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arXiv 1002.0799cond-mat.supr-concond-mat.mtrl-sci
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缺镁对MgB2单晶结构和超导性能的影响

Influence of Mg deficiency in MgB2 single crystals on crystal structure and superconducting properties

N. D. Zhigadlo, S. Katrych, J. Karpinski, B. Batlogg, F. Bernardini, S. Massidda, R. Puzniak

摘要:

研究了高温真空退火对高压生长的MgB 2单晶中Mg的影响。随着退火温度从800 ℃升高到975 ℃,MgB_2晶体中的平均Mg含量系统地降低,而Tc基本保持不变,超导转变从0.55 K略微加宽到1.3 K。在875 ℃退火后,超导体积分数的减少已经是明显的。在975 ℃退火的样品部分分解,Mg占位率从生长态晶体的0.98降低到0.92。在1000 ℃退火完全破坏超导性。X射线衍射分析表明,MgB_2分解的最终产物主要是多晶MgB_4,因此MgB_2的分解反应可以描述为2 MgB_2(s)= MgB_4(s)+ Mg(g)。超晶胞方法中的Mg 1-x(VMg)xB 2电子结构的第一性原理计算显示费米能级的小的下移。空位诱导的空穴进入σ带和π带。这些小的修改预计不会影响Tc,与观察结果一致。的超导体积分数的显着减少,而没有明显的Tc降低表明共存,在同一晶体内,与富镁和贫镁空位的区域相关的电子相和非电子相。

英文摘要:

The effects of high-temperature vacuum-annealing induced Mg deficiency in MgB2 single crystals grown under high pressure were investigated. As the annealing temperature was increased from 800 to 975 C, the average Mg content in the MgB2 crystals systematically decreased, while Tc remains essentially unchanged and the superconducting transition slightly broadens from 0.55 K to 1.3 K. The reduction of the superconducting volume fraction was noticeable already after annealing at 875 C. Samples annealed at 975 C are partially decomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-grown crystals. Annealing at 1000 C completely destroys superconductivity. X-ray diffraction analysis revealed that the main final product of decomposition is polycrystalline MgB4 and thus the decomposition reaction of MgB2 can be described as 2MgB2(s) = MgB4(s) + Mg(g). First-principles calculations of the Mg1-x(VMg)xB2 electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both sigma and pi bands. These small modifications are not expected to influence Tc, in agreement with observations. The significant reduction of the superconducting volume fraction without noticeable Tc reduction indicates the coexistence, within the same crystal, of superconductive and non-superconductive electronic phases, associated with regions rich and poor in Mg vacancies.

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