全文获取类型
收费全文 | 16867篇 |
免费 | 39篇 |
国内免费 | 75篇 |
专业分类
系统科学 | 53篇 |
丛书文集 | 173篇 |
教育与普及 | 67篇 |
理论与方法论 | 49篇 |
现状及发展 | 7642篇 |
研究方法 | 659篇 |
综合类 | 8180篇 |
自然研究 | 158篇 |
出版年
2012年 | 196篇 |
2011年 | 334篇 |
2009年 | 99篇 |
2008年 | 261篇 |
2007年 | 287篇 |
2006年 | 264篇 |
2005年 | 281篇 |
2004年 | 323篇 |
2003年 | 288篇 |
2002年 | 280篇 |
2001年 | 466篇 |
2000年 | 491篇 |
1999年 | 337篇 |
1994年 | 304篇 |
1992年 | 268篇 |
1991年 | 223篇 |
1990年 | 278篇 |
1989年 | 252篇 |
1988年 | 260篇 |
1987年 | 282篇 |
1986年 | 290篇 |
1985年 | 338篇 |
1984年 | 233篇 |
1983年 | 257篇 |
1982年 | 212篇 |
1981年 | 204篇 |
1980年 | 245篇 |
1979年 | 508篇 |
1978年 | 434篇 |
1977年 | 390篇 |
1976年 | 314篇 |
1975年 | 386篇 |
1974年 | 474篇 |
1973年 | 422篇 |
1972年 | 431篇 |
1971年 | 562篇 |
1970年 | 608篇 |
1969年 | 531篇 |
1968年 | 532篇 |
1967年 | 459篇 |
1966年 | 408篇 |
1965年 | 294篇 |
1964年 | 128篇 |
1959年 | 166篇 |
1958年 | 322篇 |
1957年 | 253篇 |
1956年 | 202篇 |
1955年 | 200篇 |
1954年 | 193篇 |
1948年 | 167篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
881.
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
882.
Metastasis suppressor gene KiSS-1 encodes peptide ligand of a G-protein-coupled receptor 总被引:72,自引:0,他引:72
Ohtaki T Shintani Y Honda S Matsumoto H Hori A Kanehashi K Terao Y Kumano S Takatsu Y Masuda Y Ishibashi Y Watanabe T Asada M Yamada T Suenaga M Kitada C Usuki S Kurokawa T Onda H Nishimura O Fujino M 《Nature》2001,411(6837):613-617
Metastasis is a major cause of death in cancer patients and involves a multistep process including detachment of cancer cells from a primary cancer, invasion of surrounding tissue, spread through circulation, re-invasion and proliferation in distant organs. KiSS-1 is a human metastasis suppressor gene, that suppresses metastases of human melanomas and breast carcinomas without affecting tumorigenicity. However, its gene product and functional mechanisms have not been elucidated. Here we show that KiSS-1 (refs 1, 4) encodes a carboxy-terminally amidated peptide with 54 amino-acid residues, which we have isolated from human placenta as the endogenous ligand of an orphan G-protein-coupled receptor (hOT7T175) and have named 'metastin'. Metastin inhibits chemotaxis and invasion of hOT7T175-transfected CHO cells in vitro and attenuates pulmonary metastasis of hOT7T175-transfected B16-BL6 melanomas in vivo. The results suggest possible mechanisms of action for KiSS-1 and a potential new therapeutic approach. 相似文献
883.
The size and albedo of the Kuiper-belt object (20000) Varuna 总被引:1,自引:0,他引:1
Observations over the last decade have revealed the existence of a large number of bodies orbiting the Sun beyond Neptune. Known as the Kuiper-belt objects (KBOs), they are believed to be formed in the outer reaches of the protoplanetary disk around the young Sun, and have been little altered since then. They are probably the source of short-period comets. The KBOs are, however, difficult objects to study because of their distance from earth, so even basic physical properties such as their sizes and albedos remain unknown. Previous size estimates came from assuming an albedo with the canonical value being 0.04. Here we report simultaneous measurements of the thermal emission and reflected optical light of the bright KBO (20000) Varuna, which allow us to determine independently both the size and the albedo. Varuna has an equivalent circular diameter of D = 900+129-145 km and a red geometric albedo of pR = 0.070+0.030-0.017. Its surface is darker than Pluto's, suggesting that it is largely devoid of fresh ice, but brighter than previously assumed for KBOs. 相似文献
884.
'Inverse' melting of a vortex lattice 总被引:1,自引:0,他引:1
Avraham N Khaykovich B Myasoedov Y Rappaport M Shtrikman H Feldman DE Tamegai T Kes PH Li M Konczykowski M van der Beek K Zeldov E 《Nature》2001,411(6836):451-454
Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare, and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases. Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique to 'dither' the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures. 相似文献
885.
All 'solar-like' stars are surrounded by coronae, which contain magnetically confined plasma at temperatures above 106 K. (Until now, only the Sun's corona could be observed in the optical-as a shimmering envelope during a total solar eclipse.) As the underlying stellar 'surfaces'-the photospheres-are much cooler, some non-radiative process must be responsible for heating the coronae. The heating mechanism is generally thought to be magnetic in origin, but is not yet understood even for the case of the Sun. Ultraviolet emission lines first led to the discovery of the enormous temperature of the Sun's corona, but thermal emission from the coronae of other stars has hitherto been detectable only from space, at X-ray wavelengths. Here we report the detection of emission from highly ionized iron (Fe XIII at 3,388.1 A) in the corona of the red-dwarf star CN Leonis, using a ground-based telescope. The X-ray flux inferred from our data is consistent with previously measured X-ray fluxes, and the non-thermal line width of 18.4 km s-1 indicates great similarities between solar and stellar coronal heating mechanisms. The accessibility and spectral resolution (45,000) of the ground-based instrument are much better than those of X-ray satellites, so a new window to the study of stellar coronae has been opened. 相似文献
886.
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
887.
Fernandez-Lopez S Kim HS Choi EC Delgado M Granja JR Khasanov A Kraehenbuehl K Long G Weinberger DA Wilcoxen KM Ghadiri MR 《Nature》2001,412(6845):452-455
The rapid emergence of bacterial infections that are resistant to many drugs underscores the need for new therapeutic agents. Here we report that six- and eight-residue cyclic d,l-alpha-peptides act preferentially on Gram-positive and/or Gram-negative bacterial membranes compared to mammalian cells, increase membrane permeability, collapse transmembrane ion potentials, and cause rapid cell death. The effectiveness of this class of materials as selective antibacterial agents is highlighted by the high efficacy observed against lethal methicillin-resistant Staphylococcus aureus infections in mice. Cyclic d,l-alpha-peptides are proteolytically stable, easy to synthesize, and can be derived from a potentially vast membrane-active sequence space. The unique abiotic structure of the cyclic peptides and their quick bactericidal action may also contribute to limit temporal acquirement of drug resistant bacteria. The low molecular weight d,l-alpha-peptides offer an attractive complement to the current arsenal of naturally derived antibiotics, and hold considerable potential in combating a variety of existing and emerging infectious diseases. 相似文献
888.
Herbig-Haro (HH) objects have been known for 50 years to be luminous condensations of gas in star-forming regions, but their underlying physical nature is still being elucidated. Previously suggested models encompass newborn stars, stellar winds clashing with nebular material, dense pockets of interstellar gas excited by shocks from outflows, and interstellar 'bullets' (ref. 6). Recent progress has been made with the jet-induced shock model, in which material streams out of young stellar objects and collides with the surrounding interstellar medium. A clear prediction of this model is that the most energetic Herbig-Haro objects will emit X-rays, although they have not hitherto been detected. Here we report the discovery of X-ray emission from one of the brightest and closest Herbig-Haro objects, HH2, at a level consistent with the model predictions. We conclude that this Herbig-Haro object contains shock-heated material located at or near its leading edge with a temperature of about 106 K. 相似文献
889.
890.
The neurotrophic factors in non-neuronal tissues 总被引:17,自引:0,他引:17
Sariola H 《Cellular and molecular life sciences : CMLS》2001,58(8):1061-1066
Although neurotrophic factors are defined as molecules that maintain neuronal cells, they possess a range of functions outside
the nervous system. For example, glial cell line-derived neurotrophic factor is essential for ureteric branching in kidney
morphogenesis and for regulating the fate of stem cells during spermatogenesis. Leukemia inhibitory factor, a member of the
interleukin-6 (IL-6) ciliary neurotrophic factor family, inhibits differentiation of embryonic stem cells, induces tubulogenesis
in the embryonic kidney, and regulates sperm differentiation. Other IL-6 family members are important in cardiac differentiation
and they have pleiotropic functions in the hematopoietic and immune systems. Although neurotrophin receptors have been found
on a number of non-neuronal tissues, they represent mostly truncated receptor isoforms that are incapable of signal transduction
and may have scavenger or dominant negative functions. However, several examples can be presented of essential non-neuronal
functions played by neurotrophins in e.g., cardiac, hair follicle, and vascular differentiation, and the maintenance of immune
cells. 相似文献