Infrared conductivity of metallic (III,Mn)V ferromagnets

Abstract

We present a theory of the infrared conductivity and absorption coefficients of metallic (III,Mn)V ferromagnetic semiconductors. We find that the conductivity is dominated by inter-valence-band transitions that produce peaks athω ∼ 220meV and obscure the broadened Drude peak. We demonstrate that transverse f-sum rule measurements can be used to extract accurate values for the free carrier density, bypassing the severe characterization difficulties that have till now been created by the large anomalous Hall effect in these materials.Following the discovery of carrier-induced ferromagnetism in (In,Mn)As [1] and (Ga,Mn)As [2] in the early nineties, studies of ferromagnetism in (III,Mn)V diluted magnetic semiconductors (DMS's) have yielded many surprises. The transport and optical properties of these heavily-doped semiconductors are richer than those of conventional itinerant electron ferromagnets because of strong valence-band spin-orbit coupling, and because of the sensitivity of their magnetic state to growth conditions, doping, and external fields. These novel ferromagnets are likely to have a major technological impact [3] if systems with Curie temperatures above room temperature can be created and better control of disorder effects can be achieved. We present a theory of the infrared conductivity of (III,Mn)V ferromagnets which we believe will increase the value of this powerful probe in characterizing present and future DMS ferromagnets. In particular, we demonstrate that infrared conductivity measurements in metallic samples can be used to obtain accurate measurements of the total free carrier density, likely the key to understanding the now firmly established [4] dependence of magnetic properties on growth and post-growth annealing conditions. This experimental possibility is especially important in these semiconductors because they are ferromagnetic and have a large anomalous Hall conductivity [5] which severely complicates Hall-effect carrier-density measurements, [2,6] even when they are performed in very strong magnetic fields.Our conclusions are based on a semi-phenomenological model [7][8][9][10] in which host semiconductor valence band electrons, described by a k.p model, interact with randomly distributed aligned (T = 0) Mn local-moment acceptors via Coulomb and short-range exchange interactions, and via Coulomb interactions only with the compensating charged defects known to be present. Some of our considerations are based on standard linear-response theory expressions for the ac conductivity of weakly disordered metals, in which disorder is included only through finite quasiparticle lifetimes and localization effects are ignored. In the dc limit, with quasiparticle scattering rates evaluated using Born approximation expressions for scattering off the spatial fluctuations of the screened-Coulomb and exchange potentials, these expressions imply mobilities [11] that are consistent with values measured in optimally annealed, high-T c , DMS ferromagnets. Although the predictions discussed below are int...

Electronic structure and carrier dynamics of the ferromagnetic semiconductorGa1−xMnxAs<

“…18,19 While it's origin is as yet unresolved, this feature has been attributed to a small polaron, 18 impurity band absorption, 18 -21 and intervalence band transitions. 19,[22][23][24] Additionally these experiments have found a strong growth of the far-infrared spectral weight below T C .…”

“…An alternative scenario suggests that holes are doped into the GaAs valence band ͑VB͒. 3,[22][23][24]50 Within these models, the role of Mn doping is primarily restricted to increase of the hole concentration ͑panel C in Fig. 7͒.…”

“…The expected energy and oscillator strength of the transitions, which depend on the subband masses, spin-orbit energy, and doping level, are consistent with the resonance we observe near 2000 cm Ϫ1 . 23,24 While both models can explain the absorption band in the midinfrared, critical differences can be found in the predicted band structure properties. Specifically, the itinerant carrier mass will be significantly heavier in the impurity band than the GaAs valence band.…”

“…24 At this high doping level the nonparabolic character of the valence band may be partially responsible for this discrepancy. The effects of hybridization between the Mn acceptor levels and As p levels that make up the valence band can also lead to an enhanced mass.…”

Critical temperatures of the two-band model for diluted magnetic semiconductors

“…However, much theoretical work remains to be done, particularly the consideration of the several bands active in real DMS materials beyond mean field approximations. 4 In this paper, we study a two-band model for DMS at realistic low dopings x≪1, using a combination of nonperturbative techniques that gives unique characteristics to our work. To properly analyze the intermediate and strong coupling J regime, MC simulations that correctly handle the random Mn distribution are crucial.…”