Single crystals of MgB2: synthesis, substitutions and properties

Janusz Karpinski 1Nikolai D. Zhigadlo 1Sergiy Katrych 1Roman Puzniak 2A. Wisniewski 2K. Rogacki 3

1. Laboratory for Solid State Physics ETH (ETH), Schafmatstr. 16, Zürich 8093, Switzerland
2. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
3. Polish Academy of Sciences, Institute of Low Temperature and Structure Research (INTiBS), Okólna 2, Wrocław 50-422, Poland

Abstract

Structural and superconducting properties of MgB2 are strongly anisotropic. Therefore investigations of its intrinsic properties should be performed on single crystals rather than on polycrystalline samples with randomly oriented grains. Unfortunately, conventional methods of crystal growth did not work for MgB2. High temperature solution growth in metals (Mg, Al, Cu, etc.) at normal pressure used for other borides is not possible due to very low solubility of MgB2 in these metals or formation of other compounds. The solubility of MgB2 in Mg is extremely low up to the boiling temperature of Mg (1107 °C) at ambient pressure. At higher temperature solubility increases, but partial pressure of Mg vapor above molten Mg increases with temperature, at temperature of crystal growth (1800-2200 °C) is very high and using of high pressure methods is necessary. Single crystals of MgB2 have been grown from flux with a high-pressure cubic anvil technique. Investigations of the PT phase diagram prove that the MgB2 phase is stable up to 2200 oC at high hydrostatic pressure. Superconducting and normal state properties of pure MgB2 are now well evidenced by experiments and explained by theory. However, modification of properties through chemical substitutions is still not well understood. Specific band structure of MgB2 with two bands, π and σ, involved in superconductivity is strongly influenced by chemical substitutions. Substitutions of Al for Mg and C for B dope MgB2 with electrons and lead to increase of scattering within both π and σ bands, however, with different rates for both elements. Therefore, different changes of the upper critical field, Hc2, and its anisotropy, gammaHc2, for Mg1-xAlxB2 and MgB2-xCx are observed. By introduction of C, Al, Li, Mn, Fe in the melt we were able to substitute partially Mg or B in MgB2 crystals and to dope the crystals with electrons or holes. Hole doping with Li decreases superconducting transition temperature, Tc, but in much slower rate, than electron doping with C and Al. In the crystals co-doped simultaneously with both Li and C or Li and Al one can expect compensation of electron doping with C or Al by holes from Li and increase of Tc. In this presentation the review of the results of crystal growth of pure and substituted MgB2 crystals as well as investigations of the structure and superconducting properties is given.

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Presentation: Oral at Joint Fith International Conference on Solid State Crystals & Eighth Polish Conference on Crystal Growth, by Janusz Karpinski
See On-line Journal of Joint Fith International Conference on Solid State Crystals & Eighth Polish Conference on Crystal Growth

Submitted: 2007-01-12 19:32
Revised:   2009-06-07 00:44
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