Exchange-mediated magnetic blue-shift of the band-gap energy in the antiferromagnetic semiconductor MnTe
Sprache des Titels:
Englisch
Original Kurzfassung:
In magnetic semiconductors the optical spectrum and, in particular, the absorption edgerepresenting the band-gap are strongly affected by the onset of the magnetic order. Thiscontribution to the band-gap energy has hitherto been described theoretically in terms of aHeisenberg Hamiltonian, in which a delocalized conduction carrier is coupled to the localizedmagnetic moments by the exchange interaction. Such models, however, do not take into accountthe strong correlations displayed in a wide variety of magnetic semiconductors, which areresponsible for the formation of the local moments. In particular, the itinerant carrier itselfcontributes to the spin moment. Here, we overcome this simplification in a combinedexperimental and theoretical study of the antiferromagnetic semiconductor?-MnTe. First, wepresent a spectroscopic optical investigation as a function of temperature, from which we extractthe magnetic contribution to the blue-shift of the band-gap. Second, we formulate a minimalmodel based on a Hubbard?Kondo Hamiltonian. In this model, the itinerant charge is one of theelectrons forming the localized magnetic moment, which properly captures correlation effects inthe material. Our theory reproduces the experimental findings with excellent quantitativeagreement, demonstrating that the magnetic contribution to the band-gap energy of?-MnTe ismediated solely by the exchange interaction. These results describe an intrinsic property of thematerial, independent of the thickness, substrate and capping layer of the specimen. One of the keyfindings of the model is that the basic effect, namely a blue-shift of the band-gap due to theestablishment of the magnetic order, is a general phenomenon in charge-transfer insulators. Theidentification of the relevant magnetic interaction discloses the possibility to exploit the effect herediscussed to induce a novel regime of coherent spin dynamics, in which spin oscillations on acharacteristic time-scale of 100 fs are triggered and are intrinsically coupled to charges
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