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Determination of the superconducting gap in near optimally doped Bi_2Sr_{2-x}La_xCuO_{6+δ} (x ~ 0.4) from low-temperature specific heat

Low-temperature specific heat of the monolayer high-Tc superconductor Bi_2Sr_{2-x}La_xCuO_{6+δ} has been measured close to the optimal doping point (x ~ 0.4) in different magnetic fields. The identification of both a T^2 term in zero field and a \sqrt{H} dependence of the specific heat in fields is shown to follow the theoretical prediction for d-wave pairing, which enables us to extract the slope of the superconducting gap in the vicinity of the nodes (v_Δ, which is proportional to the superconducting gap Δ_0 at the antinodes according to the standard d_{x^2-y^2} gap function). The v_Δ or Δ_0 (~ 12 meV) determined from this bulk measurement shows close agreement with that obtained from spectroscopy or tunneling measurements, which confirms the simple d-wave form of the superconducting gap.