Search for Cooper-pair Fluctuations in Severely Underdoped YBCO Films

TL;DR

This study measures the superfluid fraction in severely underdoped YBCO films to test preformed-pairs theory, finding results that contradict the predicted nonanalytic temperature dependence at low temperatures.

Contribution

The paper provides high-precision measurements of superfluid density in underdoped cuprates, challenging existing theoretical predictions about pseudogap physics.

Findings

Superfluid density follows a quadratic temperature dependence at low T.

Results disagree with the nonanalytic T-dependence predicted by theory.

Data suggest alternative mechanisms for pseudogap behavior.

Abstract

The preformed-pairs theory of pseudogap physics in high-$T_C$ superconductors predicts a nonanalytic $T$-dependence for the $ab$-plane superfluid fraction, $\rho_S$, at low temperatures in underdoped cuprates. We report high-precision measurements of $\rho_S(T)$ on severely underdoped YBa$_2$Cu$_3$O$_{6+x}$ and Y$_{0.8}$Ca$_{0.2}$Ba$_2$Cu$_3$O$_{6+x}$ films. At low $T$, $\rho_S$ looks more like $1 - T^2$ than $1 - T^{3/2}$, in disagreement with theory.