全文获取类型
收费全文 | 1543篇 |
免费 | 23篇 |
国内免费 | 51篇 |
专业分类
系统科学 | 94篇 |
丛书文集 | 37篇 |
教育与普及 | 50篇 |
理论与方法论 | 2篇 |
现状及发展 | 183篇 |
研究方法 | 64篇 |
综合类 | 1186篇 |
自然研究 | 1篇 |
出版年
2021年 | 4篇 |
2020年 | 3篇 |
2018年 | 25篇 |
2017年 | 26篇 |
2016年 | 11篇 |
2015年 | 31篇 |
2014年 | 45篇 |
2013年 | 38篇 |
2012年 | 73篇 |
2011年 | 67篇 |
2010年 | 69篇 |
2009年 | 138篇 |
2008年 | 132篇 |
2007年 | 123篇 |
2006年 | 111篇 |
2005年 | 83篇 |
2004年 | 129篇 |
2003年 | 52篇 |
2002年 | 37篇 |
2001年 | 99篇 |
2000年 | 98篇 |
1999年 | 55篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1995年 | 6篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 24篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 12篇 |
1988年 | 7篇 |
1987年 | 4篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1972年 | 2篇 |
1971年 | 6篇 |
1970年 | 7篇 |
1969年 | 8篇 |
1968年 | 3篇 |
1967年 | 4篇 |
1966年 | 2篇 |
1958年 | 2篇 |
1956年 | 2篇 |
排序方式: 共有1617条查询结果,搜索用时 15 毫秒
101.
New evidence on the earliest human presence at high northern latitudes in northeast Asia 总被引:1,自引:0,他引:1
Zhu RX Potts R Xie F Hoffman KA Deng CL Shi CD Pan YX Wang HQ Shi RP Wang YC Shi GH Wu NQ 《Nature》2004,431(7008):559-562
The timing of early human dispersal to Asia is a central issue in the study of human evolution. Excavations in predominantly lacustrine sediments at Majuangou, Nihewan basin, north China, uncovered four layers of indisputable hominin stone tools. Here we report magnetostratigraphic results that constrain the age of the four artefact layers to an interval of nearly 340,000 yr between the Olduvai subchron and the Cobb Mountain event. The lowest layer, about 1.66 million years old (Myr), provides the oldest record of stone-tool processing of animal tissues in east Asia. The highest layer, at about 1.32 Myr, correlates with the stone tool layer at Xiaochangliang, previously considered the oldest archaeological site in this region. The findings at Majuangou indicate that the oldest known human presence in northeast Asia at 40 degrees N is only slightly younger than that in western Asia. This result implies that a long yet rapid migration from Africa, possibly initiated during a phase of warm climate, enabled early human populations to inhabit northern latitudes of east Asia over a prolonged period. 相似文献
102.
Expression of lysine-rich protein gene and analysis of lysine content in transgenic wheat 总被引:2,自引:0,他引:2
MENGChaomin CHENXuqing LIANGRongqi YANGFengping ZHANGLiquan ZHANGXiaodong CHENTianyou S.S.M.Sun 《科学通报(英文版)》2004,49(19):2053-2057
Expression vector pBPC102, which carries winged bean lysine-rich protein (wblrp) gene and dihydropicolinate synthase (DHDPS) gene, was transferred into hexaploid winter wheat cv. Jinghua No.l, Jing411, You899 and Yangnongl5 explants of immature inflorescence and immature embryos by particle bombardment. More than 100 transgenic plants were obtained under the selection of s-(2-aminoethyl)-L-cysteine (AEC). Confirmed transgenic plants of To and TI generation by PCR and PCR-Southern blotting analyses showed successful integration of wblrp gene into wheat genome. Analysis of transgenic plant lines of T2 by Northern dot-blotting showed good expression of wblrp gene in offspring seed. The content of free lysine in leaves, contents of bound lysine and total proteins in seeds of T2 transgenie wheat lines were determined and analyzed. Among 34 tested transgenic lines, levels of free lysine content in leaves of 9 transgenic lines are 2~3times higher than un-trans-formed wild-type cultivars. Among 17 analyzed transgenic lines, bound lysine content of 4 transgenic lines is more than 10% higher than that of wild-type cultivars. Our research suggests that introducing wblrp gene into wheat is an effective way to improve its nutrition quality. 相似文献
103.
HAb18G/CD147-mediated calcium mobilization and hepatoma metastasis require both C-terminal and N-terminal domains 总被引:4,自引:0,他引:4
Jiang JL Chan HC Zhou Q Yu MK Yao XY Lam SY Zhu H Ho LS Leung KM Chen ZN 《Cellular and molecular life sciences : CMLS》2004,61(16):2083-2091
HAb18G/CD147 is a heavily glycosylated protein containing two immunoglobulin superfamily domains. Our previous studies have indicated that overexpression of HAb18G/CD147 enhances metastatic potentials in human hepatoma cells by disrupting the regulation of store-operated Ca2+ entry by nitric oxide (NO)/cGMP. In the present study, we investigated the structure-function of HAb18G/CD147 by transfecting truncated HAb18G/CD147 fragments into human 7721 hepatoma cells. The inhibitory effect of HAb18G/CD147 on 8-bromo-cGMP-regulated thapsigargin-induced Ca2+ entry was reversed by the expression of either C or N terminus truncated HAb18G/CD147 in T7721C and T7721N cells, respectively. The potential effect of HAb18G/CD147 on metastatic potentials, both adhesion and invasion capacities, of hepatoma cells was abolished in T7721C cells, but not affected in T7721N cells. Release and activation of matrix metalloproteinases (MMPs), MMP-2 and MMP-9, were found to be enhanced by the expression of HAb18G/CD147, and this effect was abolished by both truncations. Thapsigargin significantly enhanced release and activation of MMPs (MMP-2 and MMP-9) in non-transfected 7721 cells, and this effect was negatively regulated by SNAP. However, no effects of thapsigargin or SNAP were observed in T7721 cells, and expression of HAb18G/CD147 enhanced secretion and activation of MMPs at a stable and high level. Taken together, these results suggest that both ectodomain and intracellular domains of HAb18G/CD147 are required to mediate the effect of HAb18G/CD147 on the secretion and activation of MMPs and metastasis-related processes in human hepatoma cells by disrupting the regulation of NO/cGMP-sensitive intracellular Ca2+ mobilization although each domain may play different roles.Received 1 April 2004; received after revision 15 June 2004; accepted 22 June 2004 相似文献
104.
When a retrovirus infects a cell, its RNA genome is reverse transcribed into a double-stranded DNA, which is then permanently integrated into the host chromosome. Integration is one of the essential steps in the retroviral life cycle. Many transposable elements also move around and integrate into the host genome as part of their life cycle, some through RNA intermediates and some through 'cut and paste' mechanisms. Integration of retroviruses and transposable elements into 'sensitive areas' of the genome can cause irreparable damage. On the other hand, because of their ability to integrate permanently, and the relatively efficient rates of transgenesis, retroviruses and transposable elements are widely used as gene delivery tools in basic research and gene therapy trials. Recent events in gene therapy treatments for X-linked severe combined immunity deficiencies (X-SCID) have highlighted both the promise and some of the risks involved with utilizing retroviruses. Nine of 11 children were successfully treated for X-SCID using a retrovirus carrying the gene mutated in this disease. However, later two of these children developed leukemias because of retroviral integrations in the putative oncogene LMO2 [1]. A third child has also been demonstrated to have an integration in LMO2, but is as of yet nonsymptomatic [2]. It is a bit difficult to explain the high frequency of integrations into the same gene using a random model of retroviral integration, and there has been evidence for decades that retroviral integrations may not be random. But the data were somewhat limited in their power to determine the precise nature of the integration biases. The completion of the human genome sequence coupled with sensitive polymerase chain reaction techniques and an ever-decreasing cost of sequencing has given a powerful new tool to the study of integration site selection. In this review, we describe the findings from several recent global surveys of target site selection by retroviruses and transposable elements, and discuss the possible ramifications of these findings to both mechanisms of action and to the use of these elements as gene therapy vectors. 相似文献
105.
Larbalestier DC Cooley LD Rikel MO Polyanskii AA Jiang J Patnaik S Cai XY Feldmann DM Gurevich A Squitieri AA Naus MT Eom CB Hellstrom EE Cava RJ Regan KA Rogado N Hayward MA He T Slusky JS Khalifah P Inumaru K Haas M 《Nature》2001,410(6825):186-189
The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered. 相似文献
106.
Integration of telomere sequences with the draft human genome sequence 总被引:15,自引:0,他引:15
Riethman HC Xiang Z Paul S Morse E Hu XL Flint J Chi HC Grady DL Moyzis RK 《Nature》2001,409(6822):948-951
Telomeres are the ends of linear eukaryotic chromosomes. To ensure that no large stretches of uncharacterized DNA remain between the ends of the human working draft sequence and the ends of each chromosome, we would need to connect the sequences of the telomeres to the working draft sequence. But telomeres have an unusual DNA sequence composition and organization that makes them particularly difficult to isolate and analyse. Here we use specialized linear yeast artificial chromosome clones, each carrying a large telomere-terminal fragment of human DNA, to integrate most human telomeres with the working draft sequence. Subtelomeric sequence structure appears to vary widely, mainly as a result of large differences in subtelomeric repeat sequence abundance and organization at individual telomeres. Many subtelomeric regions appear to be gene-rich, matching both known and unknown expressed genes. This indicates that human subtelomeric regions are not simply buffers of nonfunctional 'junk DNA' next to the molecular telomere, but are instead functional parts of the expressed genome. 相似文献
107.
A primitive sarcopterygian fish with an eyestalk 总被引:3,自引:0,他引:3
The discovery of two Early Devonian osteichthyan (bony fish) fossils has challenged established ideas about the origin of osteichthyans and their divergence into actinopterygians (teleosts and their relatives) and sarcopterygians (tetrapods, coelacanths, lungfishes and related groups). Psarolepis from China and an unnamed braincase from Australia combine derived sarcopterygian and actinopterygian characters with primitive features previously restricted to non-osteichthyans, suggesting that early osteichthyan evolution may have involved substantial parallellism between sarcopterygians and actinopterygians. But interpretation of these fossils has been hampered by poor phylogenetic resolution. Here we describe a basal sarcopterygian fish, Achoania gen, et sp. nov., that fills the morphological gap between Psarolepis and higher sarcoptergyians. We also report the presence of eyestalk attachments in both Achoania and Psarolepis, showing that this supposedly non-osteichthyan feature occurs in basal sarcopterygians as well as the actinoptergyian-like Australian braincase. 相似文献
108.
High critical current density and enhanced irreversibility field in superconducting MgB2 thin films 总被引:5,自引:0,他引:5
Eom CB Lee MK Choi JH Belenky LJ Song X Cooley LD Naus MT Patnaik S Jiang J Rikel M Polyanskii A Gurevich A Cai XY Bu SD Babcock SE Hellstrom EE Larbalestier DC Rogado N Regan KA Hayward MA He T Slusky JS Inumaru K Haas MK Cava RJ 《Nature》2001,411(6837):558-560
The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications. 相似文献
109.
Evidence for ubiquitous strong electron-phonon coupling in high-temperature superconductors 总被引:3,自引:0,他引:3
Lanzara A Bogdanov PV Zhou XJ Kellar SA Feng DL Lu ED Yoshida T Eisaki H Fujimori A Kishio K Shimoyama JI Noda T Uchida S Hussain Z Shen ZX 《Nature》2001,412(6846):510-514
Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity. The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high-transition-temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle-resolved photoemission spectroscopy to probe electron dynamics-velocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80 meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity. 相似文献
110.
Khalifah P Nelson KD Jin R Mao ZQ Liu Y Huang Q Gao XP Ramirez AP Cava RJ 《Nature》2001,411(6838):669-671
Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Transition-metal oxides have proved to be particularly fruitful in this regard, especially for those materials with the perovskite structure, where the special characteristics of transition-metal-oxygen orbital hybridization determine their properties. Ruthenates have recently emerged as an important family of perovskites because of the unexpected evolution from high-temperature ferromagnetism in SrRuO3 to low-temperature superconductivity in Sr2RuO4 (refs 1, 2). Here we show that a ruthenate in a different structural family, La4Ru6O19, displays a number of highly unusual properties, most notably non-Fermi-liquid behaviour. The properties of La4Ru6O19 have no analogy among the thousands of previously characterized transition-metal oxides. Instead, they resemble those of CeCu6-xAux-a widely studied f-electron-based heavy fermion intermetallic compound that is often considered as providing the best example of non-Fermi-liquid behaviour. In the ruthenate, non-Fermi-liquid behaviour appears to arise from just the right balance between the interactions of localized electronic states derived from Ru-Ru bonding and delocalized states derived from Ru-O hybridization. 相似文献