Unchanged thermopower enhancement at the semiconductor-metal transition in correlated FeSb2−xTex

Abstract

Substitution of Sb in FeSb 2 by less than 0.5% of Te induces a transition from a correlated semiconductor to an unconventional metal with large effective charge carrier mass m ‫ء‬ . Spanning the entire range of the semiconductor-metal crossover, we observed an almost constant enhancement of the measured thermopower compared to that estimated by the classical theory of electron diffusion. Using the latter for a quantitative description one has to employ an enhancement factor of 10-30. Our observations point to the importance of electron-electron correlations in the thermal transport of FeSb 2 , and suggest a route to design thermoelectric materials for cryogenic applications. © 2011 American Institute of Physics. ͓doi:10.1063/1.3556645͔Current material science has not yet found efficient thermoelectric ͑TE͒ materials for cryogenic ͑T Ͻ 100 K͒ applications such as spot cooling microelectronic superconducting devices. 1 In this context recent observations of a colossal thermopower S of -͑6-45͒ mV/K below 30 K in FeSb 2 have stimulated a great interest in the underlying physics and also practical applications. [2][3][4] Original interest in this compound stems from its narrow energy gap and strong electronic correlations as well as the thus induced unusual physical properties. 5-8 For a long time, systems with electron correlations have been expected to be candidates for TE applications. 1 Following this line are correlated semiconductors, e.g., FeSi, and correlated metals, such as CePd 3 , which do show promising TE properties at cryogenic range. 9 Though the figure of merit zT ͑=TS 2 / , with being the electrical resistivity and the thermal conductivity͒ is only ϳ0.01 at 50 K for FeSi, it can be enhanced to slightly below 0.1 at 100 K by 5% Ir doping. 9 CePd 3 , on the other hand, an intermediate-valence metal with low carrier density, exhibits a zT = 0.23 at around 200 K. 9 Our intensive investigations on FeSb 2 have already clarified the interplay between the enhanced thermopower and electron correlations, 2,3,10,11 nevertheless the microscopic mechanism of the enhancement is far from being understood. 4 Here, we report TE properties of FeSb 2−x Te x with a narrow doping range, 0 Ͻ x Ͻ 0.16, where a semiconductormetal ͑SM͒ transition occurs. Magnetism and electrical resistivity over a wide x range ͑0-1.2͒ have already been reported by Hu et al. 12 Our emphasis is aimed at learning to what extent the enhanced S in a correlated semiconductor can be kept when crossing a SM transition. FeSb 2 and FeTe 2 are isostructural ͑marcasite type͒ semiconductors having rather different energy gaps. The former shows a transport gap of E g = 26-36 meV together with an even smaller one of ϳ6 meV, 11 contrasting to a much larger E g of the latter, 0.2-0.5 eV. 13 The most striking finding of our study is that the enhancement to S͑T͒ ͑by a factor of 10-30͒ relative to the classical expectation, is rather robust against doping, spanning the entire range of the SM transition. This feature provides a convincing link between the enhan...

Strong electron correlations in FeSb2

“…A colossal thermopower, ranging between −6 and −45 mV/K at around 10 K, was observed in FeSb 2 [27]. Our semi-quantitative analyses have revealed an unconventional enhancement of the thermopower, most likely due to many-body effects [26,28,29]. In this paper, we review the typical magnetic, transport, and optical properties as well as specific heat of FeSb 2 and its slightly Te doped variants.…”

“…Experimentally, ferromagnetism was indeed observed by doping both FeSb 2 and FeSi: doping the latter compound by 25% Ge, one observes a first order phase transition from a paramagnetic semiconductor into a ferromagnetic metal [22]; doping Te [23] or Co [24] into FeSb 2 also results in ferromagnetic metallic states. Furthermore, like FeSi can be transformed into a heavy fermion metal by Al doping [25], FeSb 2 changes into a metallic state with strongly enhanced quasiparticle mass (10 − 20 times of m 0 ) by slight Te doping [26]. Note that Te substitution for Sb is non-isoelectronic and adds one electron / Te atom to the conduction band.…”

Highly dispersive electron relaxation and colossal thermoelectricity in the correlated semiconductor FeSb2

“…This may hint at a better quality of the samples used in this work. For example, a tiny amount of electron doping by Te (0.05%) into the Sb sites can shift the TEP minimum to 30 K, with its magnitude reduced by more than one order of magnitude [22]. In support of the peculiar thermal transport in FeSb 2 , recently, in the isostructural semiconductor CrSb 2 , a huge TEP minimum of −4.5 mV/K at 18 K and thermal conductivity comparable with that of FeSb 2 were reported by Sales et al [23].…”

Phonon drag effect in nanocomposite FeSb2

“…Several authors [5,[8][9][10][11] suggested a strong electron-electron correlation as a possible cause. Other authors [12,13], however, surmised that the origin of the colossal value of the Seebeck coefficient is not due to electron-electron correlations but to the phonon drag effect.…”