全文获取类型
收费全文 | 180608篇 |
免费 | 2172篇 |
国内免费 | 569篇 |
专业分类
化学 | 95376篇 |
晶体学 | 3119篇 |
力学 | 7365篇 |
综合类 | 3篇 |
数学 | 20769篇 |
物理学 | 56717篇 |
出版年
2020年 | 1587篇 |
2019年 | 1712篇 |
2018年 | 2101篇 |
2017年 | 2175篇 |
2016年 | 3436篇 |
2015年 | 2122篇 |
2014年 | 3364篇 |
2013年 | 8126篇 |
2012年 | 6429篇 |
2011年 | 7714篇 |
2010年 | 5413篇 |
2009年 | 5258篇 |
2008年 | 7146篇 |
2007年 | 7097篇 |
2006年 | 6777篇 |
2005年 | 6184篇 |
2004年 | 5637篇 |
2003年 | 5058篇 |
2002年 | 4848篇 |
2001年 | 5317篇 |
2000年 | 3963篇 |
1999年 | 3060篇 |
1998年 | 2566篇 |
1997年 | 2478篇 |
1996年 | 2348篇 |
1995年 | 2195篇 |
1994年 | 2175篇 |
1993年 | 2066篇 |
1992年 | 2362篇 |
1991年 | 2300篇 |
1990年 | 2241篇 |
1989年 | 2151篇 |
1988年 | 2163篇 |
1987年 | 2153篇 |
1986年 | 2040篇 |
1985年 | 2699篇 |
1984年 | 2727篇 |
1983年 | 2412篇 |
1982年 | 2545篇 |
1981年 | 2360篇 |
1980年 | 2328篇 |
1979年 | 2468篇 |
1978年 | 2501篇 |
1977年 | 2424篇 |
1976年 | 2464篇 |
1975年 | 2393篇 |
1974年 | 2348篇 |
1973年 | 2521篇 |
1972年 | 1629篇 |
1971年 | 1345篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
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
83.
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
84.
We have recently demonstrated that polarization transfer using an adiabatic passage through the Hartmann-Hahn condition (APHH-CP) by a variation of the radio-frequency amplitude can substantially improve the transfer efficiency over Hartmann-Hahn cross polarization. Here we show that APHH-CP can be combined with fast magic angle sample spinning (MAS). The heteronuclear dipolar order, established in the course of the transfer, can indeed be created and preserved. 相似文献
85.
An M/G/1 retrial queueing system with additional phase of service and possible preemptive resume service discipline is considered. For an arbitrarily distributed retrial time distribution, the necessary and sufficient condition for the system stability is obtained, assuming that only the customer at the head of the orbit has priority access to the server. The steady-state distributions of the server state and the number of customers in the orbit are obtained along with other performance measures. The effects of various parameters on the system performance are analysed numerically. A general decomposition law for this retrial queueing system is established. 相似文献
86.
87.
88.
89.
Levoglucosenone reacts with α-aminoazoles to yield azolo[1,5-a]pyrimidine systems fused with a carbohydrate fragment. The reaction oocurs much more smoothly than in the case of other α,β-unsaturated
ketones. The reactions of levoglucosenone with β-dicarbonyl compounds (dimedone, barbituric acid) in the presence of a base
results in the pyran ring closure, which has never been observed earlier in reactions of β-dicarbonyl compounds with α,β-unsaturated
ketones under the conditions of basic catalysis. The structures of products were established by IR and NMR spectroscopy.
For Part 3, see Ref. 1.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 553–558, March, 1997. 相似文献
90.
Ya. V. Fattakhov M. F. Galyautdinov T. N. L’vova I. B. Khaibullin 《Technical Physics》1997,42(12):1457-1459
The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface
upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures
is discussed.
Zh. Tekh. Fiz. 67, 97–99 (December 1997) 相似文献