Nodal gap behavior of Bi2Sr2−xLnxCuO6+δ (Ln = La, Eu) investigated by specific heat measurements

We performed low-temperature specific heat measurements on slightly underdoped samples of monolayer cuprate superconductors Bi2Sr2−xLnxCuO6+δ (Ln = La, Eu, Ln-Bi2201) under magnetic fields H. In La-Bi2201, the coefficient γ of T-linear term in the electronic specific heat Cel at T ≪ Tc shows H dependence, as expected in dx2−y2 -wave superconductors. In Eu-Bi2201, γ shows almost the same H dependence as that of La-Bi2201 below H ∼ 2 T, while γ is suppressed above H ∼ 2 T and deviates downward from the H curve of La-Bi2201. This result suggests the the gap and the electronic excitation spectrum near nodes are modified in Eu-Bi2201 except the region of the Fermi surface in the immediate vicinity of nodes.