First Principles Study of the Electronic and Vibrational Properties of LiNbO2

TL;DR

This study investigates the electronic structure, vibrational properties, and potential superconductivity mechanisms in LiNbO$_2$ using first-principles calculations, highlighting electron-phonon interactions and phonon mode characteristics.

Contribution

It provides a detailed first-principles analysis of LiNbO$_2$'s electronic and vibrational properties, including phonon modes and electron-phonon coupling relevant to superconductivity.

Findings

Dominant second neighbor hopping $t_2 = 100$ meV in the tight-binding model.

Identification of all zone-center phonon modes and their IR/Raman activity.

Significant anisotropy in Born effective charges indicating interlayer coupling.

Abstract

In the layered transition metal oxide LiNbO$_2$ the Nb$^{3+}$ ($4d^2$) ion is trigonal-prismatically coordinated with O ions, with the resulting crystal field leading to a single band system for low energy properties. A tight-binding representation shows that intraplanar second neighbor hopping $t_2 = 100$ meV dominates the first neighbor interaction ($t_1 = 64$ meV). The first and third neighbor couplings are strongly modified by oxygen displacements of the symmetric Raman-active vibrational mode, and electron-phonon coupling to this motion may provide the coupling mechanism for superconductivity in Li-deficient samples (where $T_c = 5$ K). We calculate all zone-center phonon modes, identify infrared (IR) and Raman active modes, and report LO-TO splitting of the IR modes. The Born effective charges for the metal ions are found to have considerable anisotropy reflecting the degree to which the ions participate in interlayer coupling and covalent bonding. Insight into the microscopic origin of the valence band density, composed of Nb $d_{z^2}$ states with some mixing of O $2p$ states, is obtained from examining Wannier functions for these bands.