全文获取类型
收费全文 | 3907篇 |
免费 | 65篇 |
国内免费 | 44篇 |
专业分类
化学 | 2777篇 |
晶体学 | 25篇 |
力学 | 55篇 |
数学 | 672篇 |
物理学 | 487篇 |
出版年
2022年 | 18篇 |
2021年 | 34篇 |
2020年 | 21篇 |
2019年 | 45篇 |
2018年 | 23篇 |
2017年 | 32篇 |
2016年 | 73篇 |
2015年 | 52篇 |
2014年 | 61篇 |
2013年 | 192篇 |
2012年 | 176篇 |
2011年 | 203篇 |
2010年 | 102篇 |
2009年 | 81篇 |
2008年 | 244篇 |
2007年 | 263篇 |
2006年 | 271篇 |
2005年 | 271篇 |
2004年 | 209篇 |
2003年 | 146篇 |
2002年 | 143篇 |
2001年 | 55篇 |
2000年 | 61篇 |
1999年 | 53篇 |
1998年 | 39篇 |
1997年 | 47篇 |
1996年 | 73篇 |
1995年 | 49篇 |
1994年 | 53篇 |
1993年 | 39篇 |
1992年 | 40篇 |
1991年 | 30篇 |
1990年 | 34篇 |
1989年 | 26篇 |
1988年 | 32篇 |
1987年 | 35篇 |
1986年 | 47篇 |
1985年 | 60篇 |
1984年 | 64篇 |
1983年 | 52篇 |
1982年 | 43篇 |
1981年 | 47篇 |
1980年 | 54篇 |
1979年 | 33篇 |
1978年 | 47篇 |
1977年 | 37篇 |
1976年 | 31篇 |
1975年 | 27篇 |
1974年 | 25篇 |
1973年 | 33篇 |
排序方式: 共有4016条查询结果,搜索用时 0 毫秒
31.
Morvan Salez Pascal Febvre William R. Mcgrath Bruce Bumble Henry G. Leduc 《International Journal of Infrared and Millimeter Waves》1994,15(2):349-368
A waveguide SIS heterodyne receiver using a Nb/AlOX/Nb junction has been built for astronomical observations of molecular transitions in the frequency range 600 GHz - 635 GHz, and has been successfully used at the Caltech Submillimeter Observatory (CSO). We report double sideband (DSB) receiver noise temperatures as low as 245 K at 600 GHz -610 GHz, and near 300 K over the rest of the bandwidth. These results confirm that SIS quasiparticle mixers work well at submillimeter-wave frequencies corresponding to photon energies of at least 90% of the superconductor energy gap. In addition, we have systematically investigated the effect on the receiver performance of the overlap between first-order and second-order photon steps of opposite sign at these frequencies. The receiver noise increases by as much as 40% in the region of overlap. We infer potential limitations for operating submillimeter-wave Nb/AlOx/Nb mixers. 相似文献
32.
We derive the Kramers equation, namely, the Fokker-Planck equation for an oscillator, from a completely deterministic picture. The oscillator is coupled to a “booster”, i.e., a deterministic system in a fully chaotic state, wherein diffusion is derived from the sensitive dependence of chaos on initial conditions and friction is a consequence of the linear response of the booster to the action exerted on it by the oscillator. To deal with the Hamiltonian nature of the system of interest and of its coupling to the booster, we extend the earlier theoretical derivation of macroscopic transport coefficients from deterministic dynamics. We show that the frequency of the oscillator can be tuned to the microscopic frequencies of the booster without affecting the canonical nature of the “macroscopic” statistics. The theoretical predictions are supported by numerical simulations. 相似文献
33.
Multicuts in unweighted graphs and digraphs with bounded degree and bounded tree-width 总被引:2,自引:0,他引:2
Gruia C
linescu Cristina G. Fernandes Bruce Reed 《Journal of Algorithms in Cognition, Informatics and Logic》2003,48(2):333-359
The Multicut problem can be defined as: given a graph G and a collection of pairs of distinct vertices {si,ti} of G, find a minimum set of edges of G whose removal disconnects each si from the corresponding ti. Multicut is known to be NP-hard and Max SNP-hard even when the input graph is restricted to being a tree. The main result of the paper is a polynomial-time approximation scheme (PTAS) for Multicut in unweighted graphs with bounded degree and bounded tree-width. That is, for any ε>0, we present a polynomial-time (1+ε)-approximation algorithm. In the particular case when the input is a bounded-degree tree, we have a linear-time implementation of the algorithm. We also provide some hardness results: we prove that Multicut is still NP-hard for binary trees and that it is Max SNP-hard if we drop any of the three conditions (unweighted, bounded-degree, bounded tree-width). Finally we show that some of these results extend to the vertex version of Multicut and to a directed version of Multicut. 相似文献
34.
The fluorescence dynamics parameters of the fluorescent transient flavin-luciferase species from the typesVibrio fischeri andPhotobacterium leiognathi are presented. The fluorescence anisotropy decay is a single exponential function for both types. The correlation time is 70 ns for theP. leiognathi fluorescent transient intermediate (2°C, aqueous buffer, pH 7.0), consistent with the rotational correlation time of the luciferase macromolecule (77 kD) to which the flavin fluorophore is rigidly attached. In contrast, for theV. fischeri species the observed correlation time for the anisotropy decay function is 133 ns. This suggests that protein self-association occurs in theV. fischeri case and this is confirmed by filtration, where the fluorescent transient fromV. fischeri does not pass through a 100,000 molecular weight cutoff membrane, whereas theP. leiognathi species does. The filtration method also demonstrates self-association in the luciferase peroxyflavin and photoflavin fromV. fischeri. A monomer-dimer equilibrium also explains the previously reported high correlation times for theV. harveyi luciferase-flavin species. It is proposed that the self-association competes with the lumazine protein interaction in the bioluminescence reaction. 相似文献
35.
36.
Nasibulin Albert G. Kauppinen Esko I. Thomson Bruce A. Fernandez de la Mora J. 《Journal of nanoparticle research》2002,4(5):449-453
Polyethylene glycol (PEG) molecules with masses below 1300amu are electrosprayed (ES) from solution, mobility-selected at high resolution in a differential mobility analyzer (DMA), collected on a grid and imaged by transmission electron microscopy (ES–DMA–TEM). The DMA resolves individual n-mers, and selects only one out of the many present in the original sample. Ion identity is established from parallel mass spectra (ES-MS). The images reveal spherical particles 1.46nm in diameter, in good agreement with the known ion mass and bulk density. The DMA-selection technique opens new paths for the study of very small particles. 相似文献
37.
38.
The crystal structures of the high-temperature modifications of sodium and silver orthophosphates have been determined using powder neutron diffraction (PND) data. II-Na3PO4 adopts the space group Fm3m with
at 400°C. The PO3−4 group is centered around the origin, but it shows high orientational disorder. The sodium ions occupy the
and
sites. II-Ag3PO4, at 650°C, is similar with
. The structure of I-Ag3PO4 at room temperature
has been re-examined by single-crystal X-ray diffraction. The derived model, with R=0.019 for 116 independent reflections, is in agreement with the latest work reported in the literature. The structure of I-Ag3PO4 at 375°C, as determined by PND, has
, and displays no gross modifications from that observed at 25°C, although the anisotropic nature of the silver sites is markedly more pronounced at this higher temperature. The cation mobility is discussed in relation to the high-temperatures structures. 相似文献
Full-size image (<1K)
39.
Fractional-order calculus is about the differentiation and integration of non-integer orders. Fractional calculus (FC) is based on fractional-order thinking (FOT) and has been shown to help us to understand complex systems better, improve the processing of complex signals, enhance the control of complex systems, increase the performance of optimization, and even extend the enabling of the potential for creativity. In this article, the authors discuss the fractional dynamics, FOT and rich fractional stochastic models. First, the use of fractional dynamics in big data analytics for quantifying big data variability stemming from the generation of complex systems is justified. Second, we show why fractional dynamics is needed in machine learning and optimal randomness when asking: “is there a more optimal way to optimize?”. Third, an optimal randomness case study for a stochastic configuration network (SCN) machine-learning method with heavy-tailed distributions is discussed. Finally, views on big data and (physics-informed) machine learning with fractional dynamics for future research are presented with concluding remarks. 相似文献
40.
Katherine A. Koenig Ken E. Sakaie Mark J. Lowe Jian Lin Lael Stone Robert A. Bermel Erik B. Beall Stephen M. Rao Bruce D. Trapp Micheal D. Phillips 《Magnetic resonance imaging》2013