全文获取类型
收费全文 | 14781篇 |
免费 | 1478篇 |
国内免费 | 448篇 |
专业分类
化学 | 4716篇 |
晶体学 | 15篇 |
力学 | 844篇 |
综合类 | 64篇 |
数学 | 6489篇 |
物理学 | 4579篇 |
出版年
2024年 | 18篇 |
2023年 | 143篇 |
2022年 | 202篇 |
2021年 | 346篇 |
2020年 | 389篇 |
2019年 | 394篇 |
2018年 | 268篇 |
2017年 | 316篇 |
2016年 | 541篇 |
2015年 | 444篇 |
2014年 | 688篇 |
2013年 | 1131篇 |
2012年 | 656篇 |
2011年 | 709篇 |
2010年 | 626篇 |
2009年 | 867篇 |
2008年 | 1024篇 |
2007年 | 1023篇 |
2006年 | 847篇 |
2005年 | 619篇 |
2004年 | 523篇 |
2003年 | 574篇 |
2002年 | 571篇 |
2001年 | 436篇 |
2000年 | 459篇 |
1999年 | 384篇 |
1998年 | 368篇 |
1997年 | 253篇 |
1996年 | 210篇 |
1995年 | 174篇 |
1994年 | 175篇 |
1993年 | 153篇 |
1992年 | 145篇 |
1991年 | 93篇 |
1990年 | 87篇 |
1989年 | 89篇 |
1988年 | 103篇 |
1987年 | 71篇 |
1986年 | 66篇 |
1985年 | 103篇 |
1984年 | 80篇 |
1983年 | 24篇 |
1982年 | 50篇 |
1981年 | 47篇 |
1980年 | 48篇 |
1979年 | 38篇 |
1978年 | 33篇 |
1977年 | 34篇 |
1976年 | 30篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
Uncertainty about the nature and significance of nonlinearities and the manner in which dynamics affect future realizations makes model specification the most difficult aspect of modeling dynamic systems. By interpreting several popular fishery models as subcases of a nesting dynamic Taylor series approximation, we isolate the specification differences between these models in a way that accounts for commonalities. On the argument that the differences due to alternative nonlinear forms are likely to be small compared to more mundane considerations such as delay difference and general dynamic lag specification, we propose an alternative model that uses the terms from the first order approximation common to all models combined with a data-based determination of the appropriate lags using the methods of state space time series analysis. Finally, the success of the alternative models is judged in an application to Pacific halibut data. 相似文献
212.
Solveig Linko 《Accreditation and quality assurance》2002,7(2):55-59
Internal audits inherently effect quality improvement, and concrete results can be expected by careful planning. As audits
should improve the quality system in situ, the development of audits should not be neglected. ISO/IEC 17025 describes the
objectives of internal audits and management reviews in more detail compared to the former standards EN 45001 and ISO/IEC
Guide 25, although the main features are unchanged. This addresses continuous surveillance of the current practice. A brief
questionnaire on vertical audits was distributed to 74 of 120 employees at a Finnish private medical laboratory, Medix Laboratories
Ltd. A positive feed-back was received from the current management of audits. However, the study revealed that continuous
supervision of audits is needed. Many employees were not willing to perform audits if they had no previous experience. Additionally,
audits performed at the workbench were not considered as the best possible for fruitful discussions.
Received: 20 February 2001 Accepted: 30 November 2001 相似文献
213.
P. Bonville M. Rams K. Królas J.-P. Sanchez P. C. Canfield O. Trovarelli C. Geibel 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(1):77-84
We have examined the magnetic properties of the heavy electron compounds
YbAgGe and YbPtIn by 170Yb M?ssbauer spectroscopy down to 0.1 K, and
the crystal field properties of YbAgGe by Perturbed Angular Correlations
(PAC) measurements up to 900 K. In YbAgGe, we show that each of the two
magnetically ordered phases below 0.8 K involves a specific incommensurate
modulation of the Yb moment. An analysis of existing low temperature specific
heat data suggests the
persistence of fluctuations of the correlated Yb spins down to 0.1 K.
The PAC data allow to discriminate among proposed Yb3+ crystal field
level schemes. In YbPtIn, we show that the low temperature magnetic order
phase has an antiferro-para structure, where zero moment Yb ions coexist with
large moment ones, and that a 90° moment reorientation occurs at
1.4 K. 相似文献
214.
主要研究相互粒子系统中概率测度的负相关.我们得到判定概率测度是负相关的一个充分必要条件.最后证明了具有负相关的概率测度的线性组合及乘积测度仍是负相关的. 相似文献
215.
216.
We have studied the vibrational frequencies and atom displacements of one-dimensional systems formed by combinations of Thue-Morse and Rudin-Shapiro quasi-regular stackings with periodic ones. The materials are described by nearest-neighbor force constants and the corresponding atom masses. These systems exhibit differences in the frequency spectrum as compared to the original simple quasi-regular generations and periodic structures. The most important feature is the presence of separate confinement of the atom displacements in one of the parts forming the total composite structure for different frequency ranges, thus acting as a kind of phononic cavity. 相似文献
217.
利用指数二分性理论和泛函分析方法,我们研究了自治奇摄动系统的同,异宿轨道的存在性,给出了高维奇摄动系统从退化系统分支出同异宿轨道的Mel-nikov型函数。 相似文献
218.
M. Reuhl P. Nielaba K. Binder 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(2):225-232
,Static and dynamic properties of the Potts model on the simple cubic lattice with nearest neighbor -interaction are obtained from Monte Carlo simulations in a temperature range where full thermal equilibrium still can be
achieved (). For a lattice size L
= 16, in this range finite size effects are still negligible, but the data for the spin glass susceptibility agree with previous
extrapolations based on finite size scaling of very small lattices. While the static properties are compatible with a zero
temperature transition, they certainly do not prove it. Unlike the Ising spin glass, the decay of the time-dependent order
parameter is compatible with a simple Kohlrausch function, , while a power law prefactor cannot be distinguished. The Kohlrausch exponent y
(
T
) decreases from at [0pt] to at [0pt] however. The relaxation time is compatible with the exponential divergence postulated by McMillan for spin glasses at their lower critical dimension,
but the exponent that can be extracted still differs significantly from the theoretical value, . Thus the present results support the conclusion that the Potts spin glass in d
= 3 dimensions differs qualitatively from the Ising spin glass.
Received: 8 October 1997 / Accepted: 27 November 1997 相似文献
219.
K. Held D. Vollhardt 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):473-478
The importance of Hund's rule coupling for the stabilization of itinerant ferromagnetism is investigated within a two-band
Hubbard model. The magnetic phase diagram is calculated by finite-temperature quantum Monte-Carlo simulations within the dynamical
mean-field theory. Ferromagnetism is found in a broad range of electron fillings whereas antiferromagnetism exists only near
half filling. The possibility of orbital ordering at quarter filling is also analyzed.
Received: 26 February 1998 / Accepted: 17 April 1998 相似文献
220.
4-Aminophthalimide Derivatives as Environment-Sensitive Probes 总被引:2,自引:0,他引:2
The potential of 4-aminophthalimide (AP) and its derivatives as fluorescence probes for organized media is highlighted. The fluorescence response of AP, as measured from the position of the fluorescence maximum, fluorescence intensity and lifetime, is highly sensitive to the polarity of the medium. The sensitivity of the fluorescence parameters is further enhanced due to the involvement of the emitting intramolecular charge transfer state in hydrogen bonding interaction with the solvent molecules containing hydroxyl groups. It is shown that the microheterogeneous environments of organized media such as cyclodextrins and micelles can be very conveniently monitored using this probe. The results of the investigations carried out employing AP and its derivatives as fluorescence probe molecules in these media clearly suggest that a combination of the hydrophobic interaction with the host media and hydrogen bonding interaction with the solvent molecules determine the location of the fluorophore, which in all cases is found to be the interfacial region separating the nonpolar core of the micelle or the cyclodextrin cavity and the polar aqueous environment. Guidelines for the design of probes of this class of systems for the nonpolar core region of the micelles are provided and possible ways to increase the sensitivity of the fluorescence response of the systems are suggested. 相似文献