Spin fluctuations, magnetic long-range order and Fermi surface gapping in NaxCoO2

TL;DR

This paper investigates the complex magnetic and electronic phases of NaxCoO2, revealing spin fluctuations, magnetic order, and Fermi surface gapping, with a focus on high sodium content and discrepancies between experimental and theoretical density of states.

Contribution

It provides an extended low energy phase diagram for NaxCoO2, highlighting the coexistence of spin fluctuations, magnetic order, and Fermi surface gapping, especially at high x values.

Findings

Mass enhancement observed in heat capacity data

Presence of spin fluctuations without long-range magnetic order

Fermi surface gapping associated with magnetic order

Abstract

In this study an extended low energy phase diagram for NaxCoO2 is experimentally established with emphasis on the high x range. It is based on systematic heat capacity studies on both polycrystalline and single crystalline samples and on uSR measurements. Main features are the existence of mass enhancement, spin fluctuations without long-range order, and magnetic order with associated Fermi surface gapping. The latter is seen in the electronic density of states (DOS) and suppression of nuclear specific heat. While there is agreement between the band structure and the low energy DOS in the low x range, in the high x range (x > 0.6) the thermodynamically determined DOS is approximately three times that deduced from the angle-resolved photoemission spectroscopy (ARPES)-measured band dispersion or local-density approximation (LDA) calculations.