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   320篇
  免费   1篇
  国内免费   8篇
系统科学   1篇
丛书文集   1篇
教育与普及   1篇
理论与方法论   1篇
现状及发展   48篇
研究方法   49篇
综合类   202篇
自然研究   26篇
  2021年   1篇
  2019年   1篇
  2018年   2篇
  2017年   3篇
  2016年   6篇
  2015年   2篇
  2014年   1篇
  2013年   7篇
  2012年   22篇
  2011年   48篇
  2010年   3篇
  2009年   1篇
  2008年   28篇
  2007年   34篇
  2006年   21篇
  2005年   17篇
  2004年   22篇
  2003年   17篇
  2002年   26篇
  2001年   14篇
  2000年   2篇
  1999年   2篇
  1998年   1篇
  1996年   2篇
  1994年   2篇
  1992年   1篇
  1990年   1篇
  1989年   1篇
  1988年   2篇
  1986年   3篇
  1985年   1篇
  1984年   2篇
  1982年   2篇
  1979年   1篇
  1978年   3篇
  1977年   3篇
  1976年   3篇
  1975年   2篇
  1974年   1篇
  1972年   3篇
  1971年   1篇
  1970年   2篇
  1969年   1篇
  1968年   4篇
  1967年   2篇
  1965年   3篇
  1960年   1篇
  1957年   1篇
排序方式: 共有329条查询结果,搜索用时 78 毫秒
201.
Most Oryza sativa cultivars die within a week of complete submergence--a major constraint to rice production in south and southeast Asia that causes annual losses of over US 1 billion dollars and affects disproportionately the poorest farmers in the world. A few cultivars, such as the O. sativa ssp. indica cultivar FR13A, are highly tolerant and survive up to two weeks of complete submergence owing to a major quantitative trait locus designated Submergence 1 (Sub1) near the centromere of chromosome 9 (refs 3, 4, 5-6). Here we describe the identification of a cluster of three genes at the Sub1 locus, encoding putative ethylene response factors. Two of these genes, Sub1B and Sub1C, are invariably present in the Sub1 region of all rice accessions analysed. In contrast, the presence of Sub1A is variable. A survey identified two alleles within those indica varieties that possess this gene: a tolerance-specific allele named Sub1A-1 and an intolerance-specific allele named Sub1A-2. Overexpression of Sub1A-1 in a submergence-intolerant O. sativa ssp. japonica conferred enhanced tolerance to the plants, downregulation of Sub1C and upregulation of Alcohol dehydrogenase 1 (Adh1), indicating that Sub1A-1 is a primary determinant of submergence tolerance. The FR13A Sub1 locus was introgressed into a widely grown Asian rice cultivar using marker-assisted selection. The new variety maintains the high yield and other agronomic properties of the recurrent parent and is tolerant to submergence. Cultivation of this variety is expected to provide protection against damaging floods and increase crop security for farmers.  相似文献   
202.
通过那些流传在民间社会的抄本,我们可以了解中国平民社会的生活状态.大约成书于1944年的唐家家谱就是这样一种民间抄本.仔细研究这本家谱,我们会在字里行间发现很多关于执笔者的故事:它的历史脉络、经济地位、婚姻网络等等.由此,通过家谱的编写,人们把历史融人到了自己的日常生活之中,并由此实现对文化的重新阐述.所有这些,又折射出民国时期中国的平民文化.  相似文献   
203.
204.
205.
Initialization and read-out of coupled quantum systems are essential ingredients for the implementation of quantum algorithms. Single-shot read-out of the state of a multi-quantum-bit (multi-qubit) register would allow direct investigation of quantum correlations (entanglement), and would give access to further key resources such as quantum error correction and deterministic quantum teleportation. Although spins in solids are attractive candidates for scalable quantum information processing, their single-shot detection has been achieved only for isolated qubits. Here we demonstrate the preparation and measurement of a multi-spin quantum register in a low-temperature solid-state system by implementing resonant optical excitation techniques originally developed in atomic physics. We achieve high-fidelity read-out of the electronic spin associated with a single nitrogen-vacancy centre in diamond, and use this read-out to project up to three nearby nuclear spin qubits onto a well-defined state. Conversely, we can distinguish the state of the nuclear spins in a single shot by mapping it onto, and subsequently measuring, the electronic spin. Finally, we show compatibility with qubit control: we demonstrate initialization, coherent manipulation and single-shot read-out in a single experiment on a two-qubit register, using techniques suitable for extension to larger registers. These results pave the way for a test of Bell's inequalities on solid-state spins and the implementation of measurement-based quantum information protocols.  相似文献   
206.
Lange J  Pan J  Cole F  Thelen MP  Jasin M  Keeney S 《Nature》2011,479(7372):237-240
In many organisms, developmentally programmed double-strand breaks (DSBs) formed by the SPO11 transesterase initiate meiotic recombination, which promotes pairing and segregation of homologous chromosomes. Because every chromosome must receive a minimum number of DSBs, attention has focused on factors that support DSB formation. However, improperly repaired DSBs can cause meiotic arrest or mutation; thus, having too many DSBs is probably as deleterious as having too few. Only a small fraction of SPO11 protein ever makes a DSB in yeast or mouse and SPO11 and its accessory factors remain abundant long after most DSB formation ceases, implying the existence of mechanisms that restrain SPO11 activity to limit DSB numbers. Here we report that the number of meiotic DSBs in mouse is controlled by ATM, a kinase activated by DNA damage to trigger checkpoint signalling and promote DSB repair. Levels of SPO11-oligonucleotide complexes, by-products of meiotic DSB formation, are elevated at least tenfold in spermatocytes lacking ATM. Moreover, Atm mutation renders SPO11-oligonucleotide levels sensitive to genetic manipulations that modulate SPO11 protein levels. We propose that ATM restrains SPO11 via a negative feedback loop in which kinase activation by DSBs suppresses further DSB formation. Our findings explain previously puzzling phenotypes of Atm-null mice and provide a molecular basis for the gonadal dysgenesis observed in ataxia telangiectasia, the human syndrome caused by ATM deficiency.  相似文献   
207.
Palmitate activates the NF-κB pathway, and induces accumulation of lipid metabolites and insulin resistance in skeletal muscle cells. Little information is available whether and how these processes are causally related. Therefore, the objectives were to investigate whether intra-cellular lipid metabolites are involved in FA-induced NF-κB activation and/or insulin resistance in skeletal muscle and to investigate whether FA-induced insulin resistance and NF-κB activation are causally related. Inhibiting DGAT or CPT-1 by using, respectively, amidepsine or etomoxir increased DAG accumulation and sensitized myotubes to palmitate-induced insulin resistance. While co-incubation of palmitate with etomoxir increased NF-κB transactivation, co-incubation with amidepsine did not, indicating that DAG accumulation is associated with insulin resistance but not with NF-κB activation. Furthermore, pharmacological or genetic inhibition of the NF-κB pathway could not prevent palmitate-induced insulin resistance. In conclusion, we have demonstrated that activation of the NF-κB pathway is not required for palmitate-induced insulin resistance in skeletal muscle cells.  相似文献   
208.
The new venture analysis is the foundation of venturedevelopment. In this paper, 14 venture prototypesare proposed based on the attributes of venture.Then, a new venture analysis method is discussed bythe wny of matching the new venture with thecorresponding prototype. Considering the fuzziness ofhuman subjective grading, the L-R fuzzy numbers are  相似文献   
209.
210.
Hart RA  Xu X  Legere R  Gibble K 《Nature》2007,446(7138):892-895
The collision of two ultracold atoms results in a quantum mechanical superposition of the two possible outcomes: each atom continues without scattering, and each atom scatters as an outgoing spherical wave with an s-wave phase shift. The magnitude of the s-wave phase shift depends very sensitively on the interaction between the atoms. Quantum scattering and the underlying phase shifts are vitally important in many areas of contemporary atomic physics, including Bose-Einstein condensates, degenerate Fermi gases, frequency shifts in atomic clocks and magnetically tuned Feshbach resonances. Precise experimental measurements of quantum scattering phase shifts have not been possible because the number of scattered atoms depends on the s-wave phase shifts as well as the atomic density, which cannot be measured precisely. Here we demonstrate a scattering experiment in which the quantum scattering phase shifts of individual atoms are detected using a novel atom interferometer. By performing an atomic clock measurement using only the scattered part of each atom's wavefunction, we precisely measure the difference of the s-wave phase shifts for the two clock states in a density-independent manner. Our method will enable direct and precise measurements of ultracold atom-atom interactions, and may be used to place stringent limits on the time variations of fundamental constants.  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