Mesoscale flux-closure domain formation in single-crystal BaTiO3

Over 60 years ago, Charles Kittel predicted that quadrant domains should spontaneously form in small ferromagnetic platelets. He expected that the direction of magnetization within each quadrant should lie parallel to the platelet surface, minimizing demagnetizing fields,and that magnetic moments should be configured into an overall closed loop, or flux-closure arrangement. Although now a ubiquitous observation in ferromagnets, obvious flux-closure patterns have been somewhat elusive in ferroelectric materials. This is despite the analogous behaviour between these two ferroic subgroups and the recent prediction of dipole closure states by atomistic simulations research. Here we show Piezoresponse Force Microscopy images of mesoscopic dipole closure patterns in free-standing, single-crystal lamellae of BaTiO3. Formation of these patterns is a dynamical process resulting from system relaxation after the BaTiO3 has been poled with a uniform electric field. The flux-closure states are composed of shape conserving 90° stripe domains which minimize disclination stresses

Enhanced electric conductivity at ferroelectric vortex cores in BiFeO3

In many large ensembles, the property of the system as a whole cannot be understood from studying the individual entities alone ¿ these ensembles can be made up by neurons in the brain, transport users in traffic networks or data packages in the Internet. The past decade has seen important progress in our fundamental understanding of what such seemingly disparate 'complex systems' have in common; some of these advances are surveyed here

Controlling domain wall motion in ferroelectric thin films

Domain walls in ferroic materials have attracted significant interest in recent years, in particular because of the unique properties that can be found in their vicinity(1-3). However, to fully harness their potential as nanoscale functional entities(4,5), it is essential to achieve reliable and precise control of their nucleation, location, number and velocity. Here, using piezoresponse forcemicroscopy, we showthe control andmanipulation of domain walls in ferroelectric thin films of Pb(Zr, Ti)O-3 with Pt top electrodes. This high-level control presents an excellent opportunity to demonstrate the versatility and flexibility of ferroelectric domain walls. Their position can be controlled by the tuning of voltage pulses, and multiple domain walls can be nucleated and handled in a reproducible fashion. The system is accurately described by analogy to the classical Stefan problem(6), which has been used previously to describe many diverse systems and is here applied to electric circuits. This study is a step towards the realization of domain wall nanoelectronics utilizing ferroelectric thin films

Velocity Control of 180 degrees Domain Walls in Ferroelectric Thin Films by Electrode Modification

The velocity of individual 1800 domain walls in thin ferroelectric films of PbZr0.1Ti0.9O3 is strongly dependent on the thickness of the top Pt electrode made by electron-beam induced deposition (EBID). We show that when the thickness is varied in the range <100 nm the domain wall velocity is seen to change by 7 orders of magnitude. We attribute this huge range of velocities to the similarly large range of resistivities for the EBID Pt electrode as extrapolated from four-point probe measurements. The domain wall motion is governed by the supply of charges to the domain wall, determined by the top electrode resistivity, and which is described using a modified Stefan Problem model. This has significant implications for the feasibility of ferroelectric domain wall nanoelectronics, wherein the speed of operation will be limited by the maximum velocity of the propagating domain wall front. Furthermore, by introducing sections of either modified thickness or width along the length of a "line" electrode, the domain wall velocity can be changed at these locations, opening up possibilities for dynamic regimes

Room temperature concurrent formation of ultra-dense arrays of ferroelectric domain walls

Properties of ferroelectric domain walls are attractive for future nano- and optoelectronics. An important element is the potential to electrically erase/rewrite domain walls inside working devices. Dense domain wall patterns, formed upon cooling through the ferroelectric phase transition, were demonstrated. However, room temperature domain wall writing is done with a cantilever tip, one domain stripe at a time, and reduction of the inter-wall distance is limited by the tip diameter. Here, we show, at room temperature, controlled formation of arrays of domain walls with sub-tip-diameter spacing (i.e., inter-wall distance down to approximate to 10 nm). Each array contains 100-200 concurrently formed walls. Array rewriting is confirmed. The method is demonstrated in several materials. Dense domain pattern formation through a continuous electrode, practical for potential device applications, is also demonstrated. A quantitative theory of the phenomenon is provided. (C) 2015 AIP Publishing LLC

Self-Similar Nested Flux Closure Structures in a Tetragonal Ferroelectric

In specific solid-state materials, under the right conditions, collections of magnetic dipoles are known to spontaneously form into a variety of rather complex geometrical patterns, exemplified by vortex and skyrmion structures. While theoretically, similar patterns should be expected to form from electrical dipoles, they have not been clearly observed to date: the need for continued experimental exploration is therefore clear. In this Letter we report the discovery of a rather complex domain arrangement that has spontaneously formed along the edges of a thin single crystal ferroelectric sheet, due to surface-related depolarizing fields. Polarization patterns are such that nanoscale "flux-closure" loops are nested within a larger mesoscale flux closure object Despite the orders of magnitude differences in size, the geometric forms of the dual-scale flux closure entities are rather similar.</p

Piezoelectric enhancement under negative pressure

Enhancement of ferroelectric properties, both spontaneous polarization and Curie temperature under negative pressure had been predicted in the past from first principles and recently confirmed experimentally. In contrast, piezoelectric properties are expected to increase by positive pressure, through polarization rotation. Here we investigate the piezoelectric response of the classical PbTiO3, Pb(Zr,Ti)O-3 and BaTiO3 perovskite ferroelectrics under negative pressure from first principles and find significant enhancement. Piezoelectric response is then tested experimentally on free-standing PbTiO3 and Pb(Zr,Ti)O-3 nanowires under self-sustained negative pressure, confirming the theoretical prediction. Numerical simulations verify that negative pressure in nanowires is the origin of the enhanced electromechanical properties. The results may be useful in the development of highly performing piezoelectrics, including lead-free ones

Maximized electron interactions at the magic angle in twisted bilayer graphene

The electronic properties of heterostructures of atomically thin van der Waals crystals can be modified substantially by moiré superlattice potentials from an interlayer twist between crystals1,2. Moiré tuning of the band structure has led to the recent discovery of superconductivity3,4 and correlated insulating phases5 in twisted bilayer graphene (TBG) near the ‘magic angle’ of twist of about 1.1 degrees, with a phase diagram reminiscent of high-transition-temperature superconductors. Here we directly map the atomic-scale structural and electronic properties of TBG near the magic angle using scanning tunnelling microscopy and spectroscopy. We observe two distinct van Hove singularities (VHSs) in the local density of states around the magic angle, with an energy separation of 57 millielectronvolts that drops to 40 millielectronvolts with high electron/hole doping. Unexpectedly, the VHS energy separation continues to decrease with decreasing twist angle, with a lowest value of 7 to 13 millielectronvolts at a magic angle of 0.79 degrees. More crucial to the correlated behaviour of this material, we find that at the magic angle, the ratio of the Coulomb interaction to the bandwidth of each individual VHS (U/t) is maximized, which is optimal for electronic Cooper pairing mechanisms. When doped near the half-moiré-band filling, a correlation-induced gap splits the conduction VHS with a maximum size of 6.5 millielectronvolts at 1.15 degrees, dropping to 4 millielectronvolts at 0.79 degrees. We capture the doping-dependent and angle-dependent spectroscopy results using a Hartree–Fock model, which allows us to extract the on-site and nearest-neighbour Coulomb interactions. This analysis yields a U/t of order unity indicating that magic-angle TBG is moderately correlated. In addition, scanning tunnelling spectroscopy maps reveal an energy- and doping-dependent three-fold rotational-symmetry breaking of the local density of states in TBG, with the strongest symmetry breaking near the Fermi level and further enhanced when doped to the correlated gap regime. This indicates the presence of a strong electronic nematic susceptibility or even nematic order in TBG in regions of the phase diagram where superconductivity is observed