全文获取类型
收费全文 | 5754篇 |
免费 | 58篇 |
国内免费 | 76篇 |
专业分类
系统科学 | 97篇 |
丛书文集 | 119篇 |
教育与普及 | 175篇 |
理论与方法论 | 11篇 |
现状及发展 | 581篇 |
研究方法 | 991篇 |
综合类 | 3908篇 |
自然研究 | 6篇 |
出版年
2018年 | 8篇 |
2017年 | 22篇 |
2016年 | 18篇 |
2015年 | 13篇 |
2014年 | 24篇 |
2013年 | 18篇 |
2012年 | 463篇 |
2011年 | 542篇 |
2010年 | 125篇 |
2009年 | 47篇 |
2008年 | 427篇 |
2007年 | 437篇 |
2006年 | 451篇 |
2005年 | 474篇 |
2004年 | 411篇 |
2003年 | 417篇 |
2002年 | 355篇 |
2001年 | 283篇 |
2000年 | 409篇 |
1999年 | 102篇 |
1998年 | 15篇 |
1997年 | 15篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 24篇 |
1992年 | 23篇 |
1991年 | 19篇 |
1990年 | 30篇 |
1989年 | 23篇 |
1988年 | 12篇 |
1987年 | 26篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 30篇 |
1983年 | 25篇 |
1982年 | 22篇 |
1981年 | 19篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1971年 | 10篇 |
1970年 | 13篇 |
1966年 | 15篇 |
1959年 | 48篇 |
1958年 | 101篇 |
1957年 | 54篇 |
1956年 | 46篇 |
1955年 | 51篇 |
1954年 | 55篇 |
1948年 | 24篇 |
排序方式: 共有5888条查询结果,搜索用时 93 毫秒
71.
Lai CW Kim NY Utsunomiya S Roumpos G Deng H Fraser MD Byrnes T Recher P Kumada N Fujisawa T Yamamoto Y 《Nature》2007,450(7169):529-532
The effect of quantum statistics in quantum gases and liquids results in observable collective properties among many-particle systems. One prime example is Bose-Einstein condensation, whose onset in a quantum liquid leads to phenomena such as superfluidity and superconductivity. A Bose-Einstein condensate is generally defined as a macroscopic occupation of a single-particle quantum state, a phenomenon technically referred to as off-diagonal long-range order due to non-vanishing off-diagonal components of the single-particle density matrix. The wavefunction of the condensate is an order parameter whose phase is essential in characterizing the coherence and superfluid phenomena. The long-range spatial coherence leads to the existence of phase-locked multiple condensates in an array of superfluid helium, superconducting Josephson junctions or atomic Bose-Einstein condensates. Under certain circumstances, a quantum phase difference of pi is predicted to develop among weakly coupled Josephson junctions. Such a meta-stable pi-state was discovered in a weak link of superfluid 3He, which is characterized by a 'p-wave' order parameter. The possible existence of such a pi-state in weakly coupled atomic Bose-Einstein condensates has also been proposed, but remains undiscovered. Here we report the observation of spontaneous build-up of in-phase ('zero-state') and antiphase ('pi-state') 'superfluid' states in a solid-state system; an array of exciton-polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These in-phase and antiphase states reflect the band structure of the one-dimensional polariton array and the dynamic characteristics of metastable exciton-polariton condensates. 相似文献
72.
73.
74.
75.
Base-excision repair of oxidative DNA damage 总被引:1,自引:0,他引:1
Maintaining the chemical integrity of DNA in the face of assault by oxidizing agents is a constant challenge for living organisms. Base-excision repair has an important role in preventing mutations associated with a common product of oxidative damage to DNA, 8-oxoguanine. Recent structural studies have shown that 8-oxoguanine DNA glycosylases use an intricate series of steps to locate and excise 8-oxoguanine lesions efficiently against a high background of undamaged bases. The importance of preventing mutations associated with 8-oxoguanine is shown by a direct association between defects in the DNA glycosylase MUTYH and colorectal cancer. The properties of other guanine oxidation products and the associated DNA glycosylases that remove them are now also being revealed. 相似文献
76.
How long does it take a random walker to reach a given target point? This quantity, known as a first-passage time (FPT), has led to a growing number of theoretical investigations over the past decade. The importance of FPTs originates from the crucial role played by first encounter properties in various real situations, including transport in disordered media, neuron firing dynamics, spreading of diseases or target search processes. Most methods of determining FPT properties in confining domains have been limited to effectively one-dimensional geometries, or to higher spatial dimensions only in homogeneous media. Here we develop a general theory that allows accurate evaluation of the mean FPT in complex media. Our analytical approach provides a universal scaling dependence of the mean FPT on both the volume of the confining domain and the source-target distance. The analysis is applicable to a broad range of stochastic processes characterized by length-scale-invariant properties. Our theoretical predictions are confirmed by numerical simulations for several representative models of disordered media, fractals, anomalous diffusion and scale-free networks. 相似文献
77.
78.
The known galaxies most dominated by dark matter (Draco, Ursa Minor and Andromeda IX) are satellites of the Milky Way and the Andromeda galaxies. They are members of a class of faint galaxies, devoid of gas, known as dwarf spheroidals, and have by far the highest ratio of dark to luminous matter. None of the models proposed to unravel their origin can simultaneously explain their exceptional dark matter content and their proximity to a much larger galaxy. Here we report simulations showing that the progenitors of these galaxies were probably gas-dominated dwarf galaxies that became satellites of a larger galaxy earlier than the other dwarf spheroidals. We find that a combination of tidal shocks and ram pressure swept away the entire gas content of such progenitors about ten billion years ago because heating by the cosmic ultraviolet background kept the gas loosely bound: a tiny stellar component embedded in a relatively massive dark halo survived until today. All luminous galaxies should be surrounded by a few extremely dark-matter-dominated dwarf spheroidal satellites, and these should have the shortest orbital periods among dwarf spheroidals because they were accreted early. 相似文献
79.
80.