Huaxiang Fu

Polarization rotation mechanism for ultrahigh electromechanical response in single-crystal piezoelectrics

Piezoelectric materials, which convert mechanical to electrical energy (and vice versa), are crucial in medical imaging, telecommunication and ultrasonic devices. A new generation of single-crystal materials, such as Pb(Zn1/3Nb2/3)O3–PbTiO 3 (PZN–PT) and Pb(Mg1/3Nb2/3)O3–PbTiO 3 (PMN–PT), exhibit a piezoelectric effect that is ten times larger than conventional ceramics, and may revolutionize these applications. However, the mechanism underlying the ultrahigh performance of these new materials—and consequently the possibilities for further improvements—are not at present clear. Here we report a first-principles study of the ferroelectric perovskite, BaTiO3, which is similar to single-crystal PZN–PT but is a simpler system to analyse. We show that a large piezoelectric response can be driven by polarization rotation induced by an external electric field. Our computations suggest how to design materials with better performance, and may stimulate further interest in the fundamental theory of dielectric systems in finite electric fields.

Unusual phase transitions in ferroelectric nanodisks and nanorods

Bulk ferroelectrics undergo structural phase transformations at low temperatures, giving multi-stable (that is, multiple-minimum) degenerate states with spontaneous polarization. Accessing these states by applying, and varying the direction of, an external electric field is a key principle for the operation of devices such as non-volatile ferroelectric random access memories (NFERAMs). Compared with bulk ferroelectrics, low-dimensional finite ferroelectric structures promise to increase the storage density of NFERAMs 10,000-fold. But this anticipated benefit hinges on whether phase transitions and multi-stable states still exist in low-dimensional structures. Previous studies have suggested that phase transitions are impossible in one-dimensional systems, and become increasingly less likely as dimensionality further decreases. Here we perform ab initio studies of ferroelectric nanoscale disks and rods of technologically important Pb(Zr,Ti)O3 solid solutions, and demonstrate the existence of previously unknown phase transitions in zero-dimensional ferroelectric nanoparticles. The minimum diameter of the disks that display low-temperature structural bistability is determined to be 3.2 nm, enabling an ultimate NFERAM density of 60 × 1012 bits per square inch—that is, five orders of magnitude larger than those currently available. Our results suggest an innovative use of ferroelectric nanostructures for data storage, and are of fundamental value for the theory of phase transition in systems of low dimensionality.

Ultrathin films of ferroelectric solid solutions under a residual depolarizing field.

A first-principles-derived approach is developed to study the effects of depolarizing electric fields on the properties of Pb(Zr,Ti)O3 ultrathin films for different mechanical boundary conditions. A rich variety of ferroelectric phases and polarization patterns is found, depending on the interplay between strain and the amount of screening of surface charges. Examples include triclinic phases, monoclinic states with in-plane and/or out-of-plane components of the polarization, homogeneous and inhomogeneous tetragonal states, as well as peculiar laminar nanodomains.

Atomistic treatment of depolarizing energy and field in ferroelectric nanostructures

An atomistic approach allowing an accurate and efficient treatment of depolarizing energy and field in any low-dimensional ferroelectric structure is developed. Application of this approach demonstrates the limits of the widely used continuum model (even) for simple test cases. Moreover, implementation of this approach within a first-principles-based model reveals an unusual phase transition---from a state exhibiting a spontaneous polarization to a phase associated with a toroid moment of polarization---in a ferroelectric nanodot for a critical value of the depolarizing field.

Vortex-to-polarization phase transformation path in ferroelectric Pb(ZrTi)O3 nanoparticles

Phase transformation in finite-size ferroelectrics is of fundamental relevance for understanding collective behaviors and balance of competing interactions in low-dimensional systems. We report a first-principles effective Hamiltonian study of vortex-to-polarization transformation in Pb(Zr0.5Ti0.5)O3 nanoparticles, caused by homogeneous electric fields normal to the vortex plane. The transformation is shown to (1) follow an unusual macroscopic path that is symmetry nonconforming and characterized by the occurrence of a previously unknown structure as the bridging phase, and (2) lead to the discovery of a striking collective phenomenon, revealing how ferroelectric vortex is annihilated microscopically. Interactions underlying these behaviors are discussed.

Comparison of the k⋅p and the direct diagonalization approaches for describing the electronic structure of quantum dots

It is shown that the standard (decoupled) 6×6 k⋅p effective-mass approach for semiconductor quantum dots overestimates significantly the hole and electron confinement energies, and, for dots made of materials with small spin-orbit coupling (e.g., phosphides, sulphides) produces a reverse order of s- and p-like valence states. By contrasting the electronic structures of dots as obtained by a direct diagonalization (multiband) pseudopotential approach and by its k⋅p approximation, we are able to trace the systematic errors of k⋅p in dots to the k⋅p errors in the underlying bulk solids. This suggests a “diagnostic tool” and a strategy for improving the k⋅p.

Cooperative response of Pb(ZrTi)O3 nanoparticles to curled electric fields.

We investigate cooperative responses, as well as a microscopic mechanism for vortex switching, in Pb(Zr0.5Ti0.5)O3 nanoparticles under curled electric fields. We find that the domain coexistence mechanism is not valid for toroid switching. Instead dipoles display unusual collective behavior by forming a new vortex with a perpendicular (not opposite) toroid moment. The correlation between the new and original vortices is revealed to be critical for reversing the toroid moment. We further describe a technological approach that is able to drastically reduce the curled electric field needed for vortex switching.

Density-functional study of organic-inorganic hybrid single crystal ZnSe(C2H8N2)(1/2).

Unusual properties (i.e., strong band dispersion, high carrier mobility, wide absorption-energy window, and sharp band-edge transition) that are desirable for hybrid-material electronics and for solar electric energy conversion are predicted to exist in the organic-inorganic chalcogenide single crystal ZnSe(C2H8N2)(1/2) by using density-functional calculations. A simple mechanism, namely that the band-edge electronic states of the hybrid composite is predominantly determined by the inorganic constituent, is revealed to be responsible for governing these properties. Suggestions for further engineering hybrid semiconductors are also provided

Properties of Pb ( Zr , Ti ) O 3 ultrathin films under stress-free and open-circuit electrical boundary conditions

A first-principles-based scheme is developed to simulate properties of (001) PbO-terminated

Collective Dipole Behavior and Unusual Morphotropic Phase Boundary in Ferroelectric Pb(Zr0.5Ti0.5)O3 Nanowires

\mathrm{Pb}({\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}){\mathrm{O}}_{3}