An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca2Co2O5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 µV·K-1 at 100 K and increases with temperature up to 210 µV·K-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (Z T) of the Co-225 whiskers is estimated 1.2–2.7 at T≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.

An anomalous negative temperature dependence of electrical resistivity has been observed in ͑Fe 12x V x ͒ 3 Al alloys with V compositions up to x 0.35. In particular, the Heusler-type Fe 2 VAl compound is found to be on the verge of magnetic ordering and to exhibit a semiconductorlike behavior with the resistivity reaching 3000 mV cm at 2 K, in spite of the possession of a clear Fermi cutoff as revealed in photoemission valence-band spectra. A substantial mass enhancement deduced from specific heat measurements suggests that Fe 2 VAl is a possible candidate for a 3d heavy-fermion system. [S0031-9007(97)03978-1] PACS numbers: 72.15.Eb, 75.50.Bb, 79.60.Bm, 81.40.Rs Intermetallic compounds Fe 3 Si and Fe 3 Al are wellordered ferromagnets with a D0 3 crystal structure. Recently, Nishino et al. [1,2] found an anomalous temperature dependence of electrical resistivity in a series of the pseudobinary alloys in which Fe atoms are partly replaced by other 3d transition elements. These alloys possess common features characterized by (1) a resistance maximum near the Curie point T c in contrast to an angular change observed in ordinary ferromagnets, and (2) a negative resistivity slope at higher temperatures up to 1000 K and above. The substitution of 3d elements to the left of Fe in the periodic table seems to be responsible for the occurrence of the negative temperature dependence of the resistivity [1,2]. In other words, the elements with less than half-filled d states are more effective for the anomaly than those with more than half-filled ones. Since the substitution of Ti, V, Cr, and Mn always causes a sharp reduction in T c and in magnetization, the anomalous resistance behavior could be attributed to a weakening of ferromagnetism compelled by the substituents [1].The most spectacular feature of the resistance anomaly has been found for ͑Fe 12x V x ͒ 3 Al [2], as well as for. Among them, the ͑Fe 12x V x ͒ 3 Al system is of greater interest because a single phase of the D0 3 structure remains stable over a wide V composition range [2,4]. In this Letter, we demonstrate the occurrence of the negative temperature dependence of the electrical resistivity in ͑Fe 12x V x ͒ 3 Al with V compositions up to x 0.35. In particular, the Heuslertype Fe 2 VAl compound ͑x 0.33͒ exhibits a strong anomaly in a manner similar to a semiconductor. It may be worthwhile mentioning here that such a semiconductorlike negative resistivity slope can also be found for heavyfermion compounds [5], all of which are of f-electron systems. Furthermore, as will be discussed later, the present Fe 2 VAl compound is proved to be in a marginally magnetic state. In this regard, Fe 2 VAl shows an apparent similarity to a nonmagnetic narrow-gap semiconductor, FeSi, which has been classified by Fisk et al.[6] as a unique d-electron system among the family of "strongly correlated" or "Kondo" insulators. In addition to the resistivity measurements, the electronic structure of Fe 2 VAl has been investigated by means of high-resolution photoemission spectrosco...

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Introduction 1 1.1 What is the electron theory of metals? 1 1.2 Historical survey of the electron theory of metals 3 1.3 Outline of this book 8 2 Bonding styles and the free-electron model 10 2.1 Prologue 2.2 Concept of an energy band 2.3 Bonding styles 2.4 Motion of an electron in free space 2.5 Free electron under the periodic boundary condition 2.6 Free electron in a box 2.7 Construction of the Fermi sphere Exercises 3 Electrons in a metal at finite temperatures 29 3.1 Prologue 3.2 Fermi-Dirac distribution function (I) 3.3 Fermi-Dirac distribution function (II) 3.4 Electronic specific heat 3.5 Low-temperature specific heat measurement 3.6 Pauli paramagnetism 3.7 Thermionic emission Exercise 53 4 Periodic lattice, and lattice vibrations in crystals 54 4.1 Prologue 54 4.2 Periodic structure and reciprocal lattice vectors 54 4.3 Periodic lattice in real space and in reciprocal space 57 4.