BL7U@UVSOR, U-NIM@ Synchrotron radiation center, and Shin-Lab.@ISSPRole of coherent part and incoherent part in the electron transport
properties of the materials characterized by strong electron correlation
Tsunehiro Takeuchi
EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan

1

Recent progress in angle resolved photoemission
spectroscopy (ARPES) allows us to gain deep insight
into the momentum dependent electronic structure ofthe materials. The self-energy of quasiparticles can bealso investigated by ARPES as a function of energy
and momentum, in which information about thevariety of many body effects including the electron
correlations is involved. The real part of theself-energy indicates the energy shift from that of thebare electrons free from the many body effects, while
the imaginary part represents the lifetime of thequasiparticles. A large number of groups including us
have employed this highly sophisticated experimental
technique to investigate the electronic structure andthe many body effects of materials, such as thehigh-Tc superconductors.
We realized by using ARPES measurements thatthe electron transport properties could bequantitatively analyzed, because all factors, thenumber of electrons at a given energy, momentum
dependent group velocity, and momentum dependent
relaxation time, are obtainable from the ARPES
measurements, provided that the lifetime of thequasiparticles is considered as the relaxation time ofconducting wave packets. For the high-Tc
superconductors, in which almost of all quasiparticles
behave as a coherent wave, this method worked welland the electrical resistivity, thermoelectric power,and Hall coefficient were quantitatively reproducedby calculation using the information obtained fromthe ARPES measurement. [1]The layered cobalt oxide NaxCoO2 is widely
known to possess variety of unusual properties, suchas superconductivity, the large magnitude ofthermoelectric power, the Curie-Weiss magnetic
susceptibility behavior coexisting together with themetallic electrical conduction, the charge density
wave oriented insulating phase, and the spin density
wave behaviors. Those characteristics are presumablycaused by the strong electron correlation. Recently,
we have intensively investigated the effect of strong
electron correlation upon the electronic structure andthe electron transport properties of NaxCoO2 by using
high-resolution ARPES measurements, and found thatthe incoherent part as well as the coherent part,strongly affects the electron transport properties.
The electronic structure of layered cobalt oxides,
including the present material NaxCoO2, ischaracterized by the simultaneous possession ofcoherent part and incoherent part shown as Fig.1.
Generally speaking, the incoherent part is caused by
the strong electron correlation, and this strong

electron correlation drastically reduces theenergy-width of the “coherent band”. Indeed, theobserved energy-width of the “coherent band” isreduced to 1/3 of that of the band calculated by theFLAPW-LDA method. Note here that thesecharacteristics in electronic structure cannot be
predicted by the first principle calculation using themean field theory. The use of the high-resolution
ARPES, therefore, is of great importance in order toreveal the electronic structure of materials under thestrong many body effects.

Fig. 1.
An energy distribution curve of theNa0.7CoO2 at a Fermi wave vector. The spectrum isdefinitely characterized by the coherent andincoherent parts.
The temperature dependence of the thermoelectric
power and the electrical resistivity was wellaccounted for with the Boltzmann transport theory
provided that the contributions of coherent andincoherent part are properly taken into account. Bothproperties are dominantly determined by the coherent
part at low temperature below 150 K and by theincoherent part at high temperature above 300K,
respectively. The measured and calculated
thermoelectric power of Na0.7CoO2 is shown in Fig.2
as one of the typical examples.

Fig. 2. Measured (markers) and calculated (lines)
thermoelectric power of Na0.7CoO2.
[1] T. Kondo et al., Phys. Rev. B 72 (2005) 024533.,
T. Kondo et al., Phys. Rev. B 74 (2006) 225411. , T.
Kondo et al., J. Elec. Spec. Relat. Phenom. 144-147(2005) 1249-1252., T. Takeuchi et al., J. Elec. Spec.
Relat. Phenom. 156-158 (2007) 452-456., H. Komoto
and T. Takeuchi, J. Elec. Mater. 38 (2009) 1365-1370.