Spectroscopic evidence for preformed Cooper pairs in the pseudogap phase of cuprates

¹  Swiss Light Source, Paul Scherrer Institute - CH-5232 Villigen PSI, Switzerland
²  Paul Scherrer Institute, ETH Zurich and EPF Lausanne - 5232 Villigen PSI, Switzerland
³  Materials Science Division, Argonne National Laboratory - Argonne, IL 60439 USA
⁴  Department of Physics, University of Illinois at Chicago - Chicago, IL 60607 USA
⁵  Laboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institute - CH-5232 Villigen PSI, Switzerland
⁶  Materials Physics, Royal Institute of Technology KTH - S-164 40 Kista, Sweden, EU
⁷  Department of Physics, Hokkaido University - Sapporo 060-0810, Japan
⁸  Condensed Matter Theory Group, Paul Scherrer Institute - CH-5232 Villigen PSI, Switzerland

Corresponding author: ming.shi@psi.ch

Received: 31 July 2009
Accepted: 6 October 2009

Abstract

Angle-resolved photoemission on underdoped La_1.895Sr_0.105CuO₄ reveals that in the pseudogap phase, the dispersion has two branches located above and below the Fermi level with a minimum at the Fermi momentum. This is characteristic of the Bogoliubov dispersion in the superconducting state. We also observe that the superconducting and pseudogaps have the same d-wave form with the same amplitude. Our observations provide direct evidence for preformed Cooper pairs, implying that the pseudogap phase is a precursor to superconductivity.