Origins of large critical temperature variations in single-layer cuprates

Abstract

We study the electronic structures of two single-layer superconducting cuprates, ${\text{Tl}}_{2}{\text{Ba}}_{2}{\text{CuO}}_{6+\ensuremath{\delta}}$ (Tl2201) and $({\text{Bi}}_{1.35}{\text{Pb}}_{0.85})$ $({\text{Sr}}_{1.47}{\text{La}}_{0.38})$ ${\text{CuO}}_{6+\ensuremath{\delta}}$ (Bi2201) which have very different maximum critical temperatures (90 K and 35 K, respectively) using angular-resolved photoemission spectroscopy (ARPES). We are able to identify two main differences in their electronic properties. First, the shadow band that is present in double-layer and low ${\text{T}}_{c,\text{max}}$ single-layer cuprates is absent in Tl2201. Recent studies have linked the shadow band to structural distortions in the lattice and the absence of these in Tl2201 may be a contributing factor in its ${\text{T}}_{c,\text{max}}$. Second, Tl2201's Fermi surface (FS) contains long straight parallel regions near the antinode, while in Bi2201 the antinodal region is much more rounded. Since the size of the superconducting gap is largest in the antinodal region, differences in the band dispersion at the antinode may play a significant role in the pairing and therefore affect the maximum transition temperature.