全文获取类型
收费全文 | 9312篇 |
免费 | 79篇 |
国内免费 | 113篇 |
专业分类
系统科学 | 174篇 |
丛书文集 | 252篇 |
教育与普及 | 396篇 |
理论与方法论 | 22篇 |
现状及发展 | 892篇 |
研究方法 | 1232篇 |
综合类 | 6527篇 |
自然研究 | 9篇 |
出版年
2018年 | 12篇 |
2017年 | 27篇 |
2016年 | 20篇 |
2015年 | 14篇 |
2014年 | 32篇 |
2013年 | 32篇 |
2012年 | 639篇 |
2011年 | 729篇 |
2010年 | 202篇 |
2009年 | 122篇 |
2008年 | 732篇 |
2007年 | 735篇 |
2006年 | 781篇 |
2005年 | 855篇 |
2004年 | 643篇 |
2003年 | 594篇 |
2002年 | 538篇 |
2001年 | 396篇 |
2000年 | 588篇 |
1999年 | 181篇 |
1998年 | 53篇 |
1997年 | 32篇 |
1996年 | 27篇 |
1995年 | 17篇 |
1994年 | 23篇 |
1993年 | 32篇 |
1992年 | 33篇 |
1991年 | 32篇 |
1990年 | 45篇 |
1989年 | 48篇 |
1988年 | 30篇 |
1987年 | 45篇 |
1986年 | 38篇 |
1985年 | 40篇 |
1984年 | 55篇 |
1983年 | 36篇 |
1982年 | 45篇 |
1981年 | 36篇 |
1980年 | 19篇 |
1979年 | 24篇 |
1971年 | 16篇 |
1970年 | 31篇 |
1966年 | 22篇 |
1959年 | 97篇 |
1958年 | 200篇 |
1957年 | 104篇 |
1956年 | 104篇 |
1955年 | 98篇 |
1954年 | 115篇 |
1948年 | 47篇 |
排序方式: 共有9504条查询结果,搜索用时 31 毫秒
951.
Soibel A Zeldov E Rappaport M Myasoedov Y Tamegai T Ooi S Konczykowski M Geshkenbein VB 《Nature》2000,406(6793):282-287
General arguments suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can transform them into second-order transitions; but the atomic level details of this process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition on an inter-particle scale, as well as over a wide range of particle densities. Here we use a differential magneto-optical technique to obtain direct experimental visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid-liquid interface coarsening and pinning. Although the local melting is found to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temperatures, at scales down to the mesoscopic level. We also resolve local hysteretic supercooling of microscopic liquid domains, a non-equilibrium process that occurs only at selected sites where the disorder-modified melting temperature has a local maximum. By revealing the nucleation process, we are able to experimentally evaluate the solid-liquid surface tension, which we find to be extremely small. 相似文献
952.
953.
Teemu J Makinen T Bertaux JL Laakso H Pulkkinen T Summanen T Kyrola E Schmidt W Quemerais E Lallement R 《Nature》2000,405(6784):321-322
Several searches for near-Earth objects have recently been initiated, as a result of increased awareness of the hazard of impacts on the Earth. These programs mainly search for asteroids, so amateur astronomers can still contribute to the discovery of comets, especially out of the orbital plane of the Solar System. An ideal way to search for comets would be to use a spaceborne instrument capable of imaging the whole sky on a daily basis in a systematic and repeatable way. Such an instrument already exists on the solar observatory SOHO; it operates at the Lyman-alpha wavelength of neutral hydrogen, which is the main component of the emission cloud of a comet. Here we report the discovery, using archival data from this satellite, of a hitherto unnoticed comet which reached a perihelion of 1.546 a.u. on 26 June 1997. We derive the water production rate of the comet as a function of time and find that it increases after perihelion, like that of comet Halley. 相似文献
954.
955.
956.
Temperature trends over the past five centuries reconstructed from borehole temperatures 总被引:2,自引:0,他引:2
For an accurate assessment of the relative roles of natural variability and anthropogenic influence in the Earth's climate, reconstructions of past temperatures from the pre-industrial as well as the industrial period are essential. But instrumental records are typically available for no more than the past 150 years. Therefore reconstructions of pre-industrial climate rely principally on traditional climate proxy records, each with particular strengths and limitations in representing climatic variability. Subsurface temperatures comprise an independent archive of past surface temperature changes that is complementary to both the instrumental record and the climate proxies. Here we use present-day temperatures in 616 boreholes from all continents except Antarctica to reconstruct century-long trends in temperatures over the past 500 years at global, hemispheric and continental scales. The results confirm the unusual warming of the twentieth century revealed by the instrumental record, but suggest that the cumulative change over the past five centuries amounts to about 1 K, exceeding recent estimates from conventional climate proxies. The strength of temperature reconstructions from boreholes lies in the detection of long-term trends, complementary to conventional climate proxies, but to obtain a complete picture of past warming, the differences between the approaches need to be investigated in detail. 相似文献
957.
Although the crystal structures of the copper oxide high-temperature superconductors are complex and diverse, they all contain some crystal planes consisting of only copper and oxygen atoms in a square lattice: superconductivity is believed to originate from strongly interacting electrons in these CuO2 planes. Substituting a single impurity atom for a copper atom strongly perturbs the surrounding electronic environment and can therefore be used to probe high-temperature superconductivity at the atomic scale. This has provided the motivation for several experimental and theoretical studies. Scanning tunnelling microscopy (STM) is an ideal technique for the study of such effects at the atomic scale, as it has been used very successfully to probe individual impurity atoms in several other systems. Here we use STM to investigate the effects of individual zinc impurity atoms in the high-temperature superconductor Bi2Sr2CaCu2O8+delta. We find intense quasiparticle scattering resonances at the Zn sites, coincident with strong suppression of superconductivity within approximately 15 A of the scattering sites. Imaging of the spatial dependence of the quasiparticle density of states in the vicinity of the impurity atoms reveals the long-sought four-fold symmetric quasiparticle 'cloud' aligned with the nodes of the d-wave superconducting gap which is believed to characterize superconductivity in these materials. 相似文献
958.
959.
Seismic hazard in the Marmara Sea region following the 17 August 1999 Izmit earthquake 总被引:3,自引:0,他引:3
Hubert-Ferrari A Barka A Jacques E Nalbant SS Meyer B Armijo R Tapponnier P King GC 《Nature》2000,404(6775):269-273
On 17 August 1999, a destructive magnitude 7.4 earthquake occurred 100 km east of Istanbul, near the city of Izmit, on the North Anatolian fault. This 1,600-km-long plate boundary slips at an average rate of 2-3 cm yr(-1), and historically has been the site of many devastating earthquakes. This century alone it has ruptured over 900 km of its length. Models of earthquake-induced stress change combined with active fault maps had been used to forecast that the epicentral area of the 1999 Izmit event was indeed a likely location for the occurrence of a large earthquake. Here we show that the 1999 event itself significantly modifies the stress distribution resulting from previous fault interactions. Our new stress models take into account all events in the region with magnitudes greater than 6 having occurred since 1700 as well as secular interseismic stress change, constrained by GPS data. These models provide a consistent picture of the long term spatio-temporal behaviour of the North Anatolian fault and indicate that two events of magnitude equal to, or greater than, the Izmit earthquake are likely to occur within the next decades beneath the Marmara Sea, south of Istanbul. 相似文献
960.
Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds. Such properties include high-temperature superconductivity, heavy fermion behaviour and metal-to-insulator transitions. High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths (< or = 5 A) associated with the low excitation energies conventionally used (< or = 120 eV) make this a surface-sensitive technique. Now that high-resolution PES is possible at much higher energies, with mean free paths as long as 15 A (ref. 6), it should become feasible to probe the bulk electronic states in these materials. Here we demonstrate the power of this technique by applying it to the cerium compounds CeRu2Si2 and CeRu2. Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands. Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences. 相似文献