全文获取类型
收费全文 | 348291篇 |
免费 | 4321篇 |
国内免费 | 1151篇 |
专业分类
化学 | 190438篇 |
晶体学 | 5681篇 |
力学 | 14709篇 |
综合类 | 18篇 |
数学 | 35632篇 |
物理学 | 107285篇 |
出版年
2021年 | 2313篇 |
2020年 | 2680篇 |
2019年 | 2939篇 |
2018年 | 3245篇 |
2017年 | 3348篇 |
2016年 | 5550篇 |
2015年 | 3873篇 |
2014年 | 5709篇 |
2013年 | 15778篇 |
2012年 | 11945篇 |
2011年 | 14848篇 |
2010年 | 9851篇 |
2009年 | 10048篇 |
2008年 | 13436篇 |
2007年 | 13331篇 |
2006年 | 12681篇 |
2005年 | 11590篇 |
2004年 | 10528篇 |
2003年 | 9368篇 |
2002年 | 9057篇 |
2001年 | 11031篇 |
2000年 | 8408篇 |
1999年 | 6417篇 |
1998年 | 4974篇 |
1997年 | 4947篇 |
1996年 | 4792篇 |
1995年 | 4380篇 |
1994年 | 4181篇 |
1993年 | 3921篇 |
1992年 | 4804篇 |
1991年 | 4697篇 |
1990年 | 4401篇 |
1989年 | 4400篇 |
1988年 | 4428篇 |
1987年 | 4387篇 |
1986年 | 4169篇 |
1985年 | 5710篇 |
1984年 | 5708篇 |
1983年 | 4527篇 |
1982年 | 4767篇 |
1981年 | 4727篇 |
1980年 | 4483篇 |
1979年 | 4732篇 |
1978年 | 4827篇 |
1977年 | 4883篇 |
1976年 | 4689篇 |
1975年 | 4412篇 |
1974年 | 4304篇 |
1973年 | 4317篇 |
1972年 | 2777篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
161.
C. Waldtmann H.-U. Everts B. Bernu C. Lhuillier P. Sindzingre P. Lecheminant L. Pierre 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):501-507
We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagomé lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis
of Néel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather
unusual. The magnetic () excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic () excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless
continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these
states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg
antiferromagnet on the kagomé lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably
they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets.
Received: 27 November 1997 / Accepted: 29 January 1998 相似文献
162.
E. Jackiewicz M. Lukasiak M. Kopcewicz K. Szpila N. Bakun-Czubarow 《Hyperfine Interactions》1992,70(1-4):993-996
Terrestrial basalt rocks considered to be a good analogues of the Martian regolith were studied by using the Mössbauer spectroscopy and X-ray diffraction. The weathering of basalts was followed by the changes of the Fe2+/Fe3+ ratio. The basalt rock undergoes alteration from primary mineral composition (olivine+pyroxene+plagioclase) through smectite to kaolinite clay with oxides and hydroxides with the increase of the Fe2+/Fe3+ ratio. 相似文献
163.
Ron C. Blei 《Probability Theory and Related Fields》1993,95(2):141-153
Summary Multi-parameter stochastic integrators are described and classified according to directions of integrability. Sets of directions are distinguished precisely by the combinatorial dimension of corresponding fractional Cartesian products. The main theorem establishes existence of stochastic processes which are integrators in prescribed directions but not others. 相似文献
164.
Stephen C. Littlechild 《The Journal of the Operational Research Society》1996,47(5):601-611
In this Blackett Lecture, Professor Littlechild, Director General of Electricity Supply in the UK's Office of Electricity Regulation, renews acquaintance with some models he has developed over the years, introduces some models he has recently met, and discusses models that it would seem to be worthwhile developing. He concludes that for all concerned in the electricity industry, success depends on ever improving decision-making, in which OR has a vital role. 相似文献
165.
Highly sensitive optical reflectivity measurements are used to investigate the layer-by-layer transition in extremely thin
and thick N-(4-n-hexyloxybenzylidene)-4-n-hexylaniline (6O.6) films. The simple power-law form, N=N
0
t
−1/3, for the penetration of the crystal-B order is found to describe the transitions only near the surface. A deviation from
the power law is observed for the interior layers.
Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 266–269 (25 February 1996)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
166.
C. Fallnich B. Ruffing Th. Herrmann A. Nebel R. Beigang R. Wallenstein 《Applied physics. B, Lasers and optics》1995,60(5):427-436
We report on a cw mode-locked non-critically phase matched KTP optical parametric oscillator synchronously pumped by a picosecond Ti:Sapphire laser. High average signal output power of up to 950 mW over a large tuning range has been achieved. For this OPO the influence of resonator-length detuning on the output power, pulse duration and spectral bandwidth has been investigated. The measured data are in good agreement with the results of a numerical simulation using a split-step Fourier method which considers the group-velocity mismatch, the group-velocity dispersion and the self-phase modulation. The numerical simulation also describes the measured strong pump depletion and its influence on the OPO output and efficiency. 相似文献
167.
168.
NaY zeolite samples loaded with sodium metal by vapor phase deposition have been investigated using129Xe NMR spectroscopy. At low sodium concentration, the129Xe NMR spectrum showed three resonance lines which clearly indicate the existence of distinct domains in the zeolite sample. Such an observation suggests that the diffusion of the xenon atoms into each domain only occurs with respect to the NMR time scale (2.9 ms). As the sodium concentration increases, observation of a single broad line indicate a macroscopic homogenization of the system. The shift of this line is explained in part due to a paramagnetic interaction between the xenon atoms and the unpaired electrons of particles containing an odd number of sodium atoms. The linewidth is due to the distribution of the local magnetic fields partially averaged by the rapid motion of the xenon atoms and to the statistical distribution of the sodium particles in the supercage cavities. The paramagnetic interaction vanishes with the oxidation of the sample leading to a narrowing and a shift of the line to higher magnetic fields. 相似文献
169.
We study spectral properties of discrete Schrödinger operators with potentials obtained via dimerization of a class of aperiodic sequences. It is shown that both the nature of the autocorrelation measure of a regular sequence and the presence of generic (full probability) singular continuous spectrum in the hull of primitive and palindromic (four block substitution) potentials are robust under dimerization. Generic results also hold for circle potentials. We illustrate these results with numerical studies of the quantum mean square displacement as a function of time. The numerical techniques provide a very fast algorithm for the time evolution of wave packets. 相似文献
170.
In this paper we consider a wavelet algorithm for the piecewise constant collocation method applied to the boundary element solution of a first kind integral equation arising in acoustic scattering. The conventional stiffness matrix is transformed into the corresponding matrix with respect to wavelet bases, and it is approximated by a compressed matrix. Finally, the stiffness matrix is multiplied by diagonal preconditioners such that the resulting matrix of the system of linear equations is well conditioned and sparse. Using this matrix, the boundary integral equation can be solved effectively. 相似文献