全文获取类型
收费全文 | 524831篇 |
免费 | 4126篇 |
国内免费 | 1409篇 |
专业分类
化学 | 223571篇 |
晶体学 | 7470篇 |
力学 | 25262篇 |
综合类 | 11篇 |
数学 | 66346篇 |
物理学 | 152157篇 |
无线电 | 55549篇 |
出版年
2020年 | 3883篇 |
2019年 | 4618篇 |
2018年 | 13155篇 |
2017年 | 13629篇 |
2016年 | 11804篇 |
2015年 | 5185篇 |
2014年 | 8231篇 |
2013年 | 18411篇 |
2012年 | 17343篇 |
2011年 | 26722篇 |
2010年 | 18721篇 |
2009年 | 19394篇 |
2008年 | 24984篇 |
2007年 | 28211篇 |
2006年 | 15879篇 |
2005年 | 19215篇 |
2004年 | 16090篇 |
2003年 | 15048篇 |
2002年 | 13212篇 |
2001年 | 13583篇 |
2000年 | 10904篇 |
1999年 | 8522篇 |
1998年 | 7399篇 |
1997年 | 7348篇 |
1996年 | 6933篇 |
1995年 | 6344篇 |
1994年 | 6300篇 |
1993年 | 6206篇 |
1992年 | 6544篇 |
1991年 | 6735篇 |
1990年 | 6423篇 |
1989年 | 6320篇 |
1988年 | 6166篇 |
1987年 | 5612篇 |
1986年 | 5414篇 |
1985年 | 6953篇 |
1984年 | 7225篇 |
1983年 | 5894篇 |
1982年 | 5991篇 |
1981年 | 5869篇 |
1980年 | 5611篇 |
1979年 | 5930篇 |
1978年 | 5988篇 |
1977年 | 5987篇 |
1976年 | 6079篇 |
1975年 | 5582篇 |
1974年 | 5532篇 |
1973年 | 5730篇 |
1972年 | 4166篇 |
1971年 | 3623篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
191.
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
192.
A modification to the `shape-invariant' sinusoidal speech model is proposed, whereby the phases of the component sinewaves used for the excitation are made to add coherently at each glottal closure. Applied to pitch and time-scale modification, higher quality synthetic speech is produced when large changes are required 相似文献
193.
194.
Based on a variational analysis, the authors demonstrate that the noise-induced Gordon-Haus timing jitter in a dispersion-managed soliton transmission system can be substantially reduced by appropriate placement of the amplifiers 相似文献
195.
P. Seelig A. Dax S. Faber M. Gerlach G. Huber T. Kühl D. Marx P. Merz W. Quint F. Schmitt H. Winter M. Würtz 《Hyperfine Interactions》1998,114(1-4):135-139
The investigation of the 1s HFS provides a good possibility for testing QED effects in a combination of a strong electric
and magnetic field. Here, we report about the laserspectroscopic measurements of the ground state hyperfine splitting in 207Pb81+. To handle this M1-transition in the infrared optical regime with its long lifetime, we developed a new detection technique
using a bunched ion beam. For the observation of fluorescence light, a new mirror system is adapted to the emission characteristics
from an ion beam at relativistic velocities.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
196.
Presents the Satellite Communications Network Expert (SaNE), a knowledge-based aid designed to provide engineering support for fault diagnosis. The SaNE is composed of two elements: a satcom network model, which simulates the structure and functionality of a system based loosely upon a large military satcom network, and a diagnostic component, which uses knowledge- and model-based reasoning techniques to analyse system anomalies and diagnose possible causes for the alarms such networks generate. The development cycle is described, emphasising lessons learnt during development and testing and the advantages and disadvantages of the techniques applied. The goal of the SaNE project is primarily commercial acceptance rather than innovation. The authors illustrate how novel concepts can be implemented in a practical system without compromising this goal 相似文献
197.
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
198.
C. Waldtmann H.-U. Everts B. Bernu C. Lhuillier P. Sindzingre P. Lecheminant L. Pierre 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):501-507
We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagomé lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis
of Néel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather
unusual. The magnetic () excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic () excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless
continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these
states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg
antiferromagnet on the kagomé lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably
they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets.
Received: 27 November 1997 / Accepted: 29 January 1998 相似文献
199.
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
200.
Kringlebotn J.T. Morkel P.R. Reekie L. Archambault J.-L. Payne D.N. 《Photonics Technology Letters, IEEE》1993,5(10):1162-1164
We report a 7.6-mW single-frequency fiber laser operating at 1545 nm, using for the first time an Er3+:Yb3+ doped fiber and a fiber grating output coupler. The laser did not exhibit self-pulsation, which is a typical problem in short three-level fiber lasers, and had a relative intensity noise (RIN) level below -145.5 dB/Hz at frequencies above 10 MHz. The linewidth of the laser was limited by the relaxation oscillation sidebands in the optical spectrum and was typically less than 1 MHz 相似文献