Study of electronic, magnetic and thermoelectric properties of oxide heterostructures from density functional theory and beyond

Designing novel electronic properties in artificial transition metal oxide (TMO) based heterostructures, that are distinct from their respective bulk counterparts, has become a new paradigm enabled by the advent of modern layer-by-layer growth techniques. Further understanding and enhancement of applications in the area of TMO heterostructures rely on the knowledge of their electronic structure at the atomic scale. Apart from the electronic properties, TMO based heterostructures are known to exhibit promisingly high thermoelectric response, in addition to their environmental friendliness and high stability. This thesis employs density functional theory (DFT) and methods beyond it to explore the effects of confinement, strain, charge transfer, and heterostructuring on the electronic, magnetic, and thermoelectric properties of TMO heterostructures.

In a systematic study, the effect of t2g orbital occupation and confinement on electronic, magnetic and thermoelectric properties of non-polar (SrXO3)1/(SrTiO3)1(001) superlat- tices (SLs), X = V, Cr or Mn, was explored. To disentangle the effect of confinement and octahedral rotations, tetragonal (P4/mmm) and monoclinic P21/c symmetries are considered. DFT calculations combined with an on-site Coulomb repulsion term U find that ground state SL geometries always display finite octahedral rotations, which drive an orbital reconstruction and a metal-to-insulator transition (MIT) in confined SrVO3 and SrCrO3 single layers with ferro- and antiferromagnetic spin alignments. On the other hand, the confined SrMnO3 single layer exhibits electronic properties similar to bulk. Using Boltzmann transport theory within the constant relaxation time approxima- tion, it is shown that confinement enhances the thermoelectric properties, particularly for SrVO3 and SrCrO3 due to the emergent Mott phase. Large room-temperature See- beck coefficients are obtained for tilted SLs, ranging from 500 to 600 µV/K near band edges. The estimated attainable power factors of 27.9 (26.6) µW/K2 cm (in-plane) for (SrCrO3)1/(SrTiO3)1(001) SL with P4/mmm (P21/c) symmetry and 28.1 µW/K2 cm (cross-plane) for (SrMnO3)1/(SrTiO3)1(001) SL with P21/c symmetry compare favor- ably with some of the best-performing oxide thermoelectrics.

Further focussing on (SrVO3)1/(SrTiO3)1(001) SL, based on DFT calculations with meta-GGA exchange-correlation functional SCAN, this thesis reports distinct mechanisms of MIT due to coupling between electronic and structural properties. The study reveals that DFT + U always favors monoclinic (P21/c) symmetry in (SrXO3)1/(SrTiO3)1(001) SLs, X being a transition metal cation, irrespective of in-plane lattice constant and X, beyond a critical value of U which depends on the exchange-correlation functional. Notably, the SCAN functional is sufficient to determine the
ground state structure and possible symmetry breaking at a given lateral lattice constant and best describes the electronic and magnetic properties of weakly correlated (SrRuO3)1/(SrTiO3)1(001) SL and its constituents by minimizing the self-interaction error. However, an additional Hubbard U term is necessary for the strongly correlated (SrVO3)1/(SrTiO3)1(001) SLs. For orthorhombic (SrVO3)1/(SrTiO3)1(001) SL (Cmmm) at a SrTiO3 (tensile strain +1.7%), strong correlation and confinement drives Mott-Hubbard
type MIT via long-range stripe antiferromagnetic ordering, whereas, under compressive strain (3.6%) at a YAlO3, interplay of confinement, correlation, and finite octahedral tilts and rotations lead to monoclinic (P21/c) symmetry, which drives an orbital reconstruction and a concomitant MIT with ferromagnetic spin alignment. Further, for (SrVO3)1/(SrTiO3)1(001) SL at a SrTiO3, large n-type Seebeck coefficients S of 566 (in-plane) and 454 µV/K (cross-plane), respectively, along with an in-plane (cross-plane)
power factor of 31.4 (8.5) µW/K2 cm (assuming τ =4 fs) are obtained at 300 K.

Apart from demonstrating novel ways to maximize the electronic power factor in (SrXO3)1/(SrTiO3)1(001) SLs, this thesis further assesses the impact of heterostructuring on the lattice thermal conductivity, a key ingredient in determining the overall figure of merit ZT . For (SrMnO3)1/(SrTiO3)1(001) SL, DFT + U predicts an insulating phase along with G-type AFM order, and tetragonal (P4/mbm) symmetry in the ground state. The lattice dynamics study within the harmonic approximation in conjunction with DFT + U highlights the presence of competing tetragonal (P4/mbm) and monoclinic (P21/c) symmetries in (SrMnO3)1/(SrTiO3)1(001) SL. Next, by including anharmonic contributions arising from many-body phonon-phonon interactions in the lattice dynamics calculation, a significantly reduced cross-plane lattice thermal conductivity (κzz) of 11 W/m K in (SrMnO3)1/(SrTiO3)1(001) SL, compared to the cross-plane lattice ther-
mal conductivity (κzz) of low T tetragonal (I4/mcm) phase of bulk SrTiO3 (18.2 W/mK), were obtained at 100 K. However, the value of κzz in (SrMnO3)1/(SrTiO3)1(001) SL is similar to the lattice thermal conductivity of cubic (Pm3m) bulk SrMnO3, at 100 K. This augments the role of heterostructuring in reducing the lattice thermal conductivity, compared to their bulk counterparts. Finally, doping (SrMnO3)1/(SrTiO3)1(001) SL by a
p-type dopant (Scandium), the Fermi level shifts to the valence band edge, still preserving the antiferromagnetic electronic phase along with a promisingly high cross-plane power factor of 20.8 µW/K2 cm (assuming τ =4 fs) at 300 K.

Besides SrTiO3 based SLs, this thesis explores the effect of oxygen va-
cancies on the electronic, magnetic, and thermoelectric properties in ultrathin (LaNiO3−δ)1/(LaAlO3)1(001) SLs using a combination of DFT + U and Boltzmann transport theory within the constant relaxation time approximation. At δ = 0.125 and 0.25, the interplay of confinement and strain triggers a varying degree of charge disproportionation, resulting in ferrimagnetic insulators. However, at δ = 0.5, an antiferromagnetic charge-
disproportionated insulating phase emerges with alternating stripes of Ni2+ (high-spin) and Ni2+ (low-spin) and oxygen vacancies ordered along the [110] direction. This phase shows a robust n-type in-plane power factor of 12.4 µW/K2 cm (assuming τ =4 fs) for δ =0.5, at 300 K.

In addition, this thesis presents a DFT + U study of the electronic and magnetic properties of La0.67Ca0.33MnO3 subject to a uniform biaxial tensile strain up to 5%. Calculations reveal a distinct change in electronic and magnetic behavior with a transition from the ferromagnetic metallic ground state to an antiferromagnetic insulating phase. Furthermore,
the strained insulating phase of La0.67Ca0.33MnO3 consists of charge-ordered stripes of Mn4+ and Mn3+ ions within the plane.

Next, based on DFT + U calculations, this thesis explores the mechanism behind the formation of a two-dimensional electron gas (2DEG) at (001) surface of EuTiO3 and compares it with the 2DEG at the structurally similar SrTiO3(001) surface. The calculations reveal that oxygen divacancies, located at the topmost and subsurface layers of TiO2 terminated (001) surfaces of EuTiO3 and SrTiO3, respectively, play a fundamental role in the appearance of spin-polarized ferromagnetic 2DEG. Finally, the origin of
2DEG at the LaAlO3/EuTiO3/SrTiO3(001) oxide interface has been discussed. DFT +U calculations show that an FM spin-polarized q2DEG is formed at defect-free interfaces in (001) LAO/ETO/STO heterostructure. The results demonstrate that the filling of 3dxz/3dyz bands is simultaneous to the emergence of FM state. Moreover, spin-polarized 3dxz/3dyz electrons created at the LAO/ETO interface leak also into the first few layers of
STO, explaining the contribution from STO to the Ti-3d magnetic moment.




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