High density of states in the pseudogap phase of the cuprate superconductor HgBa2CuO4+δ from low-temperature normal-state specific heat

The specific heat C of the single-layer cuprate superconductor HgBa2CuO4+δ was measured in an underdoped crystal with Tc=72 K at temperatures down to 2 K in magnetic fields up to 35 T, a field large enough to suppress superconductivity at that doping (p≃0.09). In the normal state at H=35 T, a residual linear term of magnitude γ=12±2 mJ/K2mol is observed in C/T as T→0, a direct measure of the electronic density of states. This high value of γ has two major implications. First, it is significantly larger than the value measured in overdoped cuprates outside the pseudogap phase (p>p∗), such as La2-xSrxCuO4 and Tl2Ba2CuO6+δ at p≃0.3, where γ≃7 mJ/K2mol. Given that the pseudogap causes a loss of density of states and assuming that HgBa2CuO4+δ has the same γ value as other cuprates at p≃0.3, this implies that γ in HgBa2CuO4+δ must peak between p≃0.09 and p≃0.3, namely, at (or near) the critical doping p∗ where the pseudogap phase is expected to end (p∗≃0.2). Second, the high γ value implies that the Fermi surface must consist of more than the single electronlike pocket detected by quantum oscillations in HgBa2CuO4+δ at p≃0.09, whose effective mass m∗=2.7m0 yields only γ=4.0 mJ/K2mol. This missing mass imposes a revision of the current scenario for how pseudogap and charge order, respectively, transform and reconstruct the Fermi surface of cuprates.