全文获取类型
收费全文 | 27160篇 |
免费 | 1432篇 |
国内免费 | 1380篇 |
专业分类
化学 | 2074篇 |
晶体学 | 154篇 |
力学 | 747篇 |
综合类 | 200篇 |
数学 | 2278篇 |
物理学 | 4125篇 |
综合类 | 20394篇 |
出版年
2024年 | 64篇 |
2023年 | 135篇 |
2022年 | 459篇 |
2021年 | 460篇 |
2020年 | 375篇 |
2019年 | 323篇 |
2018年 | 280篇 |
2017年 | 388篇 |
2016年 | 478篇 |
2015年 | 590篇 |
2014年 | 1150篇 |
2013年 | 1013篇 |
2012年 | 1252篇 |
2011年 | 1564篇 |
2010年 | 1449篇 |
2009年 | 1612篇 |
2008年 | 1911篇 |
2007年 | 2449篇 |
2006年 | 2283篇 |
2005年 | 2122篇 |
2004年 | 1779篇 |
2003年 | 1453篇 |
2002年 | 1149篇 |
2001年 | 891篇 |
2000年 | 647篇 |
1999年 | 580篇 |
1998年 | 480篇 |
1997年 | 386篇 |
1996年 | 321篇 |
1995年 | 332篇 |
1994年 | 276篇 |
1993年 | 225篇 |
1992年 | 199篇 |
1991年 | 145篇 |
1990年 | 143篇 |
1989年 | 150篇 |
1988年 | 108篇 |
1987年 | 82篇 |
1986年 | 52篇 |
1985年 | 36篇 |
1984年 | 24篇 |
1983年 | 15篇 |
1982年 | 17篇 |
1981年 | 19篇 |
1980年 | 9篇 |
1979年 | 25篇 |
1978年 | 20篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1974年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Influence theory is a foundational theory of physics that is not based on traditional empirically defined concepts, such as positions in space and time, mass, energy, or momentum. Instead, the aim is to derive these concepts, and their empirically determined relationships, from a more primitive model. It is postulated that there exist things, which are call particles, that influence one another in a discrete and directed fashion resulting in a partially ordered set of influence events. The problem of consistent quantification of the influence events is considered. Observers are modeled as particle chains (observer chains) as if an observer were able to track a particle and quantify the influence events that the particle experiences. From these quantified influence events, consistent quantification of the universe of events based on the observer chains is studied. Herein, the kinematics and dynamics of particles from the perspective of influence theory are both reviewed and further developed. 相似文献
122.
An experimentally feasible strong coupling system between a spin ensemble and a superconducting qubit is studied. The coupling strength can be exponentially enhanced by applying the squeezing transformations to the system. By means of the two spin ensembles commonly coupled to a superconducting qubit, a set of universal nonadiabatic holonomic single‐qubit quantum gates can be realized in a decoherence‐free subspace. Furthermore, this proposal is robust with respect to decay of the system parameters, and it is experimentally feasible with currently available technology. 相似文献
123.
Tunable wavelength filters using polymer long-period waveguide gratings based on metal-cladding directly defined technique 下载免费PDF全文
In this work, long-period waveguide grating-based tunable wavelength filters using organic–inorganic grafting poly(methyl methacrylate)(PMMA) materials are designed and fabricated by metal-cladding directly defined technique.The thermal stabilities and optical properties of the organic–inorganic grafting PMMA core materials are analyzed. Structures and performance parameters of the waveguide gratings and self-electrode heaters are designed and simulated. The contrast of the filter is about 15 d B and the resonant wavelength can be tuned by different electric powers applied to the metal-cladding self-electrode heaters. The temperature sensitivity is 3.5 nm/℃ and the switching time is about 1 ms. The technique is very suitable for realizing the optoelectronic integrated wavelength-division-multiplexing systems. 相似文献
124.
We study the possible advantages of adopting quantum strategies in multi-player evolutionary games. We base our study on the three-player Prisoner’s Dilemma (PD) game. In order to model the simultaneous interaction between three agents we use hypergraphs and hypergraph networks. In particular, we study two types of networks: a random network and a SF-like network. The obtained results show that in the case of a three-player game on a hypergraph network, quantum strategies are not necessarily stochastically stable strategies. In some cases, the defection strategy can be as good as a quantum one. 相似文献
125.
We introduce the Conditional Mutual Information (CMI) for the estimation of the Markov chain order. For a Markov chain of K symbols, we define CMI of order m, Ic(m), as the mutual information of two variables in the chain being m time steps apart, conditioning on the intermediate variables of the chain. We find approximate analytic significance limits based on the estimation bias of CMI and develop a randomization significance test of Ic(m), where the randomized symbol sequences are formed by random permutation of the components of the original symbol sequence. The significance test is applied for increasing m and the Markov chain order is estimated by the last order for which the null hypothesis is rejected. We present the appropriateness of CMI-testing on Monte Carlo simulations and compare it to the Akaike and Bayesian information criteria, the maximal fluctuation method (Peres–Shields estimator) and a likelihood ratio test for increasing orders using ?-divergence. The order criterion of CMI-testing turns out to be superior for orders larger than one, but its effectiveness for large orders depends on data availability. In view of the results from the simulations, we interpret the estimated orders by the CMI-testing and the other criteria on genes and intergenic regions of DNA chains. 相似文献
126.
Let A
1,…,A
N
be complex self-adjoint matrices and let ρ be a density matrix. The Robertson uncertainty principle
gives a bound for the quantum generalized covariance in terms of the commutators [A
h
,A
j
]. The right side matrix is antisymmetric and therefore the bound is trivial (equal to zero) in the odd case N=2m+1.
Let f be an arbitrary normalized symmetric operator monotone function and let 〈⋅,⋅〉
ρ,f
be the associated quantum Fisher information. Based on previous results of several authors, we propose here as a conjecture
the inequality
whose validity would give a non-trivial bound for any N∈ℕ using the commutators i[ρ,A
h
]. 相似文献
127.
从层流动能出发提出数值模拟原则;综合考虑自然、旁路和分离流转捩的因素构建实用的层流动能湍流模型,结合预处理和基本求解技术发展出适于转捩流动的数值模拟方法和程序.针对预处理技术,以Weiss-Smith矩阵为基础,考虑湍流粘性的影响;针对基本离散格式和边界条件,结合模型方程进行对角占优强化等特殊处理.最后通过平板边界层和典型翼型,特别是低雷诺数翼型的数值模拟,验证数值方法的有效性和鲁棒性.算例表明本文的方法能够为求解更复杂的流动提供参考. 相似文献
128.
重金属铅离子胁迫下玉米叶片光谱弱差信息的DSAT甄别模型 总被引:2,自引:0,他引:2
光谱间微弱信息测度是当今高光谱遥感研究难点之一,传统光谱测度方法难以区分光谱信息的微弱差异。研究设计了不同浓度的铅(Pb)污染实验,并测量了不同浓度铅离子(Pb2+)胁迫下玉米叶片的高光谱反射率、叶绿素含量及Pb2+含量,但是从所测结果得出,不同浓度Pb2+胁迫下的光谱相似性相关系数均达到0.999,难以区分不同浓度Pb2+胁迫引发的光谱间微弱信息差异和污染程度。针对这一情况,基于光谱微分处理、正切函数增强、光谱角量度与波谱分段检测等,提出了一种新型的相似光谱测度方法,即微分光谱角正切(derivative spectral angle tangent,DSAT)法。为了验证DSAT在区分相关系数达0.99以上相似光谱的可行性和有效性,将DSAT用于不同浓度Pb2+胁迫玉米叶片的整体波形与光谱区间子波形的信息差异性度量与检测。实验结果得到,波形差异信息与玉米叶片中叶绿素相对浓度与Pb2+含量显著相关。进而也证明DSAT法在甄别较高相似性光谱间差异上具有更好的实用性和优越性。 相似文献
129.
130.
针对老人意外跌倒的及时救助问题,设计了一种基于加速度矢量特征的老人跌倒检测装置。该装置结合加速度传感器MMA8452Q、GSM通信和GPS定位技术,通过分析人体姿态变化时的加速度矢量特征,利用加速度Z轴分量和加速度幅值作为跌倒判定的基础准则,并且考虑到跌倒后是否严重未起来,以加速度幅值的瞬时变化值为辅助提高了对于跌倒检测求助事件的判定准确性,据此远程报警并发送老人位置信息以及时施救。实测结果表明,该装置能准确地辨别出老人日常行为和意外跌倒需要求助状态并进行报警,降低跌倒带来的风险,实用性强。 相似文献