首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2484篇
  免费   95篇
  国内免费   23篇
化学   1686篇
晶体学   15篇
力学   66篇
数学   349篇
物理学   486篇
  2023年   27篇
  2022年   51篇
  2021年   54篇
  2020年   65篇
  2019年   78篇
  2018年   54篇
  2017年   56篇
  2016年   96篇
  2015年   54篇
  2014年   69篇
  2013年   132篇
  2012年   162篇
  2011年   189篇
  2010年   107篇
  2009年   68篇
  2008年   134篇
  2007年   177篇
  2006年   157篇
  2005年   121篇
  2004年   104篇
  2003年   78篇
  2002年   53篇
  2001年   29篇
  2000年   28篇
  1999年   39篇
  1998年   20篇
  1997年   27篇
  1996年   25篇
  1995年   14篇
  1994年   10篇
  1993年   17篇
  1992年   19篇
  1991年   10篇
  1990年   7篇
  1989年   15篇
  1988年   18篇
  1987年   7篇
  1985年   18篇
  1984年   14篇
  1983年   6篇
  1982年   9篇
  1981年   17篇
  1980年   13篇
  1979年   10篇
  1978年   15篇
  1977年   9篇
  1976年   10篇
  1974年   7篇
  1973年   6篇
  1887年   6篇
排序方式: 共有2602条查询结果,搜索用时 15 毫秒
1.
We extract some properties of Mahlo’s operation and show that some other very natural operations share these properties. The weakly compact sets form a similar hierarchy as the stationary sets. The height of this hierarchy is a large cardinal property connected to saturation properties of the weakly compact ideal.  相似文献   
2.
Spin-transfer driven switching was observed in MgO based magnetic tunnelling junctions (MTJ) with tunnelling magnetoresistance ratio of up to 160% and the average intrinsic switching current density (Jc0) down to 2 MA/cm2, which are the best known results reported in spin-transfer switched MTJ nanostructures. Based on a comparison of results both from MgO and AlOx MTJs, further switching current decrease via MgO dual structures with two pinned layers is discussed.  相似文献   
3.
We study property (T) and the fixed-point property for actions on L p and other Banach spaces. We show that property (T) holds when L 2 is replaced by L p (and even a subspace/quotient of L p ), and that in fact it is independent of 1≤p<∞. We show that the fixed-point property for L p follows from property (T) when 1<p< 2+ε. For simple Lie groups and their lattices, we prove that the fixed-point property for L p holds for any 1< p<∞ if and only if the rank is at least two. Finally, we obtain a superrigidity result for actions of irreducible lattices in products of general groups on superreflexive spaces. Bader partially supported by ISF grant 100146; Furman partially supported by NSF grants DMS-0094245 and DMS-0604611; Gelander partially supported by NSF grant DMS-0404557 and BSF grant 2004010; Monod partially supported by FNS (CH) and NSF (US).  相似文献   
4.
Facility-location problems have several applications, such as telecommunications, industrial transportation and distribution. One of the most well-known facility-location problems is the p-median problem. This work addresses an application of the capacitated p-median problem to a real-world problem. We propose a genetic algorithm (GA) to solve the capacitated p-median problem. The proposed GA uses not only conventional genetic operators, but also a new heuristic “hypermutation” operator suggested in this work. The proposed GA is compared with a tabu search algorithm.  相似文献   
5.
6.
Resonant-cavity-enhanced photodetectors and LEDs in the mid-infrared   总被引:1,自引:0,他引:1  
In this paper we outline the use of resonant-cavity enhancement for increasing the exterior coupling efficiency of photodetectors and light-emitting diodes (LEDs) in the mid-infrared (MIR) spectral region. This method is potentially very important in the MIR because encapsulation is not presently feasible due to the lack of suitable materials. Among other potential applications, resonant-cavity-enhanced (RCE) photodetectors and LEDs could be particularly suitable for greenhouse gas detection because of their ‘pre-tunable’ spectrally narrowed resonantly enhanced peaks. We also present the optical characterization of an InAs RCE photodetector aimed at the detection of methane gas (λ≈3.3 μm), and an InAs/InAs0.91Sb0.09 resonant-cavity LED (RCLED) aimed at carbon dioxide gas (λ≈4.2 μm). The high peak responsivity of the RCE photodetector was 34.7 A/W at λ=3.14 μm, and the RCLED peaked at λ=3.96 μm. These are among the longest operating wavelengths for III–V RCE photodetectors and RCLEDs reported in the literature.  相似文献   
7.
Let G = (V,E) be a graph or digraph and r : VZ+. An r‐detachment of G is a graph H obtained by ‘splitting’ each vertex ν ∈ V into r(ν) vertices. The vertices ν1,…,νr(ν) obtained by splitting ν are called the pieces of ν in H. Every edge uν ∈ E corresponds to an edge of H connecting some piece of u to some piece of ν. Crispin Nash‐Williams 9 gave necessary and sufficient conditions for a graph to have a k‐edge‐connected r‐detachment. He also solved the version where the degrees of all the pieces are specified. In this paper, we solve the same problems for directed graphs. We also give a simple and self‐contained new proof for the undirected result. © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 67–77, 2003  相似文献   
8.
9.
The Kasner metric is one of the more widely studied metrics. Its usefulness for the construction of cosmological models and its utility for certain studies of elementary particles has made it particularly attractive for exploitation. Because of its simplicity it has been rediscovered many times and is itself very closely related to metrics given several years earlier by Weyl, Levi-Civita, and Wilson. The form in which Kasner presented it has been virtually forgotten in favor of the dynamic form of the synchronous Bianchi I metric. Kasner metrics may be either static or dynamic. To each static metric there is acognate dynamic metric andvice versa, and the two are related by a complex coordinate transformation. The characterization of a Kasner metric is by means of a set of parameters. It is possible for these to take on complex values with interesting consequences, In the discussion a curious error in the original paper is rectified and some new Kasner metrics are presented. The discussion throughout is confined to the vacuum metric.  相似文献   
10.
The H2 and CH4 chemical ionization mass spectra of a series of series of substituted benzoic acids and substituted benzyl alcohols have been determined. For the benzoic acids the major fragmentation reactions of the protonated molecule involve elimination of H2O or elimination of CO2, the latter reaction involving migration of the carboxylic hydrogen to the aromatic ring. For the benzyl alcohols the major fragmentation reactions of [MH]+ involve loss of H2O or CH2O, analogous to the CO2 elimination reaction for the benzoic acids. It is shown that the CO2 and CH2O elimination reactions occur only when a conjugated aromatic ring system is present, and that for the carboxylic acid systems, methyl groups and, to a lesser extent, phenyl groups are capable of migrating. The only discernible effect of substituents on the fragmentation of [MH]+ is an enhancement of the H2O loss reaction in the benzoic acid system when an amino, hydroxyl, or halogen substituent is ortho to the carboxyl function. This ‘ortho’ effect, which differs in scope from that observed in electron impact mass spectra, is attributed to an intramolecular catalysis by the ortho substituent of the 1,3 hydrogen migration in the carbonyl protonated acid followed by H2O elimination. Apparently, this route is favoured over the direct elimination of H2O from the carbonyl protonated acid, since the latter has a high activation energy barrier because of unfavourable orbital symmetry restrictions.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号