Phase-fluctuation induced reduction of the kinetic energy at the superconducting transition

TL;DR

This paper demonstrates that phase fluctuations of the superconducting order parameter can explain the observed reduction in in-plane kinetic energy at the superconducting transition in underdoped cuprates, aligning with optical sum rule violations.

Contribution

It introduces a phase fluctuation model that accounts for kinetic energy changes at T_c, providing a theoretical explanation for experimental sum rule violations.

Findings

Phase fluctuations cause a transition from phase-incoherent to phase-coherent Cooper pairs.

The model explains the observed optical sum rule violation in underdoped cuprates.

Kinetic energy decreases at T_c due to phase coherence onset.

Abstract

Recent reflectivity measurements provide evidence for a "violation" of the in-plane optical integral in the underdoped high-T_c compound Bi_2Sr_2CaCu_2O_{8+\delta} up to frequencies much higher than expected by standard BCS theory. The sum rule violation may be related to a loss of in-plane kinetic energy at the superconducting transition. Here, we show that a model based on phase fluctuations of the superconducting order parameter can account for this change of in-plane kinetic energy at T_c. The change is due to a transition from a phase-incoherent Cooper-pair motion in the pseudogap regime above T_c to a phase-coherent motion at T_c.