The rate of quasiparticle recombination probes the onset of coherence in cuprate superconductors

TL;DR

This study investigates quasiparticle recombination rates in underdoped cuprate superconductors, revealing deviations from BCS theory that suggest a transition from phase-fluctuating order to a coherent superfluid involving mixed particle pairs.

Contribution

It provides new insights into the quasiparticle dynamics and the nature of the superfluid condensate in underdoped cuprates, highlighting the role of charge order and phase fluctuations.

Findings

Quasiparticle recombination lifetime shows a maximum near $T_c$ in underdoped samples.

Application of a magnetic field suppresses the maximum in $ au_{qp}(T)$.

Evidence for a transition from phase-fluctuating order to a coherent condensate involving particle-hole pairs.

Abstract

The condensation of an electron superfluid from a conventional metallic state at a critical temperature $T_c$ is described well by the BCS theory. In the underdoped copper-oxides, high-temperature superconductivity condenses instead from a nonconventional metallic "pseudogap" phase that exhibits a variety of non-Fermi liquid properties. Recently, it has become clear that a charge density wave (CDW) phase exists within the pseudogap regime, appearing at a temperature $T_{CDW}$ just above $T_c$. The near coincidence of $T_c$ and $T_{CDW}$, as well the coexistence and competition of CDW and superconducting order below $T_c$, suggests that they are intimately related. Here we show that the condensation of the superfluid from this unconventional precursor is reflected in deviations from the predictions of BSC theory regarding the recombination rate of quasiparticles. We report a detailed investigation of the quasiparticle (QP) recombination lifetime, $\tau_{qp}$, as a function of temperature and magnetic field in underdoped HgBa$_{2}$CuO$_{4+\delta}$ (Hg-1201) and YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO) single crystals by ultrafast time-resolved reflectivity. We find that $\tau_{qp}(T)$ exhibits a local maximum in a small temperature window near $T_c$ that is prominent in underdoped samples with coexisting charge order and vanishes with application of a small magnetic field. We explain this unusual, non-BCS behavior by positing that $T_c$ marks a transition from phase-fluctuating SC/CDW composite order above to a SC/CDW condensate below. Our results suggest that the superfluid in underdoped cuprates is a condensate of coherently-mixed particle-particle and particle-hole pairs.