Properties of the donor impurity band in mixed valence insulators

Abstract

In traditional semiconductors with large effective Bohr radius, an electron donor creates a hydrogen-like bound state just below the conduction band edge. The properties of the impurity band arising from such hydrogenic impurities have been studied extensively during the last 70 years. In this paper we consider whether a similar bound state and a similar impurity band can exist in mixed-valence insulators, where the gap arises at low temperature due to strong electron-electron interactions. We find that the structure of the hybridized conduction band leads to an unusual bound state that can be described using the physics of the one-dimensional hydrogen atom. The properties of the resulting impurity band are also modified in a number of ways relative to the traditional semiconductor case; most notably, the impurity band can hold a much larger concentration without inducing an insulator-to-metal transition. We estimate the critical doping associated with this transition, and then proceed to calculate the dc and ac conductivities and the specific heat. We discuss our results in light of recent measurements on the mixed-valence insulator SmB 6 , and find them to be consistent with the experiments.

Mapping out the emergence of topological features in the highly alloyed topological Kondo insulators Sm1−xMxB6(M=EuXu1, Kotta2, Song3 et al. 2021Phys. Rev. B2000View full textAdd to dashboardCite

“…The compound SmB 6 has been of great ongoing research interest as a Kondo insulator [1,2], a mixed-valent system [3][4][5], and as the prototypical example of a strongly correlated topological insulator [6][7][8][9][10]. A topologically nontrivial interpretation of the band structure is supported by angle resolved photoemission spectroscopy (ARPES) investigations [10][11][12][13][14][15], and is consistent with measurements of surface state spin texture [16].…”

“…4(c)]. A likely explanation is that rather than generating incoherent scattering, impurity doping results in strong coherent local screening as suggested in recent theoretical work [5,31], and should therefore be thought of as contributing to real self-energy rather than imaginary [32]. As such, the self-energy components would not be linearly additive.…”

Bulk transport paths through defects in floating zone and Al flux grown SmB6

“…These local moments in the lattice would be screened, and the amount of screening, and thus the magnetization, would oscillate in magnetic field [46]. Still another report focused on nonmagnetic impurities, which were found to form a deep impurity band as in a metal as well as an in-gap band [47], and another proposal revisited the idea of in-gap impurity states [48]. Historically, hydrogenic in-gap impurity states like those found in doped semiconductors were proposed in SmB 6 .…”

“…Instead, Ref. [48] showed that the hybrid band structure of SmB 6 has its own model of hydrogenlike in-gap impurity states. Interestingly, the density of defects required for an insulator-to-metal transition is orders of magnitude higher than the required density for an insulator-metal transition in parabolic semiconductors.…”

Intrinsic Bulk Quantum Oscillations in a Bulk Unconventional Insulator SmB6

“…Our surprising finding of quantum oscillations in the magnetic torque of floating zone-grown single crystals of SmB 6 ( Tan et al., 2015 ), despite the bulk insulating gap, has given rise to multiple proposals for their potential origin. Physical mechanisms proposed as potential explanations for the observed quantum oscillations include bulk in-gap low energy excitations ( Tan et al., 2015 ; Hartstein et al., 2018 ), two-dimensional surface conduction states ( Li et al., 2014 ), tunneling across the bulk insulating gap ( Knolle and Cooper, 2015 , 2017 ), and impurity inclusions ( Thomas et al., 2019 ; Fuhrman et al., 2018 ; Fuhrman and Nikolić, 2020 ), among others ( Baskaran, 2015 ; Knolle and Cooper, 2015 , 2017 ; Erten et al., 2016 , 2017 ; Chowdhury et al., 2018 ; Sodemann et al., 2018 ; Thomson and Sachdev, 2016 ; Skinner, 2019 ; Peters et al., 2019 ; Liu and Balents, 2017 ; Shen and Fu, 2018 ; Valentine et al., 2016 ; Harrison, 2018 ; Zhang et al., 2016 ; Pal, 2017 ; Riseborough and Fisk, 2017 ; Grubinskas and Fritz, 2018 ; Sakhya and Maiti, 2020 ; Pixley et al., 2018 ; Ram and Kumar, 2017 ; Anderson, 1992 ; Coleman et al., 1993 ; Motrunich, 2006 ; Grover et al., 2010 ; Paul et al., 2007 ; Kishigi and Hasegawa, 2014 ).…”