Compressibility of the MgB₂ superconductor

Considerable excitement has been caused recently by the discovery that the binary-boride system with stoichiometry MgB₂ is superconducting at the remarkably high temperature of 39 K [J. Nagamatsu, N, Nakagawa, T. Muranaka, Y. Zenitani, and J. Akimitsu, Nature 410, 63 (2001)]. This potentially opens the way to even higher-T_c values in a new family of superconductors with unexpectedly simple composition and structure. The simplicity in the electronic and crystal structures could allow the understanding of the physics of high-T_c superconductivity without the presence of the multitude of complicated features, associated with the cuprates. Synchrotron x-ray diffraction was used to measure the isothermal compressibility of MgB₂, revealing a stiff tightly packed incompressible solid with only moderate bonding anisotropy between intralayer and interlayer directions. These results, combined with the pressure evolution of the superconducting transition temperature, T_c, establish its relation to the B and Mg bonding distances over a broad range of values.