Title: Thermoelectrics in stronglycorrelated metals: Towards the nanoscale energy conversion in selforganiz
1Thermoelectrics in strongly-correlated
metalsTowards the nano-scale energy conversion
in self-organized systems
- Ichiro Terasaki
- Department of Applied Physics, Waseda
UniversityTokyo
2Outline
- Brief introduction to thermoelectrics
- Layered cobalt oxide NaxCoO2
- Large thermopower due to large entropy at lattice
sites - Layered rhodium oxide CuRhO2
- Self-organization of doped carriers
- Towards the nano-scale energy conversion
3What is Thermoelectrics?
- Thermoelectrics
- Conversion between heat
- and electricity via ther-moelectric phenomena
- Thermoelectric Devices
- long life,no maintenance
- no waste matter
- power from waste heat
- A key to Energy and Ecological issues
4Thermoelectric Material
- Thermoelectric figure of merit Z
- Z STEP2 / ?? ZT gt1 is a goal
- high thermo(electric)power STEP ? large
voltage - low resistivity ? ? low internal resistance
- low thermal conductivity ? ? large ?T
5Strongly correlated system
- A strongly correlated electron system is a system
in which each electron moves with the other
electrons in a correlated way owing to strong
electron-electron Coulomb repulsion. - Electrons are nearly localized, and show
intermediate properties between metal and
insulator. - Typical examples are conducting transition-metal
oxides.
6Intermediate between metal and insulator
We need large themopower like an insulator and
low resistivity like a metal
7Layered cobalt oxide NaxCoO2
8Thermoelectric properties of NaxCoO2
Resistivity In-plane 200 ??cm at 300 K
Out-of-plane 8 m?cm at 300 K Themopower
In-plane 100 ?V/K at 300 K (I. T. PRB56
(1997) R12685) Thermal conductivity(Data are
scattered from sample to sample) In-plane 40
mW/cmK at 300 K (Satake JAP 96 (2004) 931)
STEP
9ZT of the layered Co oxides
10The Boltzmann equation for electrons
Electric current density (particle flow)
Temperature gradient
Electric field( E)
Thermal current density (Heat flow)
11Physical meaning of thermopower
Entropy current density
Electric current density
Thermopower is the ratio of the entropy current
to the electric current, i.e. Entropy per
carrier.
12Origin of large thermopower
Degeneracy 6Entropy kBln6
Degeneracy 1Entropy 0
NaxCoO2 x0.5 Co3Co411
Charge of e flows with an entropy of kBln6
Koshibae et al.PRB 62(2000)6869
13Layered rhodium oxide CuRhO2
- Rh is located below Co in the periodic table
- CuRhO2 has the hexagonal RhO2 layer that is
isomorphic to the hexagonal CoO2 layer in NaxCoO2 - Kuriyama et al. found that the substitution of Mg
for Rh supplies carries.
14CuRh1-yMgyO2
STEP
eSTEP
Shibasaki, Kobayashi, IT
15Doping-independent thermopower
- The thermopower S is roughly written as
- If the thermopower is independent of carrier
concentration, then we get - This implies ??/?n0, and the compressibility of
the electron system diverges ? a sign for
phase separation
16Electronic Phase Separation
cond-mat/0011293
Phys. Rev. B61 (2000) 15515
17Self-organization of carrier and spin
Bi-stripe order in Mn oxides
Stripe order in high-Tc Cu oxides
18Towards nano-scale energy conversion
- Strongly correlated systems are at the verge of
electronic phase separation (nano-scale
self-organization of carriers) - This is a nature-made modulation doping
- The mobility of CuRh2-xMgxO2 is independent of Mg
content for xlt0.2 - Each Co4 (Rh4) cite includes a large entropy
kBlog6. - The large thermopower from Co4 should be in
principle effective at nano scale