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991.
S. Heinz E. Berdermann F. Heine O. Joeres P. Kienle I. Koenig W. Koenig C. Kozhuharov U. Leinberger M. Rhein A. Schröter H. Tsertos The ORANGE Collaboration at GSI 《The European Physical Journal A - Hadrons and Nuclei》2000,9(1):55-61
We present new results from measurements and simulations of positron spectra, originating from 238U + 181Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental
setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion
(IPC) processes in the measured e+-energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by
Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission
from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron
spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities
of the observed γ-transitions ( E
γ∼ 1250-1600 keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons,
caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines
have been observed, is presented and the implications are discussed.
Received: 27 April 2000 / Accepted: 14 September 2000 相似文献
992.
H. Kachkachi A. Ezzir M. Noguès E. Tronc 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(4):681-689
We present a microscopic model for nanoparticles, of the maghemite (-Fe2O3) type, and perform classical Monte Carlo simulations of their magnetic properties. On account of M?ssbauer spectroscopy and
high-field magnetisation results, we consider a particle as composed of a core and a surface shell of constant thickness.
The magnetic state in the particle is described by the anisotropic classical Dirac-Heisenberg model including exchange and
dipolar interactions and bulk and surface anisotropy. We consider the case of ellipsoidal (or spherical) particles with free
boundaries at the surface. Using a surface shell of constant thickness ( nm) we vary the particle size and study the effect of surface magnetic disorder on the thermal and spatial behaviors of the
net magnetisation of the particle. We study the shift in the surface “critical region” for different surface-to-core ratios
of the exchange coupling constants. It is also shown that the profile of the local magnetisation exhibits strong temperature
dependence, and that surface anisotropy is responsible for the non saturation of the magnetisation at low temperatures.
Received 1 September 1999 and Received in final form 3 November 1999 相似文献
993.
The dynamics defined by the Hamiltonian
, where the
m are fixed random phases, is investigated for large values of A, and for
. For a given P
* and for
, this Hamiltonian is transformed through a rigorous perturbative treatment into a Hamiltonian where the sum of all the nonresonant terms, having a Q dependence of the kind cos(kQ – nt +
m) with
\Delta \upsilon$$
" align="middle" border="0">
, is a random variable whose r.m.s. with respect to the
m is exponentially small in the parameter
. Using this result, a rationale is provided showing that the statistical properties of the dynamics defined by H, and of the reduced dynamics including at each time t only the terms in H such that
, can be made arbitrarily close by increasing . For practical purposes close to 5 is enough, as confirmed numerically. The reduced dynamics being nondeterministic, it is thus analytically shown, without using the random-phase approximation, that the statistical properties of a chaotic Hamiltonian dynamics can be made arbitrarily close to that of a stochastic dynamics. An appropriate rescaling of momentum and time shows that the statistical properties of the dynamics defined by H can be considered as independent of A, on a finite time interval, for A large. The way these results could generalize to a wider class of Hamiltonians is indicated. 相似文献
994.
Tristan Rivière 《Letters in Mathematical Physics》1998,45(3):229-238
We introduce a formulation of the Skyrme problem using differential forms. By means of this formulation, we prove first that the homothetic map between the standard three-sphere of radius R, S3
r R4, and S3
1 is the unique minimizer, modulo isometries, of the Skyrme energy in its homotopy class, for any R less than some critical value R0 (3/2, 2]. We then establish a stability result for this Skyrme-form problem from which we can recover the result of M. Loss and N. S. Manton which states that this homothetic map is stable only up to R = 2. 相似文献
995.
Aaraki Huzihiro Flato Moshé Michéa Sébastien Sternheimer Daniel 《Letters in Mathematical Physics》1998,43(2):155-171
We indicate similarities in the structure of two types of infinite-dimensional algebras, one introduced 28 years ago in connection with the mass problem of elementary particles and the other seven years ago in connection with spin systems (XY models). We show that these algebras can be considered as representations of a single Grand Algebra, the enveloping algebra of an affine Kac–Moody algebra built on the Poincaré Lie algebra. As an associative and coassociative bialgebra of operators, the latter representation of the grand algebra is a preferred nontrivial deformation of the Ising case bialgebra. 相似文献
996.
997.
The particle swarm optimization (PSO) technique is a powerful stochastic evolutionary algorithm that can be used to find the global optimum solution in a complex search space. This paper presents a variation on the standard PSO algorithm called the rank based particle swarm optimizer, or PSOrank, employing cooperative behavior of the particles to significantly improve the performance of the original algorithm. In this method, in order to efficiently control the local search and convergence to global optimum solution, the γ best particles are taken to contribute to the updating of the position of a candidate particle. The contribution of each particle is proportional to its strength. The strength is a function of three parameters: strivness, immediacy and number of contributed particles. All particles are sorted according to their fitness values, and only the γ best particles will be selected. The value of γ decreases linearly as the iteration increases. A time-varying inertia weight decreasing non-linearly is introduced to improve the performance. PSOrank is tested on a commonly used set of optimization problems and is compared to other variants of the PSO algorithm presented in the literature. As a real application, PSOrank is used for neural network training. The PSOrank strategy outperformed all the methods considered in this investigation for most of the functions. Experimental results show the suitability of the proposed algorithm in terms of effectiveness and robustness. 相似文献
998.
999.
采用水热法合成了系列双掺杂的萤石结构Ce1-xLnxCa0.2O1.8-0.4x固溶体,并研究了3种稀土离子对固溶体导电性的影响,发现掺杂离子半径接近Ce^4 时,体系电导率增大而活化能降低,同时发现水热合成的Ce1-xLnxCa0.2O1.8-0.4x样品的平均粒度按着掺入离子Y^3 ,Gd^3 ,La^3 的顺序逐渐减小,分别为32,20和15nm,这种变化是由于Y^3 半径比La^3 和G ^3 更接近Ce^4 ,因而在水热合成过程中, 掺Y^3 的体系更有利于晶粒的生长,得到的晶体粒度较大。 相似文献
1000.