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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Recent transport, specific heat and magnetization measurements indicated that the Heusler-type compound is a candidate for a 3d heavy-fermion system. As a first step towards a detailed theoretical understanding of the observed anomalous electronic properties of this compound, we have performed first-principles electronic structure and total energy calculations for and . The calculated lattice constants and magnetic moments are in good agreement with experiments. Remarkably, we find that, unlike , Heusler-type is a nonmagnetic semimetal with a narrow pseudogap at the Fermi level. Furthermore, our calculations suggest that the observed large enhancement of electronic specific heat coefficient is largely caused by the mechanisms such as spin-fluctuations rather than electron - phonon coupling in . The relations between the existence of the Fermi surface and negative temperature dependence of the low-temperature electrical resistivity in are discussed and further experimental and theoretical work is suggested.

We report on the effect of off-stoichiometry on the temperature dependence of electrical resistivity, the Seebeck coefficient, and the Hall coefficient in the Heusler-type Fe 2 VAl compound. While the stoichiometric Fe 2 VAl exhibits a semiconductorlike resistivity behavior, a small deviation of the Al content from stoichiometry causes a significant decrease in the low-temperature resistivity and a large enhancement in the Seebeck coefficient. Substantial enhancements for the Seebeck coefficient are in reasonable accord with changes in the Hall coefficient and can be explained on the basis of the electronic structure, where the Fermi level shifts slightly from the center of a pseudogap due to off stoichiometry.

Optical conductivity data of the intermetallic compounds (Fe1-xVx)3Al ( 0