全文获取类型
收费全文 | 360篇 |
免费 | 22篇 |
专业分类
化学 | 224篇 |
晶体学 | 2篇 |
力学 | 7篇 |
数学 | 59篇 |
物理学 | 90篇 |
出版年
2023年 | 5篇 |
2022年 | 3篇 |
2021年 | 10篇 |
2020年 | 12篇 |
2019年 | 11篇 |
2018年 | 3篇 |
2017年 | 15篇 |
2016年 | 16篇 |
2015年 | 19篇 |
2014年 | 20篇 |
2013年 | 28篇 |
2012年 | 23篇 |
2011年 | 32篇 |
2010年 | 18篇 |
2009年 | 12篇 |
2008年 | 24篇 |
2007年 | 18篇 |
2006年 | 27篇 |
2005年 | 20篇 |
2004年 | 9篇 |
2003年 | 13篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1994年 | 2篇 |
1991年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
排序方式: 共有382条查询结果,搜索用时 187 毫秒
41.
Antti Perälä 《Integral Equations and Operator Theory》2011,71(1):113-128
We study Toeplitz operators between analytic Bloch-type spaces of the unit disk. We construct suitable classes of distributions
that generate bounded Toeplitz operators between these spaces. The classes are naturally connected to the corresponding results
in the reflexive Bergman space setting and previously known results on A
1 and the Bloch space. We also study distributional symbols satisfying logarithmic BMO-condition. In addition, sufficient compactness
criteria are provided. 相似文献
42.
We have performed a comprehensive theoretical investigation of the structural principles of semiconducting clathrate frameworks composed of the Group 14 elements carbon, silicon, germanium, and tin. We have investigated the basic clathrate frameworks, together with their polytypes, intergrowth clathrate frameworks, and extended frameworks based on larger icosahedral building blocks. Quantum chemical calculations with the PBE0 hybrid density functional method provided a clear overview of the structural trends and electronic properties among the various clathrate frameworks. In agreement with previous experimental and theoretical studies, the clathrate II framework proved to be the energetically most favorable, but novel hexagonal polytypes of clathrate II also proved to be energetically very favorable. In the case of silicon, several of the studied clathrate frameworks possess direct and wide band gaps. The band structure diagrams and simulated powder X-ray patterns of the studied frameworks are provided and systematic preliminary evaluation of guest-occupied frameworks is conducted to shed light on the characteristics of novel, experimentally feasible clathrate compositions. 相似文献
43.
Feyer V Plekan O Kivimäki A Prince KC Moskovskaya TE Zaytseva IL Soshnikov DY Trofimov AB 《The journal of physical chemistry. A》2011,115(26):7722-7733
Core-level X-ray photoemission and near-edge X-ray absorption fine structure spectra of 5-methylcytosine, 5-fluorocytosine, and isocytosine are presented and discussed with the aid of high-level ab initio calculations. The effects of the methylation, halogenation, and isomerization on the relative stabilities of cytosine tautomers are clearly identified spectroscopically. The hydroxy-oxo tautomeric forms of these molecules have been identified, and their quantitative populations at the experimental temperature are calculated and compared with the experimental results and with previous calculations. The calculated values of Gibbs free energy and Boltzmann population ratios are in good agreement with the experimental results characterizing tautomer equilibrium. 相似文献
44.
Pekka Kekäläinen Mikko Voutilainen Antti Poteri Pirkko Hölttä Aimo Hautojärvi Jussi Timonen 《Transport in Porous Media》2011,87(1):125-149
A model transport system is considered in which a pulse of tracer molecules is advected along a flow channel with porous walls.
The advected tracer thus undergoes diffusion, matrix-diffusion, inside the walls, which affects the breakthrough curve of
the tracer. Analytical solutions in the form of series expansions are derived for a number of situations which include a finite
depth of the porous matrix, varying aperture of the flow channel, and longitudinal diffusion and Taylor dispersion of the
tracer in the flow channel. Novel expansions for the Laplace transforms of the concentration in the channel facilitated closed-form
expressions for the solutions. A rigorous result is also derived for the asymptotic form of the breakthrough curve for a finite
depth of the porous matrix, which is very different from that for an infinite matrix. A specific experimental system was created
for validation of matrix-diffusion modeling for a matrix of finite depth. A previously reported fracture-column experiment
was also modeled. In both cases model solutions gave excellent fits to the measured breakthrough curves with very consistent
values for the diffusion coefficients used as the fitting parameters. The matrix-diffusion modeling performed could thereby
be validated. 相似文献
45.
We consider Hermitian and symmetric random band matrices H in
d \geqslant 1{d \geqslant 1} dimensions. The matrix elements H
xy
, indexed by
x,y ? L ì \mathbbZd{x,y \in \Lambda \subset \mathbb{Z}^d}, are independent and their variances satisfy
sxy2:=\mathbbE |Hxy|2 = W-d f((x - y)/W){\sigma_{xy}^2:=\mathbb{E} |{H_{xy}}|^2 = W^{-d} f((x - y)/W)} for some probability density f. We assume that the law of each matrix element H
xy
is symmetric and exhibits subexponential decay. We prove that the time evolution of a quantum particle subject to the Hamiltonian
H is diffusive on time scales t << Wd/3{t\ll W^{d/3}} . We also show that the localization length of the eigenvectors of H is larger than a factor Wd/6{W^{d/6}} times the band width W. All results are uniform in the size |Λ| of the matrix. This extends our recent result (Erdős and Knowles in Commun. Math.
Phys., 2011) to general band matrices. As another consequence of our proof we show that, for a larger class of random matrices satisfying
?xsxy2=1{\sum_x\sigma_{xy}^2=1} for all y, the largest eigenvalue of H is bounded with high probability by 2 + M-2/3 + e{2 + M^{-2/3 + \varepsilon}} for any ${\varepsilon > 0}${\varepsilon > 0}, where M : = 1 / (maxx,ysxy2){M := 1 / (\max_{x,y}\sigma_{xy}^2)} . 相似文献
46.
Jyrki T. Louhi Antti V. R?is?nen 《International Journal of Infrared and Millimeter Waves》1997,18(11):2063-2075
A frequency doubler for 200 GHz utilising a planar surface channel Schottky varactor was designed, constructed and tested.
The doubler employes novel split-waveguide mount design with two sliding backshorts at both input and output waveguides. The
theoretical maximum efficiency of the doubler is 44.0 % with input power level of 32 mW and the maximum output power is 16.5
mW with input power level of 50 mW. The measured maximum efficiency of the doubler was 7.1 % and the maximum output power
was 2.6 mW 相似文献
47.
The molecular approach to heterogeneous nucleation 总被引:2,自引:0,他引:2
A molecular approach to heterogeneous nucleation has been developed. The expressions for the equilibrium cluster distribution, the reversible work of the cluster formation, and the nucleation rate have been derived. Two separate statements for the work of formation were formulated. If the equilibrium cluster distribution is normalized on the monomer concentration near the substrate surface, the reversible work of formation is expressed by DeltaG(het) (I) = (F(n) (het)-F(n) (hom))-(F(1) (het)-F(1) (hom)) + DeltaG(hom) where F(n) (het) and F(n) (hom) are the Helmholtz free energies of a cluster interacting with a substrate and a cluster not interacting with the substrate, respectively. If the equilibrium cluster distribution is normalized on the monomer concentration far from the substrate surface, the work of cluster formation is given by DeltaG(het) (II) = (F(n) (het)-F(n) (hom)) + DeltaG(hom). The former expression corresponds to the approach of the classical heterogeneous nucleation theory. The cluster partition function appears to be dependent on the location of a virtual plane, which separates the volume, where the interaction of the clusters with the substrate is effective from the one where interaction is negligible. Our Monte Carlo simulations have shown that the dependence is rather weak and thus the location of the plane is not very important. According to the simulations the variation of the plane position in the range from 20 to 50 Angstroms does not lead to a considerable change of the heterogeneous nucleation rate. 相似文献
48.
The interactions of Na(+) and Ag(+) cations with an Ag(111) surface in the presence and absence of water molecules were investigated with cluster models and ab initio methods. The Ag surface was described with two-layered Ag(10) and Ag(18) cluster models, and MP2/RECP/6-31+G(d) was used as the computational method. The effect of the basis set superposition error (BSSE) was taken into account with counterpoise (CP) correction. The interactions between Na(+) and Ag(111) surface were found to be primarily electrostatic, and the interaction energies and equilibrium distances at the different adsorption sites were closely similar. The largest CP-corrected MP2 adsorption energy for Na(+) was -190.2 kJ/mol. Owing to the electrostatic nature of the Na(+)-Ag(111) interaction, Na(+) prefers to be completely surrounded by water molecules rather than directly adsorbed to the surface. Ag(+)-Ag(111) interactions were much stronger than Na(+)-Ag(111) interactions because they were dominated by orbital contributions. The largest CP-corrected MP2 adsorption energy for Ag(+) was -358.9 kJ/mol. Ag(+) prefers to adsorb on sites where it can bind to several surface atoms, and in the presence of water molecules, it remains adsorbed to the surface while the water molecules form hydrogen bonds with one another. The CP correction had an effect on the interaction energies but did not change the relative trends. 相似文献
49.
Alvarez Ruiz J Kivimäki A Stankiewicz M Melero García E Coreno M Ali S Koperski J Rachlew E Vall-Llosera I Serrano G Feyer V Tuckett R 《Physical chemistry chemical physics : PCCP》2006,8(44):5199-5206
The interaction of SF(5)CF(3) with vacuum-UV radiation has been investigated by photon induced fluorescence spectroscopy. Total fluorescence yield and dispersed fluorescence spectra of SF(5)CF(3) were recorded in the 200-1000 nm fluorescence window. In all cases, the fluorescence spectra resemble those of CF(3)X (X = H, F, Cl, and Br) molecules. At photon energies below 20 eV, the emission is attributed to the excited CF(3) and CF(2) fragments. The threshold for the CF(3) emission is 10.2 +/- 0.2 eV, giving an upper limit estimate for the SF(5)-CF(3) bond dissociation energy of 3.9 +/- 0.3 eV. The excitation functions of the CF(3) and CF(2) emissions were measured in the photon energy range 13.6-27.0 eV. The resonant structures observed in SF(5)CF(3) are attributed to electronic transitions from valence to Rydberg orbitals, following similar assignments in CF(3)X molecules. The photoabsorption spectrum of SF(5)CF(3) shows features at the same energies, indicating a strong contribution from Rydberg excitations. 相似文献
50.
Esa Järvenpää Maarit Järvenpää Antti Käenmäki Tapio Rajala Sari Rogovin Ville Suomala 《Mathematische Zeitschrift》2010,266(1):83-105
Let X be a metric measure space with an s-regular measure μ. We prove that if A ì X{A\subset X} is r{\varrho} -porous, then dimp(A) £ s-crs{{\rm {dim}_p}(A)\le s-c\varrho^s} where dimp is the packing dimension and c is a positive constant which depends on s and the structure constants of μ. This is an analogue of a well known asymptotically sharp result in Euclidean spaces. We illustrate by an example that the
corresponding result is not valid if μ is a doubling measure. However, in the doubling case we find a fixed N ì X{N\subset X} with μ(N) = 0 such that
dimp(A) £ dimp(X)-c(log\tfrac1r)-1rt{{\rm {dim}_p}(A)\le{\rm {dim}_p}(X)-c(\log \tfrac1\varrho)^{-1}\varrho^t} for all r{\varrho} -porous sets A ì X\ N{A \subset X{\setminus} N} . Here c and t are constants which depend on the structure constant of μ. Finally, we characterize uniformly porous sets in complete s-regular metric spaces in terms of regular sets by verifying that A is uniformly porous if and only if there is t < s and a t-regular set F such that A ì F{A\subset F} . 相似文献