Phonon softening and metallization of a narrow-gap semiconductor by thermal disorder

Abstract

The vibrations of ions in solids at finite temperature depend on interatomic force-constants that result from electrostatic interactions between ions, and the response of the electron density to atomic displacements. At high temperatures, vibration amplitudes are substantial, and electronic states are affected, thus modifying the screening properties of the electron density. By combining inelastic neutron scattering measurements of Fe 1−x Co x Si as a function of temperature, and finite-temperature first-principles calculations including thermal disorder effects, we show that the coupling between phonons and electronic structure results in an anomalous temperature dependence of phonons. The strong concomitant renormalization of the electronic structure induces the semiconductor-to-metal transition that occurs with increasing temperature in FeSi. Our results show that for systems with rapidly changing electronic densities of states at the Fermi level, there are likely to be significant phonon-electron interactions, resulting in anomalous temperature-dependent properties.electron-phonon coupling | metal-insulator transition | thermoelectrics B ecause many properties of the solid state derive from the electronic structure (1), understanding finite temperature effects on the band structure is crucial to accurately describe materials in realistic operating conditions. The effects of thermal disorder on the electronic structure of materials at high temperature are largely unexplored, however, and the role of the electron-phonon interaction above room temperature has remained controversial (2-5). We performed detailed investigations of the phonons and electronic structure in Fe 1−x Co x Si and found that an adiabatic coupling can lead to pronounced anomalies in the temperature dependence of both phonons and electron states. The mechanism is general and could affect a broad class of materials, including narrow-gap semiconductors, superconductors, heavy-Fermion compounds, and thermoelectrics.FeSi has attracted a great deal of interest as it exhibits an insulator-metal transition with increasing temperature, and many of its physical properties show anomalous temperature dependences, including the magnetic susceptibility, heat capacity, Seebeck coefficient, thermal expansion, and elastic properties (6-11). Recently, it has been argued that doping FeSi with Al can lead to a surprising heavy-Fermion metal (12). FeSi has also attracted attention as a possible reaction product at Earth's core-mantle boundary (13-16), and as a candidate thermoelectric material for refrigeration applications (9). The compounds FeSi and CoSi are isostructural, crystallizing in the cubic B20 structure, with similar ion coordinates (17, 18). Although FeSi undergoes a gradual transition from narrow-gap semiconductor (E gap ∼ 0.1 eV) to metal with increasing temperature, the additional d electron in CoSi leads to a metallic state at all temperatures. The anomalous temperature dependences observed in FeSi are absent in CoSi.Here, we show that the adia...

Heavy-impurity resonance, hybridization, and phonon spectral functions inFe1−xMxSi (M=Ir, Delaire1, Al-Qasir2, May3 et al. 2015Phys. Rev. B412190View full textAdd to dashboardCite

“…The Si on-site diagonal force-constant in FeSi is 12.595 eV/Å 2 , close to the corresponding value in diamond-Si (13.445 eV/Å 2 ) 64 . Our results for phonon dispersion curves of pure FeSi are in good agreement with previous calculations 65 , as well as with our previously reported inelastic neutron scattering measurements 28 . …”

“…A similar effect was observed in the T dependence of phonons in FeSi, with T 300 K leading to metallization with a pronounced concomitant phonon softening 24,25,28 .…”

“…We have reported in a previous study that the phonon DOS of FeSi softens dramatically more than is expected from the quasiharmonic (QH) theory, while the isostructural compound CoSi (metallic at all temperatures) follows a typical QH softening 28 . This behavior was explained by the sensitivity of the electronic structure of FeSi to thermal disorder, as phonon excitations perturb the lattice sufficiently to cause the metallization of the system at T 200 K, in turn leading to strong phonon softening 24,28 .…”

“…These have been related to the very narrow gap in the electronic band structure 15,16,[18][19][20][21][22][23] . In addition, it has been shown that phonon excitations at finite temperature can close the narrow gap, accounting for some of the observed anomalous temperature dependences [24][25][26][27][28][29][30] , in particular the experimental observation of the gap closing [31][32][33] . Possibly strong electronic correlations have also be noted 34,35 , and it has been proposed that that FeSi could be an unusual d−electron Kondo insulator, although this point remains controversial 32,34,[36][37][38][39] .…”

“…This behavior was explained by the sensitivity of the electronic structure of FeSi to thermal disorder, as phonon excitations perturb the lattice sufficiently to cause the metallization of the system at T 200 K, in turn leading to strong phonon softening 24,28 . Electron-phonon coupling was also investigated in Refs.…”