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   12431篇
  免费   1099篇
  国内免费   705篇
化学   8391篇
晶体学   121篇
力学   496篇
综合类   51篇
数学   1491篇
物理学   3685篇
  2023年   155篇
  2022年   224篇
  2021年   272篇
  2020年   315篇
  2019年   335篇
  2018年   254篇
  2017年   264篇
  2016年   401篇
  2015年   413篇
  2014年   481篇
  2013年   715篇
  2012年   892篇
  2011年   1026篇
  2010年   651篇
  2009年   677篇
  2008年   769篇
  2007年   734篇
  2006年   647篇
  2005年   563篇
  2004年   532篇
  2003年   455篇
  2002年   404篇
  2001年   330篇
  2000年   270篇
  1999年   206篇
  1998年   162篇
  1997年   139篇
  1996年   175篇
  1995年   131篇
  1994年   150篇
  1993年   124篇
  1992年   117篇
  1991年   96篇
  1990年   106篇
  1989年   82篇
  1988年   90篇
  1987年   61篇
  1986年   69篇
  1985年   95篇
  1984年   69篇
  1983年   68篇
  1982年   49篇
  1981年   46篇
  1980年   47篇
  1979年   34篇
  1977年   37篇
  1976年   37篇
  1975年   34篇
  1974年   48篇
  1973年   44篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Square-shaped two-dimensional optical microcavities (micro-cavities) were investigated for possible applications as filters for dense wavelength-division multiplexing. Multimode cavity resonances were observed in the elastic scattering of approximately 200-microm square-shaped micro-cavities in fused silica. Based on a two-dimensional k-space representation, we accounted for the multimode spectrum by different normal modes with rays confined by total internal reflection. The cavity-mode trajectories need not be closed after each round trip. Single-mode spectra are expected from smaller square-shaped micro-cavities.  相似文献   
962.
Qu J  Chang H  Xiong S 《Optics letters》2001,26(16):1268-1270
We describe an optical processing method for characterizing tissue pathology that is based on principal-component analysis of light-induced autofluorescence. A set of optical spectral filters, which are related to the principal-component loading vectors, is designed to process the autofluorescence signal optically and to generate principal-component scores from the autofluorescence spectra. The scores are then correlated with the tissue pathology. An optical processing system is designed that uses the in vivo fluorescence spectra recorded from nasopharyngeal tissues. We demonstrate that the system can differentiate nasopharyngeal carcinoma from normal tissue with a high degree of sensitivity and specificity and that the optical filters used in the system can be manufactured.  相似文献   
963.
The effects of anomalous dispersion (resonance) on multiple reflection of x rays and their interference in crystals at atomic absorption edges are studied. Intensity ratios of two inversion-symmetry-related multiple diffractions at or near absorption edges exhibit highly phase-sensitive profiles with strong asymmetric characteristics, unlike those far from the edges. A new resonance perturbation Bethe approach is developed to explain this behavior. This leads to direct determination of the phase change for x-ray reflections at resonance.  相似文献   
964.
Baryon rapidity loss in relativistic Au + Au collisions   总被引:4,自引:0,他引:4  
An excitation function of proton rapidity distributions for different centralities is reported from AGS Experiment E917 for Au+Au collisions at 6, 8, and 10.8 GeV/nucleon. The rapidity distributions from peripheral collisions have a valley at midrapidity which smoothly change to distributions that display a broad peak at midrapidity for central collisions. The mean rapidity loss increases with increasing beam energy, whereas the fraction of protons consistent with isotropic emission from a stationary source at midrapidity decreases with increasing beam energy. The data suggest that the stopping is substantially less than complete at these energies.  相似文献   
965.
Two-dimensional lead (Pb) islands of varying heights have been grown on the Si(111)-(7 x 7) surface at low temperature. Individual islands are investigated concurrently with real-space and local-probe scanning tunneling microscopy and spectroscopy. Quantum size effects, manifested in the formation of new electronic bound states, redistribution of surface charge density, and oscillatory relaxations in island thickness are found to be perfectly correlated to each other.  相似文献   
966.
We report a determination of the B(0)(d)-&B_(0)(d) mixing parameter Deltam(d) based on the time evolution of dilepton yields in Upsilon(4S) decays. The measurement is based on a 5.9 fb(-1) data sample collected by the Belle detector at KEKB. The proper-time difference distributions for same-sign and opposite-sign dilepton events are simultaneously fitted to an expression containing Deltam(d) as a free parameter. Using both muons and electrons, we obtain Deltam(d) = 0.463+/-0.008 (stat)+/-0.016 (syst) ps(-1). This is the first determination of Deltam(d) from time evolution measurements at the Upsilon(4S). We also place limits on possible CPT violations.  相似文献   
967.
We present a measurement of the standard model CP violation parameter sin2 phi(1) based on a 29.1 fb(-1) data sample collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. One neutral B meson is fully reconstructed as a J/psi K(S), psi(2S)K(S), chi(c1)K(S), eta(c)K(S), J/psi K(L), or J/psi K(*0) decay and the flavor of the accompanying B meson is identified from its decay products. From the asymmetry in the distribution of the time intervals between the two B meson decay points, we determine sin2 phi(1) = 0.99+/-0.14(stat)+/-0.06(syst). We conclude that we have observed CP violation in the neutral B meson system.  相似文献   
968.
With the aid of the Friedel sum rule, we perform first-principles calculations of conductances through monatomic Na wires, taking into account the sharp tip geometry and discrete atomic structure of electrodes. We find that conductances (G) depend on the number (L) of atoms in the wires; G is G(0)( = 2e(2)/h) for odd L, independent of the wire geometry, while G is generally smaller than G(0) and sensitive to the wire structure for even L. This even-odd behavior is attributed to the charge neutrality and resonant character due to the sharp tip structure. We suggest that similar even-odd behavior may appear in other monovalent atomic wires.  相似文献   
969.
The radial profiles of KT-1 tokamak (major radius of 27 cm, minor radius of 4.25 cm, two poloidal stainless-steel limiters) edge plasma parameters are measured using single and triple electric probes. The particle transport parameters are calculated from the measured edge plasma parameters, and the results are analyzed by the simple fluid approximations. The cross-field particle diffusion coefficient (D) in the boundary plasma of the KT-1 is calculated from the density scrape-off length (λn) measured by using a triple probe. The particle density and electron temperature fall exponentially in the radial direction with the e-folding length of λn=0.13 cm and λe=0.41 cm, respectively. From the scrape-off layer (SOL) model, the experimental values of scrape-off length (λn) is used to calculate the cross-field diffusion coefficient (D=1.2×103cm2/s), roughly corresponding to one third of the typical Bohm value. A simple SOL model with the contribution of recombination is introduced to evaluate the Bohm diffusion in the KT-1 tokamak edge plasma. Cross-field heat conductivity calculated from these deduced values is 5.2D in the SOL of KT-1 edge plasma. These results provide the finally certain information for edge particle transport in the KT-1 boundary plasmas.  相似文献   
970.
Let be a Noetherian local ring. In this paper we supply formulae for computing the ranks of syzygy and Betti numbers of -modules of essentially monomial type. These modules are defined with respect to various -regular sequences. For example, finite length modules of monomial type over regular local rings of dimension are modules of essentially monomial type with respect to -regular sequences of length . If a module is of essentially monomial type with respect to an -regular sequence of length , then the rank of its -th syzygy is at least and its -th Betti number is at least .

  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