全文获取类型
收费全文 | 374495篇 |
免费 | 3970篇 |
国内免费 | 1511篇 |
专业分类
化学 | 195312篇 |
晶体学 | 6004篇 |
力学 | 17366篇 |
综合类 | 18篇 |
数学 | 42022篇 |
物理学 | 119254篇 |
出版年
2020年 | 2816篇 |
2019年 | 3049篇 |
2018年 | 3740篇 |
2017年 | 3680篇 |
2016年 | 5985篇 |
2015年 | 4090篇 |
2014年 | 6177篇 |
2013年 | 16632篇 |
2012年 | 12889篇 |
2011年 | 16135篇 |
2010年 | 10818篇 |
2009年 | 10872篇 |
2008年 | 14777篇 |
2007年 | 14718篇 |
2006年 | 14087篇 |
2005年 | 12835篇 |
2004年 | 11768篇 |
2003年 | 10238篇 |
2002年 | 10136篇 |
2001年 | 11704篇 |
2000年 | 8979篇 |
1999年 | 6971篇 |
1998年 | 5659篇 |
1997年 | 5579篇 |
1996年 | 5372篇 |
1995年 | 4903篇 |
1994年 | 4724篇 |
1993年 | 4718篇 |
1992年 | 5053篇 |
1991年 | 5168篇 |
1990年 | 4703篇 |
1989年 | 4663篇 |
1988年 | 4705篇 |
1987年 | 4536篇 |
1986年 | 4291篇 |
1985年 | 6035篇 |
1984年 | 6183篇 |
1983年 | 5030篇 |
1982年 | 5379篇 |
1981年 | 5183篇 |
1980年 | 5019篇 |
1979年 | 5131篇 |
1978年 | 5239篇 |
1977年 | 5162篇 |
1976年 | 5213篇 |
1975年 | 4882篇 |
1974年 | 4854篇 |
1973年 | 4962篇 |
1972年 | 3285篇 |
1971年 | 2570篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
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
12.
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
13.
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
14.
This paper deals mainly with generalizations of results in finitary combinatorics to infinite ordinals. It is well-known that for finite ordinals ∑bT<αβ is the number of 2-element subsets of an α-element set. It is shown here that for any well-ordered set of arbitrary infinite order type α, ∑bT<αβ is the ordinal of the set M of 2-element subsets, where M is ordered in some natural way. The result is then extended to evaluating the ordinal of the set of all n-element subsets for each natural number n ≥ 2. Moreover, series ∑β<αf(β) are investigated and evaluated, where α is a limit ordinal and the function f belongs to a certain class of functions containing polynomials with natural number coefficients. The tools developed for this result can be extended to cover all infinite α, but the case of finite α appears to be quite problematic. 相似文献
15.
L. K. Aminov I. N. Kurkin D. A. Lukoyanov K. P. Chernov 《Physics of the Solid State》1997,39(8):1184-1186
Results are reported for measurements of the spin-lattice relaxation times of E′1 centers in quartz glass, produced by neutron irradiation, with the measurements made at two frequencies 9.25 and 24.0 GHz
over a wide temperature interval 1.5–300 K. The experimental data are interpreted on the basis of interaction mechanisms of
the spins with two-level systems with excitation energies ∼6, ∼26, and ∼420 cm−1. A small modification of the existing theory allows us to explain a number of features of the observed temperature and frequency
dependence of the relaxation rate. The results are compared with the data available in the literature on spin-lattice relaxation
of irradiation centers in crystalline quartz and quartz glass.
Fiz. Tverd. Tela (St. Petersburg) 39, 1335–1337 (August 1997) 相似文献
16.
The effect of iodine sorption on the free volume of polycarbonate is investigated by Positron Annihilation Lifetime method.
The observed results are interpreted in terms of the Charge Transfer Complex formation and precipitation of iodine at the
initial and final stages of sorption, respectively. At higher levels of sorption, changes in the second lifetime and its intensity
seem to suggest a conformational transformation, probably due to the net change in the amorphous-crystalline boundary regions
of the polymer matrix. The free volume cavities have been predominantly occupied by I-
3 species and the diffusion process obeys Fick’s law. An exponential type correlation has been observed between fractional
free volume and the diffusion coefficient.
Received: 13 June 1996/Accepted: 20 September 1996 相似文献
17.
J. E. Fischer E. Werwa P. A. Heiney 《Applied Physics A: Materials Science & Processing》1993,56(3):193-196
We have used both reflection-geometry and grazing-incidence-geometry X-ray scattering to study thin films of C60 evaporated onto mica substrates via a hot-wall technique. The growth mode yields close-packed C60 planes, which are parallel to the substrate surface and which exhibit out-of-plane correlation lengths of 850 Å. In the film plane the C60 is at best pseudo-epitaxial, with a 0.9° distribution of crystallite orientations, a 450 Å in-plane correlation length, and a 3.7% lattice mismatch, better than obtained by other thin film techniques but far from the accepted definition of single crystal thin film epitaxy. 相似文献
18.
19.
20.
In this paper, the authors studied certain properties of the estimate of Liang and Krishnaiah (1985, J. Multivariate Anal. 16, 162–172) for multivariate binary density. An alternative shrinkage estimate is also obtained. The above results are generalized to general orthonormal systems. 相似文献