Ferroelectric control of metal–insulator transition

Highlights

• A strategy to control electronic structure with ferroelectric distortion.

• Control of bandwidth and metal–insulator transition can be reached.

• The orbital polarization can also be manipulated.

• The local spin polarization can also be tuned.

Abstract

We propose a method of controlling the metal–insulator transition of one perovskite material at its interface with another ferroelectric material based on first principle calculations. The operating principle is that the rotation of oxygen octahedra tuned by the ferroelectric polarization can modulate the superexchange interaction in this perovskite. We designed a tri-color superlattice of (BiFeO₃)_N/LaNiO₃/LaTiO₃, in which the BiFeO₃ layers are ferroelectric, the LaNiO₃ layer is the layer of which the electronic structure is to be tuned, and LaTiO₃ layer is inserted to enhance the inversion asymmetry. By reversing the ferroelectric polarization in this structure, there is a metal–insulator transition of the LaNiO₃ layer because of the changes of crystal field splitting of the Ni e_g orbitals and the bandwidth of the Ni in-plane e_g orbital. It is highly expected that a metal-transition can be realized by designing the structures at the interfaces for more materials.

Introduction

In transitional metal oxides, the strong correlation between lattice, charge, orbital, and spin leads to many novel properties. At their interfaces, even richer physics bring about emerging properties. Technical advances in the atomic-scale synthesis of oxide heterostructures make it possible for these interfaces to be artificially designed [1], [2], [3]. In perovskite oxide heterostructures where the oxygen octahedra share their vertices, the interplay among different distortion of octahedral units can dictate many novel functional properties [4]. One kind of interfaces between ferroelectric and other materials, of which the electronic properties are to be controlled, is very interesting because of the bi-stable property, with which two states can be reached by reversing the ferroelectric polarization with an electric field, with both changes from structure and from electric polarization involved. The controlling of the electronic structure can be through either the change of electric boundary condition [5], or structural distortion [6]. In this paper, we propose a strategy to control the electronic bandwidths and to achieve a metal–insulator transition in a material by stacking it onto a ferroelectric layer resulting in a modulation of structural distortion.

In this work, we use LaNiO₃ as the electronic active material, of which the band gap is to be controlled. Nickelates, with the chemical formula RNiO₃, have a prominent feature that there is a metal–insulator transition connected to the sizes of the R site ions by the rotation angles of octahedra [7]. The rotation of the octahedra can also be controlled by a strain, causing many works on the straining control of the electronic structure of nickelates such as in Refs. [8], [9], [10]. In addition, nickelate heterostructures have drawn great attentions since Chaloupka and Khaliullin proposed the possible superconductivity by modulating the orbitals [11]. With the proposed structure, the orbital occupations can also be tuned. We chose BiFeO₃ as the ferroelectric material because of its large polarization. It has a relative small band gap, which enables it to be used as semiconductor materials such as in switchable diodes [12], [13], [14], [15], therefore it is easier for charges to transfer from or to BiFeO₃. One can make use of this effect to manipulate the charge transfer.

By stacking a ferroelectric and non-ferroelectric layers together, it is natural to think that the electronic properties can be tuned by reversing the ferroelectric polarization. While it is true for the asymmetric thin films, in a periodic superlattice, the structures of the two polarization states are the same (or very similar) by a 180° rotation if the two interfaces of a non-ferroelectric to the ferroelectric are the same (or very similar). For example, in a (BiFeO₃)_N/LaNiO₃ superlattice, the two sides of LaNiO₃ are the LaO and BiO planes. Since the La and Bi ions are close in radius and the same in valence states, large difference in electronic properties caused by reversing the polarization is not expected.

A method to enlarge the difference is to make the structure more asymmetric by inserting another layer. By inserting a LaTiO₃ layer, tri-color superlattices of (BiFeO₃)_N/LaNiO₃/LaTiO₃ are formed as shown in Fig. 1. According to Chen et al. [16], in LaTiO₃, the Ti³⁺ ion has a 3d electron with energy higher than that of the unoccupied Ni e_g band in LaNiO₃. Therefore electrons can transfer from Ti to Ni, forming a Ti d⁰ and a Ni d⁸ configuration [17], so there is an electric polarization pointing from LaNiO₃ to LaTiO₃ at their interface. Thus the structures with opposite ferroelectric polarizations also differ in polarization continuity, which offers another way of controlling the electronic property by reversing the ferroelectric polarization. In the LaNiO₃/LaTiO₃ heterostructures, the crystal field splitting of the Ni e_g orbitals is largely affected by the distortion of the oxygen octahedra caused by the polarity of the structure [16]. Thus the control of the ferroelectric polarization in the tri-color superlattice can also tune the electronic structure by influencing the octahedra distortion.