Percolative Superconductivity inMg1−xB2

Our results from various transport experiments on ${\mathrm{M}\mathrm{g}}_{1-x}{\mathrm{B}}_2$ indicate a surprising effect associated with the presence of a Mg deficiency in $\mathrm{M}\mathrm{g}{\mathrm{B}}_2$: the phase separation between Mg-vacancy rich and Mg-vacancy poor phases. The Mg-vacancy poor phase is superconducting, but the insulating nature of the Mg-vacancy rich phase probably originates from the Anderson (disorder-induced) localization of itinerant carriers. Furthermore, electron diffraction measurements indicate that within vacancy-rich regions these defects tend to order with intriguing patterns. This electronic phase separation in ${\mathrm{M}\mathrm{g}}_{1-x}{\mathrm{B}}_2$ shows similar, but also distinct characteristics compared with that observed in ${\mathrm{L}\mathrm{a}}_2{\mathrm{C}\mathrm{u}\mathrm{O}}_{4+\delta }$.