5 research outputs found
Exchange-mediated magnetic blue-shift of the band-gap energy in the antiferromagnetic semiconductor MnTe
In magnetic semiconductors the optical spectrum and, in particular, the absorption edge representing the band-gap are strongly affected by the onset of the magnetic order. This contribution to the band-gap energy has hitherto been described theoretically in terms of a Heisenberg Hamiltonian, in which a delocalized conduction carrier is coupled to the localized magnetic moments by the exchange interaction. Such models, however, do not take into account the strong correlations displayed in a wide variety of magnetic semiconductors, which are responsible for the formation of the local moments. In particular, the itinerant carrier itself contributes to the spin moment. Here, we overcome this simplification in a combined experimental and theoretical study of the antiferromagnetic semiconductor α-MnTe. First, we present a spectroscopic optical investigation as a function of temperature, from which we extract the magnetic contribution to the blue-shift of the band-gap. Second, we formulate a minimal model based on a HubbardâKondo Hamiltonian. In this model, the itinerant charge is one of the electrons forming the localized magnetic moment, which properly captures correlation effects in the material. Our theory reproduces the experimental findings with excellent quantitative agreement, demonstrating that the magnetic contribution to the band-gap energy of α-MnTe is mediated solely by the exchange interaction. These results describe an intrinsic property of the material, independent of the thickness, substrate and capping layer of the specimen. One of the key findings of the model is that the basic effect, namely a blue-shift of the band-gap due to the establishment of the magnetic order, is a general phenomenon in charge-transfer insulators. The identification of the relevant magnetic interaction discloses the possibility to exploit the effect here discussed to induce a novel regime of coherent spin dynamics, in which spin oscillations on a characteristic time-scale of 100 fs are triggered and are intrinsically coupled to charges
Femtosecond phononic coupling to both spins and charges in a room-temperature antiferromagnetic semiconductor
Spintronics is postulated on the possibility to employ the magnetic degree of freedom of electrons for computation and couple it to charges. In this view the combination of the high frequency of spin manipulations offered
by antiferromagnets, with the wide tunability of the electronic properties peculiar of semiconductors, provides
a promising and intriguing platform. Here we explore this scenario in α-MnTe, which is a semiconductor
antiferromagnetically ordered at room temperature. Relying on a Raman mechanism and femtosecond laser
pulses, we drive degenerate modes of coherent optical phonons, which modulate the chemical bonds involved
in the superexchange interaction. The spectrally resolved measurements of the transient reflectivity reveal a
coherent modulation of the band gap at the frequency of 5.3 THz. The detection of the rotation of the polarization,
typically associated with magneto-optical effects, shows coherent and incoherent contributions. Modeling how
the ionic motion induced by the phonons affects the exchange interaction in the material, we calculate the
photoinduced THz spin dynamics: the results predict both a coherent and incoherent response, the latter of which
is consistent with the experimental observation. Our work demonstrates that the same phonon modes modulate
both the charge and magnetic degree of freedom, suggesting the resonant pumping of phonons as a viable way
to link spin and charge dynamics even in nonlinear regimes
Temperature dependence of the picosecond spin Seebeck effect
We performed temperature-dependent optical pumpâTHz emission measurements in YâFeâ
Oââ (YIG)|Pt from 5âK to room temperature in the presence of an externally applied magnetic field. We study the temperature dependence of the spin Seebeck effect and observe a continuous increase as temperature is decreased, opposite to what is observed in electrical measurements, where the spin Seebeck effect is suppressed as 0âK is approached. By quantitatively analyzing the different contributions, we isolate the temperature dependence of the spin-mixing conductance and observe features that are correlated with the bands of magnon spectrum in YIG