Anomalously high quasiparticle thermal conductivity in the underdoped cuprate superconductor HgBa 2 CuO 4+ delta
Researchers measured thermal conductivity in underdoped Hg1201 to find the nodal quasiparticle velocity ratio ($v_F/v_ ext{Δ}$). The result, $v_F/v_ ext{Δ} = 23 ± 3$, is much larger than expected for a single Fermi surface sheet. This suggests the presence of multiple Fermi surface sheets crossing the nodal line, which is supported by high specific heat measurements.