全文获取类型
收费全文 | 406791篇 |
免费 | 3836篇 |
国内免费 | 1283篇 |
专业分类
化学 | 218421篇 |
晶体学 | 6138篇 |
力学 | 18403篇 |
综合类 | 16篇 |
数学 | 45626篇 |
物理学 | 123306篇 |
出版年
2020年 | 3008篇 |
2019年 | 3292篇 |
2018年 | 4098篇 |
2017年 | 4103篇 |
2016年 | 6442篇 |
2015年 | 4293篇 |
2014年 | 6366篇 |
2013年 | 17166篇 |
2012年 | 13212篇 |
2011年 | 16331篇 |
2010年 | 11111篇 |
2009年 | 11022篇 |
2008年 | 15191篇 |
2007年 | 15444篇 |
2006年 | 14567篇 |
2005年 | 13353篇 |
2004年 | 12150篇 |
2003年 | 10875篇 |
2002年 | 10739篇 |
2001年 | 12060篇 |
2000年 | 9329篇 |
1999年 | 7132篇 |
1998年 | 5961篇 |
1997年 | 5926篇 |
1996年 | 5662篇 |
1995年 | 5176篇 |
1994年 | 5242篇 |
1993年 | 4827篇 |
1992年 | 5537篇 |
1991年 | 5547篇 |
1990年 | 5225篇 |
1989年 | 5105篇 |
1988年 | 5188篇 |
1987年 | 5052篇 |
1986年 | 4770篇 |
1985年 | 6577篇 |
1984年 | 6837篇 |
1983年 | 5663篇 |
1982年 | 5908篇 |
1981年 | 5761篇 |
1980年 | 5672篇 |
1979年 | 5644篇 |
1978年 | 5986篇 |
1977年 | 5908篇 |
1976年 | 5927篇 |
1975年 | 5586篇 |
1974年 | 5517篇 |
1973年 | 5750篇 |
1972年 | 3877篇 |
1971年 | 3065篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
The Wheeler–DeWitt equation of arbitrary Hartle–Hawking factor ordering for several minisuperspace universe models, such as the pure gravity Friedmann–Robertson–Walker and Taub ones, is mapped onto the dynamics of corresponding classical oscillators. The latter ones are studied by the classical Ermakov invariant method, which is a natural approach in this context. For the more realistic case of a minimally coupled massive scalar field, one can study, within the same type of approach, the corresponding squeezing features as a possible means of describing cosmological evolution. Finally, we comment on the analogy with the accelerator physics. 相似文献
32.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 5, pp. 815–819, May, 1991. 相似文献
33.
34.
A. L. Apanasenko A. V. Kuznichenko Yu. B. Govyadovskii V. G. Yakunin 《Journal of Applied Spectroscopy》1991,54(3):271-276
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 3, pp. 438–444, March, 1991. 相似文献
35.
Aidan Mooney John G. Keating Daniel M. Heffernan 《Chaos, solitons, and fractals》2006,30(5):1088-1097
A digital watermark is a visible, or preferably invisible, identification code that is permanently embedded in digital media, to prove owner authentication and provide protection for documents. Given the interest in watermark generation using chaotic functions a detailed study of one chaotic function for this purpose is performed. In this paper, we present an approach for the generation of watermarks using the logistic map. Using this function, in conjunction with seed management, it is possible to generate chaotic sequences that may be used to create highpass or lowpass digital watermarks. In this paper we provide a detailed study on the generation of optically detectable watermarks and we provide some guidelines on successful chaotic watermark generation using the logistic map, and show using a recently published scheme, how care must be taken in the selection of the function seed. 相似文献
36.
V. G. Osmolovski 《Journal of Mathematical Sciences》2006,132(3):304-312
Dependence of the phase transition temperature on the domain size is investigated for a double-well quadratic potential. It
is shown that for a domain whose boundary is subjected to a hydrostatical pressure, the temperature of phase transitions is
independent of the domain and the surface tension coefficient and depends exclusively on the properties of the elastic media.
If the displacement field vanishes on the boundary, then for sufficiently small domains, the temperature also does not depend
on the surface tension and domain size and is determined by properties of the elastic media only. Bibliography: 8 titles.
__________
Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 310, 2004, pp. 98–113. 相似文献
37.
The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies ωmw and ωrf, respectively, is analyzed. The effective time-independent Hamiltonian describing the “dressed” spin states of the “spin + bichromatic field” system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances ω0 = ωmw + rωrf (ω0 is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies. 相似文献
38.
We consider the radiation from nonoscillating dipoles traveling with constant velocity directed parallel or antiparallel to
the velocity of a homogeneous transparent moving medium. It is assumed that the medium in its rest frame is isotropic and
has no spatial dispersion. We obtain expressions for the radiative energy losses and estimate the polarization energy losses
of electric and magnetic dipoles of different orientations. In particular, it is shown that the energy loss of a source is
negative if it moves in the direction of the medium motion and the source velocity is less than the medium velocity. Estimates
for the energy losses of dipoles in the cases of an electron beam and a flow of a weakly dispersive medium are given.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 6, pp. 502–512, June 2006. 相似文献
39.
We study oscillation in a gyrotron with allowance for reflections from an output horn. Regions with different system behaviors,
such as stationary oscillation, self-modulation, and complex-dynamics regimes are found in the parameter plane. The scenarios
of appearance of chaotic oscillations are considered. It is shown that they can emerge via either a sequence of period-doubling
bifurcations or destruction of quasiperiodic motion. For chaotic attractors, Lyapunov exponents are calculated and their dimensions
are estimated on the basis of the Kaplan-Yorke formula. The dimension values turn out to be anomalously large, which is stipulated
by the presence of a large number of high-Q eigenmodes in the gyrotron cavity due to operation near the cutoff frequency of
an electrodynamic system.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 10, pp. 887–899, October 2006. 相似文献
40.
A successful application of ultrasound Doppler velocimetry in liquid sodium flows is described. To obtain sufficient Doppler
signals, different problems had to be solved: the transmission of the ultrasonic beam through the channel wall made of stainless
steel, the acoustic coupling between the transducer and the channel wall, and the wetting of the inner surface of the wall
by the liquid metal, respectively. A sodium flow in a square duct exposed to a transverse magnetic field is investigated.
In accordance with the existing knowledge about MHD channel flows, we found that the velocity profiles modified to a M-shape
owing to the effect of an inhomogeneous magnetic field.
Received: 12 June 2001/Accepted: 27 October 2001 相似文献