Quasiparticle coherence and the nature of the metal-insulator phase transition in Na$_x$CoO$_2$

TL;DR

This study investigates the quasiparticle behavior and phase transition mechanisms in insulating Na$_x$CoO$_2$, revealing that quasiparticle coherence onset is crucial for the metal-insulator transition, differing from traditional density-wave models.

Contribution

It provides direct experimental evidence linking quasiparticle coherence to the metal-insulator transition in layered cobaltates, highlighting a novel mechanism beyond conventional density-wave theories.

Findings

Observation of single-particle gap opening and band-folding.

Quasiparticle coherence occurs below the disorder-order transition temperature.

Fermi surface topology influences charge-ordering and insulating behavior.

Abstract

Layered cobaltates embody novel realizations of correlated quantum matter on a spin-1/2 triangular lattice. We report a high-resolution systematic photoemission study of the insulating cobaltates (Na1/2CoO2 and K1/2CoO2). Observation of single-particle gap opening and band-folding provides direct evidence of anisotropic particle-hole instability on the Fermi surface due to its unique topology. Kinematic overlap of the measured Fermi surface is observed with the $\sqrt{3}$x$\sqrt{3}$ cobalt charge-order Brillouin zone near x=1/3 but not at x=1/2 where insulating transition is actually observed. Unlike conventional density-waves, charge-stripes or band insulators, the on-set of the gap depends on the quasiparticle's quantum coherence which is found to occur well below the disorder-order symmetry breaking temperature of the crystal (the first known example of its kind).