全文获取类型
收费全文 | 1405篇 |
免费 | 21篇 |
国内免费 | 18篇 |
专业分类
化学 | 816篇 |
晶体学 | 17篇 |
力学 | 17篇 |
数学 | 128篇 |
物理学 | 466篇 |
出版年
2016年 | 15篇 |
2015年 | 12篇 |
2014年 | 11篇 |
2013年 | 37篇 |
2012年 | 29篇 |
2011年 | 61篇 |
2010年 | 33篇 |
2009年 | 34篇 |
2008年 | 51篇 |
2007年 | 44篇 |
2006年 | 63篇 |
2005年 | 46篇 |
2004年 | 50篇 |
2003年 | 53篇 |
2002年 | 44篇 |
2001年 | 32篇 |
2000年 | 35篇 |
1999年 | 24篇 |
1998年 | 19篇 |
1997年 | 19篇 |
1996年 | 25篇 |
1995年 | 28篇 |
1994年 | 32篇 |
1993年 | 25篇 |
1992年 | 27篇 |
1991年 | 24篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 21篇 |
1987年 | 19篇 |
1986年 | 19篇 |
1985年 | 21篇 |
1984年 | 27篇 |
1983年 | 21篇 |
1982年 | 13篇 |
1981年 | 10篇 |
1980年 | 21篇 |
1979年 | 19篇 |
1978年 | 20篇 |
1977年 | 10篇 |
1976年 | 22篇 |
1975年 | 20篇 |
1974年 | 28篇 |
1973年 | 22篇 |
1972年 | 15篇 |
1970年 | 11篇 |
1969年 | 16篇 |
1968年 | 16篇 |
1967年 | 19篇 |
1966年 | 16篇 |
排序方式: 共有1444条查询结果,搜索用时 0 毫秒
961.
West AG Patrickson SJ Ehleringer JR 《Rapid communications in mass spectrometry : RCM》2006,20(8):1317-1321
Stable isotopic analysis of water for many ecological applications commonly requires extractions of water from dozens to hundreds of plant and soil samples. With recent advances in mass spectrometry, water extraction, rather than the isotopic analysis itself, is the bottleneck in sample processing. Using cryogenic vacuum distillation, we have created extraction timing curves to determine how much time (T(min)) is required to extract an unfractionated water sample. Our results indicated that T(min) values are 60 to 75 min for stems, 40 min for clay soils, 30 min for sandy soils and 20 to 30 min for leaves. While the extraction times reported here may allow for some reductions relative to times reported in the literature, the extraction process will continue to be a rate-limiting step in plant water analyses. Ultimately, technological advances eliminating the need for extraction are required to greatly increase throughput rates in water isotope analysis for ecological research. 相似文献
962.
D.V. West I.D. Posen Q. Huang H.W. Zandbergen T.M. McQueen R.J. Cava 《Journal of solid state chemistry》2009,182(9):2461-2467
PbMn(SO4)2 has been synthesized in an evacuated quartz tube. The nuclear and magnetic crystal structures have been determined using powder X-ray and neutron diffraction. This material crystallizes in the enantiomorphic space group pair P41212(92) and P43212(96), forming a double-helical arrangement of Pb2+ and Mn2+ cations. The Mn2+O6 octahedra are distorted. Each 3d5 Mn2+ has four nearest-neighbors and four next-nearest-neighbors adopting a frustrating arrangement. The compound orders antiferromagnetically at 5.5 K. Field dependent specific heat and magnetization measurements show that TN is suppressed to 3.3 K when μ0H=9 T. 相似文献
963.
In this Letter we consider the previously proposed generalised space–time and investigate the structure of the field theory upon which it is based. In particular, we derive a SO(D,D) formulation of the bosonic string as a non-linear realisation at lowest levels of E11s⊗l1 where l1 is the first fundamental representation. We give a Hamiltonian formulation of this theory and carry out its quantisation. We argue that the choice of representation of the quantum theory breaks the manifest SO(D,D) symmetry but that the symmetry is manifest in a non-commutative field theory. We discuss the implications for the conjectured E11 symmetry and the role of the l1 representation. 相似文献
964.
965.
Kohli KK Davies G Vinh NQ West D Estreicher SK Gregorkiewicz T Izeddin I Itoh KM 《Physical review letters》2006,96(22):225503
By simply changing the isotopes of the Si atoms that neighbor an oxygen Oi atom in crystalline silicon, the measured decay rate tau of the asymmetric-stretch vibration (nu3=1136 cm-1) of oxygen (Oi) in silicon changes by a factor of approximately 2.5. These data establish that nu3 decays by creating one nu1 symmetric-stretch, local-vibrational mode of the Si-Oi-Si structure. If the residual energy (nu3-nu1) is less than the maximum frequency num of the host lattice, as for 28Si-16O-28Si in natural silicon, then it is emitted as one lattice mode, and tau depends on the density of one-phonon states at nu3-nu1. If (nu3-nu1)>num, as for 16O in single-isotope 30Si silicon, two lattice modes are created in addition to nu1, increasing tau. Prediction of tau for a particular defect clearly requires a detailed knowledge of that defect. 相似文献
966.
S.A. Studenikin M. Byszewski D.K. Maude M. Potemski A. Sachrajda Z.R. Wasilewski M. Hilke L.N. Pfeiffer K.W. West 《Physica E: Low-dimensional Systems and Nanostructures》2006,34(1-2):73
Microwave induced resistance oscillations (MIROs) were studied experimentally over a very wide range of frequencies ranging from 20 GHz up to 4 THz, and from the quasi-classical regime to the quantum Hall effect regime. At low frequencies regular MIROs were observed, with a periodicity determined by the ratio of the microwave to cyclotron frequencies. For frequencies below 150 GHz the magnetic field dependence of MIROs waveform is well described by a simplified version of an existing theoretical model, where the damping is controlled by the width of the Landau levels. In the THz frequency range MIROs vanish and only pronounced resistance changes are observed at the cyclotron resonance. The evolution of MIROs with frequency is presented and discussed. 相似文献
967.
We have investigated the influence of impurities on the possible supersolid transition in 4He by systematically enriching isotopically pure samples with 3He. The addition of 3He broadens the onset of nonclassical rotational inertia and shifts it to higher temperature, suggesting that the phenomenon is correlated with the condensation of 3He atoms onto the dislocation network in solid 4He. 相似文献
968.
Dean CR Piot BA Hayden P Das Sarma S Gervais G Pfeiffer LN West KW 《Physical review letters》2008,101(18):186806
Using a tilted-field geometry, the effect of an in-plane magnetic field on the even denominator nu=5/2 fractional quantum Hall state is studied. The energy gap of the nu=5/2 state is found to collapse linearly with the in-plane magnetic field above approximately 0.5 T. In contrast, a strong enhancement of the gap is observed for the nu=7/3 state. The radically distinct tilted-field behavior between the two states is discussed in terms of Zeeman and magneto-orbital coupling within the context of the proposed Moore-Read Pfaffian wave function for the 5/2 fractional quantum Hall effect. 相似文献
969.
Joanna Drews Helene Launay Keld West Peter Kingshott 《Applied Surface Science》2008,254(15):4720-4725
The stability of plasma polymerised layers has become important because of their widespread use. This study explored the hydrolysis and degradation stability of coatings of plasma polymerised maleic anhydride. Coatings made with different plasma parameters were exposed to aqueous media of different pH as a function of time. ATR-FTIR was used for structure analysis and a toluidine blue staining method allowed quantitative analysis of the hydrolysis of anhydride groups to acid groups. Coatings with constant thickness were obtained at different plasma powers and layers with varying thickness were obtained at constant plasma power by adjusting the polymerisation time. The results show that the hydrolysis resistance of the modified layer is determined by the power used in the plasma polymerisation, while changes in the chemistry of the modified layer are insignificant. 相似文献
970.
K. G. Sanjaya Ranmohotti Wendy L. Queen J. Palmer West Don VanDerveer Shiou-Jyh Hwu 《Journal of chemical crystallography》2009,39(4):303-307
Abstract A new barium chlorovanadate, Ba5(V2O7)2Cl2, was isolated by a high-temperature (850 °C) reaction employing a CsCl/RbCl flux. The structure was determined by single
crystal X-ray diffraction methods. This compound crystallizes in an orthorhombic crystal system, Pmmn (No. 59), with a = 11.558(2) ?, b = 15.164(3) ?, c = 10.023(2) ?, Z = 4 and V = 1756.7(6) ?3. The structure of Ba5(V2O7)2Cl2 was determined by full-matrix, least-squares methods with R
1 = 0.0398, wR
2 = 0.1069 and GOF = 1.048 for all data. This new structure can be described as a composite lattice made up of mixed covalent
and ionic moities. The extended framework is orchestrated by stacked [Ba(V2O7)Cl]3− slabs that are interconnected by Ba2+ cations through Ba–O bonds to the [V2O7] units. The Ba2+ and Cl- ions form BN-type “[BaCl]” sheets with pseudo-hexagonal windows that are centered by [V2O7]4− pyrovanadate units.
Graphical Abstract The structure of a new chlorovanadate, Ba5(V2O7)2Cl2, exhibits an interesting BN-type salt lattice that consists of an extended [BaCl] sheet containing pseudo-hexagonal windows
that are centered by [V2O7] pyrovanadate units.
相似文献