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121.
Y. Kafri D. Mukamel L. Peliti 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(1):135-146
Existing experimental studies of the thermal denaturation of DNA yield sharp steps in the melting curve suggesting that the
melting transition is first order. This transition has been theoretically studied since the early sixties, mostly within an
approach in which the microscopic configurations of a DNA molecule consist of an alternating sequence of non-interacting bound
segments and denaturated loops. Studies of these models neglect the repulsive, self-avoiding, interaction between different
loops and segments and have invariably yielded continuous denaturation transitions. In the present study we take into account
in an approximate way the excluded-volume interaction between denaturated loops and the rest of the chain. This is done by
exploiting recent results on scaling properties of polymer networks of arbitrary topology. We also ignore the heterogeneity
of the polymer. We obtain a first-order melting transition in d = 2 dimensions and above, consistent with the experimental results. We also consider within our approach the unzipping transition,
which takes place when the two DNA strands are pulled apart by an external force acting on one end. We find that the under
equilibrium condition the unzipping transition is also first order. Although the denaturation and unzipping transitions are
thermodynamically first order, they do exhibit critical fluctuations in some of their properties. For instance, the loop size
distribution decays algebraically at the transition and the length of the denaturated end segment diverges as the transition
is approached. We evaluate these critical properties within our approach.
Received 21 August 2001 and Received in final form 26 January 2002 相似文献
122.
Karnaukhov V. G. Kozlov V. I. Rasskazov A. O. Karnaukhova O. V. 《Mechanics of Composite Materials》2003,39(1):17-26
Based on the Kirchhoff-Love hypotheses and adequate supplementary hypotheses for the distribution of electric field quantities, a model for parametric vibrations of composite shells of revolution made of a passive (without a piezoeffect) middle layer and two active (with a piezoeffect) surface layers under the action of harmonic mechanical and electric loads is developed. The dissipative material properties are taken into account by linear viscoelastic models. Since the vibrations on the boundary of the main domain of dynamic instability (MDDI) are harmonic, the investigation of this domain, in a first approximation, is reduced to generalized eigenvalue problems, which are solved by the finite-element method. The problem on parametric vibrations of a three-layer conical shell under harmonic mechanical loading is considered. The influence of the shell thickness, dissipation, and electric boundary conditions on the MDDI is investigated. Two limiting cases of electric boundary conditions are considered, where the electrodes are short-circuited or not. The curves presented show a considerable influence of the electric boundary conditions on the characteristics of the MDDI, namely on its width and position on the frequency axis and on the critical parameter of excitation. 相似文献
123.
S. M. Arabei 《Journal of Applied Spectroscopy》1998,65(4):560-566
Monochromatic photoexcitation and selective registration in bands of purely electronic S1↔S0 transitions resulted in fine fluorescence and absorption (fluorescence excitation) spectra of hypericin in polyvinylbutyral
at a temperature of 4.2 K. Vibration analysis of the spectra is done and conclusions are made about the form of many normal
vibrations. It is found that the formation of intramolecular hydrogen bonds in the hypericin structure has a substantial effect
on the relative position of the electronic vibrational levels of the pigment molecule.
Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina ave., Minsk 200072,
Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 539–545, July–August, 1998. 相似文献
124.
Using the CIS/6-311+G** method, the normal vibrations of the benzene molecule in the first singlet electron-excited state 1
B
2u
have been calculated. The algorithm of calculation of the force field in dependent coordinates by the method of generalized inversion is described. A method of autoscaling in dependent natural coordinates is suggested. For six groups of natural coordinates scaling factors have been obtained, the use of which has led to agreement between the calculated and experimental frequencies of the vibrations of benzene. The frequencies of the B
1u
-symmetry-type vibrations, for which there is no experimental assignment, have been calculated. The problem of selection of the force field in dependent coordinates is discussed. 相似文献
125.
In this paper, we study the exact controllability and boundary stabilization of the torsional vibrations of a flexible space structure (such as a solar cell array) modeled by a rectangular panel, incorporating the material damping of the structure. The panel is hoisted at one end by a rigid hub and the other end is totally free. For the attachment of this hub on one side of the panel, the hub dynamics leads to a nonstandard boundary condition. To incorporate internal damping of the material, we assume Voigt-type viscoelasticity of the structure. Exact controllability theory is established using the Hilbert uniqueness method by means of a control torque applied only on the rigid hub of the panel. At the same time, uniform exponential energy decay rate is obtained directly for the solution of this problem. 相似文献
126.
127.
Temperature-dependent surface X-ray diffraction experiments have been performed on the K/Ag(001)-(2 × 1) adsorption system. The structure is characterized by a missing-row geometry in which alternate Ag rows along [1
0] are missing. The K atoms reside within the large grooves coordinated by six Ag atoms at a distance of 3.44(5) Å, corresponding to an effective K-radius of 2.00(5) Å. Large anisotropic disorder is observed for both the K-atoms and the top-layer (“ridge”) Ag atoms. The K-atom displacements are largest in the direction along the grooves, whereas for the Ag atoms the vibrations along [110] are significantly larger. The temperature dependence of the Ag vibrations is in accordance with Debye theory for the [110] direction, but deviates from it for the [1
0] vibrations at high temperature. In contrast to the K-atoms, the out-of-plane vibrations of the top-layer Ag atoms are larger than the in-plane vibrations. The inclusion of anharmonic contributions to describe the Ag disorder significantly improves the fits. It is shown that if anharmonicity is neglected the interlayer contraction is overestimated (Δd12/d12 only −3.2%, instead of −12.7% if anharmonicity is neglected). Due to the anharmonicity, different definitions of the atomic position arise (mean, mode and equilibrium position), which are discussed on the basis of the results. 相似文献
128.
In this paper, a model equation is presented for the study of rain-wind-induced vibrations of a simple oscillator. As will be shown the presence of raindrops in the wind-field may have an essential influence on the dynamic stability of the oscillator. In this model equation the influence of the variation of the mass of the oscillator due to an incoming flow of raindrops hitting the oscillator and a mass flow which is blown and shaken off is investigated. The time-varying mass is modeled by a time harmonic function whereas simultaneously also time-varying lift and drag forces are considered. 相似文献
129.
G. Tonini F. Werner Y. Castin 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2006,39(2):283-294
We investigate theoretically the formation of a vortex lattice in
a superfluid two-spin component Fermi gas in a rotating harmonic trap, in a BCS-type
regime of condensed non-bosonic pairs.
Our analytical solution of the superfluid
hydrodynamic equations, both for the 2D BCS equation of state
and for the 3D unitary quantum gas, predicts that the vortex free gas is
subject to a dynamic instability for fast enough rotation.
With a numerical solution of the full
time dependent BCS equations in a 2D model, we confirm the existence of this dynamic
instability and we show that it leads to the formation of a regular pattern
of quantum vortices in the gas. 相似文献
130.
Igor E. Protsenko 《Journal of Russian Laser Research》2006,27(5):414-436
A new method for calculating the total power of the spontaneous emission of a few motionless two-level atoms located at a distance of the order of the resonance-radiation wavelength with dipole-dipole interaction is suggested. The method is based on the Schrödinger representation. It is shown, as an example, that two trapped atoms cannot emit a pulse of superradiance under any conditions, while four atoms can emit such a pulse at certain conditions. Different ways of introducing the quasi-stationary states of atoms along with the generalization of the proposed method to other resonance systems are discussed. 相似文献