arXiv subjects

Axiel Yaël Birenbaum

Publications and source records attributed to Axiel Yaël Birenbaum.

5 recordsLinked to original sources

arXiv2018

Oxygen vacancy formation energies in PbTiO$_3$/SrTiO$_3$ superlattice

The defect stability in a prototypical perovskite oxide superlattice consisting of SrTiO$_3$ and PbTiO$_3$ (STO/PTO) is determined using first principles density functional theory calculations. Specifically, the oxygen vacancy formation energies E$_v$ in the paraelectric and ferroelectric phases of a superlattice with four atomic layers of STO and four layers of PTO (4STO/4PTO) are determined and compared. The effects of charge state, octahedral rotation, polarization, and interfaces on the E$_v$ are examined. The formation energies vary layer-by-layer in the superlattices, with E$_v$ being higher in the ferroelectric phase than that in the paraelectric phase. The two interfaces constructed in these oxide superlattices, which are symmetrically equivalent in the paraelectric systems, exhibit very different formation energies in the ferroelectric superlattices and this can be seen to be driven by the coupling of ferroelectric and rotational modes. At equivalent lattice sites, E$_v$ of charged vacancies is generally lower than that of neutral vacancies. Octahedral rotations (a$^0$a$^0$c$^-$) in the FE superlattice have a significant effect on the E$_v$, increasing the formation energy of vacancies located near the interface but decreasing the formation energy of the oxygen vacancies located in the bulk-like regions of the STO and PTO constituent parts. The formation energy variations among different layers are found to be primarily caused by the difference in the local relaxation at each layer. These fundamental insights into the defect stability in perovskite superlattices can be used to tune defect properties via controlling the constituent materials of superlattices and interface engineering.

cond-mat.mtrl-sci↗

arXiv2018

Intrinsic interfacial van der Waals monolayers and their effect on the high-temperature superconductor FeSe/SrTiO$_3$

The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a SrTiO$_3$ substrate exhibits a much higher superconducting critical temperature T$_C$ than the bulk material. Here, we examine the interfacial structure of FeSe / SrTiO$_3$ and the effect of an interfacial Ti$_{1+x}$O$_2$ layer on the increased T$_C$ using a combination of scanning transmission electron microscopy and density functional theory. We find Ti$_{1+x}$O$_2$ forms its own quasi-two-dimensional layer, bonding to both the substrate and the FeSe film by van der Waals interactions. The excess Ti in this layer electron-dopes the FeSe monolayer in agreement with experimental observations. Moreover, the interfacial layer introduces symmetry-breaking distortions in the FeSe film that favor a T$_C$ increase. These results suggest that this common substrate may be functionalized to modify the electronic structure of a variety of thin films and monolayers.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic order in 4-layered Aurivillius Phases

We determine the viability of 4-layered Aurivillius phases to exhibit long-range magnetic order above room temperature. We use Monte Carlo simulations to calculate transition temperatures for an effective Heisenberg model containing a minimal set of required couplings. The magnitude of the corresponding coupling constants has been determined previously from electronic structure calculations for $\mathrm{Bi_5FeTi_3O_{15}}$, for which we obtain a transition temperature far below room temperature. We analyze the role of further neighbor interactions within our Heisenberg model, in particular that of the second-nearest-neighbor coupling within the perovskite-like layers of the Aurivillius structure, as well as that of the weak inter-layer coupling, in order to identify the main bottleneck for achieving higher magnetic transition temperatures. Based on our findings, we show that the most promising strategy to obtain magnetic order at higher temperatures is to increase the concentration of magnetic cations within the perovskite-like layers, and we propose candidate compounds where magnetic order could be achieved above room temperature.

cond-mat.mtrl-sci↗

arXiv2016

Controlling the cation distribution and electric polarization with epitaxial strain in Aurivillius-phase Bi$_5$FeTi$_3$O$_{15}$

This work explores the impact of in-plane bi-axial (epitaxial) strain on the cation distribution and electric polarization of the Aurivillius-phase compound Bi$_5$FeTi$_3$O$_{15}$ using first-principles electronic structure calculations. Our calculations indicate that the site preference of the Fe$^{3+}$ cation can be controlled via epitaxial strain. Tensile strain enhances the preference for the inner sites within the perovskite-like layers of the Aurivillius-phase structure, whereas compressive strain favors occupation of the outer sites within the perovskite-layers, i.e., the sites close to the Bi$_2$O$_2$ layer. Controlling the distribution of the magnetic cations offers the possibility to control magnetic order in this magnetically dilute system. Furthermore, the magnitude of the electric polarization is strongly strain-dependent, increasing under tensile strain and decreasing under compressive strain. Analysis of the Born effective charges reveals strongly anomalous charges, both of the Bi$^{3+}$ cations and the Ti$^{4+}$ cations.

cond-mat.mtrl-sci↗

arXiv2016

The potentially multiferroic Aurivillius phase Bi$_5$FeTi$_3$O$_{15}$: cation site preference, electric polarization, and magnetic coupling from first principles

We study the structural, ferroelectric, and magnetic properties of the potentially multiferroic Aurivillius phase material Bi$_5$FeTi$_3$O$_{15}$ using first principles electronic structure calculations. Calculations are performed both with PBE and PBEsol exchange correlation functionals. We conclude that PBE systematically overestimates the lattice constants and the magnitude of the ferroelectric distortion, whereas PBEsol leads to good agreement with available experimental data. We then assess a potential site preference of the Fe$^{3+}$ cation by comparing 10 different distributions of the perovskite $B$-sites. We find a slight preference for the "inner" site, consistent with recent experimental observations. We obtain a large value of $\sim$55 $μ$C/cm$^2$ for the spontaneous electric polarization, which is rather independent of the specific Fe distribution. Finally, we calculate the strength of the magnetic coupling constants and find strong antiferromagnetic coupling between Fe$^{3+}$ cations in nearest neighbor positions, whereas the coupling between further neighbors is rather weak. This poses the question whether magnetic long range order can occur in this system in spite of the low concentration of magnetic ions.

cond-mat.mtrl-sci↗