LAUSR

Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable $\ensuremath{\beta}$-Ga with a critical temperature ${T}_{\mathrm{c}}=6.04(5)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, while stable $\ensuremath{\alpha}$-Ga has a much lower ${T}_{\mathrm{c}}l1.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Here we use a membrane-based nanocalorimeter to initiate the transition from $\ensuremath{\alpha}$-Ga to $\ensuremath{\beta}$-Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in situ transformation is initiated by bringing the temperature to about $10\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ above the melting temperature of $\ensuremath{\alpha}$-Ga. After such treatment, the liquid supercools down to $232\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where $\ensuremath{\beta}$-Ga solidifies. We find that $\ensuremath{\beta}$-Ga is a strong-coupling type-I superconductor with $\mathrm{\ensuremath{\Delta}}(0)/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=2.00(5)$ and a Sommerfeld coefficient ${\ensuremath{\gamma}}_{\mathrm{n}}=1.53(4)\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{molK}}^{2}$, 2.55 times higher than that in the $\ensuremath{\alpha}$ phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.