全文获取类型
收费全文 | 904篇 |
免费 | 53篇 |
国内免费 | 7篇 |
专业分类
化学 | 686篇 |
晶体学 | 6篇 |
力学 | 32篇 |
数学 | 60篇 |
物理学 | 180篇 |
出版年
2022年 | 15篇 |
2021年 | 29篇 |
2020年 | 14篇 |
2019年 | 10篇 |
2018年 | 16篇 |
2017年 | 10篇 |
2016年 | 24篇 |
2015年 | 19篇 |
2014年 | 28篇 |
2013年 | 54篇 |
2012年 | 55篇 |
2011年 | 75篇 |
2010年 | 45篇 |
2009年 | 34篇 |
2008年 | 59篇 |
2007年 | 60篇 |
2006年 | 54篇 |
2005年 | 43篇 |
2004年 | 36篇 |
2003年 | 26篇 |
2002年 | 44篇 |
2001年 | 15篇 |
2000年 | 19篇 |
1999年 | 8篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 13篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 10篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 4篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1970年 | 3篇 |
排序方式: 共有964条查询结果,搜索用时 15 毫秒
901.
Two phase-locked pulses are used to coherently excite excitonic polarizations. It is shown that the second pulse can either be strongly amplified by taking up energy gained from the destruction of the exciton polarization or can be decreased drastically by giving up all its energy to excitons. Both the temporal and the spectral signatures of the transmitted pulse shapes agree well with model calculations. 相似文献
902.
Yee Yee Oo K.N. Nagendra G. Ramachandran 《Journal of Quantitative Spectroscopy & Radiative Transfer》2007,108(2):161-179
The polarization of radiation by scattering on an atom embedded in combined external quadrupole electric and uniform magnetic fields is studied theoretically. Limiting cases of scattering under Zeeman effect, and Hanle effect in weak magnetic fields are discussed. The theory is general enough to handle scattering in intermediate magnetic fields (Hanle-Zeeman effect) and for arbitrary orientation of magnetic field. The quadrupolar electric field produces asymmetric line shifts, and causes interesting level-crossing phenomena either in the absence of an ambient magnetic field, or in its presence. It is shown that the quadrupolar electric field produces an additional depolarization in the Q/I profiles and rotation of the plane of polarization in the U/I profile over and above that arising from magnetic field itself. This characteristic may have a diagnostic potential to detect steady-state and time-varying electric fields that surround radiating atoms in solar atmospheric layers. 相似文献
903.
Muhammad Afiq Ngadni Muhammad Tayyab Akhtar Intan Safinar Ismail Anis Irfan Norazhar Soo Yee Lee Maulidiani Maulidiani Khozirah Shaari 《Molecules (Basel, Switzerland)》2021,26(14)
Clitorea ternatea has been used in Ayurvedic medicine as a brain stimulant to treat mental illnesses and mental functional disorders. In this study, the metabolite profiles of crude C. ternatea root extract (CTRE), ethyl acetate (EA), and 50% aqueous methanol (50% MeOH) fractions were investigated using ultrahigh-performance liquid chromatography–diode array detector–tandem mass spectrometry (UHPLC–DAD–MS/MS), while their effect on the stress-like behavior of zebrafish, pharmacologically induced with reserpine, was investigated. A total of 32 compounds were putatively identified, among which, a series of norneolignans, clitorienolactones, and various flavonoids (flavone, flavonol, isoflavone, and isoflavanone) was found to comprise the major constituents, particularly in the EA and 50% MeOH fractions. The clitorienolactones, presently unique to the species, were present in both the free and glycosylated forms in the roots. Both the EA and 50% MeOH fractions displayed moderate effects on the stress-induced zebrafish model, significantly decreasing freezing duration and elevating the total distance travelled and average velocity, 72 h post-treatment. The results of the present study provide further evidence that the basis for the use of C. ternatea roots in traditional medicine to alleviate brain-related conditions, such as stress and depression, is attributable to the presence of clitorienolactones and the isoflavonoidal constituents. 相似文献
904.
Mixtures of linear mixed models (MLMMs) are useful for clustering grouped data and can be estimated by likelihood maximization through the Expectation–Maximization algorithm. A suitable number of components is then determined conventionally by comparing different mixture models using penalized log-likelihood criteria such as Bayesian information criterion. We propose fitting MLMMs with variational methods, which can perform parameter estimation and model selection simultaneously. We describe a variational approximation for MLMMs where the variational lower bound is in closed form, allowing for fast evaluation and develop a novel variational greedy algorithm for model selection and learning of the mixture components. This approach handles algorithm initialization and returns a plausible number of mixture components automatically. In cases of weak identifiability of certain model parameters, we use hierarchical centering to reparameterize the model and show empirically that there is a gain in efficiency in variational algorithms similar to that in Markov chain Monte Carlo (MCMC) algorithms. Related to this, we prove that the approximate rate of convergence of variational algorithms by Gaussian approximation is equal to that of the corresponding Gibbs sampler, which suggests that reparameterizations can lead to improved convergence in variational algorithms just as in MCMC algorithms. Supplementary materials for the article are available online. 相似文献
905.
Kwang‐Choong Kim Mathew C. Schmidt Hitoshi Sato Feng Wu Natalie Fellows Makoto Saito Kenji Fujito James S. Speck Shuji Nakamura Steven P. DenBaars 《固体物理学:研究快报》2007,1(3):125-127
Improved nonpolar m ‐plane light emitting diodes (LEDs) with a thick InGaN multi‐quantum‐well (MQW) structure have been fabricated on low extended defect bulk m ‐plane GaN substrates using metal organic chemical vapor deposition (MOCVD). The peak wavelength of the electroluminescence emission from the packaged LEDs was 402 nm, which is in the blue‐violet region. The output power and EQE were 28 mW and 45.4%, respectively, at a pulsed driving current of 20 mA. With increasing current, the output power increased linearly, and fairly flat EQE was observed with increasing drive current. At 200 mA, the power and EQE were 250 mW and 41%, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
907.
Song JJ Tan Z Xu J Reeves JT Yee NK Ramdas R Gallou F Kuzmich K Delattre L Lee H Feng X Senanayake CH 《The Journal of organic chemistry》2007,72(1):292-294
A practical stereoselective synthesis is reported for an alpha-trifluoromethyl-alpha-alkyl epoxide (1), which is an important pharmaceutical intermediate. The key step involves a chiral auxiliary-controlled asymmetric trifluoromethylation reaction for the introduction of the unique trifluoromethyl-substituted tertiary alcohol stereogenic center in the target molecule. The fluoride-initiated CF3 addition to chiral keto ester 6a proceeded with a diastereoselectivity up to 86:14. The major diastereomer was readily obtained with a >99.5:0.5 dr through a simple crystallization of the crude product mixture. 相似文献
908.
909.
910.
Nicolaou KC Lim YH Piper JL Papageorgiou CD 《Journal of the American Chemical Society》2007,129(13):4001-4013
The total syntheses of 2,2'-epi-cytoskyrin A, rugulosin, and the alleged structure of rugulin are described. These naturally occurring bisanthraquinones and their relatives are characterized by novel molecular architectures at the core, at which lies a more or less complete, cage-like structural motif termed "skyrane". The strategies developed for their total synthesis feature a cascade sequence called the "cytoskyrin cascade" and deliver these molecules in short order and in a stereoselective manner. 相似文献