Two Energy Scales and two Quasiparticle Dynamics in the Superconducting State of Underdoped Cuprates

TL;DR

This study reveals two distinct energy scales and quasiparticle dynamics in underdoped cuprate superconductors, challenging the single-scale paradigm and showing universal ratios to critical temperature.

Contribution

It demonstrates the existence of two energy scales with opposite doping dependence and links quasiparticle coherence to momentum space regions, supported by a new theoretical sum-rule.

Findings

Two energy scales depend oppositely on doping.

Nodal quasiparticles remain coherent at low doping.

Antinodal quasiparticles lose coherence as doping decreases.

Abstract

The superconducting state of underdoped cuprates is often described in terms of a single energy-scale, associated with the maximum of the (d-wave) gap. Here, we report on electronic Raman scattering results, which show that the gap function in the underdoped regime is characterized by two energy scales, depending on doping in opposite manners. Their ratios to the maximum critical temperature are found to be universal in cuprates. Our experimental results also reveal two different quasiparticle dynamics in the underdoped superconducting state, associated with two regions of momentum space: nodal regions near the zeros of the superconducting gap and antinodal regions. While antinodal quasiparticles quickly loose coherence as doping is reduced, coherent nodal quasiparticles persist down to low doping levels. A theoretical analysis using a new sum-rule allows us to relate the low-frequency-dependence of the Raman response to the temperature-dependence of the superfluid density, both controlled by nodal excitations.