全文获取类型
收费全文 | 215993篇 |
免费 | 2205篇 |
国内免费 | 574篇 |
专业分类
化学 | 118446篇 |
晶体学 | 3743篇 |
力学 | 8464篇 |
综合类 | 11篇 |
数学 | 21177篇 |
物理学 | 66931篇 |
出版年
2020年 | 1880篇 |
2019年 | 2025篇 |
2018年 | 2350篇 |
2017年 | 2448篇 |
2016年 | 3702篇 |
2015年 | 2396篇 |
2014年 | 3793篇 |
2013年 | 9583篇 |
2012年 | 7053篇 |
2011年 | 8700篇 |
2010年 | 6080篇 |
2009年 | 5998篇 |
2008年 | 7742篇 |
2007年 | 7709篇 |
2006年 | 7336篇 |
2005年 | 6715篇 |
2004年 | 6067篇 |
2003年 | 5538篇 |
2002年 | 5267篇 |
2001年 | 6500篇 |
2000年 | 4979篇 |
1999年 | 3733篇 |
1998年 | 2929篇 |
1997年 | 2882篇 |
1996年 | 2827篇 |
1995年 | 2654篇 |
1994年 | 2530篇 |
1993年 | 2416篇 |
1992年 | 3056篇 |
1991年 | 2947篇 |
1990年 | 2836篇 |
1989年 | 2862篇 |
1988年 | 2912篇 |
1987年 | 2898篇 |
1986年 | 2735篇 |
1985年 | 3485篇 |
1984年 | 3494篇 |
1983年 | 2723篇 |
1982年 | 2873篇 |
1981年 | 2902篇 |
1980年 | 2671篇 |
1979年 | 3008篇 |
1978年 | 3011篇 |
1977年 | 3092篇 |
1976年 | 2944篇 |
1975年 | 2661篇 |
1974年 | 2619篇 |
1973年 | 2557篇 |
1972年 | 1759篇 |
1968年 | 1731篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
911.
We calculate the exclusive semileptonic bottom meson decays \(B \to D(D*) + l^ - + \bar v_l \) in the spectator quark model. The helicity structure of the mesonic current transitionsB→D(D *) is matched to the helicity structure of the free quark current transitionsb→c at minimum momentum transferq 2=0. The results are continued toq 2≠=0 by pole-dominated form factors. Our results are compared to recent calculations that use quark model dynamics at maximum momentum transferq max 2 = (M 1 ?M 2)2. We find agreement atq max 2 . Atq 2=0 there are significant differences between the predictions of the two approaches leading to marked differences in the predictions for the shape of the lepton energy spectrum, the shape of theq 2-distribution, and the helicity composition of the transition measurable in the angular distributions of the decaysD *→Dπ and \(W_{virtual}^ - \to l^ - + \bar v_l \) . 相似文献
912.
B. Dorner D. Visser U. Steigenberger K. Kakurai M. Steiner 《Zeitschrift für Physik B Condensed Matter》1988,72(4):487-496
We have measured the magnetic excitations in CsFeBr3 along the chain direction (z-axis) and perpendicular to it by inelastic neutron scattering. The measured dispersion curves can be reproduced by the formula $$\omega ^2 (q) = A^2 - 8A[J\cos (\pi q_c ) + J'\gamma (2\pi q_ \bot )]R(T)$$ which is very different from dispersion relations for usual 1 D antiferromagnets, because of the singlet groundstate of this system. The large value of the anisotropy energyA/k=29.8±0.5 K, which is independent of temperature, causes the singlet groundstate. The intra-and inter-chain exchange parameters areJ/k=?3.2±0.15K andJ'/k=?0.32±0.02 K determined atT=1.4 K. At 1.4 K the renormalisation factor is taken to beR(T)=1. AtT=30 K the value forR(T) was found to be 0.5±0.1 keepingJ andJ' unchanged. The excitations at (1/3 1/31) show soft mode behaviour but no phase transition. The observed intensities are interpreted by a heuristic model for the eigenvectors in the excited state including antiferro-as well as ferromagnetic configurations of the fluctuations. 相似文献
913.
K. Bolwin W. Schnurnberger G. Schiller 《Zeitschrift für Physik B Condensed Matter》1988,72(2):203-209
Investigation on core level and valence band photoelectron spectra of lanthanide perovskite samples (Ag
x
La1–x
CoO3;x<0,3) and reinvestigations of metallic lanthanum and La-oxide layers are reported. Differences in preparation method during perovskite formation are leading to different quality of sample purity and different electronic properties. This could be followed by changes in the density of states near the Fermi energy. On the other hand the screening mechanism after ionization of a La 3d core hole is affected by the relative energy position of occupied valence band states with respect to the empty La 4f
* level in the core ionized state. Changes in La 3d line profile are discussed as changes in multi electron processes (e.g. shake up or energy gain) during photoionization. We attribute the two electron process during photoemission of La 3d electrons to the well screened energy gain transition in LaCoO3, whereas the two electron process results in a shake up transition in La2O3 and La(OH)3. 相似文献
914.
T. S. Baller G. N. A. van Veen H. A. M. van Hal 《Applied Physics A: Materials Science & Processing》1988,46(3):215-220
Thin layers YBa2Cu3O7–x. are deposited by a laser ablation technique using a pulsed excimer laser operating at 308 nm. The influence of the substrate material and the annealing procedure on the superconducting behaviour of the 123 film and the reactions between the film and the substrate are studied by resistance, X-ray patterns and TEM measurements. The best results are obtained for deposition on (100) SrTiO3 substrates. The resistance of the 1 m thick film shows a metallic behaviour, an onset in superconductivity at a temperature of 90 K, and has zero resistance at 86 K. The 123 material has a preferential oriented c-axis perpendicular to the surface plane. 相似文献
915.
L. Käubler W. Enghardt H. Prade P. Carlé L. O. Norlin K. -G. Rensfelt U. Rosengård 《Zeitschrift für Physik A Hadrons and Nuclei》1988,329(2):143-150
Using the reaction138Ba(α,2n)140Ce the magnetic moment of the 10 1 + isomer atE x =3714.7 keV in theN=82 nucleus140Ce has been determined by means of the TDPAD method toμ=+10.3(4)μ N . Measuredg-factors in140Ce are compared to calculations within the shell model with configuration mixing. For the 10 1 + isomer in140Ce the four proton configuration π(1g 7 2/2 ,2d 5 2/2 ) has been found to be dominant. From theg-factor measurement strong contributions of multiparticle excitations to thegp2d 3/2,π3s 1 2 or π1h 11 2 shells and admixtures of neutron excitations to the wave function of the 10 1 + state could be excluded. The strongE1γ-branch of the deexcitation of the 10 1 + isomer in140Ce can be explained by means of small admixtures of configurations which contain the outer subshell excitationsπ2f 7/2 andπ1h 9/2. On this basisE1 transitions experimentally observed in theN=82 nuclei140Ce,141Pr and145Eu may be understood. 相似文献
916.
K. -H. Speidel M. Knopp W. Karle U. Kilgus M. -L. Dong H. -J. Simonis F. Hagelberg J. Gerber 《Zeitschrift für Physik A Hadrons and Nuclei》1988,331(1):29-36
Transient field precessions have been measured with the first excited 2 1 + -state as probe for ions of28Si traversing Fe at vion?1v0 and 13v0(v0=c/137) and62Ni being stopped in Fe. The degree of polarization deduced for the Si ions, p1s=0.19(6), is consistent with low-velocity data. There is clear evidence that the field strength is attenuated by heavy ion beams. For the62Ni(2 1 + ) state at 1.173 MeV a g-factor value of g=0.34(7) was obtained in good agreement with a previous result. 相似文献
917.
J. Dobaczewski W. Nazarewicz A. Plochocki K. Rykaczewski J. Zylicz 《Zeitschrift für Physik A Hadrons and Nuclei》1988,329(3):267-273
Selected properties of the Gamow-Teller beta decay of even nuclei near100Sn have been studied. Two theoretical models were used: a self-consistent Hartree-Fock-Bogolyubov approach with the Skyrme interaction and a shell-correction model with a Woods-Saxon average field and monopole pairing residual interaction. Calculations reproduce fairly well both theQ EC values and the average energies of theI π=1+ states fed directly in the decay. It is shown that the observed energy splitting between the 1+ states cannot be associated with a stable deformation of a final doubly-odd nucleus. The calculated pairing factors are used to estimate the 0+ → 1+ GT transition strength in the neighbourhood of doubly-magic 50 100 Sn50. 相似文献
918.
The reaction between thiocarbamate herbicides and 2,6-dichlorobenzoquinone-N-chloroimine or 2,6-dibromobenzoquinone-N-chloroimine is suitable for the detection of these herbicides on thin-layer plates with high sensitivity. The reactions were followed by infrared, nuclear magnetic resonance and mass spectrometry. We have established the formation of 2,6-dichlorobenzoquinone-S-alkyl sulphenylimines. In the case of the bromo-derivative, halogen exchange and substitution on the quinone ring took place simultaneously leading to the formation of mixed halogenated 2,6-dihalo- and, in addition, 2,3,6-trihalobenzoquinone-S-alkyl sulphenylimines. The final product of the detection reaction, i.e. 2,6-dichlorobenzoquinone-S-alkyl sulphenylimine was reacted with 2,6-dibromobenzoquinone-N-chloroimine where 2,6-dichloro-3-bromobenzoquinone-S-alkyl sulphenylimine formed as a consequence of the looser bromine-carbon linkage on the 2 and 6 positions of the quinone ring. 相似文献
919.
Starting from ergosterol, the synthesis of the brassinosteroid 24-epiteasterone has been achieved by the use of a new scheme for introducing a 3-hydroxy-6-keto group as the result of the Birch reduction of the corresponding 5-hydroxy-7-3,6-dione.Institute of Bioorganic Chemistry, Belorussian Academy of Sciences, Minsk. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 224–227, March–April, 1992. 相似文献
920.
Reproducibility for successive determinations with a hanging mercury drop electrode is assessed in relation to solution stirring, drop size and back-diffusion to the mercury thread. The effect of experimental parameters such as drop size, deposition time and gallium concentration on the observed stripping current is investigated. The interference of zinc present in a 0.5M NaSCN + 4.2M NaClO(4) supporting electrolyte on the gallium detection limit and calibration plots is described. Formation of an intermetallic compound with a Zn:Ga ratio of 2:3 and its destruction by co-deposition of zinc with Sb are reported. A detection limit of 10(-8)M gallium was obtained in the presence of 10(-5)M Sb(III). 相似文献