全文获取类型
收费全文 | 2396篇 |
免费 | 74篇 |
国内免费 | 9篇 |
专业分类
化学 | 1639篇 |
晶体学 | 10篇 |
力学 | 76篇 |
数学 | 262篇 |
物理学 | 492篇 |
出版年
2023年 | 13篇 |
2022年 | 19篇 |
2021年 | 16篇 |
2020年 | 34篇 |
2019年 | 28篇 |
2018年 | 16篇 |
2017年 | 14篇 |
2016年 | 39篇 |
2015年 | 44篇 |
2014年 | 56篇 |
2013年 | 96篇 |
2012年 | 119篇 |
2011年 | 181篇 |
2010年 | 55篇 |
2009年 | 61篇 |
2008年 | 176篇 |
2007年 | 166篇 |
2006年 | 147篇 |
2005年 | 155篇 |
2004年 | 129篇 |
2003年 | 106篇 |
2002年 | 98篇 |
2001年 | 34篇 |
2000年 | 17篇 |
1999年 | 25篇 |
1998年 | 29篇 |
1997年 | 24篇 |
1996年 | 30篇 |
1995年 | 26篇 |
1994年 | 24篇 |
1993年 | 12篇 |
1992年 | 17篇 |
1991年 | 25篇 |
1990年 | 21篇 |
1989年 | 21篇 |
1988年 | 13篇 |
1987年 | 16篇 |
1986年 | 20篇 |
1985年 | 28篇 |
1984年 | 20篇 |
1983年 | 26篇 |
1982年 | 25篇 |
1981年 | 29篇 |
1980年 | 17篇 |
1979年 | 19篇 |
1978年 | 22篇 |
1977年 | 18篇 |
1976年 | 16篇 |
1974年 | 15篇 |
1973年 | 11篇 |
排序方式: 共有2479条查询结果,搜索用时 15 毫秒
11.
12.
Microwave transitions up to J = 53 in the ground vibrational state of deuterothioformaldehyde, CD2S, were studied between 8 and 40 GHz. A detailed centrifugal distortion analysis yields accurate constants for comparison with force field values. The isotopic species 13CH2S, CH234S, CH233S, 13CD2S, CD234S, and CD233S were studied in natural abundance. Accurate average zero-point structures were determined for both CD2S and CH2S: Changes in the zero-point geometry for deuterium substitution were established. Quadrupole fine structure arising from the 33S nucleus has been measured in CH233S and CD233S. Analysis gives the following coupling constants (for both molecules) as χaa = ?11.7 and χbb - χcc = 88.1 MHz. The dipole moment of CD2S was measured to be 1.6588(8)D and an accurate comparison with CH2S was made; the ratio of dipole moments was found to be 1.0062(4). The spectroscopic and bonding properties of CH2S will be compared with formaldehyde and other molecules. 相似文献
13.
Frank Javor Gerald F. Thomas Stuart M. Rothstein 《International journal of quantum chemistry》1977,11(1):59-71
The goodness of the local fit of an approximate wave-function, \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \psi $\end{document}, to the exact function, ψ0, is \documentclass{article}\pagestyle{empty}\begin{document}$ |\tilde \psi - \psi _0 | $\end{document}. From this quantity the global accuracy of \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \psi $\end{document} is defined and a “working supposition” is presented, which quantitatively relates the global accuracy to the accuracy of expectation values. Two criteria based on the accuracy of the reduced local energy and the density respectively, are presented as alternatives to \documentclass{article}\pagestyle{empty}\begin{document}$ |\tilde \psi - \psi _0 | $\end{document}. The relative global accuracies of eight wave-functions for H2 are determined using the two criteria. The ‘working supposition’ is applied and predictions are made concerning the relative accuracies of the expectation values of the following operators: z2, r2, x2 + y2, 3z2 ?; r2, ξ, r, r, and EL (the reduced local energy). The success rate is high (>90%) except for those operators which are sensitive to interelectron coordinates or derivatives of the wave-function. 相似文献
14.
The transition from the quantum to the classical world is not yet understood. Here, we take a new approach. Central to this is the understanding that measurement and actualization cannot occur except on some specific basis. However, we have no established theory for the emergence of a specific basis. Our framework entails the following: (i) Sets of N entangled quantum variables can mutually actualize one another. (ii) Such actualization must occur in only one of the 2N possible bases. (iii) Mutual actualization progressively breaks symmetry among the 2N bases. (iv) An emerging “amplitude” for any basis can be amplified by further measurements in that basis, and it can decay between measurements. (v) The emergence of any basis is driven by mutual measurements among the N variables and decoherence with the environment. Quantum Zeno interactions among the N variables mediates the mutual measurements. (vi) As the number of variables, N, increases, the number of Quantum Zeno mediated measurements among the N variables increases. We note that decoherence alone does not yield a specific basis. (vii) Quantum ordered, quantum critical, and quantum chaotic peptides that decohere at nanosecond versus femtosecond time scales can be used as test objects. (viii) By varying the number of amino acids, N, and the use of quantum ordered, critical, or chaotic peptides, the ratio of decoherence to Quantum Zeno effects can be tuned. This enables new means to probe the emergence of one among a set of initially entangled bases via weak measurements after preparing the system in a mixed basis condition. (ix) Use of the three stable isotopes of carbon, oxygen, and nitrogen and the five stable isotopes of sulfur allows any ten atoms in the test protein to be discriminably labeled and the basis of emergence for those labeled atoms can be detected by weak measurements. We present an initial mathematical framework for this theory, and we propose experiments. 相似文献
15.
Stuart M. Marshall Douglas G. Moore Alastair R. G. Murray Sara I. Walker Leroy Cronin 《Entropy (Basel, Switzerland)》2022,24(7)
Assembly theory (referred to in prior works as pathway assembly) has been developed to explore the extrinsic information required to distinguish a given object from a random ensemble. In prior work, we explored the key concepts relating to deconstructing an object into its irreducible parts and then evaluating the minimum number of steps required to rebuild it, allowing for the reuse of constructed sub-objects. We have also explored the application of this approach to molecules, as molecular assembly, and how molecular assembly can be inferred experimentally and used for life detection. In this article, we formalise the core assembly concepts mathematically in terms of assembly spaces and related concepts and determine bounds on the assembly index. We explore examples of constructing assembly spaces for mathematical and physical objects and propose that objects with a high assembly index can be uniquely identified as those that must have been produced using directed biological or technological processes rather than purely random processes, thereby defining a new scale of aliveness. We think this approach is needed to help identify the new physical and chemical laws needed to understand what life is, by quantifying what life does. 相似文献
16.
17.
Yiming Liu Stuart W. Bunting Shiming Luo Kunzheng Cai Qiangqiang Yang 《Natural Resource Modeling》2019,32(1)
Inappropriate development and overexploitation have seriously degraded aquatic resources in China. Stakeholders identified three fish stock enhancement and biodiversity conservation scenarios for the Beijiang River: S1, increased fish restocking; S2, no fishing season and habitat conservation; and S3, strict pollution control. Potential impacts of these actions on the livelihoods of fishers were evaluated using applied economic modeling. Baseline costs and benefits came from logbooks from 30 fishers and a survey of 90 households in three villages. The financial net benefit for a household was US$1583 (¥11,160) annually, representing a 142% and 387% return on capital and operating costs, respectively. Larger catches associated with S1 and S2 generated a net benefit of US$1651 and US$1822, respectively. Strict pollution control resulting in higher catches (+20%) and lower operating costs (?20%) would increase the net benefit by 15.9% to US$1835 annually. Pollution control would benefit other resource users and is a prerequisite for ecological restoration. Recommendations for Resource Managers
- Stringent pollution control measures are essential to conserve aquatic biodiversity and enhance the livelihoods of fishers but will require considerable public and private sector investment.
- Enhanced fish stocks in the Beijiang River could benefit poor livelihoods but may not be sufficient to lift households out of poverty, aged fishers require government assistance to diversify their livelihoods, access alternative urban employment, and survive with dignity.
- Adopting the economic modeling approach presented here could enable responsible authorities to simultaneously evaluate fish stock enhancement and biodiversity conservation options.
- Broader application of the approach presented here could help ensure small‐scale inland fisheries are managed sustainably and aquatic ecosystems are restored and protected by 2020, in line with Target 6 of the United Nations’ Convention on Biological Diversity Strategic Plan for Biodiversity.
18.
19.
20.