全文获取类型
收费全文 | 12731篇 |
免费 | 1716篇 |
国内免费 | 1446篇 |
专业分类
化学 | 8416篇 |
晶体学 | 151篇 |
力学 | 991篇 |
综合类 | 103篇 |
数学 | 1601篇 |
物理学 | 4631篇 |
出版年
2024年 | 36篇 |
2023年 | 189篇 |
2022年 | 319篇 |
2021年 | 362篇 |
2020年 | 410篇 |
2019年 | 370篇 |
2018年 | 333篇 |
2017年 | 321篇 |
2016年 | 495篇 |
2015年 | 487篇 |
2014年 | 609篇 |
2013年 | 846篇 |
2012年 | 935篇 |
2011年 | 999篇 |
2010年 | 680篇 |
2009年 | 750篇 |
2008年 | 778篇 |
2007年 | 739篇 |
2006年 | 701篇 |
2005年 | 643篇 |
2004年 | 509篇 |
2003年 | 412篇 |
2002年 | 354篇 |
2001年 | 337篇 |
2000年 | 299篇 |
1999年 | 342篇 |
1998年 | 340篇 |
1997年 | 277篇 |
1996年 | 273篇 |
1995年 | 235篇 |
1994年 | 249篇 |
1993年 | 164篇 |
1992年 | 155篇 |
1991年 | 132篇 |
1990年 | 144篇 |
1989年 | 84篇 |
1988年 | 84篇 |
1987年 | 88篇 |
1986年 | 83篇 |
1985年 | 72篇 |
1984年 | 49篇 |
1983年 | 34篇 |
1982年 | 35篇 |
1981年 | 27篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
Fractionation of the ethanolic extract of the seeds of Calophyllum inophyllum L. has resulted in the isolation of four novel pyranocoumarin derivatives, designated as inocalophyllins A (1), B (2) and their methyl esters (3, 4) in addition to the known calophyllolide. The structures of these compounds have been determined on the basis of spectroscopic analysis including MS, heteronuclear single quantum coherence (HSQC), heteronuclear multiple bond connectivity (HMBC) and two dimensional incredible natural abundance double quantum transfer experiment (2D-INADEQUATE). Two new methylated products, 5 and 6 were also prepared by methylation of compounds 1 and 2, respectively. 相似文献
982.
Chen Cs Ran S Liu W Yang Ph Peng Dk Bouwmeester HJ 《Angewandte Chemie (International ed. in English)》2001,40(4):784-786
983.
: Multiple electron transfer processes are studied for Arq+n- Ne (q= 8, 9, 11, 12) collisions by using multi-parameter coincidence techniques. Various electron transfer processes are identified experimentally and the related cross-sections are measured. The dependence of transfer ionization cross-sections on the recoil charge states is compared with the results from the modified molecular classical overbarrier model. It is found that the modified model described the experimental results reasonably. 相似文献
984.
We make a conjecture that the number of isolated local minimum points of a 2n-degree or (2n+1)-degree r-variable polynomial is not greater than n
r when n 2. We show that this conjecture is the minimal estimate, and is true in several cases. In particular, we show that a cubic polynomial of r variables may have at most one local minimum point though it may have 2r critical points. We then study the global minimization problem of an even-degree multivariate polynomial whose leading order coefficient tensor is positive definite. We call such a multivariate polynomial a normal multivariate polynomial. By giving a one-variable polynomial majored below a normal multivariate polynomial, we show the existence of a global minimum of a normal multivariate polynomial, and give an upper bound of the norm of the global minimum and a lower bound of the global minimization value. We show that the quartic multivariate polynomial arising from broad-band antenna array signal processing, is a normal polynomial, and give a computable upper bound of the norm of the global minimum and a computable lower bound of the global minimization value of this normal quartic multivariate polynomial. We give some sufficient and necessary conditions for an even order tensor to be positive definite. Several challenging questions remain open. 相似文献
985.
The Spherical Code (SC) problem has many important applications in such fields as physics, molecular biology, signal transmission, chemistry, engineering and mathematics. This paper presents a bilevel optimization formulation of the SC problem. Based on this formulation, the concept of balanced spherical code is introduced and a new approach, the Point Balance Algorithm (PBA), is presented to search for a 1-balanced spherical code. Since an optimal solution of the SC problem (an extremal spherical code) must be a 1-balanced spherical code, PBA can be applied easily to search for an extremal spherical code. In addition, given a certain criterion, PBA can generate efficiently an approximate optimal spherical code on a sphere in the n-dimensional space
n. Some implementation issues of PBA are discussed and putative global optimal solutions of the Fekete problem in 3, 4 and 5-dimensional space are also reported. Finally, an open question about the geometry of Fekete points on the unit sphere in the 3-dimensional space is posed. 相似文献
986.
Collection-mode near-field scanning optical microscopy (NSOM) is used to map nanoscopic second-harmonic generation (SHG) in N -(4-nitrophenyl)- (L) -prolinol crystals. A spatial resolution of 98 nm is achieved. Near-field polarization-dependent SHG measurement is performed, and a local effective SHG susceptibility of 224+/-18 pm/V is obtained. 相似文献
987.
Liu Qing Cao Zhuang Qi Shen Qishun Dou Xiaoming Chen Yingli 《Optical and Quantum Electronics》2001,33(6):675-683
In this paper, we propose a novel analytic transfer matrix method (ATMM) to the analysis of planar waveguides with index discontinuity or index slope discontinuity, for the cases when the conventional WKB method is no longer valid. We also analyze the physical insight of the approximations in WKB theory, and according to our research, the phase shift at the turning point is not /2, but exactly . Test calculations are done for an index profile with a known solution and the comparison shows that our method gives extremely accurate propagation constant. 相似文献
988.
Armitage NP Lu DH Feng DL Kim C Damascelli A Shen KM Ronning F Shen ZX Onose Y Taguchi Y Tokura Y 《Physical review letters》2001,86(6):1126-1129
We have performed angle resolved photoelectron spectroscopy on the electron doped cuprate superconductor Nd1.85Ce0.15CuO4. A comparison of the leading edge midpoints between the superconducting and normal states reveals a small, but finite shift of 1.5-2 meV near the ( pi,0) position, but no observable shift along the zone diagonal near ( pi/2, pi/2). This is interpreted as evidence for an anisotropic superconducting gap in the electron doped materials, which is consistent with the presence of d-wave superconducting order in this cuprate superconductor. 相似文献
989.
Fast orientational motion of molecules at a surface can affect the sum-frequency vibrational spectra of the surface. Calculation shows that the effect is significant if the molecular orientation varies over a broad range within the vibrational relaxation time. The stretch vibration of the free OH bonds at the vapor/water interface is used to illustrate the importance of the effect. 相似文献
990.
Highly emissive Re(I) complexes, hydrophobic cis-Re(CO)2(c-dppene)(dpphen) and water-soluble cis-Re(CO)2(c-dppene)(SO3-dpphen) with a negative charge, were synthesized [where c-dppene is cis-(bis(diphenylphosphino)ethylene, dpphen is 4,7-diphenyl-1,10-phenanthroline, and SO3-dpphen is its disulfonate derivative]. These Re(I) complexes have significantly higher molar absorption in the 350- to 490-nm region compared to their tricarbonyl Re(I) counterparts and their emission spectra are similar to those of Ru(II) polypyridyl complexes. The luminescence lifetimes of these Re(I) complexes approach 10 s in the absence of oxygen in both aqueous and less polar solvents. The complexes have limiting anisotropy values exceeding 0.3. As both ligands, the diimine and the bidentate phosphine, can be modified by adding different substituents, it should be possible to make cis-Re(CO)2(diimine)(P-P) derivatives which are capable of labeling biomacromolecules for biochemical and biophysical studies. 相似文献