The single-layer cuprate superconductor ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ (Hg1201) is an ideal candidate for investigating many properties of cuprates with minimal disorder and without the complication of multiple ${\mathrm{CuO}}_2$ layers. Here we measure the in-plane longitudinal thermal conductivity $\kappa$ of underdoped Hg1201 ($T_c$ = 76 K, $p$ = 0.11) at dilution refrigerator temperatures to extract the nodal quasiparticle velocity ratio $v_F/v_{\mathrm{\Delta }}$. Assuming contributions from only a single line node per quadrant on the Fermi surface leads to a value of $v_F/v_{\mathrm{\Delta }}$ = $23\pm 3$, anomalously large compared to other cuprates at similar dopings. In conjunction with the anomalously high quasiparticle specific heat of Hg1201 in the normal state reported previously at a similar doping, this points to more than one Fermi surface sheet crossing the nodal line, suggesting the presence of more than the single small electron pocket detected by quantum oscillations.

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• Fermi surface
• Superconductivity
• Thermal conductivity
• Thermal transport
• Cuprates
• High-temperature superconductors