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   5681篇
  免费   43篇
  国内免费   30篇
化学   3279篇
晶体学   58篇
力学   245篇
数学   888篇
物理学   1284篇
  2021年   77篇
  2020年   92篇
  2019年   94篇
  2018年   135篇
  2017年   162篇
  2016年   152篇
  2015年   55篇
  2014年   123篇
  2013年   248篇
  2012年   176篇
  2011年   216篇
  2010年   189篇
  2009年   204篇
  2008年   179篇
  2007年   189篇
  2006年   148篇
  2005年   114篇
  2004年   124篇
  2003年   129篇
  2002年   139篇
  2001年   102篇
  2000年   82篇
  1999年   60篇
  1998年   60篇
  1997年   60篇
  1995年   94篇
  1994年   105篇
  1993年   82篇
  1992年   59篇
  1991年   83篇
  1990年   87篇
  1989年   79篇
  1988年   62篇
  1987年   79篇
  1986年   66篇
  1985年   87篇
  1984年   116篇
  1983年   74篇
  1982年   88篇
  1981年   69篇
  1980年   70篇
  1979年   110篇
  1978年   101篇
  1977年   105篇
  1976年   113篇
  1975年   92篇
  1974年   60篇
  1973年   85篇
  1972年   65篇
  1970年   67篇
排序方式: 共有5754条查询结果,搜索用时 10 毫秒
111.
112.
Coordinate-dependent diffusion in solids, in which case the probability of presence of a vacancy, φ, and the probability of migration, γ, depend on coordinate x is analyzed. The coordinate dependence of parameters φ and γ is shown to cause the drift component of the diffusion flux. In the presence of this component, the direction of atomic motion may coincide (enhanced diffusion) or not coincide (retarded diffusion and up-diffusion) with the direction of the diffusion component. The criteria of enhanced diffusion, retarded diffusion, and up-diffusion are established. These criteria are specified by the behavior of function F(x)=φ(x)/γ(x).  相似文献   
113.
A new approach to observing the effect of electromagnetically induced transparency in gamma optics is proposed. The propagation of a resonant photon in a 57Fe magnetic medium in an applied rf field is considered for this purpose. It is shown that, under crossing-anticrossing conditions, a resonant rf field substantially changes the gamma-optical properties of the medium, which become dependent on the parameters of the field. This opens the possibility for exercising a coherent control of the photon group velocity and a controllable filtration of unpolarized gamma radiation in a sample.  相似文献   
114.
The current-voltage characteristics (CVC) at different temperatures, the temperature dependence of electric conductivity [σ(T)] and the currents of thermostimulated depolarization (TSD) have been studied in GaSe <0.05 at.% Co> on a combined basis.The location depth (Et=0.57eV) and the concentration of traps (Nt=2.7x1012cm?3) have been determined from the temperature dependence of the trapping factor. In the course of TSD investigations, levels with location depths of 0.28±0.02 and 0.57±0.03 eV have been revealed. It is noted that traps with the energy of 0.57±0.03 eV are found both with TSD measurements and on the basis of the temperature dependence of the electric conductivity and the trapping factor.It has been established that the hole centres above the valence band are responsible for the CVC, σ(T) and TSD. The location depths, concentrations and trapping cross-sections of these centres have been determined.  相似文献   
115.
Theoretical analysis of the radiation effect on transient behavior of an optoelectronic integrated device composed of a heterojunction phototransistor and a light emitting diode is studied theoretically. First, the transient behavior and the rise time of this device before radiation are investigated based on the frequency response of the constituent devices and the optical feedback inside the device. Second, the effect of neutron irradiation flux on the transient behavior of this device is theoretically studied. The results show that, by increasing the optical feedback inside the device, the rise time in the amplification mode is increased along with an increasing output, while that in the switching mode can be reduced effectively, and the neutron irradiation reduces the transient response and the rise time in both the amplification and switching modes. This type of model can be exploited as optical amplifier, optical switching device, and other applications.  相似文献   
116.
117.
Films of the solid solutions Si1 ? x Sn x (0 ≤ x ≤ 0.04) on Si substrates have been grown by liquid phase epitaxy. The structural features of the films have been investigated using X-ray diffraction. The temperature behavior of current-voltage characteristics and the spectral dependence of the photocurrent for the heterostructures p-Si-n-Si1 ? x Sn x (0 ≤ x ≤ 0.04) have been analyzed. The grown epitaxial films of the solid solutions Si1 ? x Sn x (0 ≤ x ≤ 0.04) have a perfect single-crystal structure with a (111) orientation and a subgrain size of 60 nm. In the epitaxial films at the Si-SiO2 interfaces between silicon subgrains and SiO2 nanocrystals, where there are many sites with a high potential, the Sn ions with a high probability substitute for the Si ions and encourage the formation of Sn nanocrystals with different orientations and, as follows from the analysis of the X-ray diffraction patterns, with different sizes: 8 nm (for the (101) orientation) and 12 nm (for the (200) orientation). The current-voltage characteristics of the heterostructures p-Si-n-Si1 ? x Sn x (0 ≤ x ≤ 0.04) are described by the exponential law J = J 0exp(qV/ckT) at low voltages (V < 0.2 V) and the square law J = (9qμ p τ p μ n N d /8d 3)V 2 at high voltages (V > 1 V). These results have been explained by the drift mechanism of charge carrier transport in the electrical resistance relaxation mode.  相似文献   
118.
The optical absorption coefficient γ of a thin film of a Peierls semiconductor has been calculated in the dipole approximation for direct interband transitions as a function of intensity I and central frequency ω0 of a quasi-monochromatic frequency spectrum near the lower optical transition edge. It is shown that the γ(I, ω0) dependence is strongly nonlinear, and within a certain region of I and ω0 variation, not single-valued, which indicates the onset of resonator-free optical bistability with increasing absorption.  相似文献   
119.
The formation and decay of negative molecular ions of azafullerenes and their hydrogenated derivatives are investigated by mass spectrometry. The mechanisms of resonant electron capture and the lifetimes of negative molecular ions with respect to the electron autodetachment in azafullerene molecules are discussed. A comparative analysis of the data obtained for azafullerenes and hydrogenated fullerene derivatives is carried out.  相似文献   
120.
Nonlinear optical parameters (nonlinear refractive indices and nonlinear absorption coefficients) of solutions of polyvinylpyrrolidone doped by cobalt to various concentrations are measured at the lasing and second-harmonic wavelengths of a picosecond Nd:YAG laser (λ=1064 and 532 nm, respectively, and τ=35 ps). Data on optical limitation in these solutions are presented. The absence of nonlinear absorption in the IR spectral range and its significant effect in the visible range are demonstrated. Optical limitation at a wavelength of 1064 nm is related to defocusing, whereas at 532 nm, this effect is caused by two-photon absorption and partially by inverse saturated absorption and defocusing. Nonlinear optical parameters of metal-polymer complexes are reported.  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