Superlattices are materials created by the alternating growth of two chemically different materials. The direct consequence of creating a superlattice is the folding of the Brillouin zone, which gives rise to additional electronic bands and phonon modes. This phenomenon has been successfully exploited to achieve new transport and optical properties in semiconductor superlattices. Here, we show that multiferroic BiFeO3/LaFeO3 superlattices exhibit several structural orders parallel and perpendicular to the growth direction, not existing in individual bulk materials. Using transmission electron microscopy, x-ray diffraction, and first-principles calculations, we reveal in particular a new long-range order of tilted FeO6 octahedra, with a period along the growth direction about twice that of the chemical supercell, i.e., a superorder. The effect of this new structural order on the phonon dynamics is studied with ultrafast optical pump-probe experiments. While a folded-mode at 1.2 THz is attributed solely to the chemical modulation of the superlattice, the existence of another 0.7 THz mode seems to be explained only by a double Brillouin zone folding in agreement with the structural out-of-plane superorder. Our work shows that multiferroic BiFeO3/LaFeO3 superlattices can be used to tune the spectrum of coherent THz phonons, and potentially that of magnons or electromagnons.

中文翻译：

---

多铁性 BiFeO3/LaFeO3 超晶格中的超阶和太赫兹声学模式


超晶格是由两种化学性质不同的材料交替生长产生的材料。产生超晶格的直接结果是布里渊区的折叠，这产生了额外的电子频段和声子模式。这种现象已被成功利用在半导体超晶格中实现新的传输和光学特性。在这里，我们表明多铁性 BiFeO3/LaFeO3 超晶格表现出几个平行和垂直于生长方向的结构序，不存在于单个块状材料中。使用透射电子显微镜、X 射线衍射和第一性原理计算，我们特别揭示了一个新的倾斜 FeO6 八面体的长程顺序，其沿生长方向的周期约为化学超级胞（即超序）的两倍。通过超快光泵浦探针实验研究了这种新结构顺序对声子动力学的影响。虽然 1.2 THz 的折叠模式完全归因于超晶格的化学调制，但另一种 0.7 THz 模式的存在似乎只能由与结构面外超序一致的双布里渊区折叠来解释。我们的工作表明，多铁性 BiFeO3/LaFeO3 超晶格可用于调谐相干太赫兹声子的光谱，以及可能的磁振子或电磁振子的光谱。