MgB₂ is a two-gap superconductor with several anomalous properties originating from the existence of two separate sheets of the Fermi surface, one quasi 2D (σ band) and second quasi 3D (π band). This leads to temperature and field dependent anisotropy and high critical temperature of 39K. Prospects of applications of MgB₂ depend on a success in increasing the critical parameters such as the upper critical field and critical current. This can be realized via substitutions of B or Mg. Substitutional chemistry is one of the most effective methods to modify the electronic properties of superconductors. Partial replacement of B by C, or Mg by Al in MgB₂ single crystals introduces additional electrons which influences superconducting properties, electronic structure and energy gaps. The magnetic properties investigations with torque magnetometer and the transport investigations show twice increase of the upper critical field with carbon substitution. The behavior of two gaps as a function of temperature and field has been studied by the point contact spectroscopy and the scanning tunneling spectroscopy. However, defects introduced by substitutions led to formation of domains, vacancies and inhomogeneities in the structure, which influence the properties as well. Transition electron microscopy and single crystal x-ray investigations show the influence of substitutions on defect formation in the structure.

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