From spins to charges: Nature Communications of a collaboration Konstanz, Tokyo, Dortmund
The digital economy, society and politics are increasingly shaped by cloud-based data. The advancement of transformative technologies, such as artificial intelligence, is placing unprecedented demands on data centers. This has driven an intense pursuit of a concept for data storage, manipulation and transfer able to operate even at THz rates while minimizing energy dissipation. Collective spin excitations, namely magnons, have been proposed as energy-efficient information carriers. A critical challenge concerns the integration of this approach with the ubiquitous CMOS technology. This step requires a mechanism to convert THz coherent magnons into a charge signal. Here we demonstrate the coherence transfer from optically driven THz magnons to charges in terms of an optical response. We identify the conditions necessary for this effect and formulate a microscopic model reproducing the experimental results without any fine-tuning of the parameters. These findings offer a pathway towards an energy-efficient, high-speed information technology.
This innovative result has been obtained by a close and fruitful collaboration between the experimental Emmy-Noether group of Dr. Davide Bossini at Konstanz University and the theoretical research group of Prof. Götz S. Uhrig at TU Dortmund University with important input from Dr. Takuya Satoh at the Institute of Science Tokyo. It started, as usual, with a deceptively simple question: Why do oscillations in the transmission of light occur at the frequency of a magnon, i.e., a quantized spin wave? After a lot of brain storming and discarding many ideas the researchers achieved the identification of the underlying mechanism:
By optical pumping a sizable number of coherent magnons are excited which in turn make themselves felt as a Larmor precession of the antiferromagnetic magnetization in nickel oxide, see circle in the left figure. This precessing magnetization shifts the electronic levels in the nickel ions due to the relativistic spin-orbit coupling. These shifts in turn change the optical transition frequencies slightly so that the transmittivity is modulated periodically. By the highly advanced setup, see right figure, in the Bossini-Lab in Konstanz these modulations were detected. The shifts of the transition frequencies were computed by Priv.-Doz. Jörg Bünemann in Dortmund. Thus, the groundbreaking result showing the close interplay of magnetic and electronic excitations has been possible by fully leveraging experimental-theoretical synergy.