The spintronics of the future is expected not only to operate faster, but also to function in a fundamentally different way: using excitations in the terahertz range and processes occurring on picosecond to femtosecond timescales. A new study on metallic superlattices, recently published in the renowned journal Nature Communications (link to the original publication), precisely addresses this challenge.

A team involving researchers from the University of Potsdam, Uppsala University, European XFEL, and other partner institutions has demonstrated how targeted terahertz sound waves can be generated in fully metallic nanostructures. This represents an important contribution to the Collaborative Research Centre/Transregio TRR 227 “Ultrafast Spin Dynamics,” which investigates the coupling between electrons, phonons, and spins on ultrafast timescales, with the aim of establishing the foundations for spin-based information technologies operating at terahertz clock rates.

In the experiment, platinum and copper layers only a few nanometres thick were stacked periodically on top of one another. An ultrashort optical laser pulse excited the structure, producing a coherent lattice vibration at a frequency of approximately one terahertz. This vibration has a remarkably large amplitude and appears immediately after excitation.

The researchers were able to show that the vibration is not generated by the comparatively slow heating of the crystal lattice. Instead, it is driven by the impulsive pressure of hot electrons, which becomes localized in the platinum layers and produces an ultrafast modulation of the mechanical stress.

The study therefore introduces a new mechanism for generating acoustic terahertz excitations in metallic heterostructures. Such excitations are particularly attractive for research into ultrafast magnetism because sound waves can couple directly to magnetic order through magnetoelastic interactions. This is especially relevant for antiferromagnetic systems, whose characteristic frequencies often lie in the terahertz range and which are therefore regarded as promising platforms for ultrafast spintronics.

TRR 227 explicitly investigates the control of ferromagnetic and antiferromagnetic order using specific excitations, including phonons that couple efficiently to terahertz spin dynamics.

“The crucial point is that we are not merely generating an acoustic pulse, but a specific terahertz mode with a tailored frequency in a metallic metamaterial,” says Matias Bargheer, the principal investigator responsible for the project within the TRR 227 research network. “This provides us with a tool that could ultimately be used to address magnetic systems in the terahertz frequency range, particularly antiferromagnetic orders, which are highly promising for the spintronics of the future.”

The study also explains why this approach has previously been difficult to realize. In metals, free electrons normally distribute the deposited energy extremely rapidly, smoothing out the steep stress gradients required to generate high-frequency sound waves.

The superlattice overcomes this problem: differences in the electronic band structures and densities of states of the constituent materials produce a spatial modulation of the electronic energy. This creates an electron-pressure profile that excites coherent oscillations more rapidly than energy is transferred to the phonons on average.

Ab initio calculations by Chandan Singh and Peter Oppeneer of Uppsala University -the latter a Mercator Fellow in TRR 227- confirm that conventional electron–phonon coupling would be too slow to explain the observed terahertz oscillation with its measured phase and amplitude.

For TRR 227, these results mean that, alongside electrical and optical terahertz excitations, tailored acoustic terahertz waves in fully metallic multilayer structures are now also coming into focus. In the future, they could be used to control spin currents, magnetic precession, antiferromagnetic resonances, or magnons through precisely controlled strain oscillating at terahertz frequencies.

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Article reference:

Electron pressure drives THz phonons in metal–metal superlattices 
J.-E. Pudell, M. Mattern, M. Herzog, A. von Reppert, C. K. Singh, D. Schick, M. Hehn, U. Boesenberg, A. Rodriguez-Fernandez, R. Shayduk, W. Jo, J. Möller, J. Hallmann, J. Wrigley, P. M. Oppeneer, A. Madsen, and M. Bargheer
Nat. Commun. 17, 5308 (2026) - DOI: 10.1038/s41467-026-73927-y

Contact:

Matias Bargheer, e-mail: bargheer@uni-potsdam.de