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SUMMARY:Chiral Phonons
DTSTART:20260609T120000Z
DTEND:20260609T130000Z
DTSTAMP:20260624T211300Z
UID:indico-event-18960@indico.psi.ch
CONTACT:markus.mueller@psi.ch
DESCRIPTION:Speakers: Hiroki Ueda (PSI)\n\nPhonons with angular momentum h
 ave created an emerging field of chiral phononics.\nFormally\, chiral phon
 ons are rotational phonon modes with finite momentum out of the\nrotationa
 l plane\, breaking improper rotational symmetry. Their intrinsic angular m
 omentum\ngives rise to various emergent phenomena\, including magnetism\, 
 transport\, and light-matter\ninteractions [1]. In this presentation\, I w
 ill show our recent work on chiral phonons using X-\nrays and time-resolve
 d optical probes.\n \nFirst\, I will present direct demonstrations of chi
 ral phonons in the chiral crystal quartz [2]\,\nthe polar crystal LiNbO3 
 [3]\, and the ferroelectric crystal BaTiO3 [4] using resonant inelastic\n
 X-ray scattering (RIXS) with circularly polarized X-rays at the O K edge
 . Besides momentum\naccessibility due to short wavelengths of X-rays\, ang
 ular momentum transfer between\ncircularly polarized X-ray photons and chi
 ral phonons imposes the selection rule in the RIXS\nprocess\, enabling chi
 ral phonon observation via circular dichroism in phonon excitation peaks\n
 at finite momentum. By ferroelectric polarization reversal in BaTiO3\, we 
 demonstrate in-situ\nelectrical switching of phonon chirality\, opening pa
 thways for electrical control of chiral\nphonon-related emergent phenomena
 .\n \nSecond\, we focus on low-energy acoustic phonons\, which are more r
 elevant to transport\nproperties than optical phonons due to their larger 
 thermal population and group velocity. I will\npresent recent results of r
 esonant diffuse scattering with circular polarization\, analogous to\nRIXS
  with circular polarization but without energy resolution\, on the chiral 
 crystal CsCuCl3.\nA clear circular contrast on the tail of the polarizatio
 n-independent resonant diffraction peak at\nthe Cu L3 edge exhibits sign
  reversal across the Γ point and upon switching the crystal chirality\,\n
 consistent with chiral acoustic phonons.\n \nFinally\, I will present pos
 sible observations of coherent chiral acoustic phonons in the non-\nchiral
  crystal SrTiO3\, using THz-pump\, Kerr rotation-probe measurements. Deep 
 in the\ntetragonal phase\, softening of the transverse-optical (TO) mode c
 auses its coupling to the\ntransverse-acoustic (TA) phonon mode at finite 
 out-of-plane momentum. This coupling enables\ncoherent generation of TA ph
 onons when the TO mode is driven by THz pulses [5]. When the\nTO mode is e
 xcited with circularly polarized THz pulses\, slow oscillations consistent
  with\ncoherent rotational TA phonons are observed. Since the TO-TA coupli
 ng occurs at finite\nmomentum\, the generated coherent TA mode is chiral\,
  carrying finite phonon angular\nmomentum parallel to momentum.\n \n[1] D
 . M. Juraschek\, H. Ueda et al.\, Nat. Phys. 21\, 1532-1540 (2025).\n[2
 ] H. Ueda et al.\, Nature 618\, 946-950 (2023).\n[3] H. Ueda et al\, 
 Nat. Commun. 17\, 212 (2026).\n[4] M. Grimes\, H. Ueda et al.\, arXiv: 
 2603.06144.\n[5] G. Oreenstein et al.\, Nat. Phys. 21\, 961-965 (2025).
 \n\nhttps://indico.psi.ch/event/18960/
LOCATION:WHGA/121
URL:https://indico.psi.ch/event/18960/
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