Time-dependent polarization relaxation behaviors induced by a depolarization field E d were investigated on high-quality ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors. The E d values were determined experimentally from an applied external field to stop the net polarization relaxation. These values agree with those from the electrostatic calculations, demonstrating that a large E d inside the ultrathin ferroelectric layer could cause severe polarization relaxation. For numerous ferroelectric devices of capacitor configuration, this effect will set a stricter size limit than the critical thickness issue.PACS numbers: 77.22. Ej, 77.22.Gm, 77.80.Dj, 77.55.+f With recent breakthroughs in fabricating high-quality oxide films [1,2,3], ultrathin ferroelectric (FE) films have attracted much attention from the scientific as well as application points of view. As the FE film thickness d approaches tens of unit cell length, the FE films often show significantly different physical properties from those of bulk FE materials. Some extrinsic effects, especially coming from FE film surfaces and/or interfaces with other materials, could be very important [4]. For some other cases, intrinsic physical quantities could play vital roles in determining the unique properties of ultrathin films.Many FE-based electronic devices have the capacitor configuration, where a FE layer is inserted between two conducting electrodes. Then, polarization bound charges will be induced at the surfaces of the FE layer, but compensated by free charge carriers in the conducting electrodes. In real conducting electrodes, however, the compensating charges will be induced with a finite extent, called the screening length λ. This will result in an incomplete compensation of the polarization charges. Such an incomplete charge compensation should induce a depolarization field E d inside the FE layer, with a direction opposite to that of the FE polarization P [5]. Therefore, E d will appear in every FE capacitor, and its effects will becomes larger with the decrease of d [5]. (For a FE film without electrodes, there is no compensation for the polarization bound charge, so the value of E d will become even larger than that of the FE capacitor case.) E d has been known to be important in determining the critical thickness [6] and domain structure of ultrathin FE films [7,8,9], and reliability problems of numerous FE devices [10,11].Recently, using a first principles calculation, Junquera and Ghosez investigated the critical thickness of BaTiO 3 (BTO) layers in SrRuO 3 (SRO)/BTO/SRO capacitor [6]. For calculations, they assumed that all of the BTO and SRO layers were fully strained with the SrTiO 3 substrate. By taking the real SRO/BTO interfaces into account properly, they showed that E d could make the ferroelectricity vanish for the BTO films thinner than 6 unit cells, i.e. 2.4 nm [6]. More recently, using pulsed laser deposition with a reflection high energy electron diffraction monitoring system, we fabricated high-quality fully-strained SRO/BTO/SRO capacitors...

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We investigated anisotropic optical behavior in solid-state materials using Fourier transform infrared reflection microspectroscopy in combination with polarization modulation. For a Ca1.8Sr0.2RuO4 crystal with an isotropic optical surface, we found the reflection difference to be very close to zero, independent of the azimuthal angle of the sample. A Ca1.4Sr0.6RuO4 crystal with an anisotropic optical surface, however, exhibited a large anisotropic optical response with a strong angular dependence following a sinusoidal behavior. Furthermore, we examined the spatial distribution of the reflection difference in Bi0.17Ca0.83MnO3+delta using infrared synchrotron radiation and could clearly distinguish microscopic anisotropic domains having different optical axes. These results demonstrate that our experimental scheme can be used as a powerful tool to spectrally and spatially resolve anisotropy of solid-state materials in the mid-infrared region.