全文获取类型
收费全文 | 4509篇 |
免费 | 21篇 |
国内免费 | 65篇 |
专业分类
系统科学 | 28篇 |
丛书文集 | 83篇 |
教育与普及 | 64篇 |
理论与方法论 | 5篇 |
现状及发展 | 2004篇 |
研究方法 | 205篇 |
综合类 | 2194篇 |
自然研究 | 12篇 |
出版年
2018年 | 42篇 |
2017年 | 30篇 |
2012年 | 63篇 |
2011年 | 92篇 |
2010年 | 39篇 |
2009年 | 106篇 |
2008年 | 88篇 |
2007年 | 120篇 |
2006年 | 73篇 |
2005年 | 124篇 |
2004年 | 134篇 |
2003年 | 83篇 |
2002年 | 93篇 |
2001年 | 222篇 |
2000年 | 195篇 |
1999年 | 145篇 |
1998年 | 32篇 |
1997年 | 29篇 |
1995年 | 31篇 |
1992年 | 104篇 |
1991年 | 90篇 |
1990年 | 94篇 |
1989年 | 71篇 |
1988年 | 78篇 |
1987年 | 83篇 |
1986年 | 59篇 |
1985年 | 100篇 |
1984年 | 81篇 |
1983年 | 62篇 |
1982年 | 67篇 |
1981年 | 60篇 |
1980年 | 75篇 |
1979年 | 147篇 |
1978年 | 138篇 |
1977年 | 109篇 |
1976年 | 79篇 |
1975年 | 74篇 |
1974年 | 99篇 |
1973年 | 105篇 |
1972年 | 89篇 |
1971年 | 111篇 |
1970年 | 100篇 |
1969年 | 98篇 |
1968年 | 97篇 |
1967年 | 87篇 |
1966年 | 88篇 |
1965年 | 59篇 |
1958年 | 41篇 |
1957年 | 32篇 |
1956年 | 29篇 |
排序方式: 共有4595条查询结果,搜索用时 15 毫秒
61.
A consequence of relativity is that in the presence of an electric field, the spin and momentum states of an electron can be coupled; this is known as spin-orbit coupling. Such an interaction opens a pathway to the manipulation of electron spins within non-magnetic semiconductors, in the absence of applied magnetic fields. This interaction has implications for spin-based quantum information processing and spintronics, forming the basis of various device proposals. For example, the concept of spin field-effect transistors is based on spin precession due to the spin-orbit coupling. Most studies, however, focus on non-spin-selective electrical measurements in quantum structures. Here we report the direct measurement of coherent electron spin precession in zero magnetic field as the electrons drift in response to an applied electric field. We use ultrafast optical techniques to spatiotemporally resolve spin dynamics in strained gallium arsenide and indium gallium arsenide epitaxial layers. Unexpectedly, we observe spin splitting in these simple structures arising from strain in the semiconductor films. The observed effect provides a flexible approach for enabling electrical control over electron spins using strain engineering. Moreover, we exploit this strain-induced field to electrically drive spin resonance with Rabi frequencies of up to approximately 30 MHz. 相似文献
62.
The Wnt/beta-catenin/TCF4 pathway plays critical roles in the maintenance of small intestinal epithelium; however, downstream
targets of the beta-catenin/TCF4 complex are not extensively characterized. We identified miR-30e as an immediate target activated
by the beta-catenin/TCF4 complex. miR-30e was detected in the peri-nuclear region of the intestinal crypt IEC-6 cells. Bioinformatics
analysis revealed clustered beta-catenin/TCF4 binding sites within the miR-30e promoter region. This promoter region was cloned
into pGL3-control luciferase reporter vector, with the enhancer region removed. Transfection of pCMV-SPORT6-beta-catenin expression
vector dose-dependently increased luciferase activity, and co-transfection of pCMV-SPORT6-TCF4 expression vector further enhanced
the promoter activity. Dexamethasone-induced IEC-6 cells differentiation caused a 2.5-fold increase in miR-30e expression,
and upon beta-catenin siRNA transfection, miR-30e increased 1.3-fold. Electrophoretic mobility shift assay and chromatin immunoprecipitation
assay confirmed the binding between beta-catenin/TCF4 complexes from IEC-6 nuclear extracts and the putative sequences in
the miR-30e promoter. These results demonstrate that beta-catenin/TCF4 transactivates miR-30e during intestinal cell differentiation. 相似文献
63.
This paper presents a new spatial dependence model with an adjustment of feature difference. The model accounts for the spatial autocorrelation in both the outcome variables and residuals. The feature difference adjustment in the model helps to emphasize feature changes across neighboring units, while suppressing unobserved covariates that are present in the same neighborhood. The prediction at a given unit incorporates components that depend on the differences between the values of its main features and those of its neighboring units. In contrast to conventional spatial regression models, our model does not require a comprehensive list of global covariates necessary to estimate the outcome variable at the unit, as common macro-level covariates are differenced away in the regression analysis. Using the real estate market data in Hong Kong, we applied Gibbs sampling to determine the posterior distribution of each model parameter. The result of our empirical analysis confirms that the adjustment of feature difference with an inclusion of the spatial error autocorrelation produces better out-of-sample prediction performance than other conventional spatial dependence models. In addition, our empirical analysis can identify components with more significant contributions. 相似文献
64.
The enzymic activity of Mg2+- or Ca2+-stimulated ATPase from Escherichia coli was inhibited by one of the troponin components, TN-I, and by mitochondrial ATPase inhibitor (F1-inhibitor). The inhibitory ability of component TN-I against Mg2+-stimulated AtPase activity was lost after digestion of component TN-I with trypsin. The Mg2+-stimulated ATPase activity inhibited by component TN-I was completely restored by the addition of another troponin component TN-C. 相似文献
65.
The response of serum luteinizing hormone to glycine has been studied during the estrous cycle in adult female rats. I.p. administration of 200 mg of glycine significantly elevated serum luteinizing hormone levels at all stages of the estrous cycle. 相似文献
66.
M L Kashyap S G de Mendoza M Campbell C Y Chen R F Lutmer C J Glueck 《Experientia》1978,34(8):1044-1045
Marked urinary loss of lipoprotein lipase activator in experimental rat nephrotic syndrome may be partly responsible for its deficiency in plasma very low density lipoproteins. 相似文献
67.
Pigment granules in choroidal melanophores of the albino goldfish contained fine particulate materials which were in various degrees aggregated in clumps. Tyrosinase was considered to be present in an inhibited state in these pigment granules. 相似文献
68.
The activity of dopamine-beta-hydroxylase was measured in the serum before and immediately after electroconvulsive treatments. No significant difference was observed, suggesting that the seizures did not cause an increase in the peripheral sympathetic tone. 相似文献
69.
Hamster chondrocytes could be transformed in a quantitative assay system which used X-irradiated feeder layer cells. Morphological transformation occurred on addition of, 4NQO, but not in control cultures. Differentiation was classified into 3 types (good, poor and none); normal and transformed colonies contained similar proportions of the 3 types. 相似文献
70.