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

D. Qian·L. Wray·D. Hsieh·D. Wu·L. J. Luo·N. L. Wang·A. Fedorov·R. J. Cava·L. Viciu·M. Z. Hasan
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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).

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D. Qian, L. Wray, D. Hsieh, D. Wu, L. J. Luo, N. L. Wang, A. Fedorov, R. J. Cava, L. Viciu, M. Z. Hasan. 2006-05-13. Quasiparticle coherence and the nature of the metal-insulator phase transition in Na$_x$CoO$_2$. https://doi.org/10.1103/physrevlett.96.046407

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