全文获取类型
收费全文 | 1046篇 |
免费 | 53篇 |
国内免费 | 3篇 |
专业分类
化学 | 739篇 |
晶体学 | 8篇 |
力学 | 15篇 |
数学 | 168篇 |
物理学 | 172篇 |
出版年
2023年 | 9篇 |
2022年 | 4篇 |
2021年 | 26篇 |
2020年 | 21篇 |
2019年 | 17篇 |
2018年 | 16篇 |
2017年 | 23篇 |
2016年 | 47篇 |
2015年 | 39篇 |
2014年 | 40篇 |
2013年 | 73篇 |
2012年 | 68篇 |
2011年 | 76篇 |
2010年 | 58篇 |
2009年 | 35篇 |
2008年 | 71篇 |
2007年 | 69篇 |
2006年 | 68篇 |
2005年 | 52篇 |
2004年 | 28篇 |
2003年 | 31篇 |
2002年 | 22篇 |
2001年 | 13篇 |
2000年 | 9篇 |
1999年 | 17篇 |
1998年 | 8篇 |
1997年 | 4篇 |
1996年 | 13篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 13篇 |
1992年 | 8篇 |
1991年 | 10篇 |
1990年 | 3篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1986年 | 7篇 |
1985年 | 11篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 10篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有1102条查询结果,搜索用时 31 毫秒
991.
We demonstrate the preservation of entanglement of an energy-time entangled biphoton through a slow light medium. Using the D(1) and D(2) fine structure resonances of Rubidium, we delay one photon of the 1.5 THz biphoton by approximately 1.3 correlation lengths and measure the fourth order correlation fringes. After the group delay the fringe visibility is reduced from 97.0+/-4.4% to 80.0+/-4.8%, but is still sufficient to violate a Bell inequality. We show that temporal broadening is the primary mechanism for reducing the fringe visibility and that smaller bandwidths lead to greatly reduced broadening. 相似文献
992.
Nanostructured sol-gel TiO2 thin films spin coated on silicate glass plates are subjected to excimer (KrF*) pulsed laser irradiation in order to tailor their structure and photocatalytic properties. The surface morphology of virgin and laser-processed films are followed applying electron optical imaging and atomic force microscopy. The evolution of the surface roughness and pore formation are shown to be accompanied by optical absorption edge shift to infrared wavelength range. Conventional X-ray diffraction analysis and high-resolution transmission electron imaging are applied in order to obtain information on the phase composition. Co-existence of amorphous and anatase TiO2 phases in nonirradiated sol-gel films is found. It is established that after laser processing the most intense XRD anatase peak is shifted to lower 2θ range. The analysis of high-resolution transmission electron images of film profiles evidences for the laser induced phase transitions. Formation of rutile and brookite TiO2 accompanied by evolution of oxygen deficient TinO2n−1 phases are identified in the subsurface region. The contribution of laser processing for increasing the photocatalytic efficiency of laser-modified films toward the oxidation of methylene blue water solution is demonstrated. The results obtained reveal a novel-processing route for designing sol-gel titania films with improved photocatalytical activity. 相似文献
993.
994.
de Koning M Cai W Sadigh B Oppelstrup T Kalos MH Bulatov VV 《The Journal of chemical physics》2005,122(7):074103
We develop a general theoretical framework for the recently proposed importance sampling method for enhancing the efficiency of rare-event simulations [W. Cai, M. H. Kalos, M. de Koning, and V. V. Bulatov, Phys. Rev. E 66, 046703 (2002)], and discuss practical aspects of its application. We define the success/fail ensemble of all possible successful and failed transition paths of any duration and demonstrate that in this formulation the rare-event problem can be interpreted as a "hit-or-miss" Monte Carlo quadrature calculation of a path integral. The fact that the integrand contributes significantly only for a very tiny fraction of all possible paths then naturally leads to a "standard" importance sampling approach to Monte Carlo (MC) quadrature and the existence of an optimal importance function. In addition to showing that the approach is general and expected to be applicable beyond the realm of Markovian path simulations, for which the method was originally proposed, the formulation reveals a conceptual analogy with the variational MC (VMC) method. The search for the optimal importance function in the former is analogous to finding the ground-state wave function in the latter. In two model problems we discuss practical aspects of finding a suitable approximation for the optimal importance function. For this purpose we follow the strategy that is typically adopted in VMC calculations: the selection of a trial functional form for the optimal importance function, followed by the optimization of its adjustable parameters. The latter is accomplished by means of an adaptive optimization procedure based on a combination of steepest-descent and genetic algorithms. 相似文献
995.
Alexandre?Carlos?Camacho?RodriguesEmail author 《Journal of mathematical chemistry》2005,37(4):347-351
An intuitive and computationally non-intensive model for the classification of Hydrotalcite-like compounds (HTLCs) based simply on the chemical composition using the Sanderson Method led to good prediction of basicity and different basic sites (oxygen atoms with different charge). That model was evaluated at different M3+ /(M2++M3+) ratio and with different divalent and trivalent metallic cations.AMS subject classification: 92E10 相似文献
996.
A novel synthesis of 1,2-disubstituted 1,2-dihydroquinazoline 3-oxides 8 and the first ever examples of 1,3-dipolar trapping of these nitrones to homonuclear dipolarophiles is described. The new dipoles 8 reacted with N-methyl maleimide, generating diastereomeric adducts 14-16. In the reaction between 8 and dimethyl acetylenedicarboxylate, primary cycloadducts 17 and/or stable rearrangement products, azomethine ylides 18, are formed depending on the substitution pattern of the dipole. The structure of 18c is unambiguously assigned by X-ray crystallographic analysis. An X-ray crystal structure determination is also presented for the cyclopropylisoxazoloquinazoline 22 formed by a [3 + 2] addition of 8a to 21, the dimethyl acetylenedicarboxylate tetramer. 相似文献
997.
A. Camacho 《General Relativity and Gravitation》2002,34(11):1839-1845
Considering the existence of nonconformal stochastic fluctuations in the metric tensor a generalized uncertainty principle and a deformed dispersion relation (associated to the propagation of photons) are deduced. Matching our model with the so called quantum -Poincaré group will allow us to deduce that the fluctuation-dissipation theorem could be fulfilled without needing a restoring mechanism associated with the intrinsic fluctuations of spacetime. In other words, the loss of quantum information is related to the fact that the spacetime symmetries are described by the quantum -Poincaré group, and not by the usual Poincaré symmetries. An upper bound for the free parameters of this model will also be obtained. 相似文献
998.
Robust patterns in food web structure 总被引:1,自引:0,他引:1
999.
Effects of freeze-drying on partitioning patterns of major elements and trace metals in lake sediments 总被引:1,自引:0,他引:1
Tomas Hjorth 《Analytica chimica acta》2004,526(1):95-102
The effect of freeze-drying on elemental partitioning pattern in four lake sediments was evaluated using the sequential extraction method developed under the auspices of Community Bureau of Reference (BCR) of the Commission of European Communities. Redox-sensitive elements like iron, manganese, phosphorus, and sulfur were analyzed simultaneously with several trace metals. This allows a qualitative determination of which trace metal-scavenging phases commonly found in freshwater sediments that are most susceptible to freeze-drying.The comparison of a surface layer and a deeper more reduced layer collected from four sediment cores ranging from sub-oxic to anoxic condition indicate the changes in partitioning pattern due to freeze-drying. Two of the lakes exhibiting clearly reduced environments are most affected by freeze-drying in terms of change in partitioning patterns. However, this effect is element specific and varies with sediment depth along the redox gradient. Elements commonly associated with a sulfide phase (S, Fe, Cu, Zn, and Pb) and extracted in BCR step 3 are the most affected elements, but at the same time also dependent on the organic matter content in the sediment. Si and Al that are often considered as conservative elements show a shift from step 3 to step 2, and it is interpreted as a release of Si and Al from organic matter and the subsequent formation of oxides during freeze-drying. Calcium and manganese, elements that are often associated with a carbonate phase, do not seem to be severely affected by this kind of treatment. Carbonate-associated iron (operationally defined), however does show a clear tendency to oxidize during the freeze-drying process. In fact, the shift in iron (carbonate to oxide fraction) corresponds to 3-4% (w/w) of the total sediment in some cases. Surprisingly, a significant fraction of iron originally extracted in step 1 (as “carbonate”) seems to be immobilized during freeze-drying.These results clearly show that freeze-drying does not preserve the speciation pattern of major elements, trace metals, phosphorus, and sulfur in anoxic lake sediment core sections during storage prior to BCR sequential extraction procedures. 相似文献
1000.
Tomas Caraballo Antonio M. Marquez-Duran Jose Real 《Applied Mathematics and Optimization》2006,53(2):141-161
The long-time behaviour of a stochastic 3D LANS-α model on a bounded domain is analysed. First, we reformulate the model as
an abstract problem. Next, we establish sufficient conditions ensuring the existence of stationary (steady state) solutions
of this abstract nonlinear stochastic evolution
equation, and study the stability properties of the model. Finally, we analyse
the effects produced by stochastic perturbations in the deterministic version
of the system (persistence of exponential stability as well as possible
stabilisation effects produced by the noise). The general results are applied
to our stochastic LANS-α system throughout the paper. 相似文献