全文获取类型
收费全文 | 691588篇 |
免费 | 10739篇 |
国内免费 | 4028篇 |
专业分类
化学 | 316049篇 |
晶体学 | 8987篇 |
力学 | 26978篇 |
综合类 | 81篇 |
数学 | 73069篇 |
物理学 | 185034篇 |
无线电 | 96157篇 |
出版年
2021年 | 5825篇 |
2020年 | 6361篇 |
2019年 | 6708篇 |
2018年 | 8089篇 |
2017年 | 8030篇 |
2016年 | 12266篇 |
2015年 | 8084篇 |
2014年 | 12380篇 |
2013年 | 30402篇 |
2012年 | 23359篇 |
2011年 | 28061篇 |
2010年 | 19981篇 |
2009年 | 20924篇 |
2008年 | 27249篇 |
2007年 | 28183篇 |
2006年 | 26596篇 |
2005年 | 23992篇 |
2004年 | 22060篇 |
2003年 | 19756篇 |
2002年 | 19491篇 |
2001年 | 21497篇 |
2000年 | 17131篇 |
1999年 | 14107篇 |
1998年 | 12100篇 |
1997年 | 11747篇 |
1996年 | 11339篇 |
1995年 | 10330篇 |
1994年 | 9972篇 |
1993年 | 9697篇 |
1992年 | 10371篇 |
1991年 | 10393篇 |
1990年 | 9806篇 |
1989年 | 9272篇 |
1988年 | 9143篇 |
1987年 | 8133篇 |
1986年 | 7733篇 |
1985年 | 10244篇 |
1984年 | 10343篇 |
1983年 | 8555篇 |
1982年 | 8861篇 |
1981年 | 8590篇 |
1980年 | 8275篇 |
1979年 | 8536篇 |
1978年 | 8595篇 |
1977年 | 8504篇 |
1976年 | 8433篇 |
1975年 | 8155篇 |
1974年 | 7884篇 |
1973年 | 8071篇 |
1972年 | 5280篇 |
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
991.
Boutros J. Caire G. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(7):1772-1793
We present a framework for iterative multiuser joint decoding of code-division multiple-access (CDMA) signals, based on the factor-graph representation and on the sum-product algorithm. In this framework, known parallel and serial, hard and soft interference cancellation algorithms are derived in a unified way. The asymptotic performance of these algorithms in the limit of large code block length can be rigorously analyzed by using density evolution. We show that, for random spreading in the large-system limit, density evolution is considerably simplified. Moreover, by making a Gaussian approximation of the decoder soft output, we show that the behavior of iterative multiuser joint decoding is approximately characterized by the stable fixed points of a simple one-dimensional nonlinear dynamical system 相似文献
992.
993.
It is essential in the simulation of power electronics applications to model magnetic components accurately. In addition to modeling the nonlinear hysteresis behavior, eddy currents and winding losses must be included to provide a realistic model. In practice the losses in magnetic components give rise to significant temperature increases which can lead to major changes in the component behavior. In this paper a model of magnetic components is presented which integrates a nonlinear model of hysteresis, electro-magnetic windings and thermal behavior in a single model for use in circuit simulation of power electronics systems. Measurements and simulations are presented which demonstrate the accuracy of the approach for the electrical, magnetic and thermal domains across a variety of operating conditions, including static thermal conditions and dynamic self heating 相似文献
994.
A quantum well (QW) in the simultaneous presence of a terahertz field polarized in the growth direction and an incident optical field near an excitonic resonance results in substantial frequency mixing between the terahertz and optical fields. In particular, a response at new frequencies given by the input optical frequency plus or minus multiples of the terahertz frequency occurs-the terahertz sidebands. In a symmetric QW, the dominant contribution to terahertz-sideband formation is the high-frequency modulation of the overlap integral of the relevant conduction- and valence-subband envelope functions that determine the strength of the interband dipole moment. terahertz-sideband generation is shown to be strongly enhanced in a high quality-factor optical microcavity. Numerical values of the reflected intensity into the first terahertz sideband normalized with respect to the reflected intensity at the fundamental as large as /spl sim/10% are estimated. This suggests that terahertz-sideband generation in semiconductor microcavities is a promising option worthy of exploration for wavelength conversion for wavelength-division multiplexing applications. 相似文献
995.
Medard M. Lumetta S. Liuyang Li 《Selected Areas in Communications, IEEE Journal on》2002,20(4):822-833
We describe an architecture for an optical local area network (LAN) or metropolitan area network (MAN) access. The architecture allows for bandwidth sharing within a wavelength and is robust to both link and node failures. The architecture can be utilized with an arbitrary, link-redundant mesh network (node-redundancy is necessary only to handle all node failures), and assumes neither the use of a star topology nor the ability to embed such a topology within the physical mesh. Reservation of, bandwidth is performed in a centralized fashion at a (replicated) head end node, simplifying the implementation of complex sharing policies relative to implementation on a distributed set of routers. Unlike a router, however, the head end does not take any action on individual packets and, in particular, does not buffer packets. The architecture thus avoids the difficulties of processing packets in the optical domain while allowing for packetized shared access of wavelengths. We describe the route construction scheme and prove its ability to recover from single link and single node failures, outline a flexible medium access protocol and discuss the implications for implementing specific policies, and propose a simple implementation of the recovery protocol in terms of state machines for per-link devices 相似文献
996.
P. M. Igic P. A. Mawby M. S. Towers W. Jamal S. Batcup 《Microelectronics Reliability》2002,42(7):1045-1052
New compact models of the IGBTs (both non-punch through IGBT (NPTIGBT) and punch-through IGBT (PTIGBT)) are presented in this paper. The models are implemented in the SABER circuit simulator and used for a study of IGBT anode current and voltage characteristics during a device turn-off (clamped inductive load circuit with gate controlled turn-off), since these parts of the transient characteristics essentially predict the power dissipation (V×I) inside the device. It is shown that PTIGBTs are faster than NPTIGBTs, this becoming more apparent at higher clamp voltages. 相似文献
997.
J. Tabor 《Aequationes Mathematicae》2002,63(1-2):18-25
998.
F. F. Maia Jr. J. A. K. Freire G. A. Farias V. N. Freire E. F. da Silva Jr. 《Applied Surface Science》2002,190(1-4):247-251
The excitonic properties of a ZnSe/ZnSxSe1−x strained quantum well (QW) are calculated taking into account interface effects. Numerical results obtained with ZnS0.18Se0.82/ZnSe QWs show that graded interfaces can be responsible for a strong broadening of excitonic spectra. 相似文献
999.
A simple model consisting of two electronic levels and one vibrational mode (phonon) was theoretically studied. The electronic-vibrational interaction was linear in the vibrational displacement. The vibrational mode was taken in the harmonic approximation and was attached to the thermal bath formed by the ambient environment. The kinetic constants of the vibrational dissipation were of the second order in the vibrational-bath coupling and were taken in the Markovian limit. Although, depending on the parameters of the model, different curves of the non-radiative vibration-induced excited state decay were obtained, in general, three time intervals, corresponding to different physical behaviour, were found. In the short-time interval, small oscillations superimposed on the excited state decay were observed. They were determined by the vibrational frequency and influenced by electronic-vibrational coupling. In the middle-time interval, almost quasi-exponential decay was detected; its rate constant increased with stronger electronic-vibrational interaction and speed of vibrational relaxation. In the long-time interval, the decay was very slow and, under special conditions, even an asymptotic non-zero excited state population was observed. Its value increased with the strength of the off-diagonal electron-vibrational coupling. Links of the parameters of the model with quantum chemical terms were estimated. 相似文献
1000.
S. V. Prants 《JETP Letters》2002,75(12):651-658
A semiclassical study is carried out of the nonlinear interaction dynamics between two-level atoms and a standing-wave field in a high-finesse cavity. As a result of atomic movement or wave amplitude modulation, a dynamic local instability occurs in a strongly coupled atom-field system. The appearance of dynamical Hamiltonian chaos, fractals, and Lévy flights is demonstrated for the models of two experimental devices: a (micro)maser with thermal Rydberg atoms and a microlaser with cold atoms. Numerical simulation showed that the manifestations of classical chaos, atomic fractals, and flights can be observed in the appropriate real experiments. Attention is drawn to the prospects provided by work on the atom-field systems in the coupling-modulated high-finesse cavities for further investigation of the quantum-classical correspondence, quantum chaos, and decoherence. 相似文献