全文获取类型
收费全文 | 6389篇 |
免费 | 167篇 |
国内免费 | 25篇 |
专业分类
化学 | 4306篇 |
晶体学 | 65篇 |
力学 | 180篇 |
数学 | 753篇 |
物理学 | 1277篇 |
出版年
2021年 | 45篇 |
2020年 | 64篇 |
2019年 | 68篇 |
2018年 | 43篇 |
2017年 | 42篇 |
2016年 | 122篇 |
2015年 | 92篇 |
2014年 | 117篇 |
2013年 | 257篇 |
2012年 | 343篇 |
2011年 | 397篇 |
2010年 | 212篇 |
2009年 | 171篇 |
2008年 | 334篇 |
2007年 | 366篇 |
2006年 | 370篇 |
2005年 | 336篇 |
2004年 | 280篇 |
2003年 | 251篇 |
2002年 | 232篇 |
2001年 | 99篇 |
2000年 | 93篇 |
1999年 | 51篇 |
1998年 | 76篇 |
1997年 | 71篇 |
1996年 | 103篇 |
1995年 | 67篇 |
1994年 | 84篇 |
1993年 | 87篇 |
1992年 | 61篇 |
1991年 | 74篇 |
1990年 | 65篇 |
1989年 | 68篇 |
1988年 | 66篇 |
1987年 | 44篇 |
1986年 | 48篇 |
1985年 | 91篇 |
1984年 | 81篇 |
1983年 | 58篇 |
1982年 | 82篇 |
1981年 | 88篇 |
1980年 | 78篇 |
1979年 | 71篇 |
1978年 | 78篇 |
1977年 | 73篇 |
1976年 | 63篇 |
1975年 | 66篇 |
1974年 | 65篇 |
1973年 | 70篇 |
1972年 | 38篇 |
排序方式: 共有6581条查询结果,搜索用时 671 毫秒
991.
We introduce a method for optical characterization of hollow-core optical waveguides. Radiation pressure exerted by the waveguide modes on dielectric microspheres is used to analyze salient properties such as propagation loss and waveguide mode profiles. These quantities were measured for quasi-single-mode and multimode propagation in on-chip liquid-filled hollow-core antiresonant reflecting optical waveguides. Excellent agreement with analytical and numerical models is found, demonstrating that optically induced particle transport provides a simple, inexpensive, and nondestructive alternative to other characterization methods. 相似文献
992.
993.
The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena such as the renowned Kohn anomaly. Here we report the observation in bilayer graphene of an unusual phonon softening that provides the first experimental proof for another type of phonon anomaly. Similar to the Kohn anomaly, which is a logarithmic singularity in the phonon group velocity [W. Kohn, Phys. Rev. Lett. 2, 393 (1959)], the observed phonon anomaly exhibits a logarithmic singularity in the optical-phonon energy. Arising from a resonant electron-phonon coupling effect, the anomaly was also expected, albeit not observed, in monolayer graphene. We propose an explanation for why it is easier to observe in bilayer samples. 相似文献
994.
Lundblad N Lee PJ Spielman IB Brown BL Phillips WD Porto JV 《Physical review letters》2008,100(15):150401
We load cold atoms into an optical lattice dramatically reshaped by radio-frequency coupling of state-dependent lattice potentials. This radio-frequency dressing changes the unit cell of the lattice at a subwavelength scale, such that its curvature and topology departs strongly from that of a simple sinusoidal lattice potential. Radio-frequency dressing has previously been performed at length scales from mm to tens of mum, but not at the single-optical-wavelength scale. At this length scale significant coupling between adiabatic potentials leads to nonadiabatic transitions, which we measure as a function of lattice depth and dressing amplitude. We also investigate the dressing by measuring changes in the momentum distribution of the dressed states. 相似文献
995.
A. H. Phillips N. A. I. Aly K. Kirah H. E. El-Sayes 《中国物理快报》2008,25(1):250-253
The transport property of a quantum dot under the influence of external time-dependent field is investigated. The mesoscopic device is modelled as semiconductor quantum dot coupled weakly to superconducting leads via asymmetric double tunnel barriers of different heights. An expression for the current is deduced by using the Landauer-Buttiker formula, taking into consideration of both the Coulomb blockade effect and the magnetic field. It is found that the periodic oscillation of the current with the magnetic field is controlled by the ratio of the frequency of the applied ac-field to the electron cyclotron frequency. Our results show that the present device operates as a radio-frequency single electron transistor. 相似文献
996.
Michal P. Dlubek Andrew J. Phillips Eric C. Larkins 《Optical and Quantum Electronics》2008,40(8):561-575
A new and straightforward optical signal quality metric, which is based on the approximation of the analytical probability
density function by its Laguerre expansion, is proposed. This expansion uses statistical moments to approximate the true statistics,
but, unlike the well-known Gaussian approximation, higher order moments can be used as well. The quality of the proposed metric
is discussed and the metric is compared with the Gaussian approximation. It is shown that the new metric provides not only
a good estimation of the bit error rate in the range of practical interest but, contrary to the Gaussian approximation, correctly
estimates optimum decision threshold and accurately pictures the shape of the noise distribution. 相似文献
997.
998.
Emily K. Corlett Helen Blade Leslie P. Hughes Philip J. Sidebottom David Walker Richard I. Walton Steven P. Brown 《Magnetic resonance in chemistry : MRC》2020,58(11):1026-1035
Single-crystal X-ray diffraction structures of the 5-amino-2-methylpyridinium hydrogen fumarate salt have been solved at 150 and 300 K (CCDC 1952142 and 1952143). A base–acid–base–acid ring is formed through pyridinium-carboxylate and amine-carboxylate hydrogen bonds that hold together chains formed from hydrogen-bonded hydrogen fumarate ions. 1H and 13C chemical shifts as well as 14N shifts that additionally depend on the quadrupolar interaction are determined by experimental magic angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) and gauge-including projector-augmented wave (GIPAW) calculation. Two-dimensional homonuclear 1H-1H double-quantum (DQ) MAS and heteronuclear 1H-13C and 14N-1H spectra are presented. Only small differences of up to 0.1 and 0.6 ppm for 1H and 13C are observed between GIPAW calculations starting with the two structures solved at 150 and 300 K (after geometry optimisation of atomic positions, but not unit cell parameters). A comparison of GIPAW-calculated 1H chemical shifts for isolated molecules and the full crystal structures is indicative of hydrogen bonding strength. 相似文献
999.
Nadine Wellington Sabrina Macklai Prof. Philip Britz-McKibbin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(67):15277-15280
A zwitterionic heterocyclic boronic acid based on 4-isoquinolineboronic acid (IQBA) exhibits the highest reported binding affinity for sialic acid or N-acetylneuraminic acid (Neu5Ac, K=5390±190 m −1) through the formation of a cyclic boronate ester complex under acidic conditions (pH 3). This anomalous pH-dependent binding enhancement does not occur with common neutral saccharides (e.g., glucose, fructose, sorbitiol), because it is mediated via selective complexation to a α-hydroxycarboxylate moiety forming a stable ion pair and ternary complex with Neu5Ac in phosphate buffer. IQBA expands biorecognition beyond classical vicinal diols under neutral or alkaline buffer conditions, which enables the direct analysis of Neu5Ac by native fluorescence with sub-micromolar detection limits. 相似文献
1000.
Philip Boehm Sven Roediger Dr. Alessandro Bismuto Prof. Dr. Bill Morandi 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(41):18043-18052
An efficient palladium-catalyzed chlorocarbonylation of aryl (pseudo)halides that gives access to a wide range of carboxylic acid derivatives has been developed. The use of butyryl chloride as a combined CO and Cl source eludes the need for toxic, gaseous carbon monoxide, thus facilitating the synthesis of high-value products from readily available aryl (pseudo)halides. The combination of palladium(0), Xantphos, and an amine base is essential to promote this broadly applicable catalytic reaction. Overall, this reaction provides access to a great variety of carbonyl-containing products through in situ transformation of the generated aroyl chloride. Combined experimental and computational studies support a reaction mechanism involving in situ generation of CO. 相似文献