Dynamical magnetic susceptibility in the lamellar cobaltate superconductor Na_xCoO_2$\cdot y$H_2O

TL;DR

This paper investigates how the superconducting gap symmetry and electronic structure influence the dynamical spin susceptibility in Na_xCoO_2·yH_2O, revealing dominant antiferromagnetic fluctuations and constraining possible pairing symmetries.

Contribution

It provides a comprehensive analysis of the spin susceptibility across different models and clarifies the likely superconducting gap symmetry and the unlikelihood of a spin-resonance peak in this material.

Findings

Normal state dominated by incommensurate antiferromagnetic fluctuations.

Superconducting d-wave symmetries are consistent with experimental data.

Resonance peak is unlikely and confined to the antiferromagnetic wave vector.

Abstract

We systematically analyze the influence of the superconducting gap symmetry and the electronic structure on the dynamical spin susceptibility in superconducting Na_xCoO_2$\cdot y$H_2O within a three different models: the single a_{1g}-band model with nearest-neighbor hoppings, the realistic three-band t_{2g}-model with, and without e'_g pockets present at the Fermi surface. We show that the magnetic response in the normal state is dominated by the incommensurate antiferromagnetic spin density wave fluctuations at large momenta in agreement with experimental temperature dependence of the spin-lattice relaxation rate. Also, we demonstrate that the presence or the absence of the e'_g-pockets at the Fermi surface does not affect significantly this conclusion. In the superconducting state our results for d_{x^2-y^2}- or d_{xy}-wave symmetries of the superconducting order parameter are consistent with experimental data and exclude nodeless $d_{x^2-y^2} + id_{xy}$-wave symmetry. We further point out that the spin-resonance peak proposed earlier is improbable for the realistic band structure of Na_xCoO_2$\cdot y$H_2O. Moreover, even if present the resonance peak is confined to the antiferromagnetic wave vector and disappears away from it.