The specific heat of (Na,Ca)Co2O4 is measured at low-temperatures to determine the magnitude of the electronic specific-heat coefficient γ, in an attempt to gain an insight into the origin of the unusually large thermoelectric power of this compound. It is found that γ is as large as ∼48 mJ/molK 2 , which is an order of magnitude larger than γ of simple metals. This indicates that (Na,Ca)Co2O4 is a strongly-correlated electron system, where the strong correlation probably comes from the lowdimensionality and the frustrated spin structure. We discuss how the large thermopower and its dependence on Ca doping can be understood with the strong electron correlations.PACS numbers: 65.40.+g, 71.27.+a, 71.20.Be Recently, coexistence of a large thermopower (∼100 µV/K at 300 K) and a low resistivity was found in a transition-metal oxide NaCo 2 O 4 [1], which made this compound an attractive candidate for thermoelectric (TE) applications. Normally, large thermopower is associated with materials with low carrier densities and the thermoelectric properties are optimized for systems with typical carrier concentration of 10 19 cm −3 [2]; on the other hand, NaCo 2 O 4 has two-orders-of-magnitude larger carrier density (∼10 21 cm −3 ) and yet shows a thermopower comparable to that of the usual low-carrierdensity TE materials [1]. The origin of the large thermopower in NaCo 2 O 4 is yet to be understood.In NaCo 2 O 4 , Co ion has a mixed valence between 3+ and 4+. Since NaCo 2 O 4 is a layered system with a triangular lattice and Co 4+ has spins [1], it is expected that the interplay between charges and spins is playing a major role in producing the peculiar electronic properties of this compound, as in the case of high-T c cuprates. In those systems where Coulomb interactions or spin fluctuations are important, it is often found that the electrons become strongly correlated and thus the simple band picture is not well applicable. In fact, magnetotransport studies of NaCo 2 O 4 found that the Hall coefficient has an opposite sign to the thermopower and is strongly temperature dependent [3], which suggests the presence of a strong correlation in this system. Therefore, to elucidate the origin of the large thermopower in NaCo 2 O 4 , it would be illuminating to determine the strength of the electron correlations in NaCo 2 O 4 by measuring the electronic specific heat.

The resistivity and thermopower of Na1+xCo2O4 and Na1.1−xCaxCo2O4 are measured and analyzed. In Na1+xCo2O4, whereas the resistivity increases with x, the thermopower is nearly independent of x. This suggests that the excess Na is unlikely to supply carriers, and decreases effective conduction paths in the sample. In Na1.1−xCaxCo2O4, the resistivity and the thermopower increase with x, and the Ca 2+ substitution for Na + reduces the majority carriers in NaCo2O4. This means that they are holes, which is consistent with the positive sign of the thermopower. Strong correlation in this compound is evidenced by the peculiar temperature dependence of the resistivity.PACS numbers: 72.15. Jf, 72.80.Ga, 72.15.LhThere appears a growing interest to a hunt for new thermoelectric (TE) materials, 1 reflecting urgent needs for a new energy-conversion system in harmony with our environments. A TE material generates electric power in the presence of temperature gradient through the Seebeck effect, and pumps heat in the presence of electric current through the Peltier effect. A serious drawback is the low conversion efficiency: It is characterized by the so-called "figure of merit" Z = S 2 /ρκ, where S, ρ and κ are the thermopower, resistivity and thermal conductivity of a TE material, respectively. In other words, a good TE material is a material that shows large S, low ρ and low κ. However, a high value of Z is difficult to realize, because the three parameters cannot be changed independently. To overcome this difficulty, a number of new concepts and new materials have been examined.Recently we have observed that a layered cobalt oxide NaCo 2 O 4 exhibits unusually large S (100 µV/K at 300 K) accompanied by low ρ (200 µΩcm at 300 K) along the direction parallel to the CoO 2 plane. 2 NaCo 2 O 4 belongs to a layered Na bronze Na x CoO 2 , which was studied as a cathode for sodium batteries. 3 During the characterization, Molenda et al. 4 first found a large S in Na 0.7 CoO 2 . Although they noticed that S was anomalously large, they did not mention a possibility for a TE material. Their samples were polycrystals, the resistivity of which was 2-4 mΩcm at 300 K, much higher than that of our crystals. Our finding is that the carrier density (n) is of the order of 10 21 − 10 22 cm −3 , and is two orders of magnitude larger than n of conventional TE materials. This is difficult to understand in the framework of a conventional one-electron picture, and may indicate a way to get a good TE material other than the conventional approach. We have proposed that strong electron-electron correlation plays an important role in the enhancement of the thermopower of NaCo 2 O 4 .Even in a correlated system, we can expect that a conductor of low n will have a large S, because the diffusive part of S is the transport entropy, of the order of k B T /E F , where E F is the Fermi energy. 5 Thus it would be tempting to improve the TE properties in NaCo 2 O 4 by decreasing n. We easily think of three ways to change n in NaCo 2 O 4 , i.e., (i) doping of e...

The resistivity and thermopower of polycrystalline Na 1.1−x Ca x Co 2 O 4 were measured and analyzed. Both the quantities increase with x, suggesting that the carrier density is decreased by the substitutions of Ca 2+ for Na + . Considering that the temperature dependence of the resistivity show a characteristic change with x, the conduction mechanism is unlikely to come from a simple electronphonon scattering. As a reference for NaCo 2 O 4 , single crystals of a two-dimensional Co oxide Bi 2−x Pb x M 3 Co 2 O 9 (M =Sr and Ba) were studied. The Pb substitution decreases the resistivity, leaving the thermopower nearly intact.

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Non-contact measurement of torsion on a rotating shaft was done with fibre Bragg gratings and involved the use of graded index lenses for transmitting optical information between the light source and the gratings, and back to a spectrum analyser. Special Böhler steel with high yield strength was used for the shaft. For the experimental measurements, one end of the shaft was clamped in a lathe chuck, and the other end was fixed to a brake system, allowing the application of torque on the rotating shaft. Up to 97 N m of torque was applied to the shaft rotating at 190 rpm, and the results were very close to the expected values. Whereas torsion changes the differential-mode wavelength of the gratings in the proposed configuration, this wavelength difference is insensitive to temperature. For the particular system investigated experimentally, the differential wavelength change was 7.4 pm N −1 m −1 , and the corresponding temperature sensitivity was −0.547 pm • C −1 .