Fundamentals and Future Applications of NaxCoO2

W. J. Chang,1 J.-Y. Lin,2 C.-H. Hsu,3 J.-M. Chen,3 J.-M. Lee,3 Y. K. Kuo,4 H. L. Liu,5

and J. Y. Juang1, 5

1Department of Electrophysics, National Chiao-Tung University, Taiwan

2Institute of Physics, National Chiao Tung University , Taiwan3National Synchrotron Radiation Research Center (NSRRC), Taiwan

4Department of Physics, National Dong Hua University, Taiwan5Department of Electrophysics, National Chiao Tung University, Taiwan

Why quantum matter physics?

Conventional metalsConducting electrons are wave-like.Model: Fermi liquidProperties of materials change whenthe size is reduced to the nano scale.

Quantum matters (Strongly correlated electron systems)Electrons are particle-like.Model: not available yetProperties of materials remain whenthe size is reduced to the nano scale.

The Phase Diagram of NaxCoO2

Maw Lin Foo et al., Phys. Rev. Lett. 92, 247001 (2004).

Through soft-chemical modification, nonhydrated NaxCoO2 (0.5<x<0.9) was transformed to a parent layered oxide (0.3<x<0.9). These compounds had been widely researched, due to their large thermoelectric properties and rich phase diagram.

T-linear variation

5 Tesla

Nature 423, 425 (2003).

x= 1.36

Thermoelectric power generation

TE Technology, Inc. 1590 Keane Dr., Traverse City

Thermoelectricity, edited by Paul H. Egli

Refrigeration (Peltier effect)

Power Generation(Seebeck effect)

The differential Seebeck coefficient αab is defined by

T

VT

ab

0

limThe Peltier coefficient πab is given by

I

Qab

TI

QT

0lim

2

1

2

112 )(})(){( IdTIdTI abbaabab

abbaab

dT

d

2

1

2

10)( IdT

TTId baab

0 baabab

TdT

d

Tab

ab

The Thomson coefficient γ is defined by

From the conservation of energy

Differentiating, one finds that

The total change in entropy of the system due to the passage of unit charge under reversible conditions must be zero

By differentiation it is found that

Then

http://www.americool.com/moduleworking.pdf

The figure of merit Z

2SZ

Some TE materials

• Bi2Te3, Zn4Sb3, La0.9FeCoSb12, CsBi4Te6, Bi2Te3/Sb2Te3 superlattices etc.

(Terasaki et al., 1997)

Nature Materials 6, 129 (2007)

Nature Materials 5, 537 (2006)

MotivationNaxCoO2 has high thermoelectric power with low m

obility, low resistivity, and high carrier density, making this material suitable for themoelectric device applications.

The physical properties of single crystal and powder of NaxCoO2 had been widely studied but there have been few reports about the thin films, due to the high equilibrium vapor pressure of sodium.

1) Co3O4 (111) was grown on Al2O3 (0001) substrate by pulsed-laser deposition. Tsubstrate = 650~700 ºC, PO2 = 0.2 Torr, and thickness ~ 120 nm.

2) Co3O4(111) thin film was capped by Al2O3 substrate and muffled by sodium carbonate or Na0.75CoO2 powders.

3) Thermal annealing was operated at 700~800 ºC for 5~10 hours and cooled in air or oxygen flow with the rate < 10 ℃ /min..

4) After lateral diffusion of sodium, Co3O4 (111) thin films became NaxCoO2 (0001) epitaxial thin films with thickness ~250 nm.

Thin films preparation-Reactive Solid-Phase Epitaxy H. Ohta et al., Crystal Growth & Design (2005).W. J. Chang et al., Appl. Phys. Lett. (2007)

Growing NaxCoO2 films via Na Diffusion-Reactive Solid-Phase Epitaxy

Hiromochi Ohta et al., Crystal Growth & Design 5, 25 (2005).

Schematics of the encapsulation schemes for preparing NaxCoO2 thin films with x = 0.68 (specimen A) & 0.75 (specimen B).

1 mm

XRD θ-2θscans &Φ-scans of the (lĪ04) peaks

10 20 30 40 50 60 70 80

+

Na0.75CoO2

(d)

(c)

(b)

****

(000

8)

(000

6)

(000

4)

Inte

nsi

ty (

a. u

.)

2 (degree)

(000

2)

(444

)

(333

)

(222

)

(111

)

*

(a)

Hydrolyzed Na0.75CoO2

Co3O4

Na0.68CoO2

+

0 60 120 180 240 300 360

Na0.68CoO2

Na0.75CoO2

Sapphire

Inte

nsi

ty (

a. u

.)Phi (degree)

30o

(a)-(c) are the as grown samples. (d) was measured after exposing the Na0.

75CoO2 film. (c) at T = 25 ℃ and humidity 42% for 1 hour.

CharacterizationThin filmsNa0.68CoO2:

a = 2.8407(2) Å, c = 10.9328(8) Å Na0.75CoO2:

a = 2.843(1) Å, c = 10.877(3) Å

Sapphirea= 4.760 Å, c= 12.99 Å

The lattice mismatch is reduced down to ~3% with 30o rotation respected to sapphire

.

Maw Lin Foo et al., Phys. Rev. Lett. (2004).

SapphireNaxCoO2

00)1(1

ρab vs. T curves of NaxCoO2 thin films. Inset: the AFM image (5×5 μm2) of Na0.68CoO2 thin film was measured after thermal-diffusion process. The RMS roughness is about 1.67 nm.

M. L. Foo et al., PRL (2004).

Transport properties

The temperature dependence of the far-infrared conductivity of the Na0.68CoO2 thin film. The inset shows the temperature dependence of the Drude scattering rate 1/τ D.

Far-infrared conductivity

Y. Wang et al., Nature (2003).

x= 0.68

Thermoelectric Power vs. T

Fermi surface of Na0.5CoO2 in the kz = 0 (left) and kz = 0.5 (right) planes

(Singh, 2000)

Fermi surface from ARPES(Hasan et al., 2004)

W. B. Wu et al., Phys. Rev. Lett. 94, 146402 (2004).

NaxCoO2 Thin FilmsNa0.5CoO2 Single Crystal

O 1s XAS of NaxCoO2

528 530 532 534 536 538 5400.0

0.5

1.0

1.5

2.0

2.5

Na0.68CoO2

E//ab E//c

O1

s (

Mb

arn

s /

un

ite

ce

ll)

Photon Energy (eV)

One Fermi surface!

(Zhang et al., 2004)

What determines physics?

Crystal symmetry or Fermi symmetry?

The way it becomes superconducting

Crystal structures of the superconducting phase (right) and its parent phase (left).

Tc 5K

0 2 4 6 8 10-8

-6

-4

-2

0

2

4

6

8

10

0 1 2 3 4 5 6

-8

-6

-4

-2

0

s-wave, weak coupling s-wave, moderate coupling nodal lines

Na0.35

CoO2·1.3H

2O

(C(H

=0

)-C

n)/T

(m

J/m

ol K

2 )

T (K)

S (

mJ/

mol

K2 )

T (K)

Specific heat and other experiments suggest the nodal line existing in the order parameter. [Yang et al, 2005]

How to reconcile all experimental evidences?

• The existence of nodal lines from NMR, NQR, specific heat, and μSR.

• The spin singlet state observed by NMR.• The existence of s-wave pairing by impu

rity effects. coexistence of s-wave and unconventi

onal pairing in NaxCoO2·yH2O?

M. Mochizuki, Y. Yanase, M. Ogata, cond-mat/0407094

Summary

NaxCoO2 thin films with x = 0.68 and 0.75 were f

abricated, and achieved reproducibly by the present encapsulation schemes.

The superior qualities of NaxCoO2 thin films are

determined by the examination of XRD, ρab(T), a

nd far-infrared conductivity.

S(T) measurements show a large thermoelectric power, increasing with the Na concentration x.

More importantly

• Sailing to the unknown sea (of quantum matters) often bring us fortune, and sometimes very much unexpected fortune.