全文获取类型
收费全文 | 4238篇 |
免费 | 766篇 |
国内免费 | 347篇 |
专业分类
化学 | 764篇 |
晶体学 | 23篇 |
力学 | 413篇 |
综合类 | 67篇 |
数学 | 1507篇 |
物理学 | 2577篇 |
出版年
2024年 | 11篇 |
2023年 | 50篇 |
2022年 | 79篇 |
2021年 | 102篇 |
2020年 | 107篇 |
2019年 | 94篇 |
2018年 | 102篇 |
2017年 | 152篇 |
2016年 | 143篇 |
2015年 | 127篇 |
2014年 | 223篇 |
2013年 | 268篇 |
2012年 | 231篇 |
2011年 | 402篇 |
2010年 | 370篇 |
2009年 | 510篇 |
2008年 | 428篇 |
2007年 | 388篇 |
2006年 | 288篇 |
2005年 | 187篇 |
2004年 | 171篇 |
2003年 | 162篇 |
2002年 | 118篇 |
2001年 | 93篇 |
2000年 | 77篇 |
1999年 | 68篇 |
1998年 | 50篇 |
1997年 | 50篇 |
1996年 | 42篇 |
1995年 | 43篇 |
1994年 | 26篇 |
1993年 | 26篇 |
1992年 | 16篇 |
1991年 | 23篇 |
1990年 | 19篇 |
1989年 | 25篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1973年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有5351条查询结果,搜索用时 62 毫秒
51.
The noise of hybrid soliton pulse source (HSPS) with linearly chirped Gaussian apodized fiber Bragg grating is analyzed by couple-mode equations including spontaneous emission noise when the HSPS is mode-locked. Relative intensity noise is calculated using numerical solutions of these equations. It is shown that transform limited pulses are generated over a wide tuning range around the fundamental mode-locking frequency with low spontaneous noise. However, a high noise level affects the operation of device, and therefore transform-limited pulses are not obtainable over a wide tuning range. It is also shown that noise is extremely sensitive to the RF and DC currents, linewidth enhancement factor, gain saturation parameter and spontaneous coupling factor. 相似文献
52.
Out of a total statistics of 896139La+Ag(Br) interactions, 128 interactions having multiplicity of target fragments (Z⩾1)⩾8 and projectile fragments (Z⩾2)⩾4 have been selected. They correspond to quasi-peripheral interactions. Azimuthal angle correlation between sources of
target fragments (TFs) and projectile fragments (PFs) shows the existence of bounce-off effect. Using data of La+Ag(Br) and84Kr+Ag(Br) reactions it is shown that individual helium [Z=2, PFs] and heavier fragment [Z⩾3, PFs] show different emission characteristics. Further, a two prong correlation functionT(Φ
ij
) plotted for heavier fragments and helium fragments separately, indicates the possibility of existence of different physical
conditions. This observation is supported by the different momentum widths of helium fragments and heavier fragments. From
the momentum width data of Kr+Ag(Br) reactions normalized density comes out to be ≈4.7. Using quasi-elastic kinematics for
the bounce-off nuclei, the excitation energy has been computed from the experimental data of flow angles. The strength of
bounce-off seems to decrease with the increase of excitation energy or temperature. 相似文献
53.
54.
D. Nuzillard S. Bourg J.-M. Nuzillard 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,133(2):358-363
The concept of blind source separation is described and examples of its use in 1D and 2D NMR spectroscopy are presented. The goal of this data processing method is to extract the spectra of components molecules when only mixtures are available. 相似文献
55.
Zhi-Hui Su Munehiro Okamoto Osamu Tominaga Katsumi Akita Noboru Kashiwai Y?ki Imura Tooru Ojika Yoshiyuki Nagahata Syozo Osawa 《Proceedings of the Japan Academy. Series B, Physical and biological sciences》2006,82(7):232-250
An approach to deduce the mechanism of stabilization of the hybrid-derived populations in the Ohomopterus ground beetles has been made by comparative studies on the phylogenetic trees of the mitochondrial and nuclear DNA. A phylogenetic tree based on the internal transcribed spacer (ITS) of nuclear ribosomal gene roughly reflects the relations of morphological species group, while mitochondrial NADH dehydrogenase subunit 5 (ND5) gene shows a considerable different topology on the tree; there exist several geographically-linked lineages, most of which consist of more than one species. These results suggest that the replacement of mitochondria has occurred widely in the Ohomopterus species. In most cases, hybridization is unidirectional, i.e., the species A (♂) hybridized with another species B (♀) and not vice versa, with accompanied replacement of mitochondria of A by those of B. The results also suggest that partial or complete occupation of the distribution territory by a hybrid-derived morphological species. The morphological appearance of the resultant hybrid-derivatives are recognized as that of the original species A. Emergence of a morphological new species from a hybrid-derived population has been exemplified. 相似文献
56.
At the ISOLDE on-line isotope separation facility, the resonance ionisation laser ion source (RILIS) can be used to ionise
reaction products as they effuse from the target. The RILIS process of laser step-wise resonance ionisation of atoms in a
hot metal cavity provides a highly element selective stage in the preparation of the radioactive ion beam. As a result, the
ISOLDE mass separators can provide beams of a chosen isotope with greatly reduced isobaric contamination. With the addition
of a new three-step ionisation scheme for gold, the RILIS is now capable of ionising 26 of the elements. The optimal scheme
was determined during an extensive study of the atomic energy levels and auto-ionising states of gold, carried out by means
of in-source resonance ionisation spectroscopy. Details of the ionisation scheme and a summary of the spectroscopy study are
presented. 相似文献
57.
58.
We investigate the topology of the phase diagram of binary alloys on the fee lattice with first-neighbor antiferromagnetic interactions around the superdegenerate point, where the L10 and L12 phases meet. We treat the system as a hard-constraint lattice gas, following a procedure previously described by Lebowitzet al. We perform cluster variation method calculations in theT0 limit and Monte Carlo simulations directly atT=0 K on the ground states of the superdegenerate point. We find that: (i) there is no disordered phase in the neighborhood of this point; (ii) a phase L for which two of the four cubic sublattices have the same average occupancy and each of the two others are different appears between L10 and L12; (iii) the transition L/L12 is of first order. 相似文献
59.
Eugene P. Gross 《Journal of statistical physics》1989,54(1-2):405-427
A study is made of the ground-state energy of a spin-one-half particle in a fieldB and interacting with a phonon bath. The infrared-sensitive case of acoustic phonons with point coupling in three dimensions is characterized by two parameters, a coupling constant andB. Units are used where the high-momentum phonon cutoff is unity. There is a curve (B) separating a symmetry-breaking region with a long-range phonon field from a normal region. Two simple, well-known, approximations are compared. The source theory yields discontinuities in the first derivatives of the energy with respect toB and whenB>e
–1 and an infinite-order transition whenB<e
–1, but is trivial in the large- region. The classical theory yields discontinuities in the second derivatives but is trivial in the small- region. An improved variationally fixed ground-state wave function is analyzed. It gives a new (B) curve with an infinite-order transition with continuous energy derivatives whenB<e/(e
2–1/4) and with discontinuous derivatives whenB is larger than this value. It is nontrivial in the entire (B) plane. The crossover to classical behavior occurs near =1/2 forB1. But the wave function does not describe quantum fluctuations in the large- phase. A second way of combining source and classical effects is described. It yields a second-order transition (near =1/2 forB1) everywhere. These theories are special cases of a symmetry-breaking transformation together with a one-mode treatment of quantum fluctuations. The transition is viewed in terms of a single mode with a variable length, coupled dynamically to the spin. 相似文献
60.
同位素稀释-火花源质谱法分析人参样品中微量元素 总被引:2,自引:0,他引:2
本文采用同位素稀释-火花源质谱法定量分析了吉林野山参,园参及高丽参中30多种微量元素的含量。测定下限2μg/g,相对标准偏差在5~28%以内。 相似文献