Reservoir Layers In High Tc Mercury Cuprates

TL;DR

This paper hypothesizes that certain cations in high Tc cuprates act as negative-U centers, enhancing superconductivity by facilitating electron pairing with minimal mass increase, especially in Hg-based compounds.

Contribution

It introduces the idea that cations with 6s electrons in charge reservoir layers serve as negative-U centers, explaining high Tc in Hg cuprates.

Findings

Hg-cuprates can exist as two-site negative-U centers.

These centers facilitate strong pairing with less effective mass increase.

The orientation of Hg2+2 centers aligns with d-wave symmetry.

Abstract

We put forward the hypothesis that cations with 6s electrons (Hg,Tl,Pb,Bi) in the charge reservoir layers of high Tc cuprate superconductors actively participate in the pairing interaction as negative-U centers. We further argue that the Hg-cuprates are outstanding superconductors (Tc > 160 K) because they can exist as two-site negative-U centers, Hg2+2. Their electrons are less localized than in single-site centers (negative-U or bipolaron) and can have a strong pairing interaction with a smaller increase in effective mass. The Hg2+2 centers are oriented in the x and y directions and can have phase differences compatible with the d-wave symmetry of the CuO2 planes.