全文获取类型
收费全文 | 326582篇 |
免费 | 4857篇 |
国内免费 | 1204篇 |
专业分类
化学 | 178925篇 |
晶体学 | 4530篇 |
力学 | 13409篇 |
综合类 | 7篇 |
数学 | 38781篇 |
物理学 | 96991篇 |
出版年
2021年 | 1958篇 |
2020年 | 2309篇 |
2019年 | 2274篇 |
2018年 | 2173篇 |
2017年 | 2060篇 |
2016年 | 4363篇 |
2015年 | 3636篇 |
2014年 | 4886篇 |
2013年 | 14729篇 |
2012年 | 11612篇 |
2011年 | 14395篇 |
2010年 | 8822篇 |
2009年 | 8738篇 |
2008年 | 13128篇 |
2007年 | 13326篇 |
2006年 | 12971篇 |
2005年 | 11838篇 |
2004年 | 10607篇 |
2003年 | 9241篇 |
2002年 | 9110篇 |
2001年 | 10242篇 |
2000年 | 7839篇 |
1999年 | 6241篇 |
1998年 | 4994篇 |
1997年 | 4843篇 |
1996年 | 4907篇 |
1995年 | 4447篇 |
1994年 | 4208篇 |
1993年 | 4038篇 |
1992年 | 4506篇 |
1991年 | 4374篇 |
1990年 | 4073篇 |
1989年 | 3889篇 |
1988年 | 4193篇 |
1987年 | 3851篇 |
1986年 | 3735篇 |
1985年 | 5408篇 |
1984年 | 5485篇 |
1983年 | 4429篇 |
1982年 | 4856篇 |
1981年 | 4901篇 |
1980年 | 4632篇 |
1979年 | 4736篇 |
1978年 | 4758篇 |
1977年 | 4722篇 |
1976年 | 4653篇 |
1975年 | 4564篇 |
1974年 | 4404篇 |
1973年 | 4575篇 |
1972年 | 2590篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
971.
Partially supported by NSF Grant DMS-9004123 and ARO through MSI Cornell (DAAG 29-85-C-0018) 相似文献
972.
D. Guidotti J. G. Wilman A. J. Ricci 《Applied Physics A: Materials Science & Processing》1992,54(6):570-573
An instrument is described that can be used to monitor, with unprecedented sensitivity, changes in the optical reflectivity due to crystailine damage incurred during ion implantation. It is shown that at the shot-noise limit, changes in the optical reflectivity of silicon as small as 5·10–7 can be measured in a 10 Hz bandwidth with a signal-to-noise ratio of 100, corresponding to an extrapolated uniform implantation dose of 5·108 cm–2 for 11B+ at 50 keV in silicon. 相似文献
973.
E. Kapon M. Walther J. Christen M. Grundmann C. Caneau D.M. Hwang E. Colas R. Bhat G.H. Song D. Bimberg 《Superlattices and Microstructures》1992,12(4)
Quantum wire (QWR) heterostructures suitable for optoelectronic applications should meet a number of requirements, including defect free interfaces, large subband separation, long carrier lifetime, efficient carrier capture. The structural and opticl properties of GaAs/AlGaAs and InGaAs/GaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on nonplanr substrates, which satisfy many of these criteria, are described. These crescent-shaped QWRs are formed in situ during epitaxial growth resulting in virtually defect free interfaces. Effective wire widths as small as 10nm have been achieved, corresponding to electron subband separations greater than KBT at room temperature. The enhanced density of states at the QWR subbands manifests itself in higher optical absorption and emission as visualized in photoluminescence (PL), PL excitation, amplified spontaneous emission and lasing spectra of these structures. Effective carrier capture into the wires via connected quantum well regions, which is important for enhancing the otherwise extremely small capture cross section of these wires, has also been observed. Room temperature operation of GaAs/AlGaAs and strained InGaAs/GaAs QWR lasers with threshold currents as low as 0.6mA has been demonstrated. 相似文献
974.
Zboril R. Mashlan M. Machala L. Walla J. Barcova K. Martinec P. 《Hyperfine Interactions》2004,156(1-4):403-410
Hyperfine Interactions - The natural garnets from almandine (Fe3Al2Si3O12)–pyrope (Mg3Al2Si3O12) series with the iron to magnesium atomic ratio ranging from 0.2 to 1 were characterised and... 相似文献
975.
R. Kudrawiec M. Syperek J. Misiewicz R. Paszkiewicz B. Paszkiewicz M. Tlaczala 《Superlattices and Microstructures》2004,36(4-6):633
Undoped AlGaN/GaN heterostructures with different content and thickness of AlGaN layer are investigated by photoreflectance (PR) spectroscopy. We have observed PR resonances related to an absorption in both GaN and AlGaN layers. The character of these resonances has been analyzed, and PR lines associated with excitonic and band-to-band absorption in the GaN layer and band-to-band absorption in the AlGaN layer have been identified. The transition related to band-to-band absorption possesses characteristic Franz–Keldysh oscillations (FKOs) associated with a built-in electric field. The electric field in the AlGaN layer obtained on the basis of the analysis of FKOs has been found to be in the range of 244–341 kV/cm. The value of the field has been found to decrease with the increase in AlGaN thickness and to increase with the increase in Al concentration. The surface potential for AlGaN layers has been found to increase with the increase in Al mole fraction and has been estimated to be in the range of 1.0–1.7 eV. 相似文献
976.
A robust and efficient adaptive reweighted estimator of multivariate location and scatter 总被引:1,自引:0,他引:1
Daniel Gervini 《Journal of multivariate analysis》2003,84(1):116-144
This article proposes a reweighted estimator of multivariate location and scatter, with weights adaptively computed from the data. Its breakdown point and asymptotic behavior under elliptical distributions are established. This adaptive estimator is able to attain simultaneously the maximum possible breakdown point for affine equivariant estimators and full asymptotic efficiency at the multivariate normal distribution. For the special case of hard-rejection weights and the MCD as initial estimator, it is shown to be more efficient than its non-adaptive counterpart for a broad range of heavy-tailed elliptical distributions. A Monte Carlo study shows that the adaptive estimator is as robust as its non-adaptive relative for several types of bias-inducing contaminations, while it is remarkably more efficient under normality for sample sizes as small as 200. 相似文献
977.
Design theory crosses the boundary between mathematics and statistics, and includes a wide range of disparate types of design. In this paper we present a classification scheme which aims to include as many important types as possible, based on a balance among concept, representation and use. 相似文献
978.
979.
A. Heidenreich I. Schek D. Scharf J. Jortner 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,20(1):227-229
The time resolved dynamics of diffusionless cube → ring isomerization of the Na4Cl4 cluster was interrogated by constant energy molecular dynamics simulations, utilizing the first passage time method. The nonreactive isomerization induced by nonselective vibrational excitation is well accounted for in terms of the statistical RRK theory, opening avenues for experimental exploration of time-resolved cluster isomerization dynamics. 相似文献
980.