The (001) surfaces of polar perovskites BaTiO$_3$ and PbTiO$_3$ have been studied from first principles at T=0 K. For both cases of polarization, the most stable TiO-terminated interfaces show intrinsic ferroelectricity. In the topmost layer, where the O atoms are $>$0.1 Åabove Ti, this leads to metallic instead of the insulating behavior of the electronic states that may have important implications for multiferroic tunneling junctions.
Halide perovskites are perceived to be the promising class of materials for optoelectronics, spinorbitronics and topological electronics due to presence of strong spin-orbit coupling and polarized field. Here, we develop a Hamiltonian using quasi-degenerate perturbation theory to describe polarized field driven band structures and unique topological phases such as Dirac semimetallic rings for the universal class of Halide perovskites including both inorganic and hybrid members. The spin textures obtained through this theoretical model captures minute effects of polarization and hence can be us…
Ultrafast polarization control of zero-bias photocurrent and terahertz emission in hybrid organic perovskites
甲基碘化铅铵（Methylammonium lead iodide，简称MAI）是一种基准的混合有机钙钛矿材料，用于低成本的印刷太阳能电池，其功率转换效率超过20%。然而，光与物质相互作用的性质以及设备操作背后的确切物理机制目前仍有争议。在这里，我们报告室温下，超快光电流和自由空间太赫兹（THz）发射产生的无偏置的150 fs光脉冲引起的。所观察到的光响应的偏振依赖性是一致的体光伏效应（BPVE）所造成的注入和移位电流的组合。我们相信，这种观察可以揭示低复合，和长的载流子扩散长度，由于间接带隙。此外，弹道性质的偏移和注入BPVE光电流可以使第三代钙钛矿太阳能电池具有超过Shockley_Queisser极限的效率。我们的观察也为钙钛矿自旋电子学和可调谐THz源开辟了新的途径。
Exciton polaritons, hybrid quasiparticles with part-light part-matter nature in semiconductor microcavities, are extensively investigated for striking phenomena such as polariton condensation and quantum emulation. These phenomena have recently been discovered in emerging lead halide perovskites at elevated temperatures up to room temperature. For advancing these discoveries into practical applications, one critical requirement is the realization of electrically pumped exciton-polaritons. However, electrically pumped polariton light-emitting devices with perovskites have not yet been achieved …
Molecular ferroelectrics have attracted increasing interests over the past decade due to mechanical flexibility, chemical diversity, environmental friendliness, easy-processing, and lightness. The performance of molecular ferroelectrics is approaching more and more competitive to their inorganic counterparts. Despite the chemical diversity and tunability of the molecular cations or anions, molecular ferroelectrics are not abundant. In particular, physical directed design principles for new discovery or performance optimization are still lacking. Here, through first-principles calculations, we …
Recent realizations of ultrathin freestanding perovskite oxides offer a unique platform to probe novel properties in two-dimensional oxides. Here, we observed a giant flexoelectric response in freestanding BiFeO3 and SrTiO3 in their bent state arising from strain gradients up to 4x10e7/m, suggesting a promising approach for realizing extremely large polarizations. Additionally, a substantial reversible change in thickness was discovered in bent freestanding BiFeO3, which implies an unusual bending-expansion/shrinkage and thickness-dependence Poisson's ratios in this ferroelectric membrane that…
Doping ferroelectrics with carriers is often detrimental to polarization. This makes the design and discovery of metals that undergo a ferroelectric-like transition challenging. In this letter, we show from first principles that the oxygen octahedral rotations in perovskites are often enhanced by electron doping, and this can be used as a means to strengthen the structural polarization in certain hybrid-improper ferroelectrics -- compounds in which the polarization is not stabilized by the long range Coulomb interactions but is instead induced by a trilinear coupling to octahedral rotations. W…
Switchable Polarization in an A-site Deficient Perovskite through Vacancy and Cation Engineering
Kyoto University（京都大学）；Nantes Université（南特大学）；The University of Tokyo（东京大学）；Japan Fine Ceramics Center（日本先进陶瓷中心）；Kyushu University（九州大学）；Pennsylvania State University（宾夕法尼亚州立大学）
Band-gap engineering in AB(O$_x$S$_{1-x}$)$_{3}$ perovskite oxysulfides: A route to strongly polar materials for photocatalytic water splitting
极性非均相光催化剂显示出导致增强的电荷-载流子分离，这导致例如用于光催化水裂解的优异活性。有前途的光催化剂材料，如氮氧化物，可以通过外延应变呈现极性，然而，这也增加了它们的带隙，使它们不适合可见光吸收。这表明小带隙和极性扭曲之间的权衡-两者对催化剂的效率至关重要。在本文中，我们调查，使用密度泛函理论计算，应变AB（O $_x$ S $_{1-x}$）$_{3}$钙钛矿光催化分解水的适用性。这些材料通常具有对于水裂解来说太小的带隙，但是经由外延应变引起极性畸变可以将带隙增加到合适的范围。我们发现钙钛矿BaZr $_y$ Ti $_{1-y}$ O $_2$ S化合物由于大的极性畸变和合适的带隙而非常有希望用于光催化水裂解。
Materials combining the optoelectronic functionalities of semiconductors with control of the spin degree of freedom are highly sought after for the advancement of quantum technology devices. Here, we report the paramagnetic Ruddlesden-Popper hybrid perovskite Mn:(PEA)2PbI4 (PEA = phenethylammonium) in which the interaction of isolated Mn2+ ions with magnetically brightened excitons leads to circularly polarized photoluminescence. Using a combination of superconducting quantum interference device (SQUID) magnetometry and magneto-optical experiments, we find that the Brillouin-shaped polarizatio…
Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are introducing new directions in the 2D materials landscape. The coexistence of ferroelectricity and spin-orbit interactions play a key role in their optoelectronic properties. We perform a detailed study on a recently synthesized ferroelectric 2D-HOIP, (AMP)PbI$_4$ (AMP = 4-aminomethyl-piperidinium). The calculated polarization and Rashba parameter are in excellent agreement with experimental values. We report a striking new effect, i.e., an extraordinarily large Rashba anisotropy that is tunable by ferroelectric polarization:…
Octahedral rotation-induced ferroelectricity in cation ordered perovskites
对电场可调的电、磁和轨道（EMO）材料的日益增长的需求重新引起了人们对复杂钙钛矿氧化物中的铁电性及其与EMO性质的耦合的兴趣。实现自发极化的历史设计策略涉及二阶Jahn-Teller（SOJT）活性阳离子的掺入。然而，挑战在于，这种机制仅限于特定的化学物质，并且所产生的极性畸变在很大程度上与EMO特性无关，从而限制了它们作为场可调谐多功能技术材料的用途。在这里，我们报告的晶体化学标准，绕过这些限制，并使新材料的合理设计显示八面体旋转诱导铁电性-一种创新的路线，实现电极化的强度，而不需要SOJT阳离子-从非极性积木。该策略利用了中心八面体旋转模式，其强烈耦合到EMO性质，以及在非极性$AB$ O $_3$钙钛矿中通常观察到的阳离子排序。通过将可切换的电极化的强度与过渡金属氧八面体的连接性结合，可以对材料性质进行电场控制。
Stabilization of highly polar BiFeO$_3$-like structure: a new interface design route for enhanced ferroelectricity in artificial perovskite superlattices
The phenomenological theory for the polar instability giving rise to a two-dimensional electron gas at perovskite interfaces is hereby extended to include the coupling to perovskite tilts. A Landau theory for homogeneous tilts is first explored, setting the scene for the further, more realistic Landau-Ginzburg theory describing varying tilt amplitudes across a thin film. The theory is also generalized to account for the response to an applied electric field normal to the interface, which allows a finer control on phase transitions. The conventionally described physics of a single metal-insulat…
