BaTiO 3 ( BTO )/ SrTiO 3 ( STO ) artificial superlattices have been made on MgO (100) substrates. The periodicity of the BTO/STO layers in the superlattice was varied from one-unit cell to 125-unit cell thickness. The dielectric constant and its nonlinearity (or voltage tunability) showed similar behavior as the periodicity was varied. The voltage tunability of the superlattice increased with decreasing stacking periodicity of the BTO/STO within the critical thickness. Similarly, the lattice distortion, i.e., the ratio of the lattice parameter along surface normal to parallel, of the BTO and STO layers increased with decreasing the periodicity. Remarkable enhancement of the voltage tunability has been achieved. The superlattice exhibited large voltage tunability (94%, the highest value to date) at the periodicity of BTO2-unit cell/STO2-unit cell at which the maximum lattice distortion of each layer was obtained. This suggests that the nonlinear dielectric property of the superlattice is closely related with the lattice distortion of the individual layers.

Enhancement of dielectric properties has been demonstrated in BaTiO3 (BTO)/SrTiO3 (STO) strained artificial lattice. Large variation of lattice distortion in the consisting BTO and STO layers was achieved by varying a stacking sequence of BTO and STO layers. From this strain manipulation, it is found that BTO and STO lattices have a dielectric constant sensitively influenced by the lattice distortion and, more importantly, maximum dielectric constant at a certain degree of lattice distortion. An appropriate degree of lattice distortions of the consisting layers is needed to obtain the large dielectric constant and its nonlinearity of the artificial lattice. The artificial lattice exhibited the large dielectric constant (1230) and extremely large nonlinearity (94%) at the periodicity of BTO2 unit cell/STO2 unit cell. These results suggest that the strain is a macroscopically important factor to influence the dielectric properties and can be manipulated via oxide artificial lattice to obtain large dielectric constant and its nonlinearity.

Dielectric behavior on BaTiO3∕SrTiO3 artificial lattices has been investigated along with quantum mechanical simulation (first principles calculation). From the oxide artificial lattice approach, strain manipulation was performed to obtain a wide range of lattice deformation in the consisting BaTiO3 and SrTiO3 layers, which leads to two important consequences. First, we obtained enhanced dielectric constant and extremely large nonlinearity in the artificial lattices with very short stacking periods. Second, it is found that there exists a maximum dielectric constant in each BaTiO3 lattice and SrTiO3 lattice at a certain degree of lattice deformation. The first principles study successfully explains the dielectric behavior of strained BaTiO3 and SrTiO3 lattices, the existence of the maximum dielectric constant. The first principles study on BaTiO3∕SrTiO3 artificial lattices with very short stacking periods also reveals that the artificial lattice undergoes phase transition between the tetragonal and monoclinic phases with a misfit lattice strain and exhibits an anomalous dielectric behavior at the phase boundary. Optical phonon behavior of the BaTiO3∕SrTiO3 artificial lattice resembles that of strained SrTiO3 lattice and optical phonon softening primarily derives the anomaly of the dielectric tensor at the phase boundary. The lattice deformation is a primary influencing factor to phonon and dielectric behaviors rather than interface layer effect in BaTiO3∕SrTiO3 artificial lattice with very short stacking periods.

We determine the structure and static dielectric response of the BaTiO 3 1 unit cell / SrTiO 3 1 unit cell ͑BTO/ STO͒ superlattice as a function of epitaxial strain using first-principles density functional theory ͑DFT͒ calculations based on pseudopotentials and a plane-wave basis. We find a structural transition from the tetragonal phase at compressive strains to the monoclinic phase at tensile strains, with a nonzero in-plane component of polarization in the monoclinic phase as its order parameter. For the stable structures determined as a function of in-plane strain, we obtained optical phonon frequencies, Born effective charges and static dielectric constants using DFT linear response. Our calculations predict a large zero-temperature dielectric response with a strong anisotropy, whose origin is traced to soft phonons of the superlattice.

We performed first-principles calculation to investigate the effect of epitaxial strain on lattice instabilities and polarization behavior of BaTiO3∕SrTiO3 artificial lattice with very short stacking period, i.e., BaTiO31unitcell∕SrTiO31unitcell(BTO∕STO). The structural analysis of BTO∕STO artificial superlattice under in-plane compressive state showed enhanced stability of the tetragonal phase. On the other hand, the stability of monoclinic phase was enhanced when the BTO∕STO was in the in-plane tensile state. The phase transition from tetragonal to the monoclinic phase occurs at the misfit strain of −0.25%. As the misfit strain of BTO∕STO superlattice increases from −0.25%to−1.5% (in-plane compressive state), the tetragonal superlattice exhibits an increasing polarization along the [001] direction. In the monoclinic phase, the polarization of the superlattice rotates progressively toward [110] direction with increasing the misfit strain, and the magnitude of the polarization simultaneously increases with the rotation. The first-principles study shows that the phase stability and polarization vector is sensitively influenced by the lattice misfit strain.