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   337篇
  免费   8篇
化学   131篇
晶体学   3篇
力学   6篇
数学   19篇
物理学   186篇
  2016年   4篇
  2015年   3篇
  2014年   5篇
  2013年   8篇
  2012年   17篇
  2011年   20篇
  2010年   10篇
  2009年   7篇
  2008年   26篇
  2007年   24篇
  2006年   20篇
  2005年   12篇
  2004年   13篇
  2003年   13篇
  2002年   12篇
  2001年   5篇
  2000年   12篇
  1999年   3篇
  1998年   5篇
  1997年   2篇
  1996年   7篇
  1995年   3篇
  1994年   9篇
  1993年   12篇
  1992年   11篇
  1991年   9篇
  1990年   4篇
  1989年   3篇
  1988年   2篇
  1987年   3篇
  1986年   3篇
  1985年   4篇
  1984年   5篇
  1982年   6篇
  1981年   4篇
  1980年   3篇
  1979年   2篇
  1978年   2篇
  1977年   6篇
  1976年   2篇
  1975年   5篇
  1974年   1篇
  1973年   2篇
  1972年   3篇
  1971年   2篇
  1967年   1篇
  1966年   3篇
  1963年   1篇
  1942年   1篇
  1930年   2篇
排序方式: 共有345条查询结果,搜索用时 46 毫秒
21.
Blue diode‐based pulse amplitude modulation (PAM) technology can be used to measure the photosynthetic electron transport rate (ETR) in a purple nonsulfur anoxygenic photobacterium, Afifella (Rhodopseudomonas) marina. Rhodopseudomonads have a reaction center light harvesting antenna complex containing an RC‐2 type bacteriochlorophyll a protein (BChl a RC‐2‐LH1) which has a blue absorption peak and variable fluorescence similar to PSII. Absorptance of cells filtered onto glass fiber disks was measured using a blue–diode‐based absorptance meter (Blue‐RAT) so that absolute ETR could be calculated from PAM experiments. Maximum quantum yield (Y) was ≈0.6, decreasing exponentially as irradiance increased. ETR vs irradiance (P vs E) curves fitted the waiting‐in‐line model (ETR = (ETRmax × E/Eopt) × exp(1 ? E/Eopt)). Maximum ETR (ETRmax) was ≈1000–2000 μmol e? mg?1 BChl a h?1. Fe2+, bisulfite and thiosulfate act as photosynthetic electron donors. Optimum irradiance was ≈100 μmol m?2 s?1 PPFD even in Afifella grown in sunlight. Quantum efficiencies (α) were ≈0.3–0.4 mol e? mol hλ?1; or ≈11.8 ± 2.9 mol e? mol hλ?1 m2 μg?1 BChl a). An underlying layer of Afifella in a constructed algal/photosynthetic bacterial mat has little effect on the measured ETR of the overlying oxyphotoautotroph (Chlorella).  相似文献   
22.
We measure the electron escape rate from surface-acoustic-wave dynamic quantum dots (QDs) through a tunnel barrier. Rate equations are used to extract the tunneling rates, which change by an order of magnitude with tunnel-barrier-gate voltage. We find that the tunneling rates depend on the number of electrons in each dynamic QD because of Coulomb energy. By comparing this dependence to a saddle-point-potential model, the addition energies of the second and third electron in each dynamic QD are estimated. The scale ( approximately a few meV) is comparable to those in static QDs as expected.  相似文献   
23.
Imprinted diffractive optics for terahertz radiation   总被引:1,自引:0,他引:1  
Terahertz diffractive optic elements have been fabricated in polypropylene by imprinting with a silicon master. A silicon master is created with eight phase levels for high diffraction efficiency and etched using inductively coupled plasma reactive ion etching. This technique enables the rapid replication of complex optical structures in a high transmission material. Excellent replication of multilevel high efficiency Fresnel lenses is shown. The resulting lenses were tested with a 2 THz quantum cascade laser. The signal strength at the focus was 70 times the base signal strength.  相似文献   
24.
The dynamics of the spin-triplet trion state, under high magnetic field in a GaAs/AlGaAs quantum well, are studied using time resolved spectroscopy. The oscillator strength of the triplet transition is shown to rise with increasing electron density, in good agreement with a theoretical model where the trion interacts with excess electrons in the quantum well. This analysis suggests that the spin-triplet trion state, which is expected to be an optically "dark" state, is experimentally observable due to the interactions with the excess electrons, demonstrating that X- cannot be regarded as an isolated three particle complex.  相似文献   
25.
We report Kondo-like behavior in a quantum antidot (a submicron depleted region in a two-dimensional electron gas) in the quantum-Hall regime. When both spins of the lowest Landau level are present all around the antidot, the resonances between extended edge states via antidot bound states show an abnormal feature in alternate Coulomb-blockaded regions. The feature becomes suppressed when the temperature or source-drain bias is raised as for Kondo resonances in quantum dots. Although the exact mechanism is unknown, Kondo-like correlated tunneling may arise from a Skyrmion-type edge reconstruction. This observation demonstrates the generality of the Kondo phenomenon.  相似文献   
26.
The tunneling current between an electron gas with a periodic potential in two dimensions and a plain two-dimensional electron system (2DES) has been studied. The strength of the periodic potential, the subband energy of the plain 2DES, and an applied in-plane magnetic field were varied, mapping the Fourier transform of the periodic wave function. Periodic peaks were observed and explained by translations in the reciprocal lattice. When the potential was strongly modulated to form an array of antidots, commensurability peaks were seen in lateral transport, but, as expected, not in tunneling.  相似文献   
27.
Hadron gas models have proved successful in predicting particle production in relativistic nucleus-nucleus collisions. The extension of these models to the smaller systems formed in proton-nucleus collisions requires that the finite size of the system be considered. We study two features introduced by the finite size: the need to conserve strangeness and baryon number exactly by performing calculations in the canonical ensemble, and the inclusion of a finite size geometrical correction term in the single particle density of states. We find significant differences between the grand canonical and canonical ensembles and a strong dependence on the baryon number of the system.  相似文献   
28.
We have measured the spectral linewidths of three continuous-wave quantum cascade lasers operating at terahertz frequencies by heterodyning the free-running quantum cascade laser with two far-infrared gas lasers. Beat notes are detected with a GaAs diode mixer and a microwave spectrum analyzer, permitting very precise frequency measurements and giving instantaneous linewidths of less than -30 kHz. Characteristics are also reported for frequency tuning as the injection current is varied.  相似文献   
29.
On a high-mobility 2D electron gas we have observed, in strong magnetic fields (omega(c)tau>1), a parabolic negative magnetoresistance caused by electron-electron interactions in the regime of k(B)Ttau/ variant Planck's over 2pi approximately 1, which is the transition from the diffusive to the ballistic regime. From the temperature dependence of this magnetoresistance the interaction correction to the conductivity deltasigma(ee)(xx)(T) is obtained in the situation of a long-range fluctuation potential and strong magnetic field. The results are compared with predictions of the new theory of interaction-induced magnetoresistance.  相似文献   
30.
Chirped-superlattice quantum-cascade lasers are reported that emit at lambda approximately 85 microm (3.6 THz), which is to the authors' knowledge the longest wavelength demonstrated so far with this technology. Collected peak output powers of 1.5 mW per facet were measured at liquid-helium temperature, and a maximum operating temperature of 45 K was reached. Record low-threshold-current densities of 95 and 115 A cm(-2) were observed in pulsed and continuous-wave operation, respectively. For the latter, output powers of a few hundred microwatts are estimated at low temperatures.  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