MgB2 single crystals, influence of magnetic and non-magnetic ions substitution on superconducting properties and structure

Janusz Karpinski 1Nikolai D. Zhigadlo 1K. Rogacki 1,2Bertram Batlogg 1Götz Schuck 1Roman Puzniak 3A. Wisniewski 3R. Gonnelli 4

1. Laboratory for Solid State Physics ETH (ETH), Schafmatstr. 16, Zürich 8093, Switzerland
2. Polish Academy of Sciences, Institute of Low Temperature and Structure Research (INTiBS), Okólna 2, Wrocław 50-422, Poland
3. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
4. Politecnico di Torino, Torino 10129, Italy

Abstract

Pure and substituted single crystals of the two-gap MgB2 superconductor have been grown at a pressure of 30 kbar at temperature 1800-2000°C using the cubic anvil technique. High crystals' quality expresses as low residual resistivity ρo(40 K) = 0.5 μΩcm. Magnetic (Mn, Fe) and non-magnetic (Li, Al, C) ions have been substituted to study the substitution effects in a two-band superconductor and it's influence on the impurity scattering in and between σ and π bands. Mg1-xAlxB2 and MgB2-xCx crystals were grown for x=0-0.3. Al and C dope MgB2 with additional electrons, causing similar decrease of Tc. Li dopes MgB2 with holes without significant decrease of Tc. Magnetic ion Mn2+ for Mg2+ leads to very rapid decrease of Tc. This indicates strong pair breaking by magnetic scattering centers. For Mn substituted crystals superconductivity is completely suppressed for x=0.02. Fe at low concentration up to x=0.03 is nonmagnetic impurity in MgB2 therefore decreases Tc less rapidly than Mn. However, crystals with higher Fe concentration show rapid decrease of Tc, which indicates that Fe can be a magnetic impurity. Carbon substitution increases the upper critical field twice for x=0.05-0.10, while Al, Fe and Mn substitutions decrease this field. The upper critical field anisotropy γ decreases with all substitutions, but temperature dependence of γ is different, which indicates different scattering rates for different substitutions in π and σ bands. For Mn and Al, both energy gaps exist up to the highest substitution rate, while for C substitution, merging of two energy gaps was observed indicating influence of the interband scattering.

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Presentation: Invited oral at E-MRS Fall Meeting 2006, Symposium K, by Janusz Karpinski
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-05-24 10:51
Revised:   2009-06-07 00:44
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