The family of sodium cobalt oxides or oxyhydrates (cobaltites) Na_xCoO₂ (with variable x) is similar to cuprate HTSCs. The parent compounds are Mott insulators, in which a strong electrostatic repulsion blocks charge transport; they have layered crystal structures with electronic two-dimensionality; and they can be chemically doped by altering the sodium concentration, thereby changing the electron concentration in the electronically active cobalt-oxygen layers. There is one very important difference, however. While the electronically active copper-oxygen planes in HTSCs have square symmetry, in cobaltites, the symmetry is triangular, a configuration that geometrically frustrates formation of the antiferromagnetically ordered Néel state present in HTSCs and results in a less well ordered "quantum spin liquid" state.

Atomic structure models of Na_xCoO₂ before and after exposure to water adds H₂O to the sodium spacer layers between the electronically active cobalt-oxygen layers, whose electron concentration is controlled by the sodium fraction x.

The interplay of charge and spin degrees of freedom is indeed exciting in cobaltites and leads to a bouquet of changing electronic properties with doping. With increasing sodium concentration, the material progresses from a paramagnetic metal that is superconducting at low-temperature for 1/4 < x < 1/3 to a charge-ordered insulator at x = 1/2, to a Curie-Weiss metal (with the magnetic susceptibility of an insulator) at x = 2/3, and ends as a weakly magnetically ordered spin density wave (SDW) metal for x > 3/4.

Photoemission intensity (bright regions are highest intensity) maps of the valence excitations at T = 10 K along the two symmetry directions (Γ–M and Γ–K) in a two-dimensional hexagonal Brillouin zone. Solid curves are from mean-field calculations [D. J. Singh, Phys. Rev. B 61, 13397 (2000)]. Co t2g and O 2p indicate the origin of the bands is predominantly from these states.		The difference (green) between photoemission energy distribution curves (EDCs) taken at T = 18 K in resonance with the cobalt 3p → 3d transition (red) and out of resonance (blue) shows a resonance enhancement of the correlation satellite centered near 11 eV.

The Princeton/ALS group performed a detailed investigation of low-energy electronic structure and charge dynamics of the parent cobaltite compound Na_0.7CoO₂ at ALS Beamlines 7.0.1 and 12.0.1. This technique is sensitive to an electron’s quantum correlations because it directly probes the electron distribution function over a complete Brillouin zone (unit cell in momentum space) with good resolution.

An energy distribution curve (EDC) showing a quasielectron feature (blue curve) after background (red curve) subtraction locates the peak position for a particular momentum, giving one point on an energy–momentum dispersion curves.		Energy–momentum dispersion curves for the quasielectron after correction for the background along Γ–M (left) and Γ–K (right) directions. In both directions, the quasielectron bandwidth, confined to 100 meV, is extremely narrow. Red dots are extrapolations to the Brillouin zone boundary (dotted line).

ARPES spectra taken in the energy range of valence (loosely bound) electrons are in good agreement with the results of mean-field (first principles) calculations and map the dispersion (energy–momentum relationship) of several distinct bands. Most exciting, ARPES spectra also reveal an additional satellite feature centered at a much higher binding energy near 11 eV. Separation of this correlation satellite from the valence band gives an estimate of a strong on-site Coulomb repulsion (Hubbard U) between electrons of about 5 eV, which is in the same range as the Hubbard U for the cuprates and provides strong evidence for the highly correlated nature of electrons.

In addition, the researchers discovered a tiny feature (quasielectron) adjacent to the Fermi energy (zero binding energy) that was dwarfed by the presence of the valence band. This feature is essentially flat in momentum space with a dispersion less than 100 meV, which is a factor of 5 smaller than in cuprates and an order of magnitude below the mean-field (first principles) calculations. Accompanying the small bandwidth is a very weak nearest-neighbor single-particle hopping energy t of about 10 meV. That t is of the same order of magnitude as the spin exchange coupling J for this family implies that the charge dynamics are strongly perturbed by spin fluctuations.

Quasiparticle distribution in momentum space. The occupied area (blue) in the Brillouin zone (red line) forms a single large hexagonal pocket around the center of the Brillouin zone. The Fermi surface is the inner edge of the pocket.		Quasiparticles lose their coherence above 150K (well below room temperature) and a coherent-to-incoherent cross-over is observed. A combination of very small hopping and magnetic exchange energies causes the low coherence temperature

Temperature-dependent ARPES measurements revealed that quantum coherent quasielectrons exist only at low temperatures and linearly disappear as their motion becomes incoherent beyond 150 K. This value, which is well below the decoherence temperature of tens of thousands of degrees in conventional metals, is of the order of t and J, making geometrical frustration of antiferromagnetic interactions in triangular cobalt lattice planes the leading cause of quasielectron’s quantum decoherence.

Research conducted by M.Z. Hasan (Princeton University, Princeton Materials Institute, and ALS); Y.-D. Chuang (Princeton University and ALS); D. Qian, Y.W. Li, and Y. Kong (Princeton University); A. Kuprin, A.V. Fedorov, R. Kimmerling, E. Rotenberg, K. Rossnagel, Z. Hussain, and H. Koh, (ALS); and N.S. Rogado, M.L. Foo, and R.J. Cava (Princeton Materials Institute and Princeton University).

Research funding: National Science Foundation, Princeton University, and U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: M.Z. Hasan, Y.-D. Chuang, D. Qian, Y.W. Li, Y. Kong, A. Kuprin, A.V. Fedorov, R. Kimmerling, E. Rotenberg, K. Rossnagel, Z. Hussain, H. Koh, N.S. Rogado, M.L. Foo, and R.J. Cava, “Fermi surface and quasiparticle dynamics of Na_0.7CoO₂ investigated by angle-resolved photoemission spectroscopy,” Phys. Rev. Lett. 92, 246402 (2004).

ALSNews Vol. 252, April 27, 2005