Fundamentals and Future Applications of Na x CoO 2

1. Fundamentals and Future Applications of NaxCoO2 W. J. Chang,1J.-Y. Lin,2C.-H. Hsu,3 J.-M. Chen,3 J.-M. Lee,3 Y. K. Kuo,4H. 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

2. 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 nanoscale. Conventional metals Conducting electrons are wave-like. Model: Fermi liquid Properties of materials change when the size is reduced to the nano scale.

3. 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).

4. x= 1.36 Nature 423, 425 (2003). 5 Tesla T-linear variation

5. Thermoelectric power generation Thermoelectricity, edited by Paul H. Egli Power Generation(Seebeck effect) TE Technology, Inc. 1590 Keane Dr., Traverse City Refrigeration (Peltier effect) The differential Seebeck coefficient αab is defined by The Peltier coefficient πab is given by

6. 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

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

8. The figure of merit Z

9. Some TE materials • Bi2Te3, Zn4Sb3, La0.9FeCoSb12, CsBi4Te6, Bi2Te3/Sb2Te3 superlattices etc. (Terasaki et al., 1997)

10. Nature Materials 6, 129 (2007)

11. Nature Materials 5, 537 (2006)

12. 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.

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

14. Growing NaxCoO2 films via Na Diffusion-Reactive Solid-Phase Epitaxy Hiromochi Ohta et al., Crystal Growth & Design 5, 25 (2005).

15. 1 mm Schematics of the encapsulation schemes for preparing NaxCoO2thin films withx = 0.68 (specimen A) & 0.75 (specimen B).

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

17. 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 . Maw Lin Foo et al., Phys. Rev. Lett. (2004). NaxCoO2 Sapphire

18. Transport properties M. L. Foo et al., PRL (2004). 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.

19. Far-infrared conductivity 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.

20. Thermoelectric Power vs.T x= 0.68 Y. Wang et al., Nature (2003).

22. Fermi surface of Na0.5CoO2 in the kz = 0 (left) and kz = 0.5 (right) planes(Singh, 2000)

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

24. O 1s XAS of NaxCoO2 Na0.5CoO2 Single Crystal NaxCoO2 Thin Films W. B. Wu et al., Phys. Rev. Lett. 94, 146402 (2004).

25. One Fermi surface! What determines physics? Crystal symmetry or Fermi symmetry? (Zhang et al., 2004)

26. The way it becomes superconducting Tc 5K Crystal structures of the superconducting phase (right) and its parent phase (left).

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

28. 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?

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

31. 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.

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