首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   296929篇
  免费   2223篇
  国内免费   633篇
化学   143568篇
晶体学   4104篇
力学   14696篇
综合类   4篇
数学   43024篇
物理学   94389篇
  2018年   10491篇
  2017年   11414篇
  2016年   6516篇
  2015年   2635篇
  2014年   2578篇
  2013年   8102篇
  2012年   9773篇
  2011年   19603篇
  2010年   11991篇
  2009年   11978篇
  2008年   17069篇
  2007年   20768篇
  2006年   6184篇
  2005年   12782篇
  2004年   8735篇
  2003年   7960篇
  2002年   5754篇
  2001年   5954篇
  2000年   4709篇
  1999年   3572篇
  1998年   2798篇
  1997年   2805篇
  1996年   2909篇
  1995年   2632篇
  1994年   2485篇
  1993年   2379篇
  1992年   2860篇
  1991年   2744篇
  1990年   2696篇
  1989年   2735篇
  1988年   2679篇
  1987年   2582篇
  1986年   2377篇
  1985年   3275篇
  1984年   3321篇
  1983年   2720篇
  1982年   2911篇
  1981年   2868篇
  1980年   2762篇
  1979年   2939篇
  1978年   3209篇
  1977年   2950篇
  1976年   2922篇
  1975年   2756篇
  1974年   2702篇
  1973年   2761篇
  1972年   1756篇
  1968年   2026篇
  1967年   2164篇
  1966年   1972篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The voltage response of a thin-film normal-metal hot-electron bolometer based on a SINIS (superconductor-insulator-normal metal-insulator-superconductor) structure to the radiation of a high-temperature Josephson junction in the terahertz frequency region was measured. Bolometers were integrated with planar log-periodic and double-dipole antennas, and Josephson junctions were integrated with log-periodic antennas. Measurements showed that the Josephson junction at a temperature of 260 mK was overheated by the transport current, so that its electron temperature exceeded 3 K at a bias voltage of 1 mV. The maximum response of a bolometer with a double-dipole antenna was observed at a frequency of 300 GHz, which agreed well with the calculated value. The Josephson radiation was observed at frequencies up to 1.7 THz. The voltage response of a bolometer reached 4×108 V/W, and the total noise-equivalent power reached 1.5×10?17 W/Hz1/2.  相似文献   
992.
A “collective” attack on the key is considered, and its connection with the classical capacity of a quantum communication channel is analyzed. It has been shown that the allowable error probability for legitimate users to which the secure key can be extracted is less than half the value for “translucent” eavesdropping and individual measurements.  相似文献   
993.
The properties of high-energy ion beams (beamlets) observed in the boundary layer of the plasma sheet of the Earth’s magnetotail during short time intervals (1–2 min) have been considered. Beamlets are induced by nonlinear impulse accelerating processes occurring in the current sheet of the far regions of the geomagnetic tail. Then, moving toward the Earth along the magnetic field lines, they are detected in the magnetotail (in the plasma sheet boundary layer) and in the high-latitude part of the auroral zone in the form of short bursts of high-energy ions (with energies of several tens of keVs). The size of the localization region of the beamlets in the magnetotail and auroral zone has been determined by the epoch-superposition method, and it has been shown that beamlets are concentrated in a narrow region near the plasma sheet boundary, whose latitude size is no more than 0.8δ. This conclusion corroborates the theoretical prediction that the nonadiabatic resonant acceleration of ions occurs in a spatially localized region near the separatrix separating the open magnetic field lines and closed field lines, which contain the hot and isotropic plasmas of the plasma sheet. Based on the CLUSTER multisatellite measurements, the spatial structure of beamlets is analyzed and it has been found that the Alfvén wave arises due to the excitation of fire-hose instability at the instant of the exit of the ion beam from the current sheet to the high-latitude region of the far tail of the Earth’s magnetosphere. The longitudinal (along the magnetic field) and transverse sizes of a beamlet are estimated. It has been found that the beamlet is a dynamic plasma structure whose longitudinal size is several hundred times larger than its transverse size.  相似文献   
994.
We have demonstrated a passively Q-switched operation of Nd:GdVO4 laser in which a GaAs crystal is used as the saturable absorber for the first time as far as we know. A maximum average output power of 1.64 W was obtained at an incident pump power of 12 W, the corresponding optical conversion efficiency and peak power were 13.7% and 116.8 W, respectively. The maximum peak energy obtained in the experiment by 50% transmission couple was 19 μJ.  相似文献   
995.
The dynamical systems of identical particles admitting quadratic integrals of motion are classified. The relevant integrals are explicitly constructed and their relation to separation of variables in Hamilton-Jacobi equation is clarified.  相似文献   
996.
An interference method for measuring certain parameters of a substance in the vicinity of the critical state is proposed. The essence of the method is to decipher the interference pattern arising upon grazing incidence of a laser beam on a curved liquid-vapor interface. The deciphering allows one to determine the difference between the refractive indices of the two phases and the scaling factor of the wetting meniscus shape. This, in turn, makes it possible to calculate the critical exponents for the temperature dependences of the surface tension and the difference in the densities of the two phases. The ratio of these critical exponents for xenon, measured in the paper, is 3.81±0.03.  相似文献   
997.
Experimental data are obtained on the dynamics of conduction-electron relaxation at the stage preceding the melting of a silicon surface layer. The energy of a quantum of probe radiation is smaller than the band gap, making it possible to obtain information about the electron-phonon relaxation processes for an electron concentration of ~ 1021 cm?3 in the conduction band.  相似文献   
998.
Capacity Constrained Transit Assignment with Common Lines   总被引:1,自引:0,他引:1  
This paper proposes the use of absorbing Markov chains to solve the capacity constrained transit network loading problem taking common lines into account. The approach handles congested transit networks, where some passengers will not be able to board because of the absence of sufficient space. The model also handles the common lines problem, where choice of route depends on frequency of arrivals. The mathematical formulation of the problem is presented together with a numerical example. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
999.
The synthesis of a specific isotopomer, C6D4H(ortho)-H(ortho)D4C6 of biphenyl is reported. The intramolecular dipolar coupling of the protons leads to a well-resolved single-crystal proton nuclear magnetic resonance (NMR) spectrum and allows one to study the dynamics of the phenyl rings in a unique way. At room temperature and above, the most conspicuous dynamical mode consists of 180° ring flips. The present data together with previous measurements of the total flip rate allow us to conclude that the rings flip almost exclusively independently of each other. Between the incommensurate (IC) phase transition of biphenyl at 38 K andT=250 K, the prominent namical mode consists of oscillatory twists ϕ(t) of the two rings. The data allow us to infer the mean square, (φ2), of these twists. (φ2) is found to grow linearly withT for 50<T<200 K. From the slope of (φ2) vs.T the frequency (the wave number[(v)\tilde]\tilde v) is derived. The result is[(v)\tilde] = 20\tilde v = 20 cm−1. ForT<38 K, the spectra give direct evidence of the IC phase transition and its nature (stripelike rather than quiltlike). The temperature dependence of the magnitude of the order parameter of the IC phase is obtained.  相似文献   
1000.
LetB 1 be a ball of radiusr 1 inS n (ℍn), and letB 0 be a smaller ball of radiusr 0 such thatB 0 B 1 . ForS n we considerr 1 π. Let u be a solution of the problem- δm = 1 in Ω :=B 1 /B 0 vanishing on the boundary. It is shown that the associated functionalJ (Ω) is minimal if and only if the balls are concentric. It is also shown that the first Dirichlet eigenvalue of the Laplacian on Ω is maximal if and only if the balls are concentric.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号