全文获取类型
收费全文 | 542篇 |
免费 | 5篇 |
国内免费 | 7篇 |
专业分类
化学 | 230篇 |
晶体学 | 1篇 |
力学 | 10篇 |
综合类 | 1篇 |
数学 | 63篇 |
物理学 | 249篇 |
出版年
2018年 | 4篇 |
2017年 | 9篇 |
2016年 | 12篇 |
2015年 | 12篇 |
2014年 | 13篇 |
2013年 | 27篇 |
2012年 | 24篇 |
2011年 | 32篇 |
2010年 | 18篇 |
2009年 | 13篇 |
2008年 | 9篇 |
2007年 | 13篇 |
2006年 | 17篇 |
2005年 | 13篇 |
2004年 | 19篇 |
2003年 | 9篇 |
2002年 | 18篇 |
2001年 | 16篇 |
2000年 | 14篇 |
1999年 | 12篇 |
1998年 | 3篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 16篇 |
1993年 | 9篇 |
1992年 | 14篇 |
1991年 | 11篇 |
1990年 | 9篇 |
1989年 | 2篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 11篇 |
1980年 | 14篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1976年 | 11篇 |
1975年 | 5篇 |
1974年 | 7篇 |
1973年 | 10篇 |
1972年 | 7篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1968年 | 4篇 |
排序方式: 共有554条查询结果,搜索用时 15 毫秒
1.
We calculate the effect of radial confinement on the Schwinger pair production rate by solving the Dirac equation in a flux-tube cylinder containing a constant chromoelectric field in the longitudinal direction. We show how the Dirac equation separates into radial and longitudinal equations for a mass term which has an arbitrary radial dependence and introduce radial confinement by having a finite mass inside the cylinder and an infinitely large mass outside. The resulting boundary conditions are equivalent to the MIT boundary condition. The equations are solved analytically for a constant quark mass inside the flux-tube, which acts like a waveguide. The discretization of the transverse wave vector which has a continuous spectrum in the non-confined case leads to a large suppression of the Schwinger pair-production rate for small radii. The minimal radius where pairs are created decreases with increasing field strength. The suppression turns out to be larger for heavier quarks than for light quarks. 相似文献
2.
The historical development of Hensel's lemma is briefly discussed (Section 1). Using Newton polygons, a simple proof of a general Hensel's lemma for separable polynomials over Henselian fields is given (Section 3). For polynomials over algebraically closed, valued fields, best possible results on continuity of roots (Section 4) and continuity of factors (Section 6) are demonstrated. Using this and a general Krasner's lemma (Section 7), we give a short proof of a general Hensel's lemma and show that it is, in a certain sense, best possible (Section 8). All valuations here are non-Archimedean and of arbitrary rank. The article is practically self-contained. 相似文献
3.
Principles and recent developments in ultrasound contrast agents 总被引:6,自引:1,他引:5
The behaviour of gas bubbles and gas encapsulated spheres as echographic contrast agents is reviewed. Compared with rigid spheres, gas bubbles are superior scattering agents and they offer a number of useful properties which can be exploited in a variety of ways. The analysis of their velocity of sound, back-scatter intensity, second harmonic emission and resonant frequency opens up new perspectives in the development of contrast agents for echocardiographic research with potential clinical applications. 相似文献
4.
5.
Kato H Brink M Möllerstedt H Piqueras MC Crespo R Ottosson H 《The Journal of organic chemistry》2005,70(23):9495-9504
[Figure: see text]. A quantum chemical study has been performed to assess changes in aromaticity along the T1 state Z/E-isomerization pathways of annulenyl-substituted olefins. It is argued that the point on the T1 energy surface with highest substituent aromaticity corresponds to the minimum. According to Baird (J. Am. Chem. Soc. 1972, 94, 4941), aromaticity and antiaromaticity are interchanged when going from S0 to T1. Thus, olefins with S0 aromatic substituents (set A olefins) will be partially antiaromatic in T1 and vice versa for olefins with S0 antiaromatic substituents (set B olefins). Twist of the C=C bond to a structure with a perpendicular orientation of the 2p(C) orbitals (3p*) in T1 should lead to regaining substituent aromaticity in set A and loss of aromaticity in set B olefins. This hypothesis is verified through quantum chemical calculations of T1 energies, geometries (bond lengths and harmonic oscillator measure of aromaticity), spin densities, and nucleus independent chemical shifts whose differences along the T1 PES display zigzag dependencies on the number of -electrons in the annulenyl substituent of the olefin. Aromaticity changes are reflected in the profiles of the T1 potential energy surfaces (T1 PESs) for Z/E-isomerizations because olefins in set A have minima at 3p* whereas those in set B have maxima at such structures. The proper combination (fusion) of the substituents of set A and B olefins could allow for design of novel optical switch compounds that isomerize adiabatically with high isomerization quantum yields. 相似文献
6.
7.
8.
Richard W.M. Ten Hoedt Gerard van Koten Jan G. Noltes 《Journal of organometallic chemistry》1980,201(1):327-342
Thermal decomposition of configurationally pure 1,2-diarylpropenylcopper compounds Z-Vi2CU4Br2 and Z-Vi2Cu4R2 [Vi (2-Me2NC6H4)CC(Me)-(C6H4Me-4), R 2-Me2NC6H4 or 4-MeC6H4CC] predominantly results in the formation of ViH. In contrast, only dimers (ViVi) were formed on thermolysis of (Z-ViCu2OTf)η which is a further illustration of the influence of the counter anion on the reactivity of organocopper cluster compounds. However, in both cases partial inversion of configuration, giving mixtures of isomers, was observed. The thermolysis of (Z-ViCu2OTf)η is discussed in terms of OTf-enhanced intraaggregate electron-transfer processes. The formation of incipient vinyl cations which are η2-coordinated to the copper cluster can explain the observed isomerization.Also in the hydrolysis reaction of Z-Vi2Cu4Br2 mixtures of isomeric ViH compounds were obtained, the E/Z ratio being dependent on the type of reagent used. Mixtures of isomeric ViX compounds (X Br, Cl, I) were formed in the reaction of Z-Vi2Cu4Br2 with I2 and CuCl2. An explanation for the occurrence of isomerization is presented.Dimers (ViVi) were almost absent in the product mixture resulting from the reaction of Z-Vi2Cu4Br2 with CuCl2. In contrast, ViVi is obtained in about 50% yield from the reaction of ViLi with CuCl2 which is in accord with earlier observations for the reaction of aryllithium compounds with cupric halides.Highly selective E-ViBr formation was observed in the reaction of E-ViLi with AgBr. This reaction probably proceeds via a thermally unstable Z-ViAg2Br intermediate. 相似文献
9.
10.