Changes in the momentum density of polycrystalline superconductor ${\mathrm{MgB}}_2$ have been studied while cooling the sample below the critical temperature of 39 K. Compton-scattering experiments were performed using synchrotron radiation with incident photon energy of 29 keV at temperatures of $\approx 15,$ 55, and 293 K. Small changes in the ground-state electron momentum density were observed as a function of temperature. Above the critical temperature these can be explained by thermally induced changes in the lattice constant. However, the observed localization of the valence-electron momentum density when the sample is brought into the superconducting state cannot be interpreted via thermal lattice contraction but suggests changes in the electronic structure.

• Received 8 August 2003

DOI:https://doi.org/10.1103/PhysRevB.69.020501