全文获取类型
收费全文 | 5729篇 |
免费 | 265篇 |
国内免费 | 222篇 |
专业分类
化学 | 176篇 |
晶体学 | 1篇 |
力学 | 306篇 |
综合类 | 31篇 |
数学 | 4848篇 |
物理学 | 854篇 |
出版年
2024年 | 9篇 |
2023年 | 100篇 |
2022年 | 124篇 |
2021年 | 225篇 |
2020年 | 301篇 |
2019年 | 137篇 |
2018年 | 167篇 |
2017年 | 280篇 |
2016年 | 294篇 |
2015年 | 280篇 |
2014年 | 522篇 |
2013年 | 494篇 |
2012年 | 444篇 |
2011年 | 408篇 |
2010年 | 337篇 |
2009年 | 346篇 |
2008年 | 253篇 |
2007年 | 298篇 |
2006年 | 281篇 |
2005年 | 187篇 |
2004年 | 86篇 |
2003年 | 81篇 |
2002年 | 90篇 |
2001年 | 140篇 |
2000年 | 50篇 |
1999年 | 112篇 |
1998年 | 82篇 |
1997年 | 10篇 |
1996年 | 6篇 |
1995年 | 9篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1959年 | 1篇 |
1937年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有6216条查询结果,搜索用时 140 毫秒
1.
2.
3.
4.
This article presents vertically coupled, rectangular complementary split-ring resonator-shaped quad-band double-negative (DNG) metamaterial unit cells, that is, having both negative permittivity and permeability, which redirect negative refractive and also are not found in nature. The metamaterial is fabricated on magnesium zinc ferrite-based flexible microwave substrates, and the flexible substrates are chosen with two different concentrations of magnesium (Mg) denoted by Mg30 and Mg50 for 30% and 50% of Mg, which possess dielectric constants of 4.32 and 3.15 and loss tangents of 0.003 and 0.005, respectively. The proposed metamaterials are demonstrated by utilizing the CST microwave simulator, and their effective parameters are extracted according to the Nicolson-Ross-Wire method. With Mg30, the prepared, flexible metamaterial shows measured resonances at 3.70 GHz, 7 GHz, 8.60 GHz, and 9.78 GHz, whereas with Mg50 it shows the measured resonances at 4.10 GHz, 7.70 GHz, 9.33 GHz, and 10.62 GHz. Very good effective medium ratios (EMR) along with DNG properties are obtained, namely 6.5 and 5.85 for Mg30 and Mg50, respectively, with a physical dimension of 12.5 × 9.5 mm2 for both of the unit cells. Also, the electric field, magnetic field, and surface current distribution at different resonances and the polarization insensitivity at different polarization angles were observed. Thus, the designed new flexible substrate microwave materials based on DNG metamaterials are potential candidates for S-, C- and X-band applications, as well as for flexible microwave technologies. 相似文献
5.
6.
7.
9.
10.
Laser cooling of a molecule with heavy nuclei is often complicated because of the density distribution of the electronic states. Here, we evaluate the feasibility of the laser cooling of the SrI molecule by calculating the potential energy curves and transition dipole moments of the ground and low-lying excited states using the multi-reference configuration interaction plus Davidson corrections (MRCI + Q) and the all-electron basis sets of ANO-RCC. The relativistic effect and the spin-orbit coupling splits are included, because both Sr and I are heavy atoms. Based on the obtained potential energy curves, we solve the Schrödinger equation of nuclear motion to determine the rovibrational energy levels and the Franck-Condon factors. The spectroscopic parameters are obtained by fitting the rovibrational energy levels with the Dunham expression. The radiation lifetimes, the Doppler and recoil temperatures between the X2Σ+ and the 2Π1/2/2Π3/2/B2Σ+ states are calculated. 5-color laser cooling schemes for the molecule are proposed, which can lead to the total effective Franck-Condon factors being 0.99983, 0.99979, and 0.99941 for the three transitions, respectively. All the obtained results suggest that the SrI molecule is a feasible candidate for laser cooling. 相似文献