Direct observation of charge re-distribution in a MgB₂ superconductor

To study the origin of negative thermal expansion effects near the superconducting transition temperature T_C in MgB₂, low-temperature high-energy synchrotron radiation x-ray diffraction was used to probe the charge redistribution near the boron atoms. Our results reveal that the in-plane hole-distribution of B^- hops through the direct orbital overlap of Mg²⁺ along the c-axis at 50 K and is re-distributed out-of-plane. This study shows that the out-of-plane π-hole distribution plays a dominant role in the possible origin of superconductivity and negative thermal effects in MgB₂.