Chemical control of polar behavior in bicomponent short-period superlattices

Chemical control of polar behavior in bicomponent short-period superlattices Hena Das1, Nicola A. Spaldin2, Umesh V. Waghmare3 and T. Saha-Dasgupta1 1 S.N. Bose National Centre for Basic Sciences, Kolkata 700098, India 2 Materials Department, University of California, Santa Barbara, CA 93106-5050, USA and 3 Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560 064, India (Dated: May 30, 2022) Using first-principles density functional calculations, we study the interplay of ferroelectricity and polar discontinuities in a range of 1-1 oxide superlattices, built out of ferroelectric and paraelectric components. Studies have been carried out for a varied choice of chemical composition of the components. We find that, when polar interfaces are present, the polar discontinuities induce off- centric movements in the ferroelectric layers, even though the ferroelectric is only one unit cell thick. The distortions yield non-switchable polarizations, with magnitudes comparable to those of the corresponding bulk ferroelectrics. In contrast, in superlattices with no polar discontinuity at the interfaces, the off-centric movements in the ferroelectric layer are usually suppressed. The details of the behavior and functional properties are, however, found to be sensitive to epitaxial strain, rotational instabilities and second-order Jahn-Teller activity, and are therefore strongly influenced by the chemical composition of the paraelectric layer. PACS numbers: 73.20.-r, 77.84.-s, 71.15.Nc I. INTRODUCTION Superlattices formed by layer-by-layer epitaxial growth of perovskite-based oxide materials are currently a sub- ject of intense research, because of their promising tech- nological applications as well as fundamental scientific interest1. In ABO3 perovskites, the A+2B+4O3 (II- IV) structures consist of (100) layers of formally charge neutral AO and BO2, while A+3B+3O3 (III-III) or A+1B+5O3 (I-V) structures have charged planes, com- posed of +1 AO and -1 BO2 layers or -1 AO and +1 BO2 layers, respectively. By stacking two perovskite layers from different charge families along the [001] di- rection, one obtains a polar discontinuity at the inter- face. Such polar discontinuities have been reported to lead to nontrivial local structural and electronic ground states2–5, which are often not present in the parent bulk compounds1,6,7. Investigating the properties of these “exotic” local phases has been an increasingly active area of research in the past few years, particularly following a 2004 report2 of a conducting quasi-two dimensional electron gas (2DEG) at the interface between two wide- band insulators, LaAlO3 (LAO) and SrTiO3 (STO). The measured mobility and carrier density of the LAO/STO interface are an order of magnitude larger than those in analogous semiconductor-based systems8. Further- more, magnetism,3 superconductivity4 and a rich elec- tronic phase diagram5 have also been reported for this same system. These fascinating and unexpected effects have generated strong excitement, and an intense effort is currently devoted to better understanding the fun- damental mechanisms of charge compensation at polar oxide-oxide interfaces. Parallel to this thrust, from the materials-design point of view, it is also important to identify new compounds, artificial superlattices or inter- faces that might display similar (or possibly even more striking) behavior. A system can respond in several different ways to avoid a so-called polar catastrophe9 – a divergence in the poten- tial caused by the polar discontinuity – at such an inter- face between two charge-mismatched perovskites. Com- pensation by free carriers was proposed in Ref. 2, and is consistent with the observed conductivity at the in- terfaces. Other likely possibilities are direct ionic charge compensation through mixed valency of the B cation, ion intermixing or oxygen vacancies at the interface10, or po- lar distortions at the interface. These can be “induced” by the polar discontinuity if both materials are paraelec- tric (PE) in their bulk phase11, or “natural” if one or both components in the superlattice is ferroelectric (FE)12. The mechanisms underlying induced and natural polar distortions can be readily understood in terms of classical electrostatics and the modern theory of polarization13, whenever the relevant layers in the superlattice are at least three or four unit cells thick. In particular, the charge mismatch can be interpreted as a polarization dis- continuity, which produces macroscopic electric fields in one or both components, because the normal component of the electric displacement field, D = E + 4πP, is pre- served at a coherent insulating interface. For smaller thicknesses, macroscopic concepts lose their meaning, as each film is too thin to identify a well-defined local value of the electric field or the polarization. Thus, it remains an important question whether the above concepts are still valid when the layers in a superlattice ar

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