全文获取类型
收费全文 | 557篇 |
免费 | 20篇 |
国内免费 | 2篇 |
专业分类
化学 | 368篇 |
晶体学 | 2篇 |
力学 | 13篇 |
数学 | 101篇 |
物理学 | 95篇 |
出版年
2023年 | 3篇 |
2022年 | 8篇 |
2021年 | 17篇 |
2020年 | 8篇 |
2019年 | 6篇 |
2018年 | 7篇 |
2017年 | 10篇 |
2016年 | 18篇 |
2015年 | 16篇 |
2014年 | 20篇 |
2013年 | 36篇 |
2012年 | 35篇 |
2011年 | 42篇 |
2010年 | 25篇 |
2009年 | 18篇 |
2008年 | 38篇 |
2007年 | 45篇 |
2006年 | 34篇 |
2005年 | 42篇 |
2004年 | 22篇 |
2003年 | 16篇 |
2002年 | 20篇 |
2001年 | 6篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 2篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 8篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1977年 | 2篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1969年 | 1篇 |
1946年 | 1篇 |
1932年 | 1篇 |
1928年 | 1篇 |
1897年 | 1篇 |
1880年 | 1篇 |
排序方式: 共有579条查询结果,搜索用时 15 毫秒
51.
Shi W Petersen EB Nguyen DT Yao Z Chavez-Pirson A Peyghambarian N Yu J 《Optics letters》2011,36(18):3575-3577
We report a unique all fiber-based single-frequency Q-switched laser in a monolithic master oscillator power amplifier configuration at ~1920 nm by using highly Tm-doped germanate fibers for the first time. The actively Q-switched fiber laser seed was achieved by using a piezo to press the fiber in the fiber Bragg grating cavity and modulate the fiber birefringence, enabling Q-switching with pulse width and repetition rate tunability. A single-mode polarization maintaining large core 25 μm highly Tm-doped germanate fiber was used in the power amplifier stage. For 80 ns pulses with 20 kHz repetition rate, we achieved 220 μJ pulse energy, which corresponds to a peak power of 2.75 kW with transform-limited linewidth. 相似文献
52.
Oliver Wuerfel Frank Thomas Marcel Sven Schulte Reinhard Hensel Roland Arturo Diaz‐Bone 《应用有机金属化学》2012,26(2):94-101
Metal(loid)s are subject to many transformation processes in the environment, such as oxidation, reduction, methylation and hydride generation, predominantly accomplished by prokaryotes. Since these widespread processes affect the bioavailability and toxicity of metal(loid)s to a large extent, the investigation of their formation is of high relevance. Methanogenic Archaea are capable of methylating and hydrogenating Group 15 and 16 metal(loid)s arsenic, selenium, antimony, tellurium, and bismuth due to side reactions between central methanogenic cofactors, methylcobalamin (CH3Cob(III)) and cob(I)alamin (Cob(I)). Here, we present systematic mechanistic studies on methylation and hydride generation of Group 15 and 16 metal(loid)s by CH3Cob(III) and Cob(I). Pentavalent arsenical species showed neither methylation nor reduction as determined by using a newly developed oxidation state specific hydride generation technique, which allows direct determination of tri‐ and pentavalent arsenic species in a single batch. In contrast, efficient methylation of trivalent species without a change in oxidation state indicated that the methyl transfer does not proceed via a Challenger‐like oxidative methylation, but via a non‐oxidative methylation. Our findings also point towards a similar mechanism for antimony, bismuth, selenium, and tellurium. Overall, we suggest that the transfer of a methyl group does not involve a free reactive species, such as a radical, but instead is transferred either in a concerted nucleophilic substitution or in a caged radical mechanism. For hydride generation, we propose the intermediate formation of hydridocobalamin, transferring a hydride ion to the metal(loid)s. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
53.
54.
Héctor Moya-Cessa Francisco Soto-Eguibar José M. Vargas-Martínez Raúl Juárez-Amaro Arturo Zúñiga-Segundo 《Physics Reports》2012
Trapped ions are considered one of the best candidates to perform quantum information processing. By interacting them with laser beams they are, somehow, easy to manipulate, which makes them an excellent choice for the production of nonclassical states of their vibrational motion, the reconstruction of quasiprobability distribution functions, the production of quantum gates, etc. However, most of these effects have been produced in the so-called low intensity regime, this is, when the Rabi frequency (laser intensity) is much smaller than the trap frequency. Because of the possibility to produce faster quantum gates in other regimes it is of importance to study this system in a more complete manner, which is the motivation for this contribution. We start by studying the way ions are trapped in Paul traps and review the basic mechanisms of trapping. Then we show how the problem may be completely solved for trapping states; i.e., we find (exact) eigenstates of the full Hamiltonian. We show how, in the low intensity regime, Jaynes–Cummings and anti-Jaynes–Cummings interactions may be obtained, without using the rotating wave approximation and analyze the medium and high intensity regimes where dispersive Hamiltonians are produced. The traditional approach (low intensity regime) is also studied and used for the generation of non-classical states of the vibrational wavefunction. In particular, we show how to add and subtract vibrational quanta to an initial state, how to produce specific superpositions of number states and how to generate NOON states for the two-dimensional vibration of the ion. It is also shown how squeezing may be measured. The time dependent problem is studied by using Lewis–Ermakov methods. We give a solution to the problem when the time dependence of the trap is considered and also analyze a specific (artificial) time dependence that produces squeezing of the initial vibrational wave function. A way to mimic the ion–laser interaction via classical optics is also introduced. 相似文献
55.
Arturo Magidin 《Algebra Universalis》2002,48(2):133-143
We determine the dominion (in the sense of Isbell) of a subgroup H of a finite nonabelian simple group S in Var(S). We also obtain necessary and suficient conditions for H to be epimorphically embedded in S, and derive several examples, which generalize an example of B. H. Neumann. Finally, we extend the results to a variety generated
by a family of finite nonabelian simple groups.
Received May 12, 1999; accepted in final form April 9, 2002. 相似文献
56.
Arturo Quirantes 《Journal of Quantitative Spectroscopy & Radiative Transfer》2005,92(3):373-381
A computer code is described for the calculation of light-scattering properties of randomly oriented, axially symmetric coated particles, in the framework of the T-matrix theory. The underlying mathematical background is outlined briefly and convergence procedures are discussed. After outlining the input-output interaction between user and code, benchmark results are presented for two distinct shapes: coated, centered spheroids and offset coated spheres. 相似文献
57.
David Y. Song Arturo A. Pizano Patrick G. Holder JoAnne Stubbe Daniel G. Nocera 《Chemical science》2015,6(8):4519-4524
Proton-coupled electron transfer (PCET) is a fundamental mechanism important in a wide range of biological processes including the universal reaction catalysed by ribonucleotide reductases (RNRs) in making de novo, the building blocks required for DNA replication and repair. These enzymes catalyse the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs). In the class Ia RNRs, NDP reduction involves a tyrosyl radical mediated oxidation occurring over 35 Å across the interface of the two required subunits (β2 and α2) involving multiple PCET steps and the conserved tyrosine triad [Y356(β2)–Y731(α2)–Y730(α2)]. We report the synthesis of an active photochemical RNR (photoRNR) complex in which a Re(i)-tricarbonyl phenanthroline ([Re]) photooxidant is attached site-specifically to the Cys in the Y356C-(β2) subunit and an ionizable, 2,3,5-trifluorotyrosine (2,3,5-F3Y) is incorporated in place of Y731 in α2. This intersubunit PCET pathway is investigated by ns laser spectroscopy on [Re356]-β2:2,3,5-F3Y731-α2 in the presence of substrate, CDP, and effector, ATP. This experiment has allowed analysis of the photoinjection of a radical into α2 from β2 in the absence of the interfacial Y356 residue. The system is competent for light-dependent substrate turnover. Time-resolved emission experiments reveal an intimate dependence of the rate of radical injection on the protonation state at position Y731(α2), which in turn highlights the importance of a well-coordinated proton exit channel involving the key residues, Y356 and Y731, at the subunit interface. 相似文献
58.
Aurora Molinari Alfonso Oliva Marlene Arismendi Elizabeth Imbarack Cristian Glvez Javier Maldonado Arturo San Feliciano 《Journal of heterocyclic chemistry》2015,52(2):620-622
New fused pyrazolo‐1,4‐naphthoquinones were prepared from the reaction of hydrazines with 6‐(4‐methyl‐3‐pentenyl)‐1,4‐naphthoquinone. The reaction was extended to hydroxylamine to afford the corresponding isoxazolo‐1,4‐napthoquinone compound. 相似文献
59.
Two of the most challenging problems that scientists and researchers face when they want to experiment with new cutting‐edge algorithms are the time‐consuming for encoding and the difficulties for linking them with other technologies and devices. In that sense, this article introduces the artificial organic networks toolkit for LabVIEW? (AON‐TL) from the implementation point of view. The toolkit is based on the framework provided by the artificial organic networks technique, giving it the potential to add new algorithms in the future based on this technique. Moreover, the toolkit inherits both the rapid prototyping and the easy‐to‐use characteristics of the LabVIEW? software (e.g., graphical programming, transparent usage of other softwares and devices, built‐in programming event‐driven for user interfaces), to make it simple for the end‐user. In fact, the article describes the global architecture of the toolkit, with particular emphasis in the software implementation of the so‐called artificial hydrocarbon networks algorithm. Lastly, the article includes two case studies for engineering purposes (i.e., sensor characterization) and chemistry applications (i.e., blood–brain barrier partitioning data model) to show the usage of the toolkit and the potential scalability of the artificial organic networks technique. © 2015 Wiley Periodicals, Inc. 相似文献
60.
Arturo Fernández Arias 《Complex Analysis and Operator Theory》2017,11(8):1653-1668
The study of meromorphic functions without multiple values in the plane started by F. Nevanlinna is extended to meromorphic functions in the punctured plane \({\mathbb {C}}^{{*}}.\) It is a classical result that a meromorphic function \(f\left( z\right) \) can be obtained as quotient of solutions of the second order differential equation \(u^{{\prime \prime }}+\left\{ f\left( z\right) ,z\right\} u=0,\) where \(\left\{ f\left( z\right) ,z\right\} \) is the Schwarzian derivative of \(f\left( z\right) \). In our hypothesis of meromorphic functions of finite order without multiple values in the puntured plane, the Schwarzian derivative \(\left\{ f\left( z\right) , z\right\} \) turns out to be a rational function with only possible poles at 0 and \(\infty \). In these conditions the asymptotic behaviour of \(f\left( z\right) \) can be described by a result of Hille (Lectures on Ordinary Differential Equations in the complex plane, Addison Wesley, Boston, 1969) on ordinary differential equations in the complex plane. The results obtained are framed in the value distribution theory of meromorphic functions, in particular in the punctured plane we shall consider the work of Khrystiyanin and Kondratyuk (Mat Stud 23(1):19–30, 2005; Mat Stud 23(1):57–68, 2005) and Korhonen (Nevanlinna theory on an annulus. Value distribution theory and related topics. Adv. Complex analysis and applications, Kluwer Academic Publishers, Dordrecht, 2004). 相似文献