Linear magnetic field dependence of the specific heat in underdoped YBCO*

We report the observation of two distinct regimes in the field dependence of the low temperature electronic heat capacity (C) of YBa$_2$Cu$_3$O$_{6.47}$. Measurements were performed in applied magnetic fields ($H$) up to 34.5~T and at temperatures between 1 and 8 K. Below 10 T we observe C$\sim H^{1/2}$ attributable to a d-wave superconducting gap. Above 10 T, C approaches a nearly linear-in-field form. All behavior is clearly observable well below the field-induced resistive transition, and thus, likely closely tied to the nodes in the gap. We rule out the suppression of the superconducting gap by the magnetic field as a possible explanation through a comparison with specific heat data from other dopingsof underdoped YBCO.