○ 일시: 7월 12일(화) 오전 11:00

○ 장소: 기초과학연구소 세미나실 (31317호)

○ 연사: 이  상  현  박사님 (High Energy Accelerator Research Organization (KEK) )

○ Title: Spin-direction-dependent magnetoelastic coupling on MnO, CoO, and NiO

○ Abstract: 

High resolution time-of-flight neutron diffraction and group theory resolved not only the long-standing problems of magnetic structure and phase transition mechanism in MnO, CoO, and NiO but also the issue of general symmetry in magnetic moment [1, 2]. Exchangestriction and new magnetostriction, so-called dipolar-magnetostriction and roto-magnetostriction, that induce phase transition from paramagnetic rocksalt structure to common C2/m monoclinic structure with magnetic propagation vector k_m=(0 1 0.5). We classify those into two groups by Γ₁⁺(A_g)=(0, M_y, 0) and Γ₂⁺(B_g)=(M_x, 0, M_z) spin direction in the order-parameter vector diagram. Time-reversal symmetry breaking causes discontinuous phase transition with linear-quadratic coupling Δβ~M² between monoclinic angle Δβ and magnetic moment M. However, CoO and NiO show continuous phase transition with linear-cubic coupling Δβ~M³ forbidden by time-reversal symmetry. To the best of our knowledge, this is first experimental evidence that both magnetic moment and structural distortion can belong to same irreducible representation. Time-reversal symmetry prohibits same irreducible representation between them. The experimental results open a way to find new physics forbidden by time-reversal symmetry in condensed matter physics and material science.