IQIM Postdoctoral and Graduate Student Seminar

Abstract: Chiral phonons arise in crystals with broken mirror symmetries. In a magnetic field, the circular motion of lattice ions in a chiral phonon leads to a finite magnetic moment, even when the host material is purely nonmagnetic. Further, via electron-lattice and spin-orbit interactions, chiral phonons are predicted to induce novel topological phases of matter. To explore such hitherto unobserved phenomena, we studied Pb1-xSnxTe, which is a narrow-bandgap semiconductor that exhibits a topological phase transition as a function of Sn composition, x – from a trivial insulator (x < 0.32) to a topological crystalline insulator (x > 0.32). In addition, this material system possesses soft optical phonons (in the terahertz frequency range) and exhibits ferroelectric instabilities. First, I will describe our observations of novel magnetic phenomena associated with chiral phonons in PbTe [1]: magnetic circular dichroism, a Zeeman splitting, and a diamagnetic shift. I will show that these observations result from magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. Second, I will show the observation of two orders of magnitude larger values of phonon magnetic moment for films in the topological crystalline insulator phase than those in topologically trivial films [2]. These results strongly hint at the interplay between the magnetic properties of chiral phonons and the topology of the electronic band structure. Lastly, I will discuss our results on coupling PbTe phonons with quantum vacuum fluctuations in small mode-volume terahertz cavities. We observed a giant vacuum Rabi splitting on the order of the uncoupled phonon and cavity frequencies, placing this system in the deep-strong coupling regime.

References

[1] A. Baydin, F. G. G. Hernandez, M. Rodriguez-Vega, A. K. Okazaki, F. Tay, G. T. Noe II, I. Katayama, J. Takeda, H. Nojiri, P. H. O. Rappl, E. Abramof, G. A. Fiete, and J. Kono, "Magnetic Control of Soft Chiral Phonons in PbTe," Phys. Rev. Lett. 128, 075901 (2022)

[2] F. G. G. Hernandez, A. Baydin, S. Chaudhary, F. Tay, I. Katayama, J. Takeda, H. Nojiri, A. K. Okazaki, P. H. O. Rappl, E. Abramof, M. Rodriguez-Vega, G. A. Fiete, and J. Kono, "Observation of Interplay between Phonon Chirality and Electronic Band Topology," Sci. Adv., 9(50), eadj4074 (2023)

Lunch will be provided following the talk.