescence with a quantum efficiency of 75% for ultraviolet excitation.  相似文献   
998.
Sulfonated polystyrene latex particles were prepared by a two-stage shot-growth emulsion polymerization process in the absence of emulsifier. Sodium styrene sulfonate (NaSS) was used as an ionic co-monomer to produce a series of latex particles with the same particle size but with different surface charge densities. The electrophoretic mobility of this functionalized model colloid was studied in the presence of various types of inorganic electrolytes. The e curves of these latexes exhibit a pronounced maximum at high electrolyte concentrations: 5·10–2 M for 11 electrolytes and 10–2 M for 21 and 12 electrolytes. When a 31 electrolyte (LaCl3) was used, the electrophoretic mobility changed to positive values at high concentration due to the specific adsorption of lanthanum species. The experimental results for the electrokinetic characterization of these sulfonated polystyrene model colloids suggest that the surface of the particles is covered by a layer of oligomers or polymer chains which shift the shear plane toward the bulk solution, increasing the anomalous surface conductance of the polystyrene microsphere-electrolyte solution interface.  相似文献   
999.
We have studied the efficiency of parallel tempering simulations for a variety of systems including a coarse-grained protein, an atomistic model polypeptide, and the Lennard-Jones fluid. A scheme is proposed for the optimal allocation of temperatures in these simulations. The method is compared to the existing empirical approaches used for this purpose. Accuracy associated with the computed thermodynamic quantities such as specific heat is also computed and their dependence on the trial-exchange acceptance rate is reported.  相似文献   
1000.
C-H hydroxylation is a fundamental process. In Nature it is catalyzed by the enzyme cytochrome P450, in a still-debated mechanism that poses a major intellectual challenge for both experiment and theory; currently, the opinions keep swaying between the original single-state rebound mechanism, a two-oxidant mechanism (where ferric peroxide participates as a second oxidant, in addition to the primary active species, the high-valent iron-oxo species), and two-state reactivity (TSR) mechanism (where two spin states are involved). Recent product isotope effect (PIE) measurements for the trans-2-phenyl-methyl cyclopropane probe (1), led Newcomb and co-workers (Newcomb, M.; Aebisher, D.; Shen, R.; Esala, R.; Chandrasena, P.; Hollenberg, P.; Coon, M. J. J. Am. Chem. Soc. 2003, 125, 6064-6065) to rule out TSR in favor of the two-oxidant scenario, since the direction of the PIE was at odds with the one predicted from calculations on methane hydroxylation. The present report describes a density functional theoretical study of C-H hydroxylation of the Newcomb probe, 1, leading to rearranged (3) and unrearranged (2) products. Our study shows that the reaction occurs via TSR in which the high-spin pathway gives dominant rearranged products, whereas the low-spin pathway favors unrearranged products. The calculated PIE(2/3) values based on TSR are found to be in excellent agreement with the experimental data of Newcomb and co-workers. This match between experiment and theory makes a strong case that the reaction occurs via TSR mechanism.  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