Fundamentals and Future Applications of Na x CoO 2 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. Juang 1, 5 1 Department of Electrophysics, National Chiao-Tung University, Taiwan 2 Institute of Physics, National Chiao Tung University, Taiwan 3 National Synchrotron Radiation Research Center (NSRRC), Taiwan 4 Department of Physics, National Dong Hua University, Taiwan 5 Department of Electrophysics, National Chiao Tung University, Taiwan

Why quantum matter physics? Conventional metals Conducting electrons are wave-like. Model: Fermi liquid Properties of materials change when the size is reduced to the nano scale. 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.

The Phase Diagram of Na x CoO 2 Maw Lin Foo et al., Phys. Rev. Lett. 92, (2004). Through soft-chemical modification, nonhydrated Na x CoO 2 (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 Keane Dr., Traverse City Thermoelectricity, edited by Paul H. Egli Refrigeration (Peltier effect) Power Generation (Seebeck effect) The differential Seebeck coefficient α ab is defined by The Peltier coefficient π ab is given by

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

The figure of merit Z

Some TE materials Bi 2 Te 3, Zn 4 Sb 3, La 0.9 FeCoSb 12, CsBi 4 Te 6, Bi 2 Te 3 /Sb 2 Te 3 superlattices etc. (Terasaki et al., 1997)

Nature Materials 6, 129 (2007)

Nature Materials 5, 537 (2006)

Motivation  Na x CoO 2 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 Na x CoO 2 had been widely studied but there have been few reports about the thin films, due to the high equilibrium vapor pressure of sodium.

1)Co 3 O 4 (111) was grown on Al 2 O 3 (0001) substrate by pulsed-laser deposition. T substrate = 650~700 ºC, P O2 = 0.2 Torr, and thickness ~ 120 nm. 2)Co 3 O 4 (111) thin film was capped by Al 2 O 3 substrate and muffled by sodium carbonate or Na 0.75 CoO 2 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, Co 3 O 4 (111) thin films became Na x CoO 2 (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 Na x CoO 2 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 Na x CoO 2 thin films with x = 0.68 (specimen A) & 0.75 (specimen B). 1 mm

XRD θ-2θscans & Φ-scans of the (l Ī 04) peaks (a)-(c) are the as grown samples. (d) was measured after exposing the Na 0.75 CoO 2 film. (c) at T = 25 ℃ and humidity 42% for 1 hour.

Characterization Thin films  Na 0.68 CoO 2 : a = (2) Å, c = (8) Å  Na 0.75 CoO 2 : a = 2.843(1) Å, c = (3) Å Sapphire  a= Å, c= Å The lattice mismatch is reduced down to ~3% with 30 o rotation respected to sapphire. Maw Lin Foo et al., Phys. Rev. Lett. (2004). SapphireNa x CoO 2

ρ ab vs. T curves of Na x CoO 2 thin films. Inset: the AFM image (5×5 μm 2 ) of Na 0.68 CoO 2 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 Na 0.68 CoO 2 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, (2004). Na x CoO 2 Thin Films Na 0.5 CoO 2 Single Crystal O 1s XAS of Na x CoO 2

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). T c  5K

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 impurity effects.  coexistence of s-wave and unconventional pairing in Na x CoO 2 ·yH 2 O?

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

Summary Na x CoO 2 thin films with x = 0.68 and 0.75 were fabricated, and achieved reproducibly by the present encapsulation schemes. The superior qualities of Na x CoO 2 thin films are determined by the examination of XRD, ρ ab (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.