全文获取类型
收费全文 | 283709篇 |
免费 | 3183篇 |
国内免费 | 1427篇 |
专业分类
化学 | 155244篇 |
晶体学 | 4199篇 |
力学 | 11849篇 |
综合类 | 61篇 |
数学 | 31551篇 |
物理学 | 85415篇 |
出版年
2020年 | 2569篇 |
2019年 | 2804篇 |
2018年 | 3790篇 |
2017年 | 3908篇 |
2016年 | 5407篇 |
2015年 | 3277篇 |
2014年 | 4943篇 |
2013年 | 11595篇 |
2012年 | 10042篇 |
2011年 | 12030篇 |
2010年 | 8759篇 |
2009年 | 8576篇 |
2008年 | 11182篇 |
2007年 | 11183篇 |
2006年 | 10583篇 |
2005年 | 9595篇 |
2004年 | 8652篇 |
2003年 | 7692篇 |
2002年 | 7689篇 |
2001年 | 8493篇 |
2000年 | 6340篇 |
1999年 | 4694篇 |
1998年 | 3973篇 |
1997年 | 3872篇 |
1996年 | 3744篇 |
1995年 | 3295篇 |
1994年 | 3385篇 |
1993年 | 3246篇 |
1992年 | 3534篇 |
1991年 | 3609篇 |
1990年 | 3423篇 |
1989年 | 3320篇 |
1988年 | 3188篇 |
1987年 | 3089篇 |
1986年 | 3088篇 |
1985年 | 4010篇 |
1984年 | 4123篇 |
1983年 | 3481篇 |
1982年 | 3734篇 |
1981年 | 3397篇 |
1980年 | 3185篇 |
1979年 | 3429篇 |
1978年 | 3734篇 |
1977年 | 3791篇 |
1976年 | 3719篇 |
1975年 | 3403篇 |
1974年 | 3561篇 |
1973年 | 3575篇 |
1972年 | 2776篇 |
1971年 | 2258篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
M. Brunken L. Casper H. Genz C. Hessler S. Khodyachykh A. Richter 《Optics & Laser Technology》2003,35(5):349-353
A compact autocorrelator suitable for the measurement of the optical pulse length of the infrared free electron laser (IR-FEL) at the S-DALINAC is described. It allows to investigate the energy transfer process from the electrons into the laser light within the undulator of the FEL. The autocorrelator utilizes the second harmonic generation (SHG) and it provides the possibility of a background-free measurement at wavelengths between 4 and 10 μm. Simulations for three different crystals used for SHG show that the largest efficiency for SHG and the best temporal resolution is achieved with a ZnGeP2 crystal at a length of 2 mm. A test experiment at the free electron laser for infrared experiments confirmed the expected high efficiency of about 7.5% of SHG for the ZnGeP2 crystal as well as the easy handling and fast adjustment of the system. 相似文献
992.
H. J. Munkholm obtained a generalization for topological manifolds of the famous Borsuk–Ulam type theorem proved by Conner and Floyd. The purpose of this paper is to prove a version of Conner and Floyd's theorem for generalized manifolds. 相似文献
993.
Tracks made in minerals by the electronic stopping of uranium fission fragments provide a modern geological dating tool, and are believed also to yield specific information on the low-temperature thermal history of rocks. Experimental work showing that the damaged crystal lattice along a fission track recovers primarily as a function of temperature ignored the fact that the basic theory of atomic diffusion requires an exponential decrease in the intrinsic diffusion coefficient with increasing pressure. Here, fission track recovery was experimentally investigated in basic apatite under the simultaneous influences of temperature, pressure and stress. We show that track fading is a complex recovery mechanism responding to several environmental physical parameters simultaneously. In particular a strong decrease in the track recovery rate was observed as a function of increasing pressure. And a nearly temperature-independent recovery was observed in samples under stress. 相似文献
994.
O. A. Anikeenok 《Physics of the Solid State》2003,45(5):854-858
In solid-state physics, a crystal lattice is frequently approximated by an array of interacting ions. In this case, the wave functions of individual ions are assumed to be a fairly good zeroth approximation in calculating the matrix elements of the interaction Hamiltonian of electrons and nuclei of the lattice from first principles. Use of the second-quantization method is proposed for such calculations in the basis of these functions. As an example, the electron transition amplitude from a ligand to the central ion is estimated. The results agree well with the experimental data. 相似文献
995.
996.
For any self-adjoint realization S of a singular Sturm-Liouville equation on an interval (a,b) with limit-circle endpoints, we construct a family of self-adjoint realizations S r ,r ∈ (0,∞), of this equation on subintervals (a r ,b r ) of (a,b) such that every eigenvalue of S is the limit of a continuous eigenvalue branch of this family. Of particular interest are the cases when at least one endpoint is oscillatory or the leading coefficient function changes sign. In these cases, we show that the index determining each continuous eigenvalue branch has an infinite number of jump discontinuities and give an explicit characterization of these discontinuities. 相似文献
997.
Arthur Maki Robert L. Sams Jeffrey Barber Engelene t.H. Chrysostom Alfons Weber 《Journal of Molecular Spectroscopy》2004,225(2):109-122
Several new infrared absorption bands for 32S16O3 have been measured and analyzed. The principal bands observed were ν1+ν2 (at 1561 cm−1), ν1+ν4 (at 1594 cm−1), ν3+ν4 (at 1918 cm−1), and 3ν3 (at 4136 cm−1). Except for 3ν3, these bands are very complicated because of (a) the Coriolis coupling between ν2 and ν4, (b) the Fermi resonance between ν1 and 2ν4, (c) the Fermi resonance between ν1 and 2ν2, (d) ordinary l-type resonance that couples levels that differ by 2 in both the k and l quantum numbers, and (e) the vibrational l-type resonance between the A1′ and A2′ levels of ν3+ν4. The unraveling of the complex pattern of these bands was facilitated by a systematic approach to the understanding of the various interactions. Fortunately, previous work on the fundamentals permitted good estimates of many constants necessary to begin the assignments and the fit of the measurements. In addition, the use of hot band transitions accompanying the ν3 band was an essential aid in fitting the ν3+ν4 transitions since these could be directly observed for only one of four interacting states. From the hot band analysis we find that the A1′ vibrational level is 3.50 cm−1 above the A2′ level, i.e., r34=1.75236(7) cm−1. In the case of the 3ν3 band, the spectral analysis is straightforward and a weak Δk=±2, Δl3=±2 interaction between the l3=1 and l3=3 substates locates the latter A1′ and A2′ “ghost” states 22.55(4) cm−1 higher than the infrared accessible l3=1 E′ state. 相似文献
998.
We develop two implementable algorithms, the first for the solution of finite and the second for the solution of semi-infinite min-max-min problems. A smoothing technique (together with discretization for the semi-infinite case) is used to construct a sequence of approximating finite min-max problems, which are solved with increasing precision. The smoothing and discretization approximations are initially coarse, but are made progressively finer as the number of iterations is increased. This reduces the potential ill-conditioning due to high smoothing precision parameter values and computational cost due to high levels of discretization. The behavior of the algorithms is illustrated with three semi-infinite numerical examples. 相似文献
999.
1000.
Cleavage near microhardness indentations on the surface of corundum is found to appear long (20–30 days) after unloading. 相似文献