Transcript Document

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 , Taiwan
3National Synchrotron Radiation Research Center (NSRRC), Taiwan
4Department of Physics, National Dong Hua University, Taiwan
5Department of Electrophysics, National Chiao Tung University, Taiwan
Why quantum matter physics?
Quantum matters
(Strongly correlated electron systems)
Electrons are particle-like.
Model: not available yet
Properties of materials remain when
the size is reduced to the nano scale.
Conventional metals
Conducting electrons are wave-like.
Model: Fermi liquid
Properties of materials change when
the size is reduced to the nano scale.
The Phase Diagram of NaxCoO2
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.
Maw Lin Foo et al., Phys. Rev. Lett. 92, 247001 (2004).
x= 1.36
5 Tesla
Nature 423, 425 (2003).
T-linear variation
Thermoelectric power generation
Thermoelectricity, edited by Paul H. Egli
TE Technology, Inc. 1590 Keane Dr., Traverse City
Refrigeration
(Peltier effect)
The Peltier coefficient πab is given by
 ab
Q

I
Power Generation
(Seebeck effect)
The differential Seebeck coefficient αab
is defined by
V
T  0 T
 ab  lim
The Thomson coefficient γ is defined by
Q
T 0 IT
  lim
From the conservation of energy
2
2
{( ab ) 2  ( ab )1}I   ( a   b ) IdT   ab IdT
1
1
Differentiating, one finds that
d ab
  a   b   ab
dT
The total change in entropy of the system due to the passage of unit charge under
reversible conditions must be zero

2
1
Id (
 ab
T
)
2
1
a b
T
By differentiation it is found that
IdT  0
d ab  ab

a b  0
dT
T
Then
 ab 
 ab
T
http://www.americool.com/moduleworking.pdf
The figure of merit Z
Z
S
2

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)
Motivation
NaxCoO2 has high thermoelectric power with low
mobility, 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.
Thin films preparation
-Reactive Solid-Phase Epitaxy
H. Ohta et al., Crystal Growth & Design (2005).
W. J. Chang et al., Appl. Phys. Lett. (2007)
1)
2)
3)
4)
Co3O4 (111) was grown on Al2O3 (0001) substrate
by pulsed-laser deposition. Tsubstrate = 650~700 ºC,
PO2 = 0.2 Torr, and thickness ~ 120 nm.
Co3O4(111) thin film was capped by Al2O3 substrate
and muffled by sodium carbonate or Na0.75CoO2
powders.
Thermal annealing was operated at 700~800 ºC for
5~10 hours and cooled in air or oxygen flow with
the rate < 10 ℃/min..
After lateral diffusion of sodium, Co3O4 (111) thin
films became NaxCoO2 (0001) epitaxial thin films
with thickness ~250 nm.
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
Hydrolyzed Na0.75CoO2
*
*
*
*
*
Na0.75CoO2
Na0.68CoO2
(0006)
Na0.68CoO2
(0008)
(0004)
(b)
(0002)
30
0
60
Sapphire
o
120
180
240
300
360
Phi (degree)
+
10
20
30 40 50 60
2  (degree)
(444)
(222)
+
(333)
Co3O4
(a)
(111)
Intensity (a. u.)
(c)
Na0.75CoO2
Intensity (a. u.)
(d)
70
80
(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.
Characterization
Thin films
Na0.68CoO2:
a = 2.8407(2) Å, c = 10.9328(8) Å
Na0.75CoO2:
a = 2.843(1) Å, c = 10.877(3) Å
Sapphire
a= 4.760 Å, c= 12.99 Å
The lattice mismatch is
reduced down to ~3% with 30o
rotation respected to sapphire
(1 1 00) .
Maw Lin Foo et al., Phys. Rev. Lett. (2004).
NaxCoO2
Sapphire
Transport properties
1.2
 (m-cm)
1.0
0.8
x= 0.75
x= 0.68
0.6
0.4
0.2
0.0
M. L. Foo et al., PRL (2004).
0
50
100
150
200
Temperature (K)
250
300
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.
Far-infrared conductivity
7000
5000
4000
60
-1
300 K
100 K
20 K
1/ D (cm )
1() (-1cm-1)
80
x= 0.68
6000
40
20
3000
0
0
2000
100
200
300
Temperature (K)
1000
0
0
100
200
300
400
500
600
Frequency (cm-1)
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/τD.
Thermoelectric Power vs. T
100
S (V/K)
80
x = 0.75
60
x= 0.68
x = 0.68
40
20
0
0
50
100 150 200
Temperature (K)
250
300
Y. Wang et al., Nature (2003).
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)
O 1s XAS of NaxCoO2
Na0.5CoO2 Single Crystal
NaxCoO2 Thin Films
2.5
O1s (Mbarns / unite cell)
Na0.68CoO2
E//ab
E//c
2.0
1.5
1.0
0.5
0.0
528
530
532
534
536
Photon Energy (eV)
W. B. Wu et al., Phys. Rev. Lett. 94, 146402 (2004).
538
540
One Fermi surface!
What determines physics?
Crystal symmetry
or Fermi symmetry?
(Zhang et al., 2004)
The way it becomes
superconducting
Tc 5K
Crystal structures of the superconducting phase (right) and its parent phase (left).
Specific heat and other experiments suggest
the nodal line existing in the order parameter.
[Yang et al, 2005]
10
0
-2
2
S (mJ/mol K )
2
(C(H=0)-Cn)/T (mJ/mol K )
8
6
4
-4
-6
-8
0
2
1
2
3
4
5
6
T (K)
0
Na0.35CoO2·1.3H2O
-2
-4
s-wave, weak coupling
s-wave, moderate coupling
nodal lines
-6
-8
0
2
4
6
T (K)
8
10
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
impurity effects.
•
coexistence
of
s-wave
and
unconventional 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
fabricated, and achieved reproducibly by the
present encapsulation schemes.
The superior qualities of NaxCoO2 thin films are
determined by the examination of XRD, ρab(T),
and 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.