Chiral phonon pumping

• Applicant: Professor Dr. Sebastian Gönnenwein
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541503763

We propose to experimentally explore the generation, the transport and the electrical detection of angular momentum currents carried by acoustic phonons at GHz frequencies. The phonon currents will be sourced using the so-called phonon pumping scheme, proposed on theoretical grounds and experimentally demonstrated only recently. In phonon pumping, the elastic lattice distortions generated by the resonantly precessing magnetization in a magnetic thin film propagate into an adjacent, typically non-magnetic crystal lattice. Phonon pumping thus represents a magnetization relaxation mechanism, adding to the viscous (Gilbert) magnetization damping. In magnet/crystal heterostructures with highly polished and plane-parallel surfaces, phonon pumping furthermore manifests as a series of high-overtone standing acoustic modes superimposed onto the magnetic resonance response. This allows detecting the pumped phonons via high-resolution broad-band magnetic resonance experiments. Moreover, since the magnetization precession has a well-defined handedness, a flow of chiral phonons across the magnet/crystal interface is expected in phonon pumping. However, the amount of (spin) angular momentum carried by this phonon current still is controversially discussed. In particular, the Lande g-value and thus the magnitude of the magnetic moment associated with the pumped chiral phonons has not been established. We thus propose to systematically explore the phonon pumping process itself, to study the propagation of pumped (chiral) phonons in different crystalline materials, and to develop schemes for the detection of chiral phonon currents in particular regarding their angular momentum properties. More specifically, we intend to establish which material combinations and experimental geometries are most suitable for the pumping of chiral phonons into non-magnetic as well as magnetic single crystals. We also plan to explore whether and how spin Hall physics enables the electrical detection of chiral phonon currents, and which length and time scales are characteristic for chiral phonon propagation in single crystal lattices. In addition, we proposed to take advantage of birefringence effects in coherently propagating phonon currents to realize phase shifting elements for phonons. To reach these goals, we will closely work together with different partners within the research unit. These collaborations will not only enable access to several qualitatively different types of magnetic layer/single crystal heterostructures, but also allow to analyse, theoretically model, and compare our experimental results for GHz-frequency chiral phonons with those inferred at much higher frequencies or much smaller length scales.

• DFG Programme: Research Units
• Subproject of: FOR 5844: Chiral phonons for spintronics_ChiPS