Fermi Surface and Quasiparticle Dynamics of ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ Investigated by Angle-Resolved Photoemission Spectroscopy

We present the first angle-resolved photoemission study of ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$, the host material of the superconducting ${\mathrm{Na}}_x{\mathrm{CoO}}_2·n{\mathrm{H}}_2\mathrm{O}$ series. Our results show a hole-type Fermi surface, a strongly renormalized quasiparticle band, a small Fermi velocity, and a large Hubbard $U$. The quasiparticle band crosses the Fermi level from $M$ toward $\Gamma$ suggesting a negative sign of effective single-particle hopping ${\mathit{t}}_{\mathrm{eff}}$ (about 10 meV) which is on the order of magnetic exchange coupling $\mathit{J}$ in this system. Quasiparticles are well defined only in the $T$-linear resistivity (non-Fermi-liquid) regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics may have implications for understanding the phase of matter realized in this new class of a strongly interacting quantum system.

Figure 1

Valence excitations and resonance behavior: (a) Full valence band spectrum of ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ taken at 90 eV photon energy. It shows five prominent features; the feature at 0.7 eV arises from Co $t_{2g}$ states. (b) Valence band near $\mathrm{Co}:3p\to 3d$ resonant excitation ($\sim 63\mathrm{eV}$, red curve) compared with off-resonant excitation ($\sim 60\mathrm{eV}$, blue curve). Resonance behavior of valence excitations shows the enhancement of the 11 eV feature (green curve), a correlation satellite. (c) Incident energy dependence of the 11 eV feature. It shows a Fano-type interference [12]

Figure 2

Quasiparticle dispersion: (a) $\Gamma \to M$ Fermi crossing. The color red reflects the highest intensity; yellow to green to blue is in the order of decreasing intensity. (b) EDCs corresponding to the image plot in (a). (c) A single EDC for $k=0.78{\AA }^{-1}$. To extract the peak position a background is subtracted. A constant steplike background is best seen in 2a. Based on the extracted peak positions $E$ vs $k$ plots are made and shown in (d) for $\Gamma \to M$ and in (e) for $\Gamma \to K$ directions.

Figure 3

The $n(k)$ plot and Fermi surface of ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$: (a) $n(k)$ plot generated by integrating within 75 meV of Fermi level. A hole pocket is centered around the $\Gamma$ point. The Fermi surface, exhibiting weak hexagonal anisotropy, is the inner edge of the pocket as shown over the complete Brillouin zone. (b) Comparison with LDA calculation on ${\mathrm{NaCo}}_2{\mathrm{O}}_4$ [13]. Red dots indicate the locations of measured Fermi crossings in ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$.

Figure 4

Temperature dependence: (a) In-plane resistivity in ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ is linear up to 100 K [7] and then gradually crosses over to a stronger $T$ dependence. (b) Temperature dependence of quasiparticles near the $\Gamma \to M$ Fermi crossing. The quasiparticle spectral weight ceases to exist above 120–140 K, close to the temperature where the $T$-linear behavior of the resistivity disappears. (c) Background subtracted integrated quasiparticle spectral weight is plotted as a function of temperature.