The composite picture of the charge carriers in La2-xSrxCuO4 (0.063 < x < 0.11) superconductors

TL;DR

This study reveals that only a small fraction of holes in La2-xSrxCuO4 participate in charge transport and superconductivity, with free carriers strongly coupled to an electronic lattice, offering new insights into cuprate physics.

Contribution

It introduces a composite model where free carriers are coupled to an electronic lattice, based on far-infrared measurements of La2-xSrxCuO4.

Findings

Only ~0.2% of holes participate in charge transport.

Characteristic collective modes observed at ~18 and 22 cm-1.

Massive screening of free carriers by the electronic lattice.

Abstract

Through far-infrared studies of La2-xSrxCuO4 single crystals for x = 0.063, 0.07, 0.09, and 0.11, we found that only ~ 0.2 % of the total holes participated in the nearly dissipationless normal state charge transport and superconductivity. We have also observed characteristic collective modes at w ~ 18 cm-1 and 22 cm-1 due to the bound carriers in an electronic lattice (EL) state and the free carriers are massively screened by the EL. Our findings lead us to propose a composite picture of the charge system where the free carriers are coupled to and riding on the EL. This unique composite system of charge carriers may provide further insights into the understanding of the cuprate physics.