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   1670篇
  免费   71篇
  国内免费   2篇
化学   1078篇
晶体学   6篇
力学   19篇
数学   277篇
物理学   363篇
  2023年   15篇
  2022年   30篇
  2021年   54篇
  2020年   41篇
  2019年   48篇
  2018年   33篇
  2017年   33篇
  2016年   89篇
  2015年   54篇
  2014年   66篇
  2013年   79篇
  2012年   135篇
  2011年   154篇
  2010年   116篇
  2009年   66篇
  2008年   104篇
  2007年   136篇
  2006年   81篇
  2005年   71篇
  2004年   56篇
  2003年   41篇
  2002年   33篇
  2001年   13篇
  2000年   12篇
  1999年   14篇
  1998年   11篇
  1997年   5篇
  1996年   12篇
  1995年   19篇
  1994年   17篇
  1993年   9篇
  1992年   7篇
  1991年   4篇
  1990年   3篇
  1989年   7篇
  1987年   3篇
  1984年   6篇
  1982年   5篇
  1981年   8篇
  1980年   6篇
  1979年   9篇
  1978年   3篇
  1977年   4篇
  1976年   3篇
  1975年   3篇
  1969年   2篇
  1968年   2篇
  1959年   2篇
  1905年   3篇
  1904年   2篇
排序方式: 共有1743条查询结果,搜索用时 15 毫秒
991.
The dynamics of the pairwise entanglement in a qubit lattice in the presence of static imperfections exhibits different regimes. We show that there is a transition from a perturbative region, where the entanglement is stable against imperfections, to the ergodic regime, in which a pair of qubits becomes entangled with the rest of the lattice and the pairwise entanglement drops to zero. The transition is almost independent of the size of the quantum computer. We consider both the case of an initial maximally entangled and separable state. In this last case there is a broad crossover region in which the computer imperfections can be used to create a significant amount of pairwise entanglement.  相似文献   
992.
Discretization of phase space usually nullifies chaos in dynamical systems. We show that if randomness is associated with discretization dynamical chaos may survive and be indistinguishable from that of the original chaotic system, when an entropic, coarse-grained analysis is performed. Relevance of this phenomenon to the problem of quantum chaos is discussed.  相似文献   
993.
Joining steel with aluminum involving the fusion of one or both materials is possible by laser beam welding technique. This paper describes a method, called laser braze welding, which is a suitable process to realize this structure. The main problem with thermal joining of steel/aluminum assembly with processes such as TIG or MIG is the formation of fragile intermetallic phases, which are detrimental to the mechanical performances of such joints. Braze welding permits a localized fusion of the materials resulting in a limitation on the growth of fragile phases. This article presents the results of a statistical approach for an overlap assembly configuration using a filler wire composed of 85% Zn and 15% Al. Tensile tests carried on these assemblies demonstrate a good performance of the joints. The fracture mechanisms of the joints are analyzed by a detailed characterization of the seams.  相似文献   
994.
The idea of treating quantum general relativistic theories in a perturbative expansion around a topological theory has recently received attention, in the quantum gravity literature. We investigate the viability of this idea by applying it to conventional Yang–Mills theory on flat spacetime. This theory admits indeed a formulation as a modified topological theory, like general relativity. We find that the expansion around the topological theory coincides with the usual expansion around the free abelian theory, though the equivalence is non-trivial. In this context, the technique appears therefore to be viable, but not to bring particularly new insights. On the other hand, we point out that the relation of this expansion with the actual quantum BF theory is far from being transparent. Some implications for gravity are discussed.  相似文献   
995.
Cultural heritage materials are subject to continuous chemical and physical changes depending on the establishment of a dynamic equilibrium with the environment in which they are placed. In particular, different phenomena can take place, such as formation of black crusts, corrosion of the material, internal cracks. Lecce stone, a biocalcarenite mainly used for historical buildings in South Italy, has a high total porosity. In order to prevent its deterioration, different kinds of hydrophobic organic products are applied on the surface of the restored artefacts. Since the efficacy of the treatments depends mostly on the penetration depth and the distribution of the products in the pores, porosity and internal structure of the stone material were mainly investigated in this research. Micro x‐ray computed tomography (µ‐CT) has been used to study and characterize the internal structure of different samples, untreated and treated with protective products. The porosity and other parameters of the rock were then calculated and compared, before and after the conservation treatment, in order to highlight the changes due to the application of the product. On the other hand, small pieces of the untreated and treated samples were analysed by sub‐micron resolution x‐ray tomography where it was possible to see the distribution of the products inside the pores. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
996.
Summary Urea (ur), thiourea (tu) and diisopropylthiourea (diptu) form high-spin complexes with iron(II), for which57Fe Mössbauer quadrupole splitting and spectroscopic data suggest an octahedral distorted geometry. The x-ray diffraction study on Fe(tu)4Cl2 confirms this geometry. The crystals are tetragonal, space groupP42/n witha=13.71,c=8.94 Å andZ=4. The molecules are centrosymmetric with two axial chlorine atoms (Fe-Cl=2.46 Å) and four equatorial sulfur atoms (Fe-S=2.61 and 2.57 Å respectively). A similartrans-octahedral geometry is proposed for the new Fe(diptu)4Cl2 complex and an octahedral metal coordination in the new [Fe(ur)6]Br2, Fe(tu)3Br2 and Fe(ur)3Br2 complexes.  相似文献   
997.
Novel nonlinear superposition formulae are given for solutions of the class of nonlinear evolution equations solvable via the spectral transform associated with the Zakharov-Shabat spectral problem, and an algebraic procedure is provided to construct solutions of these equations.  相似文献   
998.
The oxidation of cedrol ( 1 ), β- and γ-eudesmol ( 6 and 7 , resp.), sclareol ( 14 ) manoyl oxide ( 15 ), 1,9-dideoxyforskolin ( 22 ) (±)-methyl trans-dihydrojasmonate ( 28 ), and tetrahydrolinalool ( 32 ) nearly all of natural terpenoid origin, by the ‘Gif system’ has afforded a number of novel products ( 3 , 11 , and 12 , 16/17 , 18/19 , 26 , 29–31 , and ketones corresponding to 34–35 , res.). The structures of these compounds were established by spectroscopic techniques including 2D-NMR and, where appropriate, by comparison with authentic samples.  相似文献   
999.
Curletto S  Parodi C  Trucco A 《Ultrasonics》2004,42(1-9):997-1003
This paper presents a study on the worsening caused by a random perturbation of the weighting window (modeling, in this way, the uncertainty on the channel sensitivity), or by an alteration of the shape of the transmitted pulse, for a wideband array. In the stability evaluation, main-lobe width and signal to noise ratio evaluated at different dB levels of the beam pattern play the role of quality parameters. Two different profiles of the weighting window have been used for this analysis: a typical raised cosine, and a window optimized by a Simulated Annealing procedure. Also for the pulse envelope, two distinct shapes have been chosen: perfectly Gaussian and experimentally measured. The analysis of the obtained results provides useful hints about the amount and the kind of the beam pattern worsening, when realistic fluctuations occur. Moreover, the results show that the weighting windows provided by the simulated annealing procedure are particularly robust to such fluctuations.  相似文献   
1000.
The recently developed Symanzik-improved staggered-quark discretization allows unquenched lattice-QCD simulations with much smaller (and more realistic) quark masses than previously possible. To test this formalism, we compare experiment with a variety of nonperturbative calculations in QCD drawn from a restricted set of "gold-plated" quantities. We find agreement to within statistical and systematic errors of 3% or less. We discuss the implications for phenomenology and, in particular, for heavy-quark physics.  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