全文获取类型
收费全文 | 310096篇 |
免费 | 19430篇 |
国内免费 | 698篇 |
专业分类
化学 | 195319篇 |
晶体学 | 4211篇 |
力学 | 13808篇 |
综合类 | 9篇 |
数学 | 32430篇 |
物理学 | 84447篇 |
出版年
2023年 | 4264篇 |
2021年 | 4049篇 |
2020年 | 6458篇 |
2019年 | 4477篇 |
2018年 | 5317篇 |
2017年 | 3690篇 |
2016年 | 9718篇 |
2015年 | 7885篇 |
2014年 | 8947篇 |
2013年 | 15654篇 |
2012年 | 11244篇 |
2011年 | 11021篇 |
2010年 | 10371篇 |
2009年 | 10085篇 |
2008年 | 10391篇 |
2007年 | 10348篇 |
2006年 | 9117篇 |
2005年 | 8260篇 |
2004年 | 7586篇 |
2003年 | 6862篇 |
2002年 | 6762篇 |
2001年 | 7413篇 |
2000年 | 5729篇 |
1999年 | 4410篇 |
1998年 | 3906篇 |
1997年 | 3948篇 |
1996年 | 3700篇 |
1995年 | 3510篇 |
1994年 | 3613篇 |
1993年 | 3424篇 |
1992年 | 3796篇 |
1991年 | 3765篇 |
1990年 | 3680篇 |
1989年 | 3561篇 |
1988年 | 3692篇 |
1987年 | 3580篇 |
1986年 | 3381篇 |
1985年 | 4494篇 |
1984年 | 4779篇 |
1983年 | 3995篇 |
1982年 | 4158篇 |
1981年 | 4108篇 |
1980年 | 4102篇 |
1979年 | 4087篇 |
1978年 | 4386篇 |
1977年 | 4413篇 |
1976年 | 4544篇 |
1975年 | 4283篇 |
1974年 | 4345篇 |
1973年 | 4337篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
991.
Arthur G. Maki John C. Grecu Brenda Winnewisser Manfred Winnewisser 《Journal of Molecular Spectroscopy》2003,222(2):198-212
High-resolution spectra of 15N12C12C15N and 14N13C13C14N have been measured and analyzed from 200 to 3600 cm−1. All the vibrational levels below 900 cm−1 have been observed and characterized. The Fermi resonance between ν2 and 2ν4 has been studied and the resonance constant has been determined for several cases. Several Σ− states have been directly observed for the first time for each isotopomer, the (0001111)0f, (0011111)0f, and (0002222)0f states. The pattern of the energy levels for clusters of l-type resonance coupled levels, such as 0001131,3, has been determined for cyanogen for the first time. Among other things this involved the determination of the vibrational l-type resonance constant, r45. Many of the power series constants, αi and xij, and higher order constants have been determined. 相似文献
992.
993.
V. G. Bozhkov 《Radiophysics and Quantum Electronics》2003,46(8-9):631-656
The state of the art in the development of semiconductor detectors, mixers, and frequency multipliers based on Schottky-barrier diodes (SBDs) and heterojunction structures for uncooled terahertz receivers is reviewed. The present status of this field features a transition from quasi-optical designs based on dot-matrix, whisker-contacted SBDs to the designs with hybrid-integrated and monolithic constructions on the planar SBD base, which are positioned in a waveguide mount. The high-level performance of these planar devices is achieved by partially or completely removing or changing semiconductor substrates and/or using membrane constructions incorporated in the waveguide. 相似文献
994.
A tight-binding (TB) hamiltonian for calcium is built with a high precision parametrization technique based on density functional calculations of the energy bands and the total energy at various lattice volumes. The new set of TB parameters is appropriate to study phenomena under pressures as high as 20 GPa. Specifically, both the metal to nonmetal transition at 4 GPa and the structural transition fcc to bcc at 19 GPa are well reproduced. These transitions and several static properties are in excellent agreement with experiments. Phonon frequencies, plasmon energy, melting temperature and the coefficient of thermal expansion were calculated with a molecular dynamics scheme of this TB hamiltonian. 相似文献
995.
F.O. Borges G.H. Cavalcanti A.G. Trigueiros 《Journal of Quantitative Spectroscopy & Radiative Transfer》2003,78(1):119-129
The weighted oscillator strengths (gf) and the lifetimes presented in this work were carried out in a multi configuration Hartree-Fock relativistic (HFR) approach. In this calculation, the electrostatic parameters were optimized by a least-squares procedure, in order to improve the adjustment to experimental energy levels. This method produces gf-values that are in better agreement with intensity observations and lifetime values that are closer to the experimental ones. In this work, we presented all the experimentally known electric dipole S IX and S X spectral lines. 相似文献
996.
B. G. Tagiev U. F. Kasumov N. N. Musaeva R. B. Dzhabbarov 《Physics of the Solid State》2003,45(3):426-432
The current-voltage characteristics of Ca4Ga2S7: Eu3+ single crystals are measured for the first time, and the processes affecting these characteristics are analyzed theoretically. It is demonstrated that Ca4Ga2S7: Eu3+ single crystals are high-resistance semiconductors with a resistivity of ~109 Ω cm and a relative permittivity of 10.55. The electrical properties of the studied materials are governed by traps with activation energies of 0.13 and 0.19 eV and a density ranging from 9.5×1014 to 2.7×1015 cm?3. The one-carrier injection is observed in weak electric fields. In electric fields with a strength of more than 4×103 V/cm, traps undergo thermal field ionization according to the Pool-Frenkel mechanism. At low temperatures and strong fields (160 K and 5×104 V/cm), the electric current is most likely due to hopping conduction by charge carriers over local levels in the band gap in the vicinity of the Fermi level. 相似文献
997.
The dependence of the stability of a magnetic spiral domain in a film on the parameters of the film and its domain structure and on an external magnetic field is considered within a phenomenological model. The model allows one to explain a number of experimentally observed properties of dynamic spiral domains resulting from the process of self-organization of domains and domain walls in an iron-garnet film placed in an external ac magnetic field. 相似文献
998.
A gravitational field can be seen as the anholonomy of the tetrad fields. This is more explicit in the teleparallel approach, in which the gravitational field-strength is the torsion of the ensuing Weitzenböck connection. In a tetrad frame, that torsion is just the anholonomy of that frame. The infinitely many tetrad fields taking the Lorentz metric into a given Riemannian metric differ by point-dependent Lorentz transformations. Inertial frames constitute a smaller infinity of them, differing by fixed-point Lorentz transformations. Holonomic tetrads take the Lorentz metric into itself, and correspond to Minkowski flat spacetime. An accelerated frame is necessarily anholonomic and sees the electromagnetic field strength with an additional term. 相似文献
999.
We have used the periodic quantum-mechanical method with density functional theory at the B3LYP level in order to study TiO2/Sn doped (1 1 0) surfaces and have investigated the structural, electronic and energy band properties of these oxides. Our calculated relaxation directions for TiO2 is the experimental one and is also in agreement with other theoretical results. We also observe for the doped systems relaxation of lattice positions of the atoms. Modification of Sn, O and Ti charges depend on the planes and positions of the substituted atoms. Doping can modify the Fermi levels, energy gaps as well as the localization and composition of both valence and conduction band main components. Doping can also modify the chemical, electronic and optical properties of these oxides surfaces increasing their suitability for use as gas sensors and optoelectronic devices. 相似文献
1000.