Thermal Expansion measurements on single crystals of the superconductor MgB$_2$

Thermal expansion measurements have been conducted along the a and c axes on single crystals of the superconductor MgB$_2$. The samples are platelets with typical thicknesses of 0.1 mm. The a axis lies in the plane of the platelets, this axis is typically 0.5 mm long. The measurements were conducted using a capacitive thermal expansion cell constructed entirely of fused silica; it is capable of detecting 0.1 $\AA$ changes in length. The results reveal that the linear thermal expansion $\Delta$$L/L$ = 21x10$^{-4}$ and 5.2x10$^{-4}$ along the c and a axes, respectively, in the temperature range 5 K $<$ T $<$ 300 K. At the superconducting transition temperature $T_c$, jumps in the thermal expansion coefficient $\alpha$ are observed to be positive along both axes. Regions where $\alpha$ $<$ 0 are observed along both axes. The results are in general agreement with recently published data [1] on polycrystalline MgB$_2$. [1] J. J. Neumeier, T. Tomita, M. Debassai, J. S. Schilling, P. W. Barnes, D. G. Hinks, and J. D. Jorgensen, Phys. Rev. B. Rapid Commun. December 2005.