Metal-Insulator Transitions in the Kondo Insulator FeSi and Classic Semiconductors Are Similar

Abstract

We have observed the metal-insulator transition in the strongly correlated insulator FeSi with the chemical substitution of Al at the Si site. The magnetic susceptibility, heat capacity, and field-dependent conductivity are measured for Al concentrations ranging from 0 to 0.08. For concentrations $0.01 we find metallic properties quantitatively similar to those measured in Si:P with the exception of a greatly enhanced quasiparticle mass. Below 2 K the temperature and field-dependent conductivity can be completely described by the theory of disordered Fermi liquids. [S0031-9007(97)02878-0] PACS numbers: 75.30.Mb, 71.30. + h, Lightly doped semiconductors and insulators lie at the heart of both microelectronic technologies and modern condensed matter physics. The most prevalent and best characterized of these systems continues to be carrier doped Si, a simple band insulator. The electronic and magnetic properties of carrier doped Si near the metal-insulator (MI) transition have been the subject of a large and growing literature where it is well established that these properties are determined by the disorder and electron-electron (e-e) interactions [1]. Investigations of the MI transition where the Coulomb interaction is more significant, such as the Mott-Hubbard systems V 22x O 3 , La 22x Sr x CuO 4 , and Ni͑S, Se͒ 2 [2] have revealed interesting magnetic and superconducting ground states. A key feature separating Si from the Mott-Hubbard insulators is that whereas Si is an insulator by virtue of its band structure, the latter are band metals which are insulating because of Coulomb interactions. Recently, another class of insulators has emerged, namely, insulators correctly predicted by band theory to be insulating [3] yet which also display optical and magnetic properties not understandable using band theory [4]. The extent to which such insulators, frequently labeled as "strongly correlated" or "Kondo" insulators, are fundamentally different from conventional insulators and semiconductors is unclear [5]. In the present Letter, we present strong evidence that one popular strongly correlated insulator, FeSi, is actually a renormalized version of Si in the same sense that the heavy fermion metals are Fermi liquids with vastly renormalized band masses. More specifically, we show that chemical doping by Al substitution onto the Si sites yields a metal which is very similar to Si doped just beyond its metal-insulator transition. The only obvious distinction is that in the metal derived from FeSi, the quasiparticle mass is greatly enhanced relative to that in doped Si. Thus we have carrier doped a strongly correlated insulator through the MI transition to obtain a disordered Fermi liquid ground state with a large carrier mass-a heavy fermion metal. Our data represent convincing support for the proposition that while heavy fermion metals and strongly correlated insulators have peculiar temperaturedependent properties, their ground states remain the well understood Fermi liquids and band insulators.The samples were either...

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

“…For example, the magnetic susceptibility, heat capacity, thermal expansion, and elastic properties all show anomalous temperature dependences between 0 K and 800 K [15][16][17] . 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] .…”

Effects of temperature and pressure on phonons in FeSi1−xAlx

“…The Al-doped phase FeSi 1−x Al x also forms the B20 structure, for concentrations x Al 0.2. Doping with Al leads to a fast increase in electrical conductivity, and the Al-doped FeSi has been reported to be an unusual heavy-fermion metal 6,10 . It is therefore interesting to compare the effects of hole and electron doping on the phonons.…”

“…FeSi has generated great interest as a possible d−electron Kondo insulator [3][4][5][6] , as well as a possible product from the reaction between molten iron and mantle silicates at the core-mantle boundary 2,7-9 . Many physical properties of FeSi display anomalous temperature dependences, which has been related to its peculiar electronic band structure [10][11][12][13][14][15][16]18 . For example, the magnetic susceptibility, heat capacity, thermal expansion, and elastic properties all show anomalous temperature dependences between 0 K and 800 K [15][16][17] .…”

“…Photoemission measurements on FeSi have been initially interpreted in terms of strong electronic correlations and a possible Kondo scenario 3,6,10 . However, recent angle-resolved photoemission measurements indicate that the electronic structure can be described in terms of an itinerant model, and show no Kondo peak 4,5,26 .…”

Large anomalous Hall effect in a silicon-based magnetic semiconductor

“…This means that by examining the transport and magnetic behavior across the Fe 1−x,y Mn x Co y Si series, we can study the continuous evolution from a classic weak itinerant magnet, to a metallic paramagnet, to a Kondo (or strongly correlated) insulator, and finally a fully polarized itinerant magnetic metal 7,9,12 all without a change in crystal structure (see Fig. 1).…”