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1.
A bound polaron in a spherical quantum dot   总被引:12,自引:0,他引:12  
The binding energy of a bound polaron in a spherical quantum dot has been investigated by using the variational method. The influence of LO and SO phonons have taken into consideration. Result shows that the phonon contribution to the binding energy is dependent on the size of the quantum dot as well as the position of the impurity in the quantum dot. Numerical calculation on the ZnSe quantum dot shows that such contribution is about 5% to 20% of the total binding energy. Received: 13 October 1997 / Revised: 4 March 1998 / Accepted: 26 May 1998  相似文献
2.
Interface polarons in a realistic heterojunction potential   总被引:9,自引:0,他引:9  
The ground states of interface polarons in a realistic heterojunction potential are investigated by considering the bulk and the interface optical phonon influence. A self-consistent heterojunction potential is used and an LLP-like method is adopted to obtain the polaron effect. The numerical computation has been done for the Zn1-xCdxSe/ZnSe system to obtain the polaron ground state energy, self energy and effective mass parallel to the interface. A simplified coherent potential approximation is developed to obtain the parameters of the ternary mixed crystal and the energy band offset of the heterojunction. It is found that at small Cd concentration the bulk longitudinal optical phonons give the main contribution for lower areal electron densities, whereas the interface phonon contribution is dominant for higher areal electron densities. The interface polaron effect is weaker than the effect obtained by the three dimensional bulk phonon and by the two dimensional interface phonon models. Received 17 September 1998  相似文献
3.
基于光谱指数与EM38的土壤盐分空间变异性研究   总被引:9,自引:0,他引:9  
针对目前半干旱区存在的土壤盐渍化问题,以典型半旱区封丘县为研究区,将遥感与磁感应电导率仪(EM38)技术相结合对典型半干旱区土壤盐分空间变异性进行了研究.结合田间采样对EM38测量数据进行解译,并对解译后的数据进行经典数据统计特征分析,建立了EM38解译后的土壤电导率与光谱指数(土壤指数、植被指数)之间的回归模型.利用该模型绘制了研究区土壤盐分的空间分布图.结果表明,受内在和外在因子的共同作用,研究区各土层含盐量均具有中等的变异强度.采用植被指数与土壤指数和EM38测量相结合的方法,较好地绘制了研究区土壤盐分的空间变化图.盐渍化土壤主要集中在研究区北部与南部,具有明显的趋势效应,并且在整个剖面盐分表现为表层最高,随着深度增加盐分先减小后增大的趋势.  相似文献
4.
With the use of variational method to solve the effective mass equation, we have studied the cyclotron resonance of a magnetopolaron in cylindrical quantum wires at arbitrary magnetic fields. The interaction of the electron with surface-optical (SO) phonons is used. Having calculated the ground state energy and the excited state energy of the magnetopolaron, we obtain the hybrid frequency of the magnetopolaron in quantum wires.  相似文献
5.
We study the resonant magnetopolaron effects in parabolic quantum wells in a tilted magnetic field. The renormalization of the first excited level, which is resonant with the ground state level plus one longitudinal-optical phonon is calculated at the resonance using an improved resonance approximation to be E= where is the polaron coupling constant. The exponent and the factor are calculated in dependence on the tilt angle of the magnetic field and the confinement energy.  相似文献
6.
In order to increase the soft X-ray conversion efficiency for the femtosecond-laser-produced plasma, we adopted a nanocylinder-array structure target. Gold nanocylinder-array targets with 70–90 nm cylinder diameter and 100 nm cylinder pitch were made. A continuous smooth soft X-ray spectrum adequate for X-ray absorption spectroscopy was obtained. An around 20-fold soft X-ray (7–20 nm) fluence enhancement compared with a flat-surface gold foil target was obtained when the cylinder height was 18 μm. X-ray (>0.06 keV) pulse duration was 17 ps, which is much shorter than that obtained by using the pre-pulse technique. The X-ray pulse peak intensity was 7-fold higher than that of a gold foil target. Received: 2 May 2001 / Revised version: 1 June 2001 / Published online: 18 July 2001  相似文献
7.
Interface polarons in a heterojunction with triangular bending-band   总被引:5,自引:0,他引:5  
The interface polaron states in a heterojunction are discussed by considering an energy-band bending near the interface and the influence of an image potential. The ground state energy and the effective mass of a polaron are variationally calculated. The numerical results for the GaAs/ heterojunction are given. It is shown that even though the influences from bulk longitudinal optical (LO) phonons are more important for the heterojunctions with lower Al composition, the contributions from two branches of interface optical (IO) phonons are not negligible. For the heterojunctions with higher Al composition, both the influences from LO phonons and two branches of IO phonons are important. The band-bending plays an important role for the interface localization of polarons, but the influence of the image potential is not essential. Received: 4 November 1997 / Revised and Accepted: 9 March 1998  相似文献
8.
Laser plasma thruster   总被引:4,自引:0,他引:4  
The laser plasma thruster (LPT) is a new microthruster for small satellites. We report on development and testing of a prototype LPT. Some advantages of the LPT are: thruster voltage 4 V, mass less than 1 kg, power-to-thrust ratio 10 kW/N, and Isp up to 1000 s. Typical thrust level is 250 μN with PVC fuel. Thrust of 1 mN is expected with energetic fuel. The pre-prototype continuous-thrust experiment includes the laser mount and heat sink, lens mounts, and focusing mechanism, which are coupled to the target-material transport mechanism. The target material is applied to a transparent plastic tape, and the laser is focused on a series of tracks on the tape. The tape-drive hardware and laser-drive electronics are described. Design, construction, and calibration of the thrust stand are described. During continuous operation, the exhaust plume is deflected in the direction of the moving tape. When the laser is operated in pulsed mode, the exhaust plume is perpendicular to the tape (parallel to the optical axis). This provides some thrust-vector control. Measured thrust is 70 μN with PVC fuel, and 550 μN with energetic fuel, with a specific impulse of 350 s. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-505-272-7203, E-mail: luke@iera.nmt.edu  相似文献
9.
Deep drilling of metals by femtosecond laser pulses   总被引:3,自引:0,他引:3  
Results of recent investigations on deep drilling of metals by femtosecond laser pulses are reported. At high laser fluences, well above the ablation threshold, femtosecond lasers can drill deep, high-quality holes in metals without any post-processing or special gas environment. It is shown that for high-quality drilling of metals, the following processes are important: (1) laser-induced optical breakdown of air containing metal vapor and small metal particles (debris) generated by multi-pulse femtosecond laser ablation, (2) transformation of laser pulses into light filaments, and (3) low-fluence finishing. Received: 15 November 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/2788-100, E-mail: ch@lzh.de  相似文献
10.
Although lasers are generally able to machine silicon, the major material in many microsystems applications, doing so without influencing the physical properties of the bulk material remains an important challenge. Ultrafast lasers, in particular, with their potential to precisely ablate all kinds of solid materials, are able to perform such processes with high efficiency and accuracy. This article starts with an overview of the general interaction of ultrafast laser radiation with semiconductors, explaining the absorption processes and different fluence regimes for the ablation of silicon. Major parameter influences, especially for cutting processes in thin silicon, are described. By varying pulse energies, beam shaping methods, the beam polarization, and temperatures, the cutting quality and speed can be significantly influenced. One important quality aspect, besides kerf widths and surface roughness, is the amount of back-side chipping when cutting brittle materials. Achievements in speed enhancement using linear focus shapes are presented, with cutting speeds up to five times higher than by conventional spot-focusing. On the other hand, laser processes that cut with a spot focus offer the possibility of free-shape cutting, which is explained using the example of wafers carrying silicon chips with highly increased package densities. Received: 10 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/2788-100, E-mail: nb@lzh.de  相似文献
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