全文获取类型
收费全文 | 468篇 |
免费 | 4篇 |
国内免费 | 1篇 |
专业分类
化学 | 280篇 |
晶体学 | 4篇 |
力学 | 18篇 |
综合类 | 1篇 |
数学 | 77篇 |
物理学 | 93篇 |
出版年
2021年 | 6篇 |
2020年 | 4篇 |
2016年 | 6篇 |
2014年 | 5篇 |
2013年 | 22篇 |
2012年 | 10篇 |
2011年 | 19篇 |
2010年 | 8篇 |
2008年 | 21篇 |
2007年 | 20篇 |
2006年 | 21篇 |
2005年 | 22篇 |
2004年 | 9篇 |
2003年 | 19篇 |
2002年 | 11篇 |
2001年 | 9篇 |
1999年 | 5篇 |
1998年 | 7篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 6篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 7篇 |
1982年 | 11篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 9篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1975年 | 6篇 |
1974年 | 10篇 |
1973年 | 7篇 |
1972年 | 4篇 |
1970年 | 4篇 |
1969年 | 5篇 |
1967年 | 6篇 |
1964年 | 4篇 |
1955年 | 4篇 |
1936年 | 6篇 |
1935年 | 3篇 |
1934年 | 3篇 |
1931年 | 3篇 |
1930年 | 3篇 |
排序方式: 共有473条查询结果,搜索用时 15 毫秒
171.
Summary The numerical error of standard finite-difference schemes is analyzed at free boundaries of the Grad-Schlüter-Shafranov equation of plasma physics. A simple correction strategy is devised to eliminate (to leading order) the errors which arise as the free boundary crosses the rectangular grid at irregular locations. The resulting scheme can be solved by Gauss-Newton or Inverse iterations, or by multigrid iterations. Extrapolation (from 2nd to 3rd order of accuracy) is possible for the new scheme.Dedicated to the memory of Professor Lothar Collatz 相似文献
172.
173.
Björn Håkansson Bengt Jönsson Per Linse Olle Söderman 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,124(2):343-351
The influence of a nonconstant magnetic-field gradient on the performance of pulsed-field-gradient NMR diffusion experiments was investigated by performing Brownian-dynamics simulations for two different coil designs using two different active sample volumes for each coil. The active sample volumes were chosen in order to represent different degrees of deviation from a perfectly constant magnetic-field gradient. The results show that one can actually tolerate a rather large deviation from a perfectly constant magnetic-field gradient and still obtain accurate values of the self-diffusion coefficients. The dependence of Δ (the observation time) and the dependence ofD(the self-diffusion coefficient) while keepingDand Δ constant, respectively, were also investigated for three of the active sample volumes. The simulations show that the effects due to a nonconstant field gradient depend only slightly on the values of Δ andD. In conclusion, the Brownian-dynamics computer simulations procedure is a quantitative way of investigating the performance characteristics of a gradient coil of a certain design before it is built. 相似文献
174.
175.
Ribbon phases consist of long cylindrical aggregates that have non-circular normal sections. We have recently pointed out that scattering data for a large number of different intermediate ribbon phases of lower than hexagonal symmetry found in ionic surfactant systems indicate that these phases have a structure possessing a centred rectangular symmetry. In this communication, we have investigated the aggregate dimensions for the phases with cylindrical aggregates, i.e., the hexagonal phases and the centred rectangular ribbon phases. Previously published phase diagrams, small angle X-ray and neutron scattering data and 2HNMR data for these phases in different systems have been used for this purpose. The results are that the axial ratios of the aggregates increase when the temperature decreases, when the surfactant concentration increases, and when the average surfactant charge decreases. Models that semi-quantitatively describe the thermodynamics of the micellar, hexagonal and lamellar phases, which are based on the Poisson-Boltzmann cell model approach, have previously been presented in the literature. We have extended these models to treat also the ribbon phases. The results from the calculations show the same trends with respect to changes in the dimensions of the non-circular aggregates upon changes in temperature, surfactant concentration and average surfactant charge, as those obtained experimentally. Theoretically calculated phase diagrams with ribbon phases are also presented. Based on the predictions of the model and some previously published experimental data for hexagonal phases, it is proposed that the formation of non-circular, cylindrical aggregates is a general property of single-chain, ionic surfactant/water systems, and that these aggregates in general pack on hexagonal lattices. The normal sections of these aggregates are circular on average, on account of the fact that the degree of deformation and the orientation of deformation changes along the axis of the aggregates and with time. Only for some systems, temperatures and surfactant concentrations do the asymmetric aggregates line up and ribbon phases with centred rectangular symmetry are obtained. The driving mechanisms for the transition from the hexagonal phase with asymmetric (fluctuating) cylinders and further to the centred rectangular phase with asymmetric (stiff) cylinders is also discussed. It is argued that this phase transition is of the first order. 相似文献
176.
The 10-dimensional supersymmetric Yang-Mills theory is constructed in the light-cone gauge. When the theory is dimensionally reduced to four dimensions it is shown that the corresponding N = 4 theory is conveniently described in terms of a scalar superfield. This formalism avoids the problem of auxiliary fields but is Lorentz invariant only on the mass shell. Similar formalisms in terms of scalar superfields are also sketched for the other supersymmetric Yang-Mills as well as for N = 8 supergravity. 相似文献
177.
178.
Bengt E.W. Nilsson 《Nuclear Physics B》1981,188(1):176-192
Differential geometry is used to formulate supergravity in a 10-dimensional superspace. From the knowledge of the supersymmetric set of fields in x-space we derive the constraints on the supertorsion and on a super 3-form field strength. We then solve the equations which follow from the Bianchi identities. The solution obtained, which is on the mass shell, is shown to be completely described in terms of a scalar superfield. 相似文献
179.
Bengt Karlsson Anne-Marie Pilotti Anne-Charlotte Söderholm Torbjörn Norin Stefen Sundin Masashi Sumimoto 《Tetrahedron》1978,34(15):2349-2354
The structure of verticillol diepoxide 4 has been established by direct single crystal X-ray analysis. The structure of verticillol 5 follows from the chemical correlation to the diepoxide 4 as well as from NMR-LIS studies on verticillol which also provide evidence for the conformation of this alcohol. The absolute configuration of verticillol 5 has been assigned on the basis of CD data for the verticillol norketodiepoxide 6a. 相似文献
180.
Cooper VR Thonhauser T Puzder A Schröder E Lundqvist BI Langreth DC 《Journal of the American Chemical Society》2008,130(4):1304-1308
The importance of stacking interactions for the Twist and stability of DNA is investigated using the fully ab initio van der Waals density functional (vdW-DF). Our results highlight the role that binary interactions between adjacent sets of base pairs play in defining the sequence-dependent Twists observed in high-resolution experiments. Furthermore, they demonstrate that additional stability gained by the presence of thymine is due to methyl interactions with neighboring bases, thus adding to our understanding of the mechanisms that contribute to the relative stability of DNA and RNA. Our mapping of the energy required to twist each of the 10 unique base pair steps should provide valuable information for future studies of nucleic acid stability and dynamics. The method introduced will enable the nonempirical theoretical study of significantly larger pieces of DNA or DNA/amino acid complexes than previously possible. 相似文献