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   541篇
  免费   2篇
  国内免费   3篇
系统科学   7篇
理论与方法论   4篇
现状及发展   84篇
研究方法   78篇
综合类   340篇
自然研究   33篇
  2018年   4篇
  2017年   6篇
  2015年   3篇
  2014年   2篇
  2013年   2篇
  2012年   43篇
  2011年   77篇
  2010年   7篇
  2009年   3篇
  2008年   29篇
  2007年   35篇
  2006年   35篇
  2005年   45篇
  2004年   29篇
  2003年   27篇
  2002年   36篇
  2001年   14篇
  2000年   5篇
  1999年   6篇
  1995年   2篇
  1993年   3篇
  1991年   3篇
  1989年   4篇
  1988年   8篇
  1987年   2篇
  1986年   7篇
  1985年   6篇
  1984年   8篇
  1983年   6篇
  1982年   6篇
  1981年   4篇
  1980年   1篇
  1979年   3篇
  1978年   1篇
  1977年   3篇
  1976年   7篇
  1975年   4篇
  1974年   4篇
  1973年   5篇
  1972年   2篇
  1971年   8篇
  1970年   9篇
  1969年   1篇
  1968年   4篇
  1967年   5篇
  1966年   6篇
  1965年   5篇
  1961年   2篇
  1958年   1篇
  1957年   3篇
排序方式: 共有546条查询结果,搜索用时 15 毫秒
541.
542.
Anomalous X-ray pulsars (AXPs) are slowly rotating neutron stars with very bright and highly variable X-ray emission that are believed to be powered by ultra-strong magnetic fields of >10(14) G, according to the 'magnetar' model. The radio pulsations that have been observed from more than 1,700 neutron stars with weaker magnetic fields have never been detected from any of the dozen known magnetars. The X-ray pulsar XTE J1810-197 was revealed (in 2003) as the first AXP with transient emission when its luminosity increased 100-fold from the quiescent level; a coincident radio source of unknown origin was detected one year later. Here we show that XTE J1810-197 emits bright, narrow, highly linearly polarized radio pulses, observed at every rotation, thereby establishing that magnetars can be radio pulsars. There is no evidence of radio emission before the 2003 X-ray outburst (unlike ordinary pulsars, which emit radio pulses all the time), and the flux varies from day to day. The flux at all radio frequencies is approximately equal--and at >20 GHz XTE J1810-197 is currently the brightest neutron star known. These observations link magnetars to ordinary radio pulsars, rule out alternative accretion models for AXPs, and provide a new window into the coronae of magnetars.  相似文献   
543.
Human chromosome 12 contains more than 1,400 coding genes and 487 loci that have been directly implicated in human disease. The q arm of chromosome 12 contains one of the largest blocks of linkage disequilibrium found in the human genome. Here we present the finished sequence of human chromosome 12, which has been finished to high quality and spans approximately 132 megabases, representing approximately 4.5% of the human genome. Alignment of the human chromosome 12 sequence across vertebrates reveals the origin of individual segments in chicken, and a unique history of rearrangement through rodent and primate lineages. The rate of base substitutions in recent evolutionary history shows an overall slowing in hominids compared with primates and rodents.  相似文献   
544.
Ultraviolet-light (UV)-induced tanning is defective in numerous 'fair-skinned' individuals, many of whom contain functional disruption of the melanocortin 1 receptor (MC1R). Although this suggested a critical role for the MC1R ligand melanocyte stimulating hormone (MSH) in this response, a genetically controlled system has been lacking in which to determine the precise role of MSH-MC1R. Here we show that ultraviolet light potently induces expression of MSH in keratinocytes, but fails to stimulate pigmentation in the absence of functional MC1R in red/blonde-haired Mc1r(e/e) mice. However, pigmentation could be rescued by topical application of the cyclic AMP agonist forskolin, without the need for ultraviolet light, demonstrating that the pigmentation machinery is available despite the absence of functional MC1R. This chemically induced pigmentation was protective against ultraviolet-light-induced cutaneous DNA damage and tumorigenesis when tested in the cancer-prone, xeroderma-pigmentosum-complementation-group-C-deficient genetic background. These data emphasize the essential role of intercellular MSH signalling in the tanning response, and suggest a clinical strategy for topical small-molecule manipulation of pigmentation.  相似文献   
545.
Multiferroic and magnetoelectric materials   总被引:7,自引:0,他引:7  
Eerenstein W  Mathur ND  Scott JF 《Nature》2006,442(7104):759-765
A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.  相似文献   
546.
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