全文获取类型
收费全文 | 832768篇 |
免费 | 8730篇 |
国内免费 | 2241篇 |
专业分类
化学 | 421344篇 |
晶体学 | 11708篇 |
力学 | 41873篇 |
综合类 | 26篇 |
数学 | 118687篇 |
物理学 | 250101篇 |
出版年
2021年 | 7007篇 |
2020年 | 7665篇 |
2019年 | 8575篇 |
2018年 | 14522篇 |
2017年 | 14694篇 |
2016年 | 17960篇 |
2015年 | 9863篇 |
2014年 | 15347篇 |
2013年 | 35852篇 |
2012年 | 29416篇 |
2011年 | 38565篇 |
2010年 | 27315篇 |
2009年 | 27162篇 |
2008年 | 35681篇 |
2007年 | 36977篇 |
2006年 | 28725篇 |
2005年 | 28039篇 |
2004年 | 24980篇 |
2003年 | 22724篇 |
2002年 | 21660篇 |
2001年 | 22742篇 |
2000年 | 17684篇 |
1999年 | 13694篇 |
1998年 | 11594篇 |
1997年 | 11292篇 |
1996年 | 10915篇 |
1995年 | 9845篇 |
1994年 | 9757篇 |
1993年 | 9420篇 |
1992年 | 10178篇 |
1991年 | 10542篇 |
1990年 | 10195篇 |
1989年 | 9870篇 |
1988年 | 9744篇 |
1987年 | 9579篇 |
1986年 | 9106篇 |
1985年 | 11760篇 |
1984年 | 12169篇 |
1983年 | 9988篇 |
1982年 | 10372篇 |
1981年 | 9957篇 |
1980年 | 9369篇 |
1979年 | 10074篇 |
1978年 | 10379篇 |
1977年 | 10197篇 |
1976年 | 10111篇 |
1975年 | 9590篇 |
1974年 | 9456篇 |
1973年 | 9650篇 |
1972年 | 6890篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
981.
A simple model consisting of two electronic levels and one vibrational mode (phonon) was theoretically studied. The electronic-vibrational interaction was linear in the vibrational displacement. The vibrational mode was taken in the harmonic approximation and was attached to the thermal bath formed by the ambient environment. The kinetic constants of the vibrational dissipation were of the second order in the vibrational-bath coupling and were taken in the Markovian limit. Although, depending on the parameters of the model, different curves of the non-radiative vibration-induced excited state decay were obtained, in general, three time intervals, corresponding to different physical behaviour, were found. In the short-time interval, small oscillations superimposed on the excited state decay were observed. They were determined by the vibrational frequency and influenced by electronic-vibrational coupling. In the middle-time interval, almost quasi-exponential decay was detected; its rate constant increased with stronger electronic-vibrational interaction and speed of vibrational relaxation. In the long-time interval, the decay was very slow and, under special conditions, even an asymptotic non-zero excited state population was observed. Its value increased with the strength of the off-diagonal electron-vibrational coupling. Links of the parameters of the model with quantum chemical terms were estimated. 相似文献
982.
V. V. Kulagin 《Optics and Spectroscopy》2003,94(5):779-783
The generation of nonclassical states of light with specified frequency-dependent squeezing by way of introducing a correlation between the phase and amplitude of the input field is considered. Three schemes for measuring amplitude fluctuations of the input beam, based on a nondemolition measurement of the intensity in a nonlinear cubic crystal, the effect of light rectification in a quadratic optical crystal, and the use of a non-degenerate optical parametric oscillator, are proposed. The maximum possible value of noise suppression is estimated for each scheme. The prospects for practical realization of the proposed method for generation of frequency-dependent squeezed states are discussed. 相似文献
983.
A. Guzmn E. Luna J. Miguel-Snchez E. Calleja E. Muoz 《Infrared Physics & Technology》2003,44(5-6):377-382
In this work, we report on the design, growth and characterization of GaAsN/AlAs/AlGaAs double barrier quantum well infrared detectors to achieve intraband absorption below 4 μm. Due to the high effective mass of N-dilute alloys, it is common for these N-containing double barrier quantum well structures to have more than one bound state within the quantum well, enabling the possibility of achieving multispectral absorption from these confined levels to the quasi-bound. Based on a transfer matrix calculation we will study the influence of the potential parameters, in particular the well width and the introduction of a GaAs spacer layer in between the N-well and the AlAs barriers. We will compare the case in which there are two confined levels with the case in which only one level is bound, like in the conventional AlGaAs/AlAs/GaAs structures. On the basis of the simulation, we have grown and characterized some N-containing double barrier detectors. Moreover, an optimization of the post-growth annealing treatments of the GaAsN quantum well structures has also been performed. Finally, room temperature absorption measurements of both as-grown and annealed samples are presented and analyzed. 相似文献
984.
A. D. Matveev 《Journal of Applied Mechanics and Technical Physics》2003,44(5):721-726
Modified three-dimensional formulations of bending problems of homogeneous elastic plates and beams are considered. Modification of the known three-dimensional formulations reduces to using additional constraints imposed on displacement functions. An advantage of the formulations proposed is that complex fixing conditions of plates and beams can be taken into account. 相似文献
985.
986.
A. N. Bandura V. V. Chebotarev I. E. Garkusha V. A. Makhlay A. K. Marchenko D. G. Solyakov V. I. Tereshin S. A. Trubchaninov A. V. Tsarenko I. Landman 《Czechoslovak Journal of Physics》2004,54(3):C53
The paper presents experimental investigation of energy characteristics of the plasma streams generated with quasi—steady—state plasma accelerator QSPA Kh-50 and adjustment of plasma paramenters from the point of view its applicability for simulation of transient plasma heat loads expected for ITER disruptions and type I ELMs. Possibility of generation of high-power magnetized plasma streams with ion impact energy up to 0.6 keV, pulse length 0.25 ms and heat loads varied in wide range from 0.5 to 30 MJ/m2 has been demonstrated and some features of plasma interaction with tungsten targets in dependence on plasma heat loads are discussed. 相似文献
987.
V. A. Alekseev 《Optics and Spectroscopy》2004,96(4):492-496
The absorption spectra of Xe + CF4 and Kr + CF4 mixtures in the vacuum ultraviolet region are presented. A considerable broadening of the short-wavelength wings of the atomic absorption lines is explained by a repulsive character of the Rg*-CF4 interaction potential. This repulsive character is also considered to be the most probable reason for the anomalously small cross sections of the quenching of electronically excited atoms of rare gases by CF4 molecules. The influence of CF4 on the emission rate of the trapped resonance radiation of Xe from the cell is discussed. 相似文献
988.
989.
The pure state space of Quantum Mechanics is investigated as Hermitian Symmetric Kähler manifold. The classical principles of quantum mechanics (Quantum Superposition Principle, Heisenberg Uncertainty Principle, Quantum Probability Principle) and Spectral Theory of observables are discussed in this non-linear geometrical context. 相似文献
990.
Let {Xk}k?1 be a strictly stationary time series. For a strictly increasing sampling function g:?→? define Yk=Xg(k) as the deterministic sub‐sampled time series. In this paper, the extreme value theory of {Yk} is studied when Xk has representation as a moving average driven by heavy‐tailed innovations. Under mild conditions, convergence results for a sequence of point processes based on {Yk} are proved and extremal properties of the deterministic sub‐sampled time series are derived. In particular, we obtain the limiting distribution of the maximum and the corresponding extremal index. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献