报告题目 (Title)：Nonreciprocal Phonons in PT-Symmetric Antiferromagnets（PT对称反铁磁体中的非互易声子）
报告人 (Speaker)：牛谦 教授（中国科学技术大学）
报告时间 (Time)：2025年7月17日（周四）16:00-18:00
报告地点 (Place)：宝山校区G601
邀请人(Inviter)：钟建新
主办部门：量子科技研究院/理学院物理系
报告摘要：
Phonon
nonreciprocity, indicating different transport properties along
opposite directions, has been observed in experiments under a magnetic
field. We show that nonreciprocal acoustic phonons can also exist
without a magnetic field or net magnetization. We identify crucial
contributions in phenomenological elastic theory. In PT symmetric
antiferromagnets, we find two terms, dubbed flexoviscosity and flexo
torque, that induce phonon nonreciprocity. The microscopic origin of
these terms is attributed to the derivatives of molecular Berry
curvature, manifested as emergent nonlocal magnetic fields on phonons.
The symmetry breaking that originated from spin order is transferred to
the phonon system through spin-orbit coupling, where the orbital degree
of freedom affects the lattice dynamics directly. By electrically
breaking inversion symmetry and modifying the spin-orbit coupling, we
find extra contributions and show that both the phonon nonreciprocity
and helicity can be controlled and enhanced. Importantly, the phonon
nonreciprocity is an odd function of the Néel vector, serving as an
indicator of the order parameter.