Search

Hafnia (HfO_{2})-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. Identifying and stabilizing the ferroelectric phases of HfO_{2} have attracted intensive research interest in recent years. In this work, first-principles calculations on (111)-oriented HfO_{2} are used to discover that imposing an in-plane shear strain on the metastable tetragonal phase drives it to a polar phase. This in-plane-shear-induced polar phase is shown to be an epitaxial-strain-induced distortion of a previously proposed metastable ferroelectric Pnm2_{1} phase of HfO_{2}. This ferroelectric Pnm2_{1} phase can account for the recently observed ferroelectricity in (111)-oriented HfO_{2}-based thin films on a SrTiO_{3} (STO) (001) substrate [Nat. Mater. 17, 1095 (2018)NMAACR1476-112210.1038/s41563-018-0196-0]. Further investigation of this alternative ferroelectric phase of HfO_{2} could potentially improve the performances of HfO_{2}-based films in logic and memory devices.

MoTe_{2} has recently attracted much attention due to the observation of pressure-induced superconductivity, exotic topological phase transitions, and nonlinear quantum effects. However, there has been debate on the intriguing structural phase transitions among various observed phases of MoTe_{2} and their connection to the underlying topological electronic properties. In this work, by means of density-functional theory calculations, we investigate the structural phase transition between the polar T_{d} and nonpolar 1T^{'} phases of MoTe_{2} in reference to a hypothetical high-symmetry T_{0} phase that exhibits higher-order topological features. In the T_{d} phase we obtain a total of 12 Weyl points, which can be created/annihilated, dynamically manipulated, and switched by tuning a polar phonon mode. We also report the existence of a tunable nonlinear Hall effect in T_{d}-MoTe_{2} and propose the use of this effect as a probe for the detection of polarity orientation in polar (semi)metals. By studying the role of dimensionality, we identify a configuration in which a nonlinear surface response current emerges. The potential technological applications of the tunable Weyl phase and the nonlinear Hall effect are discussed.

We introduce antiferroelectric topological insulators as a new class of functional materials in which an electric field can be used to control topological order and induce topological phase transitions. Using first principles methods, we predict that several alkali-MgBi orthorhombic members of an ABC family of compounds are antiferroelectric topological insulators. We also show that epitaxial strain and hydrostatic pressure can be used to tune the topological order and the band gap of these ABC compounds. Antiferroelectric topological insulators could enable precise control of topology using electric fields, enhancing the applicability of topological materials in electronics and spintronics.

The structure and properties of the 1∶1 superlattice of LaVO_{3} and SrVO_{3} are investigated with a first-principles density-functional-theory-plus-U (DFT+U) method. The lowest energy states are antiferromagnetic charge-ordered Mott-insulating phases. In one of these insulating phases, layered charge ordering combines with the layered La/Sr cation ordering to produce a polar structure with a large nonzero spontaneous polarization normal to the interfaces. This polarization, comparable to that of conventional ferroelectrics, is produced by electron transfer between the V^{3+} and V^{4+} layers. The energy of this normal-polarization state relative to the ground state is only 3 meV per vanadium. Under tensile strain, this energy difference can be further reduced, suggesting that the normal-polarization state can be induced by an electric field applied normal to the superlattice layers, yielding an antiferroelectric double-hysteresis loop. If the system does not switch back to the ground state on removal of the field, a ferroelectric-type hysteresis loop could be observed.

Using first-principles calculations, we determined the epitaxial-strain dependence of the ground state of the 1∶1 SrCrO(3)/SrTiO(3) superlattice. The superlattice layering leads to significant changes in the electronic states near the Fermi level, derived from Cr t(2g) orbitals. An insulating phase is found when the tensile strain is greater than 2.2% relative to unstrained cubic SrTiO(3). The insulating character is shown to arise from Cr t(2g) orbital ordering, which is produced by an in-plane polar distortion that couples to the superlattice d bands and is stabilized by epitaxial strain. This effect can be used to engineer the band structure near the Fermi level in transition metal oxide superlattices.

All known proper ferroelectrics are unable to polarize normal to a surface or interface if the resulting depolarization field is unscreened, but there is no fundamental principle that enforces this behavior. In this work, we introduce hyperferroelectrics, a new class of proper ferroelectrics which polarize even when the depolarization field is unscreened, this condition being equivalent to instability of a longitudinal optic mode in addition to the transverse-optic-mode instability characteristic of proper ferroelectrics. We use first-principles calculations to show that several recently discovered hexagonal ferroelectric semiconductors have this property, and we examine its consequences both in the bulk and in a superlattice geometry.

Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoO(x) (x=2.5-3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to the oxygen content x, are found in the two topotactic phases: i.e., the brownmillerite SrCoO2.5 and the perovskite SrCoO3. First-principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transition, which originate from the modification in the Co valence states and crystallographic structures. More interestingly, the two phases can be reversibly controlled by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides.

We use a first-principles rational-design approach to identify a previously unrecognized class of antiferroelectric materials in the Pnma MgSrSi structure type. The MgSrSi structure type can be described in terms of antipolar distortions of the nonpolar P6(3)/mmc ZrBeSi structure type, and we find many members of this structure type are close in energy to the related polar P6(3)mc LiGaGe structure type, which includes many members we predict to be ferroelectric. We highlight known ABC combinations in which this energy difference is comparable to the antiferroelectric-ferroelectric switching barrier of PbZrO(3). We calculate structural parameters and relative energies for all three structure types, both for reported and as-yet hypothetical representatives of this class. Our results provide guidance for the experimental realization and further investigation of high-performance materials suitable for practical applications.

We use a first-principles rational-design approach to identify a previously unrecognized class of ferroelectric materials in the P6(3)mc LiGaGe structure type. We calculate structural parameters, polarization, and ferroelectric well depths both for reported and as-yet hypothetical representatives of this class. Our results provide guidance for the experimental realization and further investigation of high-performance materials suitable for practical applications.

We use a first-principles rational-design approach to demonstrate the potential of semiconducting half-Heusler compounds as a previously unrecognized class of piezoelectric materials. We perform a high-throughput scan of a large number of compounds, testing for insulating character and calculating structural, dielectric, and piezoelectric properties. Our results provide guidance for the experimental realization and characterization of high-performance materials in this class that may be suitable for practical applications.

First-principles calculations are presented for the epitaxial-strain dependence of the ground-state phase stability of perovskite SrCoO(3). Through the combination of the large spin-phonon coupling with polarization-strain coupling and the coupling of the band gap to the polar distortion, both tensile and compressive epitaxial strain are seen to drive the bulk ferromagnetic-metallic (FM-M) phase to antiferromagnetic-insulating-ferroelectric (AFM-I-FE) phases, the latter having unusually low elastic energy. For compressive strain, there is a single coupled magnetic-ferroelectric metal-insulator transition. At this phase boundary, cross responses to applied electric and magnetic fields and stresses are expected. In particular, a magnetic field or compressive uniaxial stress applied to the AFM-FE(z) phase could induce an insulator-metal transition, and an electric field applied to the FM-M phase could induce a metal-insulator transition.

First-principles calculations reveal a large spin-phonon coupling in cubic SrMnO3, with ferromagnetic ordering producing a polar instability. Through combination of this coupling with the strain-polarization coupling characteristic of perovskites, the bulk antiferromagnetic-paraelectric ground state is driven to a previously unreported multiferroic ferroelectric-ferromagnetic state by increasing epitaxial strain. This state has a computed P(s)>54  μC/cm2 and magnetic T(c)>92  K. Large mixed magnetic-electric-elastic responses are predicted in the vicinity of the phase boundaries.

Using first-principles calculations, we identify a magnetostructural effect in the BiFeO3-BiMnO3 nanocheckerboard that is not to be found in either the bulk parent compound or in BiFeO3-BiMnO3 superlattices with (001)-oriented Fe and Mn layers. The key role of the cation arrangement is explained by a simple model of the exchange coupling between cation spins, leading to magnetic frustration in the checkerboard. We also demonstrate that the atomic-scale checkerboard has a multiferroic ground state with the desired properties of each constituent material: polar and ferrimagnetic due to BiFeO3 and BiMnO3, respectively.

We carry out first-principles calculations of the nonlinear dielectric response of short-period ferroelectric superlattices. We compute and store not only the total polarization, but also the Wannier-based polarizations of individual atomic layers, as a function of the electric displacement field, and use this information to construct a model capable of predicting the nonlinear dielectric response of an arbitrary superlattice sequence. We demonstrate the successful application of our approach to superlattices composed of SrTiO3, CaTiO3, and BaTiO3 layers.

A combined experimental and computational investigation of coupling between polarization and epitaxial strain in highly polar ferroelectric PbZr(0.2)Ti(0.8)O3 (PZT) thin films is reported. A comparison of the properties of relaxed (tetragonality c/a approximately 1.05) and highly strained (c/a approximately 1.09) epitaxial films shows that polarization, while being amongst the highest reported for PZT or PbTiO3 in either film or bulk forms P(r) approximately 82 microC/cm(2)), is almost independent of the epitaxial strain. We attribute this behavior to a suppressed sensitivity of the A-site cations to epitaxial strain in these Pb-based perovskites, where the ferroelectric displacements are already large, contrary to the case of less polar perovskites, such as BaTiO3. In the latter case, the A-site cation (Ba) and equatorial oxygen displacements can lead to substantial polarization increases.

We propose a design strategy--based on the coupling of spins, optical phonons, and strain--for systems in which magnetic (electric) phase control can be achieved by an applied electric (magnetic) field. Using first-principles density-functional theory calculations, we present a realization of this strategy for the magnetic perovskite EuTiO(3).

Multiferroicity can be induced in strontium titanate by applying biaxial strain. Using optical second harmonic generation, we report a transition from 4/mmm to the ferroelectric mm2 phase, followed by a transition to a ferroelastic-ferroelectric mm2 phase in a strontium titanate thin film. Piezoelectric force microscopy is used to study ferroelectric domain switching. Second harmonic generation, combined with phase-field modeling, is used to reveal the mechanism of coupled ferroelectric-ferroelastic domain wall motion. These studies have relevance to multiferroics with coupled polar and axial phenomena.

In insulators, the method of Marzari and Vanderbilt [Phys. Rev. B 56, 12 847 (1997)] can be used to generate maximally localized Wannier functions whose centers are related to the electronic polarization. In the case of layered insulators, this approach can be adapted to provide a natural definition of the local polarization associated with each layer, based on the locations of the nuclear charges and one-dimensional Wannier centers comprising each layer. Here, we use this approach to compute and analyze layer polarizations of ferroelectric perovskite superlattices, including changes in layer polarizations induced by sublattice displacements (i.e., layer-decomposed Born effective charges) and local symmetry breaking at the interfaces. The method provides a powerful tool for analyzing the polarization-related properties of complex layered oxide systems.

We have studied the influence of magnetic order on the optical phonons of the geometrically frustrated spinel ZnCr2O4 within density-functional theory. By mapping local spin density approximation plus Hubbard U (LSDA + U) phonon calculations onto a Heisenberg-like model, we developed a method to calculate exchange derivatives and subsequently the spin-phonon coupling parameter from first principles.

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.