全文获取类型
收费全文 | 1997篇 |
免费 | 115篇 |
国内免费 | 60篇 |
专业分类
化学 | 653篇 |
晶体学 | 44篇 |
力学 | 28篇 |
综合类 | 16篇 |
数学 | 409篇 |
物理学 | 1022篇 |
出版年
2023年 | 9篇 |
2022年 | 7篇 |
2021年 | 16篇 |
2020年 | 13篇 |
2019年 | 11篇 |
2018年 | 13篇 |
2017年 | 18篇 |
2016年 | 19篇 |
2015年 | 24篇 |
2014年 | 32篇 |
2013年 | 78篇 |
2012年 | 41篇 |
2011年 | 50篇 |
2010年 | 52篇 |
2009年 | 178篇 |
2008年 | 179篇 |
2007年 | 148篇 |
2006年 | 134篇 |
2005年 | 106篇 |
2004年 | 135篇 |
2003年 | 103篇 |
2002年 | 85篇 |
2001年 | 71篇 |
2000年 | 81篇 |
1999年 | 96篇 |
1998年 | 44篇 |
1997年 | 27篇 |
1996年 | 39篇 |
1995年 | 29篇 |
1994年 | 43篇 |
1993年 | 35篇 |
1992年 | 43篇 |
1991年 | 31篇 |
1990年 | 20篇 |
1989年 | 17篇 |
1988年 | 17篇 |
1987年 | 15篇 |
1986年 | 14篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有2172条查询结果,搜索用时 31 毫秒
11.
T. Charitat B. Fourcade 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):333-336
An o-ring takes spontaneously the shape of a chair when strong enough torsion is applied in its tangent plane. This state
is metastable, since work has to be done on the o-ring to return to the circular shape. We show that this metastable state
exists in a Hamiltonian where curvature and torsion are coupled via an intrinsic curvature term. If the o-ring is constrained to be planar (2d case), this metastable state displays a kink-anti-kink
pair. This state is metastable if the ratio is less than , where C and A are the torsion and the bending elastic constants [#!landau!#]. In three dimensions, our variational approach shows that
. This model can be generalized to the case where the bend is induced by a concentration field which follows the variations
of the curvature.
Received: 27 August 1997 / Revised: 23 October 1997 / Accepted: 12 November 1997 相似文献
12.
关于非齐次m阶马氏信源的渐近均分割性 总被引:4,自引:0,他引:4
本文研究非齐次m阶马氏信源的渐近均分割性,首先我们得到关于此种信源m 1元函数的一类强极限定理,作为推论,得到关于任意非齐次m阶马氏信源状态和熵密度的几个极限定理,最后得到一类非齐次m阶马氏信源的渐近均分割性。 相似文献
13.
We develop a lattice-dynamical model based on the screw symmetry of single-walled carbon nanotubes that allows for reducing the size of the dynamical matrix to six, for all tube chiralities. The model uses force constants derived from fitting to the phonon dispersion of 2D graphite. We present the calculation procedure in a clear and transparent way, making the model easier to follow. We calculate the phonon dispersions of a number of nanotubes of different chiralities. The splitting of two highest Raman active modes and the radial breathing mode frequency are studied by changing the tube diameter and chirality. 相似文献
14.
15.
Polycrystalline Sn1−xMnxO2 (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases. 相似文献
16.
SiC films doped with aluminum (Al) were prepared by the rf-magnetron sputtering technique on p-Si substrates with a composite target of a single crystalline SiC containing several Al pieces on the surface. The as-deposited films were annealed in the temperature range of 400-800 °C under nitrogen ambient. The thin films have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results show that the introduction of Al into films hinders crystalline formation process. And with the increase of annealing temperature, more Si particles are formed in the films, which strongly affect the optical absorption properties. The photoluminescence (PL) spectra of the samples show two peaks at 370 nm and 412 nm. The intensities of the PL peaks are evidently improved after Al doped. We attribute the origin of the two PL peaks to a kind of Si-related defect centres. The obtained results are expected to have important applications in modern optoelectronic devices. 相似文献
17.
The stress tensor of a massless scalar field satisfying a mixed boundary condition in a (1 + 1)-dimensional Reissner- Nordstrom black hole background is calculated by using Wald's axiom. We find that Dirichlet stress tensor and Neumann stress tensor can be deduced by changing the coefficients of the stress tensor calculated under a mixed boundary condition. The stress tensors satisfying Dirichlet and Neumann boundary conditions are discussed. In addition, we also find that the stress tensor in conformal flat spacetime background differs from that in flat spacetime only by a constant. 相似文献
18.
本文导出了考虑外场和束流效应后束流均方根发射度平方变化方程的一般表达式,并就仅考虑外场,仅考虑空间电荷场和仅考虑尾场等三种特殊情况进行了分析。 相似文献
19.
Physical aspects of an operation of the GaAs-based InGaAs/GaAs quantum-well (QW) VCSELs with the intentionally detuned optical
cavities have been considered in the present paper using the comprehensive three-dimensional self-consistent optical–electrical–thermal-gain
simulation. In GaAs-based structures, very good DBR resonator mirrors and a very efficient methods to confine radially both
the current spreading and the electromagnetic field with the aid of oxide apertures may be applied. It has been found using
the above simulation that even currently available immature technology enables manufacturing the above devices emitting radiation
of wavelengths over 1.20 μm. In particular, while the room-temperature 1.30-μm lasing emission is still beyond possibilities
of the InGaAs/GaAs QW VCSELs, these structures may offer analogous 1.25-μm emission, especially for the high-power and/or
high-temperature operation. 相似文献
20.
R. Guerrero F.G. Aliev R. Villar J. Hauch M. Fraune G. Güntherodt K. Rott H. Brückl G. Reiss 《Journal of magnetism and magnetic materials》2006
We report on tunnelling magnetoresistance (TMR), current–voltage (IV) characteristics and low-frequency noise in epitaxially grown Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) magnetic tunnel junctions (MTJs) with dimensions from 2×2 to 20×20 μm2. The evaluated MgO energy barrier (0.50±0.08 eV), the barrier width (13.1±0.5 Å) as well as the resistance times area product (7±1 MΩ μm2) show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. At low temperatures (T<10 K) inelastic electron tunneling spectroscopy (IETS) shows anomalies related to phonons (symmetric structures below 100 meV) and asymmetric features above 200 meV. We explain the asymmetric features in IETS as due to generation of electron standing waves in one of the Fe electrodes. The noise power, though exhibiting a large variation, was observed to be roughly anti-correlated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration. This behavior could be related to the influence of magnetostriction on the characteristics of the insulating barrier through changes in local barrier defects structure. 相似文献