Nodal gap energy in high-T_c cuprate superconductors: A new paradigm

摘要

In high-T_c cuprate superconductors, the d-wave-like superconducting gap appears to depart from the standard proportionality with critical temperature T_c, and shows different doping dependences between the nodal and antinodal regions of momentum space. The failure of the attempts so far to formulate these momentum-resolved gaps with T_c has posed questions to their roles in electron pairing and their mutual relationship. Here, we report a simple proportional relation between nodal gap and critical temperature in superconducting Bi_2Sr_2CaCu_2O_(8+δ) and Bi_2Sr_(1.6)La_(0.4)CuO_(6+δ). Using low-energy synchrotron radiation on angle-resolved photoemission spectroscopy, we observed the strongly curved gap profile in the underdoped cuprates, and found that a nodal-gap component 2Δ_N at the nodal limit of the gap slope closely follows 8.5k_BT_c and concomitantly related to the antinodal-gap energy Δ~* through a reduction factor of square-root superfluid density ρ_s. These empirical formulae provide a new gap-based paradigm on the principle governing T_c in strong-coupling superconductors.