全文获取类型
收费全文 | 430691篇 |
免费 | 5321篇 |
国内免费 | 2601篇 |
专业分类
化学 | 224480篇 |
晶体学 | 6579篇 |
力学 | 20594篇 |
综合类 | 150篇 |
数学 | 68301篇 |
物理学 | 118509篇 |
出版年
2021年 | 2917篇 |
2020年 | 3513篇 |
2019年 | 3747篇 |
2018年 | 14031篇 |
2017年 | 13947篇 |
2016年 | 12513篇 |
2015年 | 5356篇 |
2014年 | 7083篇 |
2013年 | 16350篇 |
2012年 | 16462篇 |
2011年 | 25324篇 |
2010年 | 16027篇 |
2009年 | 16226篇 |
2008年 | 20450篇 |
2007年 | 21918篇 |
2006年 | 13529篇 |
2005年 | 13256篇 |
2004年 | 12285篇 |
2003年 | 11651篇 |
2002年 | 10413篇 |
2001年 | 11161篇 |
2000年 | 8405篇 |
1999年 | 6500篇 |
1998年 | 5060篇 |
1997年 | 5077篇 |
1996年 | 4939篇 |
1995年 | 4438篇 |
1994年 | 4299篇 |
1993年 | 4016篇 |
1992年 | 4856篇 |
1991年 | 4862篇 |
1990年 | 4606篇 |
1989年 | 4576篇 |
1988年 | 4548篇 |
1987年 | 4571篇 |
1986年 | 4270篇 |
1985年 | 5550篇 |
1984年 | 5575篇 |
1983年 | 4435篇 |
1982年 | 4644篇 |
1981年 | 4493篇 |
1980年 | 4284篇 |
1979年 | 4746篇 |
1978年 | 4795篇 |
1977年 | 4818篇 |
1976年 | 4606篇 |
1975年 | 4251篇 |
1974年 | 4162篇 |
1973年 | 4154篇 |
1972年 | 2838篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Vineet Kumar Rai 《Applied physics. B, Lasers and optics》2007,88(2):297-303
The dependence of temperature on the fluorescence lifetime and fluorescence intensity ratio using Stark sublevels and thermally
coupled (close lying) levels in triply ionized rare earth ions, doped into a variety of glasses and fibers, have been reviewed.
Also, it is claimed that not only for the two closely lying levels of triply ionized rare earth ion of the same kind, but
also for two different triply ionized rare earth ions, having their excited levels very close to each other, may be used to
monitor the temperature.
PACS 42.70.Ce 相似文献
982.
The object of the present paper is to study quasi-conformally flat weakly Ricci symmetric manifolds.
相似文献
983.
In previous work, we considered the representation of human decision-making processes in closed-form simulation models of conflict. An important element of this representation is the rapid planning process that embodies the processing of information for situation assessment to support a course of action decision (eg in a military headquarters). The application of this work is in support of operational analysis models for defence procurement and balance of investment. This paper describes the application of non-linear multi-attribute utility theory in conflict scenarios in order to extend the representation of the rapid planning process to account for a wider set of subjective attributes of the decision-maker. The results show, through examination of experimental data, that decision-making can be modelled through a particular class of utility functions. These utilities embody a geometry which allows us to classify the types of decision being made when there are conflicting objectives and when decision-makers adopt very different and subjective appraisals of constraints and beliefs in outcome. The experimental results help to demonstrate that the subjective nature of the situation assessment, and the personality, training, experience and history of the decision-maker are central to the functional representations. This paper presents a way to capture this deeper representation of human decision-making in a way that is potentially useful for quantitative modelling using the rapid planning process as a basis. 相似文献
984.
We have performed structural and optical characterizations of the propolis (an organic entity of biological nature) films grown on various non-organic substrates. The films were grown from a propolis melt or a propolis alcohol solution. The crystal structure has been observed in the films precipitated from the solution onto substrates such as an amorphous glass and sapphire or semiconductor indium monoselenide. For any growth method, the propolis film is a semiconductor with the bandgap of 3.07 eV at 300 K that is confirmed by a maximum in photoluminescence spectra at 2.86 eV. We argue that propolis films might be used in various optoelectronic device applications. 相似文献
985.
Final state interaction effects inpp→ pΛK
+ andpd →3Heη reactions are explored near threshold to study the sensitivity of the cross-sections to thepΛ potential and theηN scattering matrix. The final state scattering wave functions between Λ andp andη and3He are described rigorously. The Λ production is described by the exchange of one pion and aK-meson between two protons in the incident channel. Theη production is described by a two-step model, where in the first step a pion is produced. This pion then produces anη by interacting with another nucleon 相似文献
986.
R. Guerrero F.G. Aliev R. Villar J. Hauch M. Fraune G. Güntherodt K. Rott H. Brückl G. Reiss 《Journal of magnetism and magnetic materials》2006
We report on tunnelling magnetoresistance (TMR), current–voltage (IV) characteristics and low-frequency noise in epitaxially grown Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20 μm2. The evaluated MgO energy barrier (0.50±0.08 eV), the barrier width (13.1±0.5 Å) as well as the resistance times area product (7±1 MΩ μm2) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. At low temperatures (T<10 K) inelastic electron tunneling spectroscopy (IETS) shows anomalies related to phonons (symmetric structures below 100 meV) and asymmetric features above 200 meV. We explain the asymmetric features in IETS as due to generation of electron standing waves in one of the Fe electrodes. The noise power, though exhibiting a large variation, was observed to be roughly anti-correlated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behavior could be related to the influence of magnetostriction on the characteristics of the insulating barrier through changes in local barrier defects structure. 相似文献
987.
Half-lives of the 2νββ decay are calculated in the proton-neutron QRPA for76Ge,82Se,100Mo,128,130Te,136Xe and150Nd. The strength of the particle-particle interaction, which plays a decisive role for a reliable evaluation of the half-lives, is determined from a QRPA calculation of singleβ + decays. The 2ν decay rates calculated with the interaction strength fitted in this way are strongly suppressed and found to be consistent with the existing experimental data. Effects of the ground-state correlations on the suppression are investigated. On the other hand, the present calculation indicates limitations of the QRPA approach. 相似文献
988.
Rhyd Lewis 《Journal of Heuristics》2007,13(4):387-401
In this paper we present, to our knowledge, the first application of a metaheuristic technique to the very popular and NP-complete
puzzle known as ‘sudoku’. We see that this stochastic search-based algorithm, which uses simulated annealing, is able to complete
logic-solvable puzzle-instances that feature daily in many of the UK’s national newspapers. We also introduce a new method
for producing sudoku problem instances (that are not necessarily logic-solvable) and use this together with the proposed SA
algorithm to try and discover for what types of instances this algorithm is best suited. Consequently we notice the presence
of an ‘easy-hard-easy’ style phase-transition similar to other problems encountered in operational research. 相似文献
989.
Denis V. Osin 《Archiv der Mathematik》2007,88(5):403-412
The aim of this paper is to show that free Burnside groups of sufficiently large odd exponent are non-amenable in a certain
strong sense. More precisely their left regular representations are isolated from the trivial representation uniformly on
finite generating sets. It follows that free Burnside groups are of uniform exponential growth. This answers a question of
de la Harpe.
This work has been partially supported by the NSF grant DMS-0605093 and by the RFBR Grant # 05-01-00892.
Received: 25 April 2006 相似文献
990.
Qin-long Wang Yi-rong Liu 《应用数学学报(英文版)》2007,23(3):451-466
In this paper, we study the appearance of limit cycles from the equator and isochronicity of infinity in polynomial vector fields with no singular points at infinity. We give a recursive formula to compute the singular point quantities of a class of cubic polynomial systems, which is used to calculate the first seven singular point quantities. Further, we prove that such a cubic vector field can have maximal seven limit cycles in the neighborhood of infinity. We actually and construct a system that has seven limit cycles. The positions of these limit cycles can be given exactly without constructing the Poincare cycle fields. The technique employed in this work is essentially different from the previously widely used ones. Finally, the isochronous center conditions at infinity are given. 相似文献