The chalcopyrite CuFeS 2 is a natural magnetic semiconductor, where Fe spins order antiferromagnetically at T N ¼ 853 K. We investigate the thermoelectric properties of the carrier-doped chalcopyrite alloys, Cu 1Àx Fe 1þx S 2 with x ¼ 0:03 and 0.05, and Zn 0:03 Cu 0:97 FeS 2 . All systems showed characteristic behavior of n-type degenerate semiconductors. The large Seebeck coefficient with a high carrier-density indicates enhanced carrier mass of m à ¼ 3{6m 0 , where m 0 is the electron mass. Consequently, the thermoelectric power factor exceeds 1  10 À3 W K À2 m À1 at 400 K. We propose that utilizing magnetic semiconductor can be a new effective strategy to obtain enhanced values of the power factor.

We discuss the relation between the T2 coefficient of electrical resistivity A and the T-linear specific-heat coefficient gamma for heavy-fermion systems with general N, where N is the degeneracy of quasiparticles. A set of experimental data reveals that the Kadowaki-Woods relation, A/gamma2=1 x 10(-5) muOmega cm(K mol/mJ)2, collapses remarkably for large-N systems, although this relation has been regarded to be commonly applicable to the Fermi liquids. Instead, based on the Fermi-liquid theory we propose a new relation, A /gamma2=1 x 10(-5) with A =A/1/2N(N-1) and gamma =gamma/1/2N(N-1). This new relation exhibits an excellent agreement with the data for the whole range of degenerate heavy fermions.

Magnesium diboride (MgB2) single crystals, with a maximum size of 0.5×0.5×0.02 mm3, were grown by the vapor transport method in a sealed molybdenum crucible. A superconducting transition with the onset temperature of 38.6 K was confirmed by both transport and magnetization measurements. The upper critical field anisotropy ratio, Hc2∥ab(0)/Hc2∥c(0), was estimated to be 2.6 from the magnetic field-temperature phase diagram for MgB2 single crystals.

Current high-performance thermoelectric materials require elaborate doping and synthesis procedures, particularly in regard to the artificial structure, and the underlying thermoelectric mechanisms are still poorly understood. Here, we report that a natural chalcopyrite mineral, Cu1+x Fe1-x S2 , obtained from a deep-sea hydrothermal vent can directly generate thermoelectricity. The resistivity displayed an excellent semiconducting character, and a large thermoelectric power and high power factor were found in the low x region. Notably, electron-magnon scattering and a large effective mass was detected in this region, thus suggesting that the strong coupling of doped carriers and antiferromagnetic spins resulted in the natural enhancement of thermoelectric properties during mineralization reactions. The present findings demonstrate the feasibility of thermoelectric energy generation and electron/hole carrier modulation with natural materials that are abundant in the Earth's crust.

Thermopower, which generates electricity from heat, can be greatly enhanced in weak ferromagnets even above room temperature.

The T 2 coefficient of electrical resistivity, A, is compared with the electronic specific-heat coefficient, γ, for a number of Yb-based compounds. It is revealed that many systems including YbCuAl, YbInCu 4 , YbAl 3 and YbCu 5 show A/γ 2 values close to 0.4×10 −6 µΩcm(mol · K/mJ) 2 , which is remarkably smaller than that known as the Kadowaki-Woods relation, A/γ 2 = 1.0×10 −5 µΩcm(mol · K/mJ) 2 . Empirically, the compounds with the smaller A/γ 2 values appear to show weak intersite magnetic-correlation and/or to have almost fully-degenerated (J = 5/2 or 7/2) ground states.PACS numbers: 71.27.+a, 75.30.Mb Submitted to: J. Phys.: Condens. Matter ‡ To whom correspondence should be addressed (TSUJII.Naohito@nims.go.jp) γ ∝ m * and A ∝ (m * ) 2 , respectively. The ratio A/γ 2 therefore does not depend on m * . In fact, Kadowaki and Woods showed that many U-and Ce-based compounds show a universal relation, A/γ 2 = 1.0×10 −5 µΩcm (mol·K/mJ) 2 [2]. Furthermore, this relation has proved to be applicable to d-electron systems with large γ, for example, A15-type intermetallics like V 3 Si [3], nearly-ferromagnetic itinerant-electron systems like YCo 2 [4], and metallic oxides close to metal-insulator transition like V 2 O 3 under high pressure [3] and LiV 2 O 4 [5]. These observations demonstrate that the Fermi-liquid state is a general feature for a wide range of correlated-electron systems.Large γ has also been reported in a number of Yb-based intermetallics [6]. One can naturally expects that the Kadowaki-Woods relation, A/γ 2 = 1.0×10 −5 µΩcm (mol·K/mJ) 2 , also holds for these Yb compounds. In several Yb-based compounds, however, the A/γ 2 values have been reported to be remarkably smaller than that of the Kadowaki-Woods relation. For YbCu 4 Ag [7,8] and YbCu 5−x Ag x [9, 10], the A/γ 2 values are 0.1-0.6×10 −6 µΩcm(mol·K/mJ) 2 , nearly two orders of magnitude smaller than that for the Kadowaki-Woods relation. It is also reported that the A/γ 2 for YbCu 4.5 [11,12] and YbNi 2 Ge 2 [13] is close to this value. In addition, this value well coincides with the ratio for the transition metals such as Re, Os, Ni, and so on [3,14].This deviation of A/γ 2 value from the Kadowaki-Woods relation can pose quite an interesting subject to the Fermi-liquid theory, if it is a general phenomena for a class of materials. However, there have been no systematic studies on the A/γ 2 values for Ybbased compounds. We have therefore investigated the A/γ 2 relation in several Yb-based compounds. We have measured the electrical resistivity of the Yb-based compounds, YbAl 2 , YbInAu 2 , YbInCu 4 , and YbCuAl. These systems are typical examples of Ybbased intermediate-valent or heavy-fermion compounds. We also have examined the A/γ 2 values for several other compounds using the values reported in literature. ExperimentalPolycrystalline samples of YbAl 2 and YbCuAl have been prepared by argon arc melting and subsequent annealing. To compensate the loss of Yb due to evaporation, slightly additional amount of Yb was used in synthesis. Polycryst...

We demonstrate that Mn-doping to CuGaTe2 leads to a significantly enhanced ZT via the interaction between magnetic moments and carriers.

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