Infrared Spectroscopy of the Charge Ordering Transition ofNa0.5CoO2

Abstract

We report infrared reflectivity study of charge ordering in a Na0.5CoO2 single crystal. In comparison with x=0.7 and 0.85 compounds, we found that the effective carrier density increases systematically with decreasing Na contents. The charge ordering transition only affects the optical spectra below 1000 cm(-1). A hump near 800 cm(-1) develops below 100 K, which is accompanied by the appearance of new lattice modes as well as the strong antiresonance feature of phonon spectra. These observations signify a polaronic characteristic of charge carriers. Below T(co), an optical gap develops at the magnitude of 2Delta approximately 3.5k(B)T(co) (T<<T(co)), evidencing an insulating charge density wave ground state.

Structural phase transitions and sodium ordering in Na0.5CoO2: a combined electron diffraction and Raman spectroscopy study

“…For instance, infrared spectroscopy measurements on Na 0.5 CoO 2 crystals show dramatic changes in the phonon modes above 83 K [15]; dielectric investigations on Na 0.85 CoO 2 demonstrate significant phonon modes changes between 280 and 260 K [14] and the 23 Na-NMR spectra study show only small changes of the Larmor frequencies of Na nuclei in the temperature range of 240 -260 K [16]. These observations have been partially interpreted as the evidence of the motion changes of Na ions in the low temperature range.…”

“…Numerous unusual experimental phenomena in Na x CoO 2 materials have been discussed in connection with the sodium ordering in the recent literature [14][15][16]. For instance, infrared spectroscopy measurements on Na 0.5 CoO 2 crystals show dramatic changes in the phonon modes above 83 K [15]; dielectric investigations on Na 0.85 CoO 2 demonstrate significant phonon modes changes between 280 and 260 K [14] and the 23 Na-NMR spectra study show only small changes of the Larmor frequencies of Na nuclei in the temperature range of 240 -260 K [16].…”

Ionic Hubbard model on a triangular lattice forNa0.5CoO2,Rb0.5Powell1, Merino2, McKenzie3 2009Phys. Rev. B9020View full textAdd to dashboardCite

“…A gap begins to open up over the Fermi surface below about 60 K and has a magnitude of about 6-11 meV at 10 K. 6 Measurements of the frequency dependence of the conductivity show no evidence of a Drude peak consistent with the bad metal behavior seen in the static transport quantities. 2,5 Below about 100 K there is a loss of spectral weight at frequencies below about 100 cm −1 consistent with the opening of a charge gap. This leads to a peak in ͑ ͒ at Ӎ 200 cm −1 .…”

“…This leads to a peak in ͑ ͒ at Ӎ 200 cm −1 . 5 In comparison for Na x CoO 2 , with x away from 0.5, a Drude peak is present at low temperatures and the total spectral weight of the optical conductivity scales with 1 − x. 5 …”

“…3 The phase diagram, with doping x, shows many interesting phases, 1,2,4 including a "Curie-Weiss metal," A-type antiferromagnetism ͑ferromagnetic layers stacked antiferromagnetically͒ and an "insulating" phase seen only at x = 0.5. The latter phase is particularly puzzling as many probes ͓including resistivity, 4 optical conductivity, 5 and angle-resolved photoemission spectroscopy ͑ARPES͒ 6 ͔ suggest that it is insulating, however, Shubnikov-de Haas oscillations are also observed 7 suggesting the state is metallic. The aims of this paper are to present a simple variational theory of A 0.5 CoO 2 , where A = Na, Rb, or K, and to attempt to reconcile these seemingly contradictory experiments.…”