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Title: Thermoelectrics in stronglycorrelated metals: Towards the nanoscale energy conversion in selforganiz


1
Thermoelectrics in strongly-correlated
metalsTowards the nano-scale energy conversion
in self-organized systems
  • Ichiro Terasaki
  • Department of Applied Physics, Waseda
    UniversityTokyo

2
Outline
  • 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

3
What 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

4
Thermoelectric 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

5
Strongly 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.

6
Intermediate between metal and insulator
We need large themopower like an insulator and
low resistivity like a metal
7
Layered cobalt oxide NaxCoO2
8
Thermoelectric 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
9
ZT of the layered Co oxides
10
The Boltzmann equation for electrons
Electric current density (particle flow)
Temperature gradient
Electric field( E)
Thermal current density (Heat flow)
11
Physical 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.
12
Origin 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
13
Layered 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.

14
CuRh1-yMgyO2
STEP
eSTEP
Shibasaki, Kobayashi, IT
15
Doping-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

16
Electronic Phase Separation
cond-mat/0011293
Phys. Rev. B61 (2000) 15515
17
Self-organization of carrier and spin
Bi-stripe order in Mn oxides
Stripe order in high-Tc Cu oxides
18
Towards 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
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