Electronic Raman Scattering in copper oxide Superconductors: Understanding the Phase Diagram

TL;DR

This study uses electronic Raman scattering to explore how quasiparticle coherence and energy scales evolve with doping and temperature in copper oxide superconductors, revealing a link between coherence fraction and critical temperature.

Contribution

It introduces a doping-dependent coherence fraction and a unified gap model explaining the relationship between T_c, gap amplitude, and quasiparticle coherence in cuprates.

Findings

Coherent quasiparticles develop mainly near the nodes in underdoped samples.

The critical temperature T_c scales with the product of coherence fraction and maximum gap.

A proposed 3D phase diagram links temperature, energy scales, and doping in cuprates.

Abstract

Electronic Raman scattering measurements have been performed on hole doped copper oxide superconductors as a function of temperature and doping level. In the superconducting state coherent Bogoliubov quasiparticles develop preferentially over the nodal region in the underdoped regime. We can then define the fraction of coherent Fermi surface, $f_c$ around the nodes for which quasiparticles are well defined and superconductivity sets in. We find that $f_c$ is doping dependent and leads to the emergence of two energy scales. We then establish in a one single gap shem, that the critical temperature $T_{c} \propto f_{c}\Delta_{max}$ where $\Delta_{max}$ is the maximum amplitude of the d-wave superconducting gap. In the normal state, the loss of antinodal quasiparticles spectral weight detected in the superconducting state persists and the spectral weight is only restored above the pseudogap temperature $T*$. Such a dichotomy in the quasiparticles dynamics is then responsible for the emergence of the two energy scales in the superconducting state and the appearance of the pseudogap in the normal state. We propose a 3D phase diagram where both the temperature and the energy phase diagrams have been plotted together. We anticipate that the development of coherent excitations on a restricted part of the Fermi surface only is a general feature in high $T_c$ cuprate superconductors as the Mott insulating is approaching.