PHOENICS: PHOto-inducEd femtosecoNd magneto-electrIC Spin manipulation in antiferromagnets
Final Report Abstract
The research activity carried out in the framework of PHOENICS targets the possibility to establish a coherent control of the spin-orbit coupling at THz frequency. In particular we have selected bulk materials, whose crystallographic symmetry enables a contribution to the spin-orbit coupling, additional to the atomic one, namely the so-called Rashba effect. A notable consequence thereof is the splitting of the energy bands. Symmetry considerations suggest that the Rashba effect can be coherently modulated by specific lattice modes. As a first step we focussed on establishing the generation and detection of the proper lattice modes. In the archetypal Rashba system BiTeI we conducted all-optical experiments, measuring the photoinduced dynamics of the optical reflectivity triggered by femtosecond laser pulses. Tuning the central photon energy of the excitation pulses in the visible, near- and even far-infrared spectral ranges, we succeeded in driving the coherent lattice vibration, predicted to couple with the Rashba splitting of the energy bands. We first explored the conventional light-matter interaction regime for displacive excitation of coherent phonons (DECP), as we pumped interband transitions. We observed that the lattice coherence of BiTeI is surprisingly robust against the generation of charge carriers, especially in comparison with traditional III-V semiconductors. Conventionally the microscopic mechanism behind DECP cannot disentangle two different contributions to the excitation of phonons, i.e. the increase of the carrier density and the increase of their temperature. The reason for this knowledge gap is that both processes are triggered by the photo-excitation of interband transitions. We thus set out to explore a barely addressed regime of light-matter interaction, as far as DECP is concerned. Employing a state-of-the-art home-made laser system, we relied on mid-infrared femtosecond laser pulses to drive intraband transitions solely. We note that this pumping strategy induces a modification of the carrier temperature, while their density is unaffected. This unprecedented excitation of BiTeI resulted in activating coherent lattice modes, which experimentally answers a fundamental question, open for longer than thirty years. An increase in carriers temperature is thus the main drive to induce coherent phonons via DECP. To further explore this peculiar regime of lattice dynamics we plan to perform experiments pumping BiTeI with THz laser pulses. More precisely, the ability to manipulate the temporal profile of the exciting THz pulses will enable us to attempt the excitation of coherent phonons on a timescale shorter than electronic scattering. While this experiment is still a future perspective, the required methodology required for it was already developed.
Publications
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Quantitative analysis of free-electron dynamics in InSb by terahertz shockwave spectroscopy. Physical Review B, 106(20).
Fischer, Peter; Fitzky, Gabriel; Bossini, Davide; Leitenstorfer, Alfred & Tenne, Ron
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Revealing the Microscopic Mechanism of Displacive Excitation of Coherent Phonons in a Bulk Rashba Semiconductor
P. Fischer et al.
