Interfacial Surface Energy between the Superfluid Phases ofHe3

We report the first measurements of ${\sigma }_{\mathrm{AB}}$, the surface energy associated with the interface between the $A$ and $B$ phases of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ as they coexist at $T_{\mathrm{AB}}$. At melting pressure we find ${\sigma }_{\mathrm{AB}}=6\mathrm{}\times {10}^{-6\mathrm{}}$ erg ${\mathrm{cm}}^{-2\mathrm{}}$ in no magnetic field ($\frac{T_{\mathrm{AB}}}{T_C}=0.79\mathrm{}$) rising to ${\sigma }_{\mathrm{AB}}=1.6\mathrm{}\times {10}^{-5\mathrm{}}$ erg ${\mathrm{cm}}^{-2\mathrm{}}$ in a magnetic field of 4 kOe ($\frac{T_{\mathrm{AB}}}{T_C}=0.5\mathrm{}$). Theoretical calculations that we present give an estimate about 50% larger.