全文获取类型
收费全文 | 589篇 |
免费 | 0篇 |
国内免费 | 1篇 |
专业分类
丛书文集 | 2篇 |
教育与普及 | 23篇 |
理论与方法论 | 2篇 |
现状及发展 | 23篇 |
研究方法 | 100篇 |
综合类 | 440篇 |
出版年
2012年 | 32篇 |
2011年 | 52篇 |
2010年 | 6篇 |
2008年 | 40篇 |
2007年 | 47篇 |
2006年 | 36篇 |
2005年 | 52篇 |
2004年 | 42篇 |
2003年 | 41篇 |
2002年 | 32篇 |
2001年 | 30篇 |
2000年 | 58篇 |
1999年 | 9篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 1篇 |
1991年 | 2篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1985年 | 1篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1971年 | 3篇 |
1970年 | 18篇 |
1966年 | 1篇 |
1960年 | 1篇 |
1959年 | 5篇 |
1958年 | 13篇 |
1957年 | 7篇 |
1956年 | 7篇 |
1955年 | 7篇 |
1954年 | 3篇 |
1948年 | 8篇 |
1947年 | 3篇 |
排序方式: 共有590条查询结果,搜索用时 187 毫秒
101.
102.
Haislip JB Nysewander MC Reichart DE Levan A Tanvir N Cenko SB Fox DB Price PA Castro-Tirado AJ Gorosabel J Evans CR Figueredo E MacLeod CL Kirschbrown JR Jelinek M Guziy S de Ugarte Postigo A Cypriano ES LaCluyze A Graham J Priddey R Chapman R Rhoads J Fruchter AS Lamb DQ Kouveliotou C Wijers RA Bayliss MB Schmidt BP Soderberg AM Kulkarni SR Harrison FA Moon DS Gal-Yam A Kasliwal MM Hudec R Vitek S Kubanek P Crain JA Foster AC Clemens JC Bartelme JW Canterna R Hartmann DH Henden AA Klose S 《Nature》2006,440(7081):181-183
Gamma-ray bursts (GRBs) and their afterglows are the most brilliant transient events in the Universe. Both the bursts themselves and their afterglows have been predicted to be visible out to redshifts of z approximately 20, and therefore to be powerful probes of the early Universe. The burst GRB 000131, at z = 4.50, was hitherto the most distant such event identified. Here we report the discovery of the bright near-infrared afterglow of GRB 050904 (ref. 4). From our measurements of the near-infrared afterglow, and our failure to detect the optical afterglow, we determine the photometric redshift of the burst to be z = 6.39 - 0.12 + 0.11 (refs 5-7). Subsequently, it was measured spectroscopically to be z = 6.29 +/- 0.01, in agreement with our photometric estimate. These results demonstrate that GRBs can be used to trace the star formation, metallicity, and reionization histories of the early Universe. 相似文献
103.
Continental runoff has increased through the twentieth century despite more intensive human water consumption. Possible reasons for the increase include: climate change and variability, deforestation, solar dimming, and direct atmospheric carbon dioxide (CO2) effects on plant transpiration. All of these mechanisms have the potential to affect precipitation and/or evaporation and thereby modify runoff. Here we use a mechanistic land-surface model and optimal fingerprinting statistical techniques to attribute observational runoff changes into contributions due to these factors. The model successfully captures the climate-driven inter-annual runoff variability, but twentieth-century climate alone is insufficient to explain the runoff trends. Instead we find that the trends are consistent with a suppression of plant transpiration due to CO2-induced stomatal closure. This result will affect projections of freshwater availability, and also represents the detection of a direct CO2 effect on the functioning of the terrestrial biosphere. 相似文献
104.
Bahar R Hartmann CH Rodriguez KA Denny AD Busuttil RA Dollé ME Calder RB Chisholm GB Pollock BH Klein CA Vijg J 《Nature》2006,441(7096):1011-1014
105.
Soderberg AM Kulkarni SR Nakar E Berger E Cameron PB Fox DB Frail D Gal-Yam A Sari R Cenko SB Kasliwal M Chevalier RA Piran T Price PA Schmidt BP Pooley G Moon DS Penprase BE Ofek E Rau A Gehrels N Nousek JA Burrows DN Persson SE McCarthy PJ 《Nature》2006,442(7106):1014-1017
Over the past decade, long-duration gamma-ray bursts (GRBs)--including the subclass of X-ray flashes (XRFs)--have been revealed to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 10(48) erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218. 相似文献
106.
107.
Li Y Balédent V Yu G Barišić N Hradil K Mole RA Sidis Y Steffens P Zhao X Bourges P Greven M 《Nature》2010,468(7321):283-285
The elucidation of the pseudogap phenomenon of the high-transition-temperature (high-T(c)) copper oxides-a set of anomalous physical properties below the characteristic temperature T* and above T(c)-has been a major challenge in condensed matter physics for the past two decades. Following initial indications of broken time-reversal symmetry in photoemission experiments, recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T* (refs 3, 4). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point. Here we report inelastic neutron scattering results for HgBa(2)CuO(4+δ) that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode's intensity rises below the same temperature T* and its dispersion is weak, as expected for an Ising-like order parameter. Its energy of 52-56?meV renders it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies. 相似文献
108.
Subtypes of medulloblastoma have distinct developmental origins 总被引:2,自引:0,他引:2
Gibson P Tong Y Robinson G Thompson MC Currle DS Eden C Kranenburg TA Hogg T Poppleton H Martin J Finkelstein D Pounds S Weiss A Patay Z Scoggins M Ogg R Pei Y Yang ZJ Brun S Lee Y Zindy F Lindsey JC Taketo MM Boop FA Sanford RA Gajjar A Clifford SC Roussel MF McKinnon PJ Gutmann DH Ellison DW Wechsler-Reya R Gilbertson RJ 《Nature》2010,468(7327):1095-1099
Medulloblastoma encompasses a collection of clinically and molecularly diverse tumour subtypes that together comprise the most common malignant childhood brain tumour. These tumours are thought to arise within the cerebellum, with approximately 25% originating from granule neuron precursor cells (GNPCs) after aberrant activation of the Sonic Hedgehog pathway (hereafter, SHH subtype). The pathological processes that drive heterogeneity among the other medulloblastoma subtypes are not known, hindering the development of much needed new therapies. Here we provide evidence that a discrete subtype of medulloblastoma that contains activating mutations in the WNT pathway effector CTNNB1 (hereafter, WNT subtype) arises outside the cerebellum from cells of the dorsal brainstem. We found that genes marking human WNT-subtype medulloblastomas are more frequently expressed in the lower rhombic lip (LRL) and embryonic dorsal brainstem than in the upper rhombic lip (URL) and developing cerebellum. Magnetic resonance imaging (MRI) and intra-operative reports showed that human WNT-subtype tumours infiltrate the dorsal brainstem, whereas SHH-subtype tumours are located within the cerebellar hemispheres. Activating mutations in Ctnnb1 had little impact on progenitor cell populations in the cerebellum, but caused the abnormal accumulation of cells on the embryonic dorsal brainstem which included aberrantly proliferating Zic1(+) precursor cells. These lesions persisted in all mutant adult mice; moreover, in 15% of cases in which Tp53 was concurrently deleted, they progressed to form medulloblastomas that recapitulated the anatomy and gene expression profiles of human WNT-subtype medulloblastoma. We provide the first evidence, to our knowledge, that subtypes of medulloblastoma have distinct cellular origins. Our data provide an explanation for the marked molecular and clinical differences between SHH- and WNT-subtype medulloblastomas and have profound implications for future research and treatment of this important childhood cancer. 相似文献
109.
Wheeler DA Srinivasan M Egholm M Shen Y Chen L McGuire A He W Chen YJ Makhijani V Roth GT Gomes X Tartaro K Niazi F Turcotte CL Irzyk GP Lupski JR Chinault C Song XZ Liu Y Yuan Y Nazareth L Qin X Muzny DM Margulies M Weinstock GM Gibbs RA Rothberg JM 《Nature》2008,452(7189):872-876
The association of genetic variation with disease and drug response, and improvements in nucleic acid technologies, have given great optimism for the impact of 'genomic medicine'. However, the formidable size of the diploid human genome, approximately 6 gigabases, has prevented the routine application of sequencing methods to deciphering complete individual human genomes. To realize the full potential of genomics for human health, this limitation must be overcome. Here we report the DNA sequence of a diploid genome of a single individual, James D. Watson, sequenced to 7.4-fold redundancy in two months using massively parallel sequencing in picolitre-size reaction vessels. This sequence was completed in two months at approximately one-hundredth of the cost of traditional capillary electrophoresis methods. Comparison of the sequence to the reference genome led to the identification of 3.3 million single nucleotide polymorphisms, of which 10,654 cause amino-acid substitution within the coding sequence. In addition, we accurately identified small-scale (2-40,000 base pair (bp)) insertion and deletion polymorphism as well as copy number variation resulting in the large-scale gain and loss of chromosomal segments ranging from 26,000 to 1.5 million base pairs. Overall, these results agree well with recent results of sequencing of a single individual by traditional methods. However, in addition to being faster and significantly less expensive, this sequencing technology avoids the arbitrary loss of genomic sequences inherent in random shotgun sequencing by bacterial cloning because it amplifies DNA in a cell-free system. As a result, we further demonstrate the acquisition of novel human sequence, including novel genes not previously identified by traditional genomic sequencing. This is the first genome sequenced by next-generation technologies. Therefore it is a pilot for the future challenges of 'personalized genome sequencing'. 相似文献
110.
The assimilation and mineralization of dissolved organic carbon (DOC) by marine bacterioplankton is a major process in the ocean carbon cycle. However, little information exists on the specific metabolic functions of participating bacteria and on whether individual taxa specialize on particular components of the marine DOC pool. Here we use experimental metagenomics to show that coastal communities are populated by taxa capable of metabolizing a wide variety of organic carbon compounds. Genomic DNA captured from bacterial community subsets metabolizing a single model component of the DOC pool (either dimethylsulphoniopropionate or vanillate) showed substantial overlap in gene composition as well as a diversity of carbon-processing capabilities beyond the selected phenotypes. Our direct measure of niche breadth for bacterial functional assemblages indicates that, in accordance with ecological theory, heterogeneity in the composition and supply of organic carbon to coastal oceans may favour generalist bacteria. In the important interplay between microbial community structure and biogeochemical cycling, coastal heterotrophic communities may be controlled less by transient changes in the carbon reservoir that they process and more by factors such as trophic interactions and physical conditions. 相似文献