On the Fermi Surface of YBa 2 Cu 3 O 7-δ

Abstract

We show that the signature of a Fermi surface sheet of YBa₂Cu₃O_7-δ may be obtained unambiguously from twinned crystals. Comparison of electron-positron momentum density from YBa₂Cu₃O_7-δ measured both in insulating and (superconducting) twinned phases leads to a decisive further proof of the existence of the Fermi surface in the metallic YBa₂Cu₃O_7-δ. In addition, measurements on untwinned YBa₂Cu₃O_7-δ single crystals reveal also a "ridge" Fermi surface sheet attributed by band structure calculations to CuO chains.