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1.
The five independent elastic constants of superconducting MgB2 are obtained using the first-principles plane wave method with the new relativistic analytic pseudopotential of the Hartwigsen-Goedecker-Hutter (HGH) scheme in the frame of local density approximation. The dependences of bulk modulus on temperature and pressure are also obtained. It is suggested that the HGH-type pseudopotentials are successful in investigating the ground-state mechanical properties of any solids. 相似文献
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Fast photoelectric effects have been observed in MgB2 thin film
fabricated by chemical vapour deposition. The rise time was $\sim $10
ns and the full width at half-maximum was \sim185\,ns for the photovoltaic
pulse when the film was irradiated by a 308\,nm laser pulse of 25\,ns in
duration. X-ray diffraction and the scanning electron microscope revealed
that the film was polycrystalline with preferred c-axis orientation. We
propose that nonequilibrium electron--hole pairs are excited in the grains
and grain boundary regions for MgB2 film under ultraviolet laser and
then the built-in electric field near the grain boundaries separates
carriers, which lead to the appearance of an instant photovoltage. 相似文献
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GUO Yun-dong CAI Da-feng ZHOU Li-ping CHEN Xiang - rong YANG Xiang-dong 《原子与分子物理学报》2006,23(B04):241-245
For the superconductor MgB2, we have calculated the phonon density of states (DOS), phonon dispersion and Eliashberg function throughout the Brillouin zone (BZ), using an empirical potential model. The calculated values are consistent with the theoretical and experimental values. The calculated results show our empiricalpotential model is available for MgB2. 相似文献
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GUO Yun-dong CAI Da-feng ZHOU Li-ping CHEN Xiang-rong YANG Xiang-dong 《原子与分子物理学报》2006,23(Z1):241-245
For the superconductor MgB2, we have calculated the phonon density of states (DOS), phonon dispersion and Eliashberg function throughout the Brillouin zone (BZ), using an empirical potential model. The calculated values are consistent with the theoretical and experimental values. The calculated results show our empirical potential model is available for MgB2. 相似文献
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We investigate the elecronic structures and superconductivities of new superconductor MgB2 with different lattice parameters by means of multiple-Scattered-Xa calculation.It is shown that the lattice parameters have great effect on the superconductivity of MgB2.The results of this paper show that the density of states at the Fermi level decreases with the decreasing lattice parameters.The effects are very more sensitive substantially to the change of the lattice parameter a as compared with that of the lattice paramter c.Correspondingly.the superconducting transition temperature TC decreases correspondingly mainly with deceasing the lattice parameter a .Our results are in good agreement with the experiments. 相似文献
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为观察MgO在MgB2超导样品中的影响和存在形式,通过在MgB2胚体样品之外,增加周围MgO量的实验方法,经过4 h 750℃的烧结,在制备好的MgB2超导样品内得到了壁厚是约4纳米,直径在50~80纳米,长为30~40微米以上的氧化镁(MgO)纳米管.在MgB2样品表面可以看到少量直径是200~300纳米的氧化镁(MgO)亚微米线.经过扫描电镜的能量损失谱和透射电镜EELS分析测量,证明了纳米管中没有硼元素.实验观测也表明这些氧化镁(Mgo)纳米管是由许多氧化镁(MgO)纳米晶片组成. 相似文献
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《中国物理快报》2002,19(1):122-123
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Graphite doped MgB2-xCx (x = 0.00, 0.05, 0.10) wires were fabricated via the in situ powder-in-tube method in flowing argon by using low carbon steel tubes as the sheath materials. With the increase of graphite concentration,the amount of unreacted graphite in the core area increases, and the average grain size of MgB2 decreases. It is found that the critical current density Jc can be significantly improved by graphite doping. The MgB2 wire with x = 0.05 exhibits the best Jc value of 16710 A/cm^2 at 6K, 4.5T, but the MgB1.9C0.1 wire has the highest Jc value of 2060 A/cm^2 at 6 K, 8 T. It is suggested that the enhancement of Jc is due to not only the improvement of the microstructure features but also the introduction of pinning centres. 相似文献
11.
First-Principles Calculation for the Electron-Phonon Coupling Constant of Superconducting MgB2
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We use a first-principles plane wave method with the new relativistic analytic pseudopotential of the Hartwigsen,Goedecker and Hurter (HGH) scheme and the pseudopotential of the Troullier-Martins scheme to calculate the eleetron-phonon coupling constant λ of superconducting MgB2. The calculated results show that there is a peak at about 300-350cm^-1 in the eleetron-phonon spectral functional α2F(ω), duo to the coupling of the electrons to the acoustic phonon, and the relativistic effect appears in the high frequencies zone. All the results agree well with the present and previous experimental data, 相似文献
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近来几个超导研究小组相继报导MgB2磁通线的涡旋相图具有与高温超导体相似的特性,而且该超导体电阻率在外场中展宽的超导区大部份对应涡旋液体相.本文发展了一种新的求导超导体有效钉扎势方法,并利用MgB2电阻在外场中展宽测量数据,发现在超导转变区,其有效钉扎势相应温度的函数关系分为两个区域,其中一个区的热激活能和温度的关系是线性的,另外一个区是非线性的.这明确表明与F. Bouquet et al(Nature, 411(2001), 448)发现高温超导体YBa2Cu3O7-δ具有两个涡旋液体相相类似.MgB2的涡旋液体相也同样具有两个不同的液体相. 相似文献
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MgB2超导体具有临界转变温度39 K、原材料廉价及制备工艺简单等优点,被认为是MRI中已用超导体最好的替代者.但是临界电流密度(Jc)随外加磁场增大下降较快的这一问题极大的阻碍了其实际中的应用.实验结果表明:采取掺杂的方法来提高MgB2的超导电性尤其是高场下Jc值是一条有效的途径,本文概述了用单质碳和含碳化合物对MgB2超导体进行掺杂从而提高其超导特性的最新研究工作,具体介绍了掺杂物颗粒大小、掺杂量以及烧结温度等参数对MgB2超导电性的影响.研究表明:在碳单质掺杂中纳米级碳颗粒和碳纳米管具有比较好的掺杂性能,可以大幅度提高高场下的Jc值;在含碳化合物中,用纳米级SiC进行掺杂不仅可以大幅度提高高场下的Jc值,而且相比碳单质有更高的Tc值.根据目前的研究结果,最后本文对MgB2超导体掺杂研究的未来发展趋势进行了展望. 相似文献
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《Journal of Physics and Chemistry of Solids》2004,65(8-9):1479-1484
We have investigated the effect of electron doping on the superconducting properties of MgB2. For the purpose we have synthesized several samples along the Mg1−xScxB2 section. The X-ray diffraction measurements reveal small changes in the lattice parameters suggesting that the Sc doping could be considered to simply fill the boron σ bands. Radio frequency surface resistivity measurements has been used to obtain the variation of Tc with Sc doping. Increasing the Sc content, the experimental Tc diverges from the Tc predicted by the BCS single band theory showing the key role of interchannel pairing near a shape resonance. 相似文献
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Microwave characteristics of MgB2/Al2O3 superconducting thin films were investigated by coplanar resonator technique. The thin films studied have different grain sizes resulting from different growth techniques. The experimental results can be described very well by a grain-size model which combines coplanar resonator theory and Josephson junction network model. It was found that the penetration depth and surface resistance of thin films with smaller grain sizes are larger than those of thin films with larger grain sizes. 相似文献
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用热重法(TG)、X光衍射(XRD)和带能谱的扫描电镜(SEM-EDS)研究了MgB2在空气气氛中从室温到1000℃的热稳定性和氧化情况.实验表明在400℃ MgB2开始发生氧化,在700℃以上氧化强烈.实验表明MgB2的氧化是一比较复杂的过程,同时存在Mg和B的蒸发及MgO的形成,另外蒸发的Mg和O的反应可在晶粒表面形成直径为几十纳米左右,长几个微米的MgO晶须,生长的MgO晶须的形状受环境的很大影响.用Freemen- Carroll法和多个升温速率法估算了MgB2的氧化活化能在80kJ/mol左右,表明MgB2是一较易氧化的化合物. 相似文献
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本文综述了实用化MgB2超导材料的最新研究进展.最新的理论和实验结果表明通过元素替代和掺杂,有希望进一步提高MgB2超导线带材的临界电流密度和磁通钉扎特性,完全可以达到实用化要求.目前MgB2超导磁体的制备技术研究已经开始,极有可能开发出采用闭路循环制冷机制冷的核磁共振成像仪(MRI)取代现在医学上使用的基于低温超导的核磁共振成像系统. 相似文献