全文获取类型
收费全文 | 87440篇 |
免费 | 496篇 |
国内免费 | 501篇 |
专业分类
化学 | 30052篇 |
晶体学 | 873篇 |
力学 | 6936篇 |
数学 | 33874篇 |
物理学 | 16702篇 |
出版年
2021年 | 67篇 |
2020年 | 90篇 |
2019年 | 80篇 |
2018年 | 10482篇 |
2017年 | 10304篇 |
2016年 | 6194篇 |
2015年 | 965篇 |
2014年 | 440篇 |
2013年 | 801篇 |
2012年 | 4160篇 |
2011年 | 10920篇 |
2010年 | 5894篇 |
2009年 | 6263篇 |
2008年 | 6983篇 |
2007年 | 9170篇 |
2006年 | 706篇 |
2005年 | 1762篇 |
2004年 | 1939篇 |
2003年 | 2325篇 |
2002年 | 1387篇 |
2001年 | 407篇 |
2000年 | 434篇 |
1999年 | 292篇 |
1998年 | 332篇 |
1997年 | 287篇 |
1996年 | 385篇 |
1995年 | 240篇 |
1994年 | 203篇 |
1993年 | 213篇 |
1992年 | 184篇 |
1991年 | 167篇 |
1990年 | 158篇 |
1989年 | 161篇 |
1988年 | 142篇 |
1987年 | 149篇 |
1986年 | 142篇 |
1985年 | 198篇 |
1984年 | 198篇 |
1983年 | 148篇 |
1982年 | 172篇 |
1981年 | 186篇 |
1980年 | 216篇 |
1979年 | 174篇 |
1978年 | 181篇 |
1977年 | 135篇 |
1976年 | 139篇 |
1975年 | 147篇 |
1974年 | 123篇 |
1973年 | 155篇 |
1972年 | 80篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Suppose that is a one-dimensional Brownian motion with negative drift . It is possible to make sense of conditioning this process to be in the state 0 at an independent exponential random time and if we kill the conditioned process at the exponential time the resulting process is Markov. If we let the rate parameter of the random time go to 0, then the limit of the killed Markov process evolves like conditioned to hit 0, after which time it behaves as killed at the last time visits 0. Equivalently, the limit process has the dynamics of the killed “bang–bang” Brownian motion that evolves like Brownian motion with positive drift when it is negative, like Brownian motion with negative drift when it is positive, and is killed according to the local time spent at 0.An extension of this result holds in great generality for a Borel right process conditioned to be in some state at an exponential random time, at which time it is killed. Our proofs involve understanding the Campbell measures associated with local times, the use of excursion theory, and the development of a suitable analogue of the “bang–bang” construction for a general Markov process.As examples, we consider the special case when the transient Borel right process is a one-dimensional diffusion. Characterizing the limiting conditioned and killed process via its infinitesimal generator leads to an investigation of the -transforms of transient one-dimensional diffusion processes that goes beyond what is known and is of independent interest. 相似文献
992.
A. Shumelyuk A. Volkov S. Odoulov G. Cook D. R. Evans 《Applied physics. B, Lasers and optics》2010,100(1):101-108
Coupling of two counterpropagating coherent light waves in tin hypothiodiphosphate (Sn2P2S6, SPS) is usually strongly inhibited because of severe space charge limitations. Analysis of two-beam coupling gain is performed
for different orientations of the grating vector in this monoclinic crystal that takes into account the anisotropy of dielectric
permittivity. Gain enhancements are predicted and demonstrated experimentally for several nontraditional orientations of counterpropagating
waves. 相似文献
993.
We provide an affirmative answer to the C
r
-Closing Lemma, r ≥ 2, for a large class of flows defined on every closed surface. 相似文献
994.
A. F. Isakovic A. Stein J. B. Warren S. Narayanan M. Sprung A. R. Sandy K. Evans‐Lutterodt 《Journal of synchrotron radiation》2009,16(1):8-13
Motivated by the anticipated advantageous performance of diamond kinoform refractive lenses for synchrotron X‐ray radiation studies, this report focuses on progress in designing, nanofabricating and testing of their focusing performance. The method involves using lift‐off and plasma etching to reproduce a planar definition of numerically determined kinoform refractive optics. Tests of the focusing action of a diamond kinoform refractive lens at the APS 8‐ID‐I beamline demonstrate angular control of the focal spot. 相似文献
995.
This study reveals that Fourier transform infrared is an effective analytical tool in probing the extent of hydrolysis of
cerium and gadolinium acetylacetonates dissolved in methanol. It is found that these acetylacetonates have relatively fast
hydrolysis rates at the early stage of hydrolysis (<6 h). However, their hydrolysis rates become very slow beyond 6-h of hydrolysis
and decrease to near zero after 55-h of hydrolysis. As a result, hydrolysis cannot be completed even after 170 h at room temperature.
These phenomena suggest that the first and second acetylacetone ligands chelated to Ce3+ and Gd3+ ions can actually be hydrolyzed with relative ease at room temperature, similar to alkoxide ligands. It is the last acetylacetone
ligand that is very difficult to be hydrolyzed. 相似文献
996.
On general sum-connectivity index 总被引:1,自引:0,他引:1
We report some properties of the reverse degree distance of a connected (molecular) graph, and, in particular, its relationship
with the first Zagreb index and Wiener index. We also show that the reverse degree distance satisfies the basic requirement
for a branching index. 相似文献
997.
Data for optimization problems often comes from (deterministic) forecasts, but it is naïve to consider a forecast as the only future possibility. A more sophisticated approach uses data to generate alternative future scenarios, each with an attached probability. The basic idea is to estimate the distribution of forecast errors and use that to construct the scenarios. Although sampling from the distribution of errors comes immediately to mind, we propose instead to approximate rather than sample. Benchmark studies show that the method we propose works well. 相似文献
998.
Peter Bouwknegt Jarah Evslin Varghese Mathai 《Communications in Mathematical Physics》2015,337(2):909-954
We introduce spherical T-duality, which relates pairs of the form (P, H) consisting of a principal SU(2)-bundle \({P \rightarrow M}\) and a 7-cocycle H on P. Intuitively spherical T-duality exchanges H with the second Chern class c 2(P). Unless \({dim(M) \leq 4}\), not all pairs admit spherical T-duals and the spherical T-duals are not always unique. Nonetheless, we prove that all spherical T-dualities induce a degree-shifting isomorphism on the 7-twisted cohomologies of the bundles and, when \({dim(M) \leq 7}\), also their integral twisted cohomologies and, when \({dim(M) \leq 4}\), even their 7-twisted K-theories. While spherical T-duality does not appear to relate equivalent string theories, it does provide an identification between conserved charges in certain distinct IIB supergravity and string compactifications. 相似文献
999.
Spectral measures provide invariants for braided subfactors via fusion modules. In this paper we study joint spectral measures associated to the rank two Lie group G 2, including the McKay graphs for the irreducible representations of G 2 and its maximal torus, and fusion modules associated to all known G 2 modular invariants. 相似文献
1000.
We have synthesized bismuth oxychloride (BiOCl) by wet chemical route. Characterization of the synthesized samples has been carried out using XRD, SEM with EDAX, HRTEM, FT-IR, Raman Spectroscopy, PL and UV. XRD and SEM analysis confirms crystallite size varying from 20–40 nm. FTIR spectrum indicates that the prepared material is highly pure and there is no water molecule present. Raman and photoluminescence spectrum of the bismuth oxychloridenanoplate demonstrated strong blue light emission which brings them in a special class of materials which work under visible light exposure. UV-Vis spectroscopy shows very less charge carriers transit time under visible light thus confirming excellent photocatalytic properties of material. 相似文献