Single crystals of MgB2: Synthesis, substitutions and properties

Single crystals of MgB2, with a size up to 1.5 × 1 × 0.1 mm3 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 MgB2 phase is stable up to 2200 °C at high hydrostatic pressure. 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 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, Hc2, and its anisotropy, γHc2γHc2, for Mg1−xAlxB2 and MgB2−xCx are observed. Mg1−xAlxB2 crystals show a moderate decrease of the superconducting transition temperature, Tc, for the samples with small x and, simultaneously, significant reduction of Hc2 and its anisotropy at lower temperatures, as compared to the value for unsubstituted crystals. The temperature dependence of the anisotropy is less pronounced. MgB2−xCx crystals exhibit only slight reduction of Tc with substitution and, moreover, a significant increase of Hc2 for an applied field oriented both parallel, Hc2∥ab, and perpendicular, Hc2∥c, to the ab-plane. For the single crystal with x = 0.13, Hc2∥c(0) ≈ 8.5 T is more than twice as large as that for an unsubstituted compound. The anisotropy of Hc2 decreases from 6 (MgB2) to about 4 (x = 0.13) at low temperatures. The corresponding Hc2∥ab(0) ≈ 34 T is close to the maximum possible enhancement of Hc2 due to the chemical substitutions. Hole doping with Li decreases Tc, but in much slower rate than electron doping with C and Al. For MgB2 crystals with simultaneously substituted Li for Mg and C for B, Tc decreases more rapidly than in the case when only C is substituted. The Tc reduction in co-doped crystals is a sum of Tc 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 Tc and Hc2 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.