全文获取类型
收费全文 | 3001篇 |
免费 | 103篇 |
国内免费 | 36篇 |
专业分类
化学 | 1691篇 |
晶体学 | 13篇 |
力学 | 53篇 |
数学 | 761篇 |
物理学 | 622篇 |
出版年
2023年 | 21篇 |
2022年 | 34篇 |
2021年 | 64篇 |
2020年 | 42篇 |
2019年 | 60篇 |
2018年 | 82篇 |
2017年 | 52篇 |
2016年 | 113篇 |
2015年 | 110篇 |
2014年 | 113篇 |
2013年 | 190篇 |
2012年 | 180篇 |
2011年 | 229篇 |
2010年 | 120篇 |
2009年 | 145篇 |
2008年 | 171篇 |
2007年 | 168篇 |
2006年 | 153篇 |
2005年 | 147篇 |
2004年 | 159篇 |
2003年 | 108篇 |
2002年 | 98篇 |
2001年 | 46篇 |
2000年 | 32篇 |
1999年 | 29篇 |
1998年 | 40篇 |
1997年 | 37篇 |
1996年 | 45篇 |
1995年 | 32篇 |
1994年 | 27篇 |
1993年 | 23篇 |
1992年 | 25篇 |
1991年 | 24篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 11篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 15篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1971年 | 7篇 |
1930年 | 6篇 |
排序方式: 共有3140条查询结果,搜索用时 15 毫秒
91.
Optical gap solitons, which exist due to a balance of nonlinearity and dispersion due to a Bragg grating, can couple to acoustic waves through electrostriction. This gives rise to a new species of "gap-acoustic" solitons (GASs), for which we find exact analytic solutions. The GAS consists of an optical pulse similar to the optical gap soliton, dressed by an accompanying phonon pulse. Close to the speed of sound, the phonon component is large. In subsonic (supersonic) solitons, the phonon pulse is a positive (negative) density variation. Coupling to the acoustic field damps the solitons' oscillatory instability, and gives rise to a distinct instability for supersonic solitons, which may make the GAS decelerate and change direction, ultimately making the soliton subsonic. 相似文献
92.
93.
Families of analytical solutions are found for symmetric and antisymmetric solitons in a dual-core system with Kerr nonlinearity and parity-time (PT)-balanced gain and loss. The crucial issue is stability of the solitons. A stability region is obtained in an analytical form, and verified by simulations, for the PT-symmetric solitons. For the antisymmetric ones, the stability border is found in a numerical form. Moving solitons of both types collide elastically. The two soliton species merge into one in the "supersymmetric" case, with equal coefficients of gain, loss, and intercore coupling. These solitons feature a subexponential instability, which may be suppressed by periodic switching ("management"). 相似文献
94.
Boris Baeumer Mark M. Meerschaert 《Journal of Computational and Applied Mathematics》2010,233(10):2438-2448
The space-fractional diffusion equation models anomalous super-diffusion. Its solutions are transition densities of a stable Lévy motion, representing the accumulation of power-law jumps. The tempered stable Lévy motion uses exponential tempering to cool these jumps. A tempered fractional diffusion equation governs the transition densities, which progress from super-diffusive early-time to diffusive late-time behavior. This article provides finite difference and particle tracking methods for solving the tempered fractional diffusion equation with drift. A temporal and spatial second-order Crank-Nicolson method is developed, based on a finite difference formula for tempered fractional derivatives. A new exponential rejection method for simulating tempered Lévy stables is presented to facilitate particle tracking codes. 相似文献
95.
We consider Hölder smoothness classes of surfaces for which we construct piecewise polynomial approximation networks, which are graphs with polynomial pieces as nodes and edges between polynomial pieces that are in ‘good continuation’ of each other. Little known to the community, a similar construction was used by Kolmogorov and Tikhomirov in their proof of their celebrated entropy results for Hölder classes.We show how to use such networks in the context of detecting geometric objects buried in noise to approximate the scan statistic, yielding an optimization problem akin to the Traveling Salesman. In the same context, we describe an alternative approach based on computing the longest path in the network after appropriate thresholding.For the special case of curves, we also formalize the notion of ‘good continuation’ between beamlets in any dimension, obtaining more economical piecewise linear approximation networks for curves.We include some numerical experiments illustrating the use of the beamlet network in characterizing the filamentarity content of 3D data sets, and show that even a rudimentary notion of good continuity may bring substantial improvement. 相似文献
96.
In two dimensional electron systems with Coulomb or dipolar interactions, a direct transition, whether first or second order, from a liquid to a crystalline state is forbidden. As a result, between these phases there must be other (micro-emulsion) phases which can be viewed as a meso-scale mixture of the liquid and crystalline phases. We investigate the transport properties of these new electronic phases and present arguments that they are responsible for the various transport anomalies that have been seen in experiments on the strongly correlated 2DEG in high mobility semiconductor devices with low electron densities. 相似文献
97.
Dr. Siddhartha De Dr. Alexandrine Flambard Dr. Delphine Garnier Patrick Herson Dr. Frank H. Köhler Dr. Abhishake Mondal Dr. Karine Costuas Dr. Béatrice Gillon Prof. Rodrigue Lescouëzec Dr. Boris Le Guennic Dr. Frédéric Gendron 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(52):12120-12136
The local magnetic structure in the [FeIII(Tp)(CN)3]− building block was investigated by combining paramagnetic Nuclear Magnetic Resonance (pNMR) spectroscopy and polarized neutron diffraction (PND) with first-principle calculations. The use of the pNMR and PND experimental techniques revealed the extension of spin-density from the metal to the ligands, as well as the different spin mechanisms that take place in the cyanido ligands: Spin-polarization on the carbon atoms and spin-delocalization on the nitrogen atoms. The results of our combined density functional theory (DFT) and multireference calculations were found in good agreement with the PND results and the experimental NMR chemical shifts. Moreover, the ab-initio calculations allowed us to connect the experimental spin-density map characterized by PND and the suggested distribution of the spin-density on the ligands observed by NMR spectroscopy. Interestingly, significant differences were observed between the pseudo-contact contributions of the chemical shifts obtained by theoretical calculations and the values derived from NMR spectroscopy using a simple point-dipole model. These discrepancies underline the limitation of the point-dipole model and the need for more elaborate approaches to break down the experimental pNMR chemical shifts into contact and pseudo-contact contributions. 相似文献
98.
Antonovič Valentin Sikarskas Donatas Malaiškienė Jurgita Boris Renata Stonys Rimvydas 《Journal of Thermal Analysis and Calorimetry》2019,138(6):4127-4137
Journal of Thermal Analysis and Calorimetry - The article analyses the peculiarities of the combined effect on hydration process of the following pozzolanic additives: metakaolin waste (MW),... 相似文献
99.
Laurens Kooijman Dr. Matthias Schuster Christian Baumann Simon Jurt Dr. Frank Löhr Dr. Boris Fürtig Prof. Dr. Peter Güntert Prof. Dr. Oliver Zerbe 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(47):21151-21158
To achieve efficient proton pumping in the light-driven proton pump bacteriorhodopsin (bR), the protein must be tightly coupled to the retinal to rapidly convert retinal isomerization into protein structural rearrangements. Methyl group dynamics of bR embedded in lipid nanodiscs were determined in the dark-adapted state, and were found to be mostly well ordered at the cytosolic side. Methyl groups in the M145A mutant of bR, which displays only 10 % residual proton pumping activity, are less well ordered, suggesting a link between side-chain dynamics on the cytosolic side of the bR cavity and proton pumping activity. In addition, slow conformational exchange, attributed to low frequency motions of aromatic rings, was indirectly observed for residues on the extracellular side of the bR cavity. This may be related to reorganization of the water network. These observations provide a detailed picture of previously undescribed equilibrium dynamics on different time scales for ground-state bR. 相似文献
100.
Bodil Lousen Dr. Stephan K. Pedersen Pernille Bols Kasper H. Hansen Michelle R. Pedersen Prof. Ole Hammerich Dr. Sergey Bondarchuk Prof. Boris Minaev Dr. Glib V. Baryshnikov Prof. Hans Ågren Prof. Michael Pittelkow 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(22):4935-4940
This work describes a synthetic approach where a non-planar aromatic heterocyclic [7]helicene is compressed to yield a hetero[8]circulene containing an inner antiaromatic cyclooctatetraene (COT) core. This [8]circulene consists of four benzene rings and four heterocyclic rings, and it is the first heterocyclic [8]circulene containing three different heteroatoms. The synthetic pathway proceeds via a the flattened dehydro-hetero[7]helicene, which is partially a helicene and partially a circulene: it is non-planar and helically chiral as helicenes, and contains a COT motif like [8]circulenes. The antiaromaticity of the COT core is confirmed by nucleus independent chemical shift (NICS) calculations. The planarization from a helically π-conjugated [7]helicene to a fully planar heterocyclic [8]circulene significantly alters the spectroscopic properties of the molecules. Post-functionalization of the [7]helicenes and the [8]circulenes by oxygenation of the thiophene rings to the corresponding thiophene-sulfones allows an almost complete fluorescence emission coverage of the visible region of the optical spectrum (400–700 nm). 相似文献