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   1194篇
  免费   15篇
  国内免费   3篇
化学   640篇
晶体学   80篇
力学   59篇
数学   131篇
物理学   302篇
  2022年   10篇
  2021年   16篇
  2020年   12篇
  2019年   24篇
  2018年   16篇
  2017年   19篇
  2016年   29篇
  2015年   14篇
  2014年   40篇
  2013年   86篇
  2012年   51篇
  2011年   68篇
  2010年   50篇
  2009年   37篇
  2008年   33篇
  2007年   39篇
  2006年   39篇
  2005年   42篇
  2004年   20篇
  2003年   23篇
  2002年   26篇
  2001年   15篇
  2000年   9篇
  1999年   10篇
  1998年   13篇
  1997年   17篇
  1996年   24篇
  1995年   20篇
  1994年   10篇
  1993年   13篇
  1992年   19篇
  1991年   9篇
  1990年   9篇
  1989年   20篇
  1988年   17篇
  1987年   9篇
  1986年   14篇
  1985年   23篇
  1984年   17篇
  1983年   21篇
  1982年   31篇
  1981年   17篇
  1980年   15篇
  1979年   19篇
  1978年   13篇
  1977年   21篇
  1975年   12篇
  1974年   8篇
  1972年   10篇
  1970年   8篇
排序方式: 共有1212条查询结果,搜索用时 15 毫秒
91.
The quantum density of states of the Henon-Heiles potential displays a pronounced beating pattern. This has been explained by the interference of three isolated classical periodic orbits with nearby actions and periods. A singular magnetic flux line, passing through the origin, drastically alters the beats even though the classical Lagrangian equations of motion remain unchanged. Some of the changes can be easily understood in terms of the Aharonov-Bohm effect. However, we find that the standard periodic orbit theory does not reproduce the diffraction-like quantum effects on those classical orbits which intersect the singular flux line, and argue that corrections of relative order variant Planck's over 2pi are necessary to describe these effects. We also discuss the changes in the distribution of nearest-neighbor spacings in the eigenvalue spectrum, brought about by the flux line. (c) 1995 American Institute of Physics.  相似文献   
92.
93.
94.
The 4-hydroxy buteneolide terminus 3, applicable to mucocin 1 and related annonaceous acetogenins, was prepared in an expeditious manner from the selenocarbonate 2 via an intramolecular acyl radical cyclization followed by an enantioselective Lewis-acid catalyzed Keck-allylation reaction.  相似文献   
95.
The absorption spectrum of Co2+ doped NH4Cl has been studied from the room temperature to the liquid nitrogen temperature. A sudden change in the spectrum is observed between 243° K and 233° K which is attributed to the phase transition in the crystal. From the observed spectrum it is suggested that Co2+ goes in interstitially as well as substitutionally. Both the types of centers exist at room temperature, but with decrease in temperature substitutional ions migrate to interstitial sites, the process being stimulated at the phase transformation point so that the 77° K spectrum seems to be mostly due to the interstitial centers. The 77° K spectrum is analyzed in the approximation of octahedral symmetry for interstitial ions and the band positions are fitted fairly well with B = 870 cm.?1 Dq = 850 cm.?1 and C = 4·4 B. A blue shift of about 100 cm.?1 is observed for4T1 (P) band at the phase transition which is attributed to the increase in Dq value with the anomalous lattice contraction at the phase transition. The decrease in the lattice parameter calculated from this blue shift is around 0·4% which is in good agreement with the results of X-ray measurements. Two possible models for the interstitial complex are examined and the one with fourfold chlorine coordination associated with two neutral water molecules at the first neighbour (NH4)+ site lying along < 100> direction is suggested to be more probable.  相似文献   
96.
A careful discussion is given of the definitions of “target asymmetry” and “effective neutron polarization” in the reaction d(γ,π?)2p with polarized deuterons. The connection between the analyzing powers, Tqk of the reaction and the target asymmetry, T is established taking into account the method of preparation of the polarized target. It is further shown that the concept of effective neutron polarization Peff is valid only in a very approximate way, even when the target deuteron is purely vector polarized.  相似文献   
97.
The overall Kaplan-Meier estimate of the survival curve underlying a study population does not individually account for the ages at which patients were first seen and thence followed up in the study. Since the number of patients in these age-specific subgroups varies quite arbitrarily for a given study, the overall Kaplan-Meier curve which assumes that all patients were seen from the beginning of the study usually either overestimates or underestimates the survival probability, depending on the varying sample sizes. In this paper, we therefore propose an estimate of the overall survival curve underlying a study population based on the individual age-specific Kaplan-Meier estimates and the probability distribution of the age at which patients were first seen in the study. In the particular case when there is no censoring, it is shown that the usual overall Kaplan-Meier survival curve which assumes that all patients irrespective of their age at entry into the study were followed from the beginning of the study is identical with the proposed estimate. This result does not seem to necessarily hold good in the presence of arbitrary censoring. Further, the estimate is unbiased and consistent, a property which the Kaplan-Meier estimate also enjoys. The efficiency of the proposed estimate relative to the Kaplan-Meier estimate is being investigated and will be reported along with results of its application in a separate communication.  相似文献   
98.
It is well known that the nu = 2/5 state is unpolarized at zero Zeeman energy, while it is fully polarized at large Zeeman energies. A novel state with a charge/spin density wave order for composite fermions is proposed to exist at intermediate values of the Zeeman coupling for nu = 2/5. This state has half the maximum possible polarization, and can be extended to other incompressible fractions. A Hartree-Fock calculation based on the new approach for all fractional quantum Hall states developed by R. Shankar and the author is used to demonstrate the stability of this state to single-particle excitations and to compute gaps. A very recent experiment shows direct evidence for this state.  相似文献   
99.
Effect of Co, Ru and Cu substitution at B and B’ sites on the magnetic and transport properties of LaSrCoRuO6 have been investigated. All the doped compositions crystallize in the monoclinic structure in the space group P21/n indicating a double perovskite structure. While the magnetization and conductivity increase in Co and Ru doped compounds, antiferromagnetism is seen to strengthen in the Cu doped samples. These results are explained on the basis of a competition between linear Co-O-Ru-O-Co and perpendicular Co-O-O-Co antiferromagnetic interactions and due to formation of Ru-O-Ru ferromagnetic networks.  相似文献   
100.
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