Structural modifications in bismuth cuprates: Effects on the electronic structure and Fermi surface

We compare the electronic structure of the idealized tetragonal model of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_6$ with that of the more realistic orthorhombic √2 × √2 cell as determined by Torardi and collaborators. The distortions, which involve atomic displacements as large as 0.45 Å, shift the Bi-O(3) derived bands at the Fermi level strongly, and change the number of Fermi surfaces and their topology. We emphasize that such complications, which are probably even larger in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$, must be taken into acount in the interpretation of angle-resolved photoemission spectroscopy (ARPES), since we find for this compound several Fermi surfaces in the region of the M¯ point where flat bands are seen in ARPES data. These effects will also affect the interpretation of the measured superconducting energy gap over the Fermi surface(s).