Specific-heat evidence for strong electron correlations in the thermoelectric material(Na,Ca)Co2

Abstract

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.

High-performance nanostructured thermoelectric materials

“…NaCo 2 O 4 has high electrical conductivity and a high Seebeck coeffi cient, resulting in a remarkably high power factor of 5,000 W m -1 K -2 , even higher than that of state-of-the-art Bi 2 Te 3 (4,000 W m -1 K -2 ) [76]. Th e strong electronic correlation eff ect in this material has been attributed to the unusually large Seebeck coeffi cient [77]. Many researchers subsequently started to explore high-performance oxides for TE applications, and some promising TE oxides have emerged, including Zn 1-x Al x O (n-type, ZT ≈ 0.30 at 1273 K) [78], La 1-x Sr x CoO 3 (p-type, ZT ≈ 0.18 at room temperature) [79], and La 1-x Y x CuO 4 (p-type, ZT ≈ 0.17 at 330 K) [80].…”

Structure and correlation effects in semiconductingSrTiO3

“…In addition, the thermoelectric properties have been previously associated with strong correlation. 34 The proposed polaron effects ͑Fig. 3͒ suggest that the electron-phonon coupling might play an important role for the thermoelectricity in doped STO.…”

Structural phase transitions and sodium ordering in Na0.5CoO2: a combined electron diffraction and Raman spectroscopy study

“…Structural data obtained between 200 K and 410 K, especially close to room temperature, reveal a variety of interesting structure phenomena, such as phase separation 7 and complex ordered states corresponding to different Na-ordered states [12]. It is likely that Na 0.5 CoO 2 has an intermediate (or metastable) structure within this large temperature range.…”

“…Layered deintercalatable alkali metal oxides, such as Li x CoO 2 and Na x CoO 2 , have been a subject of an intense research activity in the past years owing to their potential technological applications as a battery electrodes and thermoelectric materials [1][2][3][4][5][6][7][8][9][10][11]. The notable features of these materials, from both structural and chemical point of views, are that the cation content and crystal structure can vary over a wide range by deintercalation, and that the structural and physical properties are profoundly affected by the cation concentration and cation and vacancy ordering [4][5]11].…”