Electronic structure, disconnected Fermi surfaces and antiferromagnetism in the layered pnictide superconductor Na$_x$Ba$_{1-x}$Ti$_2$Sb$_2$O

TL;DR

This paper investigates the electronic structure of Na-doped BaTi$_2$Sb$_2$O, revealing complex Fermi surfaces, magnetic instability, and predicting a unique sign-changing s-wave superconducting state driven by spin fluctuations.

Contribution

It provides detailed electronic structure calculations and predicts a novel superconducting pairing symmetry in Na-doped BaTi$_2$Sb$_2$O, expanding understanding of layered pnictide superconductors.

Findings

Fermi surface comprises multiple sheets, including nested 2D cylinders and 3D sheets.

Magnetic instability linked to Fermi surface nesting.

Predicted a sign-changing s-wave superconducting state mediated by spin fluctuations.

Abstract

We report electronic structure calculations for BaTi$_2$Sb$_2$O and discuss the results in relation to the observed superconductivity of this material when hole doped with Na. The Fermi surface shows several sheets. These include a nested nearly 2D cylinder. There are also two sheets, which are three dimensional, in spite of the layered crystal structure. A magnetic instability associated with Fermi surface nesting is found. A sign-changing s-wave state, different from the one in the Fe-based superconductors, is predicted within a scenario of spin-fluctuation mediated superconductivity.