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171.
A relay strategy was employed to achieve an intermolecular ene-yne metathesis between 1,1-disubstituted alkenes and alkynes. The relay serves to activate an unreactive alkene which will not participate in ene-yne metathesis. The new relay cross ene-yne metathesis gives rise to 1,1,3-trisubstituted-1,3-dienes previously inaccessible by direct ene-yne metathesis methods. 相似文献
172.
The single-mode phase-sensitive parametric amplifier has been proposed as an approximation to an ideal quantum phase amplifier. We demonstrate numerically that a real, multimode parametric amplifier operated in the spatial domain, and realized by the use of three-wave mixing in nonlinear media, behaves similar to a single-mode quantum phase amplifier, but exhibits smaller phase gain. Constraints for operation in this regime are found to be consistent with small departures of phase from the ideal deamplification condition. 相似文献
173.
Crystallography indicates that molecules in crystalline cellulose either have twofold screw-axis (21) symmetry or closely approximate it, leading to short distances between H4 and H1′ across the glycosidic linkage. Therefore,
modeling studies of cellobiose often show elevated energies for 21 structures, and experimental observations are often interpreted in terms of intramolecular strain. Also, some computer models
of cellulose crystallites have an overall twist as well as twisted individual chains, again violating 21 symmetry. To gain insight on the question of inherent strain in 21 structures, modeling was employed and crystal structures of small molecules were surveyed. (Residues in a disaccharide cannot
be related by 21 symmetry because they are not identical but if their linkage geometry would lead to 21 symmetry for an infinite cellulose chain, the disaccharide would have 21 pseudo symmetry.) Several initial structures in quantum mechanics (QM) studies of cellobiose minimized to structures having
21 pseudo symmetry. Similarly, a number of relevant small molecules in experimental crystal structures have pseudo symmetry.
While the QM models of cellobiose with 21 pseudo symmetry had inter-residue hydrogen bonding, the experimental studies included cellotriose undecaacetate, a molecule
that cannot form conventional hydrogen bonds. Limitations in characterizing symmetry based on the linkage torsion angles ϕ
and ψ were also explored. It is concluded that 21 structures have little intrinsic strain, despite indications from empirical models. 相似文献
174.
Robinson T Valluri P Manning HB Owen DM Munro I Talbot CB Dunsby C Eccleston JF Baldwin GS Neil MA de Mello AJ French PM 《Optics letters》2008,33(16):1887-1889
Fluorescence lifetime imaging (FLIM) is used to quantitatively map the concentration of a small molecule in three dimensions in a microfluidic mixing device. The resulting experimental data are compared with computational fluid-dynamics (CFD) simulations. A line-scanning semiconfocal FLIM microscope allows the full mixing profile to be imaged in a single scan with submicrometer resolution over an arbitrary channel length from the point of confluence. Following experimental and CFD optimization, mixing times down to 1.3+/-0.4 ms were achieved with the single-layer microfluidic device. 相似文献
175.
Christopher French 《代数通讯》2013,41(3):1143-1170
The concept of an association scheme is a far-reaching generalization of the notion of a group. Many group theoretic facts have found a natural generalization in scheme theory. One of these generalizations is the observation that, similar to groups, association schemes of finite order are commutative if they have at most five elements and not necessarily commutative if they have six elements. While there is (up to isomorphism) only one noncommutative group of order 6, there are infinitely many pairwise non-isomorphic noncommutative association schemes of finite order with six elements. (Each finite projective plane provides such a scheme, and non-isomorphic projective planes yield non-isomorphic schemes.) In this note, we investigate noncommutative schemes of finite order with six elements which have a symmetric normal closed subset with three elements. We take advantage of the classification of the finite simple groups. 相似文献
176.
The alternating step generator is a well-known keystream generator consisting of two stop/go clocked LFSRs, LFSR1 and LFSR2, whose clocks are controlled by another LFSR, LFSR3, which is clocked regularly. A probabilistic analysis of this generator is conducted which shows that the posterior probabilites of individual bits of the first derivatives of the regularly clocked LFSR1 and LFSR2 sequences, when conditioned on a given segment of the first derivative of the keystream sequence, can be computed efficiently in a number of probabilistic models of interest. The expected values of these probabilities, for a random keystream sequence, are derived by an approximate theoretical analysis and are also verified by systematic computer experiments. It is pointed out that these posterior probabilities can be enhanced in a resynchronization scenario and thus used for a low-complexity fast correlation attack on the two LFSRs. More generally, it is argued that even without resynchronization these probabilities may be significantly different from one half for fast correlation attacks based on iterative decoding algorithms to be successful, although with incresead complexity. A related method for computing the posterior probabilities of individual bits of the LFSR3 sequence, when conditioned on both the keystream sequence and the LFSR1 and LFSR2 sequences, is also developed. As these posterior probabilities are much more different from one half, they can be used for a low-complexity fast correlation attack on LFSR3, provided that the initial states of LFSR1 and LFSR2 are previously reconstructed. 相似文献
177.
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180.
M. Ebrahimzadeh S. French W. Sibbett A. Miller 《Applied physics. B, Lasers and optics》1995,60(5):443-448
We report a new source of high-repetition rate and widely tunable picosecond pulses for the near infrared. A singly resonant, cw, picosecond optical parametric oscillator based on temperature-tuned LiB3O5 and synchronously pumped by 1.8 ps pulses from a self-mode-locked Ti:Sapphire laser is demonstrated. The oscillator can provide average output powers of up to 90 mW under non-critical type-I phase matching at a pulse repetition rate of 81 MHz. Without dispersion compensation, transform-limited signal pulses with 720 fs durations have been generated at 1.2 times threshold. With the available mirror set, signal tuning over 1.374–1.530 µm and idler tuning over 1.676–1.828 µm is demonstrated for a range of pump wavelengths and phase-matching temperatures. With additional mirrors, continuous tuning throughout 1–2.7 µm should be readily attainable with a single LiB3O5 crystal. 相似文献