全文获取类型
收费全文 | 266597篇 |
免费 | 24587篇 |
国内免费 | 17361篇 |
专业分类
化学 | 145274篇 |
晶体学 | 4174篇 |
力学 | 15073篇 |
综合类 | 30篇 |
数学 | 31071篇 |
物理学 | 112923篇 |
出版年
2021年 | 2498篇 |
2020年 | 2791篇 |
2019年 | 3871篇 |
2018年 | 4952篇 |
2017年 | 5614篇 |
2016年 | 6833篇 |
2015年 | 4586篇 |
2014年 | 6361篇 |
2013年 | 12354篇 |
2012年 | 10740篇 |
2011年 | 13275篇 |
2010年 | 14172篇 |
2009年 | 14600篇 |
2008年 | 10659篇 |
2007年 | 10092篇 |
2006年 | 8983篇 |
2005年 | 8562篇 |
2004年 | 9200篇 |
2003年 | 7999篇 |
2002年 | 7839篇 |
2001年 | 8389篇 |
2000年 | 6272篇 |
1999年 | 5504篇 |
1998年 | 4718篇 |
1997年 | 4573篇 |
1996年 | 4664篇 |
1995年 | 4698篇 |
1994年 | 4905篇 |
1993年 | 4918篇 |
1992年 | 4748篇 |
1991年 | 4553篇 |
1990年 | 4114篇 |
1989年 | 4286篇 |
1988年 | 4152篇 |
1987年 | 3533篇 |
1986年 | 3423篇 |
1985年 | 3747篇 |
1984年 | 3918篇 |
1983年 | 2909篇 |
1982年 | 3344篇 |
1981年 | 3173篇 |
1980年 | 2974篇 |
1979年 | 3104篇 |
1978年 | 3109篇 |
1977年 | 3346篇 |
1976年 | 3666篇 |
1975年 | 2798篇 |
1974年 | 2736篇 |
1973年 | 3020篇 |
1972年 | 2495篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
双掺(Tm3+,Tb3+)LiYF4激光器1.5 μm波长激光阈值分析 总被引:1,自引:0,他引:1
由速率方程推出了双掺(Tm^3 ,Tb^3 )离子准四能级系统的激光阈值解析式,讨论了Tm^3 和Tb^3 离子之间的相互作用。分析了1.5μm波长附近的激光阈值和Tm^3 、Tb^3 离子的掺杂原子数分数及晶体长度的关系。结果表明,对于对应Tm^3 离子^3H4→^3F4跃迁的约1.5μm波长的激光,激活离子Tm^3 的掺杂原子数分数过大时,交叉弛豫作用将使系统阈值迅速增加。Tb^3 离子的加入,一方面能抽空激光下能级,起到降低阈值的作用;另一方面亦减少了激光上能级的寿命,使阈值升高。故Tb^3 离子有最佳掺杂原子数分数。对于Tm原子数分数为y=0.01的Tm:LiYF4晶体,Tb^3 离子的最佳掺杂原子数分数为0.002左右,同时表明,激光阈值与晶体长度有关。最佳晶体长度与Tm^3 、Tb^3 离子的掺杂原子数分数以及晶体的衍射损耗和吸收损耗有关。 相似文献
112.
A. N. Grishkov 《Algebra and Logic》1998,37(5):294-306
Ordinary cohomologies in an algebra most often exhibit themselves either as objects that describe extensions (of algebras,
groups, or modules) or as subjects hindering the existence of algebraic constructions. We introduce the notion of nonlinear
cohomologies at a level of cochain complexes, furnish particular examples for Lie algebras, and give an interpretation of
first nonlinear cohomologies in terms of “quasiextensions” of orthogonal modules.
Translated fromAlgebra i Logika, Vol. 37, No. 5, pp. 522–541, September–October, 1998. 相似文献
113.
We investigate theoretically the effects of optical absorption and saturation of the non-linear index of refraction on the
bistability of non-linear distributed feedback structures (NLDFBs). By assuming that the Kerr non-linearity saturates in an
exponential fashion, we obtain for the first time closed-form expressions for the so-called self-phase and cross-phase modulation
terms. Our investigation shows that both absorption and, in particular, saturation significantly affect the bistable properties
of this structure and in many cases eliminate this response completely. In some cases, however, saturation alters the NLDFB's
transfer characteristics in a potentially useful manner. We find that weak levels of saturation may increase the contrast
ratio between the intensities of the high and low bistable states. At increasing levels of saturation, where bistability is
no longer observed, we find regimes where the NLDFB structure could possibly be used for optical amplification.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
114.
E. Jackiewicz M. Lukasiak M. Kopcewicz K. Szpila N. Bakun-Czubarow 《Hyperfine Interactions》1992,70(1-4):993-996
Terrestrial basalt rocks considered to be a good analogues of the Martian regolith were studied by using the Mössbauer spectroscopy and X-ray diffraction. The weathering of basalts was followed by the changes of the Fe2+/Fe3+ ratio. The basalt rock undergoes alteration from primary mineral composition (olivine+pyroxene+plagioclase) through smectite to kaolinite clay with oxides and hydroxides with the increase of the Fe2+/Fe3+ ratio. 相似文献
115.
116.
N. Yu. Netsvetaev 《Journal of Mathematical Sciences》1998,91(6):3460-3468
Some corollaries of the Hirzebruch-Thom signature theorem are discussed. The multiplicativity of the signature and the naturalness
of the Pontryagin classes for coverings in the case of ℚ-homology manifolds is proved. A geometric proof of Hirzebruch’s well-known
“functional equation” for the virtual signature is outlined. Bibliography: 25 titles.
Translated fromZapiski Nauchnykh Seminarov POMI, Vol. 231, 1995, pp. 197–209.
Translated by N. Yu. Netsvetaev. 相似文献
117.
118.
119.
Ya. V. Fattakhov M. F. Galyautdinov T. N. L’vova I. B. Khaibullin 《Technical Physics》1997,42(12):1457-1459
The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface
upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures
is discussed.
Zh. Tekh. Fiz. 67, 97–99 (December 1997) 相似文献
120.
L. K. Aminov I. N. Kurkin D. A. Lukoyanov K. P. Chernov 《Physics of the Solid State》1997,39(8):1184-1186
Results are reported for measurements of the spin-lattice relaxation times of E′1 centers in quartz glass, produced by neutron irradiation, with the measurements made at two frequencies 9.25 and 24.0 GHz
over a wide temperature interval 1.5–300 K. The experimental data are interpreted on the basis of interaction mechanisms of
the spins with two-level systems with excitation energies ∼6, ∼26, and ∼420 cm−1. A small modification of the existing theory allows us to explain a number of features of the observed temperature and frequency
dependence of the relaxation rate. The results are compared with the data available in the literature on spin-lattice relaxation
of irradiation centers in crystalline quartz and quartz glass.
Fiz. Tverd. Tela (St. Petersburg) 39, 1335–1337 (August 1997) 相似文献