Disentangling the role of bond lengths and orbital symmetries in controlling $T_c$ in YBa$_2$Cu$_3$O$_7$

TL;DR

This study uses advanced ab initio calculations to explore how bond lengths and orbital symmetries influence the critical temperature in YBa₂Cu₃O₇, revealing that structural modifications can enhance or suppress superconductivity.

Contribution

It provides a detailed analysis of how strain-induced structural changes affect spin fluctuations and orbital contributions, offering new insights into optimizing high-temperature superconductivity in cuprates.

Findings

Shortening Cu layer distance enhances spin fluctuations and increases T_c.

Relaxed structures show electron flow to d_z^2 orbital, reducing T_c.

Strain modulates hybridization and interlayer coupling, impacting superconductivity.

Abstract

Optimally doped YBCO (YBa$_{2}$Cu$_{3}$O$_{7}$) has a high critical temperature, at 92 K. It is largely believed that Cooper pairs form in YBCO and other cuprates because of spin fluctuations, the issue and the detailed mechanism is far from settled. In the present work, we employ a state-of-the-art \emph{ab initio} ability to compute both the low and high energy spin fluctuations in optimally doped YBCO. We benchmark our results against recent inelastic neutron scattering and resonant inelastic X-ray scattering measurements. Further, we use strain as an external parameter to modulate the spin fluctuations and superconductivity. We disentangle the roles of Barium-apical Oxygen hybridization, the interlayer coupling and orbital symmetries by applying an idealized strain, and also a strain with a fully relaxed structure. We show that shortening the distance between Cu layers is conducive for enhanced Fermi surface nesting, that increases spin fluctuations and drives up $T_{c}$. However, when the structure is fully relaxed electrons flow to the d$_{z^2}$ orbital as a consequence of a shortened Ba-O bond which is detrimental for superconductivity