Single crystals of MgB 2: Synthesis, substitutions and properties

Abstract

Single crystals of MgB 2, with a size up to 1.5 × 1 × 0.1 mm 3 and with a weight up to 230 μg, have been grown from flux with a high-pressure cubic anvil technique. Investigations of the P– T phase diagram prove that the MgB 2 phase is stable up to 2200 °C at high hydrostatic pressure. Specific band structure of MgB 2 with two bands (π and σ) involved in superconductivity is strongly influenced by chemical substitutions. Substitutions of Al for Mg and C for B lead to increase of scattering within both π and σ bands, however, with different rates for both substituents. Therefore, different changes of the upper critical field, H c2, and its anisotropy, γ H c 2 , for Mg 1− x Al x B 2 and MgB 2− x C x are observed. Mg 1− x Al x B 2 crystals show a moderate decrease of the superconducting transition temperature, T c, for the samples with small x and, simultaneously, significant reduction of H c2 and its anisotropy at lower temperatures, as compared to the value for unsubstituted crystals. The temperature dependence of the anisotropy is less pronounced. MgB 2− x C x crystals exhibit only slight reduction of T c with substitution and, moreover, a significant increase of H c2 for an applied field oriented both parallel, H c2∥ ab , and perpendicular, H c2∥ c , to the ab-plane. For the single crystal with x = 0.13, H c2∥ c (0) ≈ 8.5 T is more than twice as large as that for an unsubstituted compound. The anisotropy of H c2 decreases from 6 (MgB 2) to about 4 ( x = 0.13) at low temperatures. The corresponding H c2∥ ab (0) ≈ 34 T is close to the maximum possible enhancement of H c2 due to the chemical substitutions. Hole doping with Li decreases T c, but in much slower rate than electron doping with C and Al. For MgB 2 crystals with simultaneously substituted Li for Mg and C for B, T c decreases more rapidly than in the case when only C is substituted. The T c reduction in co-doped crystals is a sum of T c reductions for separate C and Li doping. This means that holes introduced with Li cannot counterbalance electrons added with C. The possible reason of this can be that holes coming from Li occupy π band and do not compensate the addition of electrons which, coming from C, fill the σ band. Substitution of magnetic Mn for Mg strongly suppresses T c and H c2 due to the magnetic pair breaking. However, this is not the case for the substitution of Fe for Mg, at least for low Fe concentration.