全文获取类型
收费全文 | 615篇 |
免费 | 25篇 |
国内免费 | 4篇 |
专业分类
化学 | 394篇 |
晶体学 | 1篇 |
力学 | 10篇 |
数学 | 110篇 |
物理学 | 129篇 |
出版年
2023年 | 9篇 |
2022年 | 6篇 |
2021年 | 7篇 |
2020年 | 13篇 |
2019年 | 14篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 25篇 |
2015年 | 15篇 |
2014年 | 18篇 |
2013年 | 43篇 |
2012年 | 38篇 |
2011年 | 51篇 |
2010年 | 28篇 |
2009年 | 32篇 |
2008年 | 38篇 |
2007年 | 35篇 |
2006年 | 36篇 |
2005年 | 33篇 |
2004年 | 36篇 |
2003年 | 17篇 |
2002年 | 21篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 14篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 7篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有644条查询结果,搜索用时 93 毫秒
41.
Markowicz PP Tiryaki H Pudavar H Prasad PN Lepeshkin NN Boyd RW 《Physical review letters》2004,92(8):083903
We have observed a dramatic enhancement of third-harmonic generation in 3D polystyrene-air photonic crystals pumped by a near infrared laser beam. As the pump wavelength is tuned, the peak of the enhancement occurs when the third-harmonic wavelength approaches the short-wavelength edge of the band gap. We show that the origin of the enhancement is phase matching provided by the periodic structure of the photonic crystals. 相似文献
42.
MntA from Lactobacillus plantarum and copA from Enterococcus hirae both encode membrane proteins that are members of the P-type family of adenosine triphosphatases (ATPases). Both transporters
act as metal importers to take up nutritionally required substrates; MntA translocates Mn(II) and CopA translocates Cu(I).
Both ATPases can also translocate secondary substrates, Cd(II) and Ag(I), respectively. Although functionally and sequentially
similar, these ATPases differ in several key residues and in their membrane topologies. The bioaccumulation properties of
these two proteins were examined by coexpressing the transporters with overexpressed metallothionein in Escherichia coli cells, a system that has previously shown high levels of substrate-specific uptake. Both strains exhibited rapid metal accumulation,
both saturated at around 50 μM metal, and both displayed temperature-sensitive uptake. However, the transporters responded differently when external conditions
were varied; MntA displayed increased sensitivity to ionic strength, while CopA was more pH sensitive and more inhibited by
chelating agents. The differences in accumulation are likely owing to structural differences in the transmembrane region of
these two ATPases. 相似文献
43.
Ultrasonic absorption in polymer gel dosimeters was investigated. An ultrasonic interferometer was used to study the frequency (f) dependence of the absorption coefficient (alpha) in a polyacrylamide gel dosimeter (PAG) in the frequency range 5-20 MHz. The frequency dependence of ultrasonic absorption deviated from that of an ideal viscous fluid. The presence of relaxation mechanisms was evidenced by the frequency dependence of alpha/f(2) and the dispersion in ultrasonic velocity. It was concluded that absorption in polymer gel dosimeters is due to a number of relaxation processes which may include polymer-solvent interactions as well as relaxation due to motion of polymer side groups.The dependence of ultrasonic absorption on absorbed dose and formulation was also investigated in polymer gel dosimeters as a function of pH and chemical composition. Changes in dosimeter pH and chemical composition resulted in a variation in ultrasonic dose response curves. The observed dependence on pH was considered to be due to pH induced modifications in the radiation yield while changes in chemical composition resulted in differences in polymerisation kinetics. 相似文献
44.
We have observed slow light propagation with a group velocity as low as 57.5+/-0.5 m/s at room temperature in a ruby crystal. A quantum coherence effect, coherent population oscillations, produces a very narrow spectral "hole" in the homogeneously broadened absorption profile of ruby. The resulting rapid spectral variation of the refractive index leads to a large value of the group index. We observe slow light propagation both for Gaussian-shaped light pulses and for amplitude modulated optical beams in a system that is much simpler than those previously used for generating slow light. 相似文献
45.
Blindauer CA Polfer NC Keiper SE Harrison MD Robinson NJ Langridge-Smith PR Sadler PJ 《Journal of the American Chemical Society》2003,125(11):3226-3227
Using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, we show that one Zn2+ ion in the Zn4 cluster of cyanobacterial metallothionein is inert to exchange with 67Zn2+. We suggest that this is zinc in site A, which together with the surrounding alpha and beta secondary structure forms a zinc-finger fold. 相似文献
46.
Thanh Nam Nguyen Monique Thual Laurent Provino Nick Traynor 《Optics Communications》2007,278(1):60-65
We present a novel and simple method to measure both the value of the second-order dispersion coefficient and the nonlinear coefficient in optical fibers. This method is based on the higher-order soliton-effect pulse compression phenomenon and is valid for dispersion values greater than 0.5 ps/km/nm. A non-zero dispersion-shifted fiber, a standard single-mode fiber and a highly-dispersive highly-nonlinear fiber have been measured using this method. 相似文献
47.
48.
49.
50.
Maria Rubega Emanuela Formaggio Franco Molteni Eleonora Guanziroli Roberto Di Marco Claudio Baracchini Mario Ermani Nick S. Ward Stefano Masiero Alessandra Del Felice 《Entropy (Basel, Switzerland)》2021,23(5)
Stroke is the commonest cause of disability. Novel treatments require an improved understanding of the underlying mechanisms of recovery. Fractal approaches have demonstrated that a single metric can describe the complexity of seemingly random fluctuations of physiological signals. We hypothesize that fractal algorithms applied to electroencephalographic (EEG) signals may track brain impairment after stroke. Sixteen stroke survivors were studied in the hyperacute (<48 h) and in the acute phase (∼1 week after stroke), and 35 stroke survivors during the early subacute phase (from 8 days to 32 days and after ∼2 months after stroke): We compared resting-state EEG fractal changes using fractal measures (i.e., Higuchi Index, Tortuosity) with 11 healthy controls. Both Higuchi index and Tortuosity values were significantly lower after a stroke throughout the acute and early subacute stage compared to healthy subjects, reflecting a brain activity which is significantly less complex. These indices may be promising metrics to track behavioral changes in the very early stage after stroke. Our findings might contribute to the neurorehabilitation quest in identifying reliable biomarkers for a better tailoring of rehabilitation pathways. 相似文献