首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   282篇
  免费   12篇
  国内免费   1篇
化学   172篇
晶体学   2篇
力学   16篇
数学   42篇
物理学   63篇
  2023年   4篇
  2022年   7篇
  2021年   8篇
  2020年   5篇
  2019年   5篇
  2018年   7篇
  2016年   7篇
  2015年   10篇
  2014年   8篇
  2013年   30篇
  2012年   13篇
  2011年   20篇
  2010年   3篇
  2009年   9篇
  2008年   9篇
  2007年   6篇
  2006年   11篇
  2005年   8篇
  2004年   12篇
  2003年   7篇
  2002年   2篇
  2001年   3篇
  2000年   4篇
  1999年   8篇
  1998年   3篇
  1997年   2篇
  1996年   5篇
  1995年   4篇
  1994年   3篇
  1993年   2篇
  1991年   2篇
  1986年   3篇
  1984年   2篇
  1981年   4篇
  1980年   5篇
  1979年   2篇
  1978年   5篇
  1965年   2篇
  1940年   2篇
  1932年   3篇
  1927年   2篇
  1926年   3篇
  1922年   2篇
  1920年   3篇
  1918年   2篇
  1912年   4篇
  1911年   2篇
  1910年   2篇
  1904年   1篇
  1899年   2篇
排序方式: 共有295条查询结果,搜索用时 15 毫秒
61.
We report time-resolved studies of ballistic phonon absorption in the fractional quantum Hall regime at Landau level filling factors of and . The technique used can resolve the interaction of the two-dimensional electron system with LA and TA phonons and has been used to measure the temperature variation of the heat capacity of a single layer of electrons at . The energy gaps at have also been measured and found to be in good agreement with theory. The roles of compressible and incompressible regions in the phonon absorption process are discussed. Angle resolved measurements at are also in good agreement with theory.  相似文献   
62.
For more than four decades, the lowest excitation in the whole landscape of atomic nuclei, the low‐lying, isomeric state of 229Th, the so‐called thorium isomer, has challenged physicists of various disciplines. Being a solitaire with its uniquely low excitation energy of <10 eV, its predicted lifetime of a few hours results in an extremely sharp relative linewidth ΔE/E as low as 10–20. While until recently the indication of its existence was based only on indirect evidence, its unique properties inspired a multitude of potential applications, like the use of the thorium isomer as a nuclear frequency standard, potentially able to outperform even the best atomic clocks and a sensitivity‐enhanced access to potential temporal variations of fundamental constants. The various proposals to exploit the unique properties of 229mTh are presented herein, in particular focusing on its ability to serve as a test bench for time variations of fundamental constants like the fine structure constant.  相似文献   
63.
A RATIO method for analysis of intensity changes in time‐resolved pump–probe Laue diffraction experiments is described. The method eliminates the need for scaling the data with a wavelength curve representing the spectral distribution of the source and removes the effect of possible anisotropic absorption. It does not require relative scaling of series of frames and removes errors due to all but very short term fluctuations in the synchrotron beam.  相似文献   
64.
In this paper we show that the cyclotomic factors of the En Coxeter polynomials depend only on the value of nmod360, and come exclusively from spherical subdiagrams.  相似文献   
65.
The concept of syncrystallization was reinvestigated by focusing on phthalic acid (PA) grown with methyl red (MR). Crystals are alternately red and yellow in adjacent growth sectors. X-ray structures of MR and its cocrystals, revealing MR in the neutral, zwitterionic, and protonated states, as well as measurements of linear birefringence and linear dichroism of mixed crystals, were used to investigate mechanisms of PA coloring. These experiments were complemented by force field calculations of the lowest energy stable surfaces of expressed facets and energies of MR on and in crystals, as well as molecular orbital calculations of MR. Two MR species were detected in PA having distinct energies, polarizations, and face selectivities. Assignments of structures to these MRs, previously thought to be neutral and protonated, required a nuanced analysis of hydrogen bonds. The essential difference between yellow and red species is whether the MR carboxylic acid proton is inter- or intramolecularly hydrogen bound. Inferences about mixed crystal structure drawn from an examination of cocrystals of PA and MR are inconsistent with polarization spectroscopy signaling caution when using stoichiometric compounds as models of dilute solid solutions. Upon heating mixed crystals, linear dichroism diminishes and oriented, elongated pools of MR separate and pass through the bulk in directions perpendicular to the direction of elongation. These batonnets subsequently crystallize leaving macroscopic oriented crystals of a MR-rich phase within PA. No evidence was found for the simultaneous crystallization of MR and PA; however, the MR reorientation on heating as well as the separation and recrystallization of a MR-rich phase are distinct processes that could be embraced by the literal meaning of syncrystallization.  相似文献   
66.
In acoustic cavitation, the relationship between the bubble dynamics on the microscale and the flow properties on the macroscale is critical in determining sonochemical reaction kinetics. A new technique was developed to measure the void fraction and estimate water mobility in the vicinity of cavitating bubbles using phase-encoded magnetic resonance imaging with short characteristic measurement timescales (0.1–1 ms). The exponential behavior of the NMR signal decay indicated the fast diffusion regime, with the relationship between local mechanical dispersion D mix and the average bubble radius R, Dmix >> \frac2R210-4s, D_{\rm mix}\gg \frac{2R^2}{10^{-4}\hbox{s}}, resulting in dispersion of orders of magnitude greater than diffusion in quiescent water. For two different samples (water and a surfactant solution), the independent measurements of three-dimensional void fraction and velocity fields permitted the calculation of compressibility, divergence and vorticity of the cavitating medium. The measured dynamics of the dissolved gas, compared with that of the surrounding liquid, reflected the difference in the bubble coalescence and lifetimes and correlated with the macroscopic flow parameters.  相似文献   
67.
Novel carotenoids genetically engineered in a heterologous host   总被引:2,自引:0,他引:2  
Carotenoids are commercially important pigments that are essential for human health. Diverse carotenoids have been identified, but availability has impeded evaluation of their pharmaceutical potential. Molecular techniques have been developed to produce specific and novel carotenoids with improved biological activities.  相似文献   
68.
The excited-state structure of [Cu(I)[(1,10-phenanthroline-N,N') bis(triphenylphosphine)] cations in their crystalline [BF(4)] salt has been determined at both 180 and 90 K by single-pulse time-resolved synchrotron experiments with the modified polychromatic Laue method. The two independent molecules in the crystal show distortions on MLCT excitation that differ in magnitude and direction, a difference attributed to a pronounced difference in the molecular environment of the two complexes. As the excited states differ, the decay of the emission is biexponential with two strongly different lifetimes, the longer lifetime, assigned to the more restricted molecule, becoming more prevalent as the temperature increases. Standard deviations in the current Laue study are very much lower than those achieved in a previous monochromatic study of a Cu(I) 2,9-dimethylphenanthroline substituted complex ( J. Am. Chem. Soc. 2009 , 131 , 6566 ), but the magnitudes of the shifts on excitation are similar, indicating that lattice restrictions dominate over the steric effect of the methyl substitution. Above all, the study illustrates emphatically that molecules in solids have physical properties different from those of isolated molecules and that their properties depend on the specific molecular environment. This conclusion is relevant for the understanding of the properties of molecular solid-state devices, which are increasingly used in current technology.  相似文献   
69.
We complete the classification of positive rank gradings on Lie algebras of simple algebraic groups over an algebraically closed field k whose characteristic is zero or not too small, and we determine the little Weyl groups in each case. We also classify the stable gradings and prove Popov’s conjecture on the existence of a Kostant section.  相似文献   
70.
The use of crystal engineering to convert liquids into crystalline solids remains a powerful method for inhibiting undesired degradation pathways. When nicotine, a liquid sensitive to both light and air, is combined with the GRAS-listed compound, gentisic acid, the resulting crystalline solid, exhibits enhanced photo and thermal stability. Despite a modest ΔTm of 42.7 °C, the melting point of 155.9 °C for the nicotinium gentisate salt is the highest reported for nicotine-containing crystalline solids. An analysis of the crystal packing and thermodynamic properties provides context for the observed properties.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号