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Register a new userAbsence of localized-spin magnetism in the narrow-gap semiconductor FeSb2
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We report inelastic neutron scattering measurements aimed at investigating the origin of the temperature-induced paramagnetism in the narrow-gap semiconductor FeSb2. We find that inelastic response for energies up to 60 meV and at temperatures 4.2 K, 300 K and 550 K is essentially consistent with the scattering by lattice phonon excitations. We observe no evidence for a well-defined magnetic peak corresponding to the excitation from the non-magnetic S = 0 singlet ground state to a state of magnetic multiplet in the localized spin picture. Our data establish the quantitative limit of S_{eff}^2 < 0.25 on the fluctuating local spin. However, a broad magnetic scattering continuum in the 15 meV to 35 meV energy range is not ruled out by our data. Our findings make description in terms of the localized Fe spins unlikely and suggest that paramagnetic susceptibility of itinerant electrons is at the origin of the temperature-induced magnetism in FeSb2.
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A study of the anisotropy in magnetic, transport and magnetotransport properties of FeSb2 has been made on large single crystals grown from Sb flux. Magnetic susceptibility of FeSb2 shows diamagnetic to paramagnetic crossover around 100K. Electrical transport along two axes is semiconducting whereas the third axis exhibits a metal - semiconductor crossover at temperature Tmin which is sensitive to current alignment and ranges between 40 and 80K. In H=70kOe semiconducting transport is restored for T<300K, resulting in large magnetoresistance [rho(70kOe)-rho(0)]/rho(0)=2200% in the crossover temperature range
We present a study of the magnetoresistance and Hall effect in the narrow-gap semiconductor FeSb2 at low temperatures. Both the electrical and Hall resistivities show unusual magnetic field dependence in the low-temperature range where a large Seebeck coefficient was observed. By applying a two-carrier model, we find that the carrier concentration decreases from 1 down to 10^-4 ppm/unit cell and the mobility increases from 2000 to 28000 cm2/Vs with decreasing temperature from 30 down to 4 K. At lower temperatures, the magnetoresistive behavior drastically changes and a negative magnetoresistance is observed at 3 K. These low-temperature behaviors are reminiscent of the low-temperature magnetotransport observed in doped semiconductors such as As-doped Ge, which is well described by a weak-localization picture. We argue a detailed electronic structure in FeSb2 inferred from our observations.
Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have received a lot of attention because they exhibit a large thermopower, as well as striking similarities to heavy fermion Kondo insulators. Many proposals have been advanced, however, lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we employ realistic many-body calculations to elucidate the impact of electronic correlation effects on FeSi. Our methodology accounts for all substantial anomalies observed in FeSi: the metallization, the lack of conservation of spectral weight in optical spectroscopy, and the Curie susceptibility. In particular we find a very good agreement for the anomalous thermoelectric power. Validated by this congruence with experiment, we further discuss a new physical picture of the microscopic nature of the insulator-to-metal crossover. Indeed, we find the suppression of the Seebeck coefficient to be driven by correlation induced incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic homologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will exhibit an increased thermopower at intermediate temperatures.
To elucidate an origin of the two energy gaps in the narrow-gap semiconductor FeSb2, we have investigated the effects of hydrostatic pressure on the resistivity, Hall resistance and magnetoresistance at low temperatures. The larger energy gap evaluated from the temperature dependence of resistivity above 100 K is enhanced from 30 to 40 meV with pressure from 0 to 1.8 GPa, as generally observed in conventional semiconductors. In the low-temperature range where a large Seebeck coefficient was observed, we evaluate the smaller energy gap from the magnetotransport tensor using a two-carrier model and find that the smaller gap exhibits a weak pressure dependence in contrast to that of the larger gap. To explain the pressure variations of the energy gaps, we propose a simple model that the smaller gap is a gap from the impurity level to the conduction band and the larger one is a gap between the valence and conduction bands, suggesting that the observed large Seebeck coefficient is not relevant to electron correlation effects.
The thermal expansion and heat capacity of FeSb2 at ambient pressure agrees with a picture of a temperature induced spin state transition within the Fe t_{2g} multiplet. However, high pressure powder diffraction data show no sign of a structural phase transition up to 7GPa. A bulk modulus B=84(3)GPa has been extracted and the temperature dependence of the Gruneisen parameter has been determined. We discuss here the relevance of a Kondo insulator description for this material.
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