Superfluid Density in a Highly Underdoped YBCO Superconductor

TL;DR

This study measures the superfluid density in highly underdoped YBCO superconductors, revealing behavior consistent with a quantum phase transition rather than critical fluctuations near T_c.

Contribution

It provides evidence that superfluid density in underdoped YBCO follows (3+1)D XY universality class predictions, challenging previous fluctuation-based models.

Findings

Superfluid density shows no critical fluctuations near T_c.

Data consistent with quantum phase transition behavior.

Superfluid density scales with doping as predicted by (3+1)D XY model.

Abstract

The superfluid density rho_s(T) = 1/lambda^2(T) has been measured at 2.64 GHz in highly underdoped YBCO, at 37 dopings with T_c between 3 K and 17 K. Within limits set by the transition width Delta T_c ~ 0.4 K, rho_s(T) shows no evidence of critical fluctuations as T goes to T_c, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that rho_s displays the behaviour expected for a quantum phase transition in the (3 + 1)-dimensional XY universality class, with rho_s0 ~ (p - p_c), T_c ~ (p - p_c)^1/2 and rho_s(T) ~ (T_c - T)^1 as T goes to T_c.