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

Abstract

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.

arXiv comment21 pages, 1 table, 6 figures, PRB accepted

References in corpus (12)

• Superconductivity of metallic boron in MgB_2
• Beyond Eliashberg superconductivity in MgB2: anharmonicity, two-phonon scattering, and multiple gaps
• Thermodynamics of the Mg-B system: Implications for the deposition of MgB2 thin films
• Band filling and interband scattering effects in MgB${}_2$: C vs Al doping
• Effect of lattice strain and defects on the superconductivity of MgB2
• Synthesis and Stoichiometry of MgB2
• Experimental study of MgB2 decomposition
• Effects of Boron Purity, Mg Stoichiometry and Carbon Substitution on Properties of Polycrystalline MgB${}_2$
• Low Temperature Fabrication of MgB2
• Influence of the starting composition on the structural and superconducting properties of MgB2 phase
• MgB2 single crystals substituted with Li and with Li-C: Structural and superconducting properties
• Nonstoichiometry, Defects and Transport Properties in MgB2

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