Transversely modulated crystal structure of charge-orbital ordered manganites Nd_1-xSr_1+xMnO₄ (x = 2/3, 3/4)

The crystallographic superstructures of the charge-orbital ordered phases were investigated for layered manganites, Nd_1-xSr_1+xMnO₄ (x=2/3, 3/4), by high-resolution electron microscopy (HREM) at low temperature. Transverse and sinusoidal structural distortion giving rise to the superstructure were successfully observed in HREM images at 80 K. We also performed the simulations of electron diffraction patterns and HREM images, which support the transverse modulations. The observation implies that a new kind of "Wignercrystal" model - accompanied with a charge-density wave (CDW) of e_g electrons - best explains the charge-orbital ordered state in the manganites.