Phonon Magnetochiral Effect in Lanthanum Cuprate*

Recently, Grissonnanche et al. observed a giant thermal Hall effect in several cuprate compounds below the critical doping p*. [1] A large, negative thermal Hall coefficient was measured independent of the direction of the applied heat current, making phonons the most likely heat carriers. [2] There is, however, no compelling explanation for why phonons become “chiral” in these cuprates. Here, we use pulse echo ultrasound to probe whether phonons couple to chiral order in the parent compound La2CuO4.
In the absence of mirror symmetry the phase velocity of sound can be sensitive to whether the sound wave-vector is parallel or antiparallel to an applied magnetic field. [3] Detection of such a phonon magnetochiral effect (MCE) suggests coupling to chiral order. By depositing thin-film transducers on opposing faces of an LCO sample, we measure the relative change in the speed of sound of an ultrasonic wave traveling with versus against an applied magnetic field. We discuss the phonon MCE as a symmetry-sensitive measurement of cuprates.

1. Grissonnanche G., et al. Nature 571, 376–380 (2019).
2. Grissonnanche G., et al. Nat. Phys. (2020).
3. Nomura T., et al. Phys. Rev. Lett. 122, 145901 (2019).