Atomic-scale observation of structural and electronic orders in the layered compound α-RuCl3.

A pseudospin-1/2 Mott phase on a honeycomb lattice is proposed to host the celebrated two-dimensional Kitaev model which has an elusive quantum spin liquid ground state, and fascinating physics relevant to the development of future templates towards topological quantum bits. Here we report a comprehensive, atomically resolved real-space study by scanning transmission electron and scanning tunnelling microscopies on a novel layered material displaying Kitaev physics, α-RuCl3. Our local crystallography analysis reveals considerable variations in the geometry of the ligand sublattice in thin films of α-RuCl3 that opens a way to realization of a spatially inhomogeneous magnetic ground state at the nanometre length scale. Using scanning tunnelling techniques, we observe the electronic energy gap of ≈0.25 eV and intra-unit cell symmetry breaking of charge distribution in individual α-RuCl3 surface layer. The corresponding charge-ordered pattern has a fine structure associated with two different types of charge disproportionation at Cl-terminated surface.

Novel Iron-based ternary amorphous oxide semiconductor with very high transparency, electronic conductivity, and mobility.

Here we report that ternary metal oxides of type (Me)2O3 with the primary metal (Me) constituent being Fe (66 atomic (at.) %) along with the two Lanthanide elements Tb (10 at.%) and Dy (24 at.%) can show excellent semiconducting transport properties. Thin films prepared by pulsed laser deposition at room temperature followed by ambient oxidation showed very high electronic conductivity (>5 × 10(4) S/m) and Hall mobility (>30 cm(2)/V-s). These films had an amorphous microstructure which was stable to at least 500 °C and large optical transparency with a direct band gap of 2.85 ± 0.14 eV. This material shows emergent semiconducting behavior with significantly higher conductivity and mobility than the constituent insulating oxides. Since these results demonstrate a new way to modify the behaviors of transition metal oxides made from unfilled d- and/or f-subshells, a new class of functional transparent conducting oxide materials could be envisioned.

Band excitation in scanning probe microscopy: recognition and functional imaging.

Field confinement at the junction between a biased scanning probe microscope's tip and solid surface enables local probing of various bias-induced transformations, such as polarization switching, ionic motion, and electrochemical reactions. The nanoscale size of the biased region, smaller or comparable to that of features such as grain boundaries and dislocations, potentially allows for the study of kinetics and thermodynamics at the level of a single defect. In contrast to classical statistically averaged approaches, this approach allows one to link structure to functionality and deterministically decipher associated mesoscopic and atomistic mechanisms. Furthermore, responses measured as a function of frequency and bias can serve as a fingerprint of local material functionality, allowing for local recognition imaging of inorganic and biological systems. This article reviews current progress in multidimensional scanning probe microscopy techniques based on band excitation time and voltage spectroscopies, including discussions on data acquisition, dimensionality reduction, and visualization, along with future challenges and opportunities for the field.

Domain wall conductivity in La-doped BiFeO3.

The transport physics of domain wall conductivity in La-doped bismuth ferrite (BiFeO3) has been probed using variable temperature conducting atomic force microscopy and piezoresponse force microscopy in samples with arrays of domain walls in the as-grown state. Nanoscale current measurements are investigated as a function of bias and temperature and are shown to be consistent with distinct electronic properties at the domain walls leading to changes in the observed local conductivity. Our observation is well described within a band picture of the observed electronic conduction. Finally, we demonstrate an additional degree of control of the wall conductivity through chemical doping with oxygen vacancies, thus influencing the local conductive state.

Deterministic control of ferroelastic switching in multiferroic materials.

Multiferroic materials showing coupled electric, magnetic and elastic orderings provide a platform to explore complexity and new paradigms for memory and logic devices. Until now, the deterministic control of non-ferroelectric order parameters in multiferroics has been elusive. Here, we demonstrate deterministic ferroelastic switching in rhombohedral BiFeO(3) by domain nucleation with a scanning probe. We are able to select among final states that have the same electrostatic energy, but differ dramatically in elastic or magnetic order, by applying voltage to the probe while it is in lateral motion. We also demonstrate the controlled creation of a ferrotoroidal order parameter. The ability to control local elastic, magnetic and torroidal order parameters with an electric field will make it possible to probe local strain and magnetic ordering, and engineer various magnetoelectric, domain-wall-based and strain-coupled devices.

Electric modulation of conduction in multiferroic Ca-doped BiFeO3 films.

Many interesting materials phenomena such as the emergence of high-Tc superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO3 through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of approximately 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO3.

Controlling polarization dynamics in a liquid environment: from localized to macroscopic switching in ferroelectrics.

The effect of disorder on polarization switching in ferroelectric materials is studied using piezoresponse force microscopy in a liquid environment. The spatial extent of the electric field created by a biased tip is controlled by the choice of medium, resulting in a transition from localized switching dictated by tip radius, to uniform switching across the film. In the localized regime, the formation of fractal domains has been observed with dimensionality controlled by the length scale of the frozen disorder. In the nonlocal regime, preferential nucleation at defect sites and the presence of long-range correlations has been observed.

High resolution electromechanical imaging of ferroelectric materials in a liquid environment by piezoresponse force microscopy.

High-resolution imaging of ferroelectric materials using piezoresponse force microscopy (PFM) is demonstrated in an aqueous environment. The elimination of both long-range electrostatic forces and capillary interactions results in a localization of the ac field to the tip-surface junction and allows the tip-surface contact area to be controlled. This approach results in spatial resolutions approaching the limit of the intrinsic domain-wall width. Imaging at frequencies corresponding to high-order cantilever resonances minimizes the viscous damping and added mass effects on cantilever dynamics and allows sensitivities comparable to ambient conditions. PFM in liquids will provide novel opportunities for high-resolution studies of ferroelectric materials, imaging of soft polymer materials, and imaging of biological systems in physiological environments on, ultimately, the molecular level.

Bioelectromechanical imaging by scanning probe microscopy: Galvani's experiment at the nanoscale.

Since the discovery in the late 18th century of electrically induced mechanical response in muscle tissue, coupling between electrical and mechanical phenomena has been shown to be a near-universal feature of biological systems. Here, we employ scanning probe microscopy (SPM) to measure the sub-Angstrom mechanical response of a biological system induced by an electric bias applied to a conductive SPM tip. Visualization of the spiral shape and orientation of protein fibrils with 5 nm spatial resolution in a human tooth and chitin molecular bundle orientation in a butterfly wing is demonstrated. In particular, the applicability of SPM-based techniques for the determination of molecular orientation is discussed.

Electromechanical imaging of biomaterials by scanning probe microscopy.

The majority of calcified and connective tissues possess complex hierarchical structure spanning the length scales from nanometers to millimeters. Understanding the biological functionality of these materials requires reliable methods for structural imaging on the nanoscale. Here, we demonstrate an approach for electromechanical imaging of the structure of biological samples on the length scales from tens of microns to nanometers using piezoresponse force microscopy (PFM), which utilizes the intrinsic piezoelectricity of biopolymers such as proteins and polysaccharides as the basis for high-resolution imaging. Nanostructural imaging of a variety of protein-based materials, including tooth, antler, and cartilage, is demonstrated. Visualization of protein fibrils with sub-10nm spatial resolution in a human tooth is achieved. Given the near-ubiquitous presence of piezoelectricity in biological systems, PFM is suggested as a versatile tool for micro- and nanostructural imaging in both connective and calcified tissues.

Spatial resolution, information limit, and contrast transfer in piezoresponse force microscopy.

Scanning probe-based ferroelectric domain imaging and patterning has attracted broad attention for use in the characterization of ferroelectric materials, ultrahigh density data storage, and nanofabrication. The viability of these applications is limited by the minimal domain size that can be fabricated and reliably detected by scanning probe microscopy. Here, the contrast transfer mechanism in piezoresponse force microscopy (PFM) of ferroelectric materials is analysed in detail. A consistent definition of resolution is developed both for the writing and the imaging processes, and the concept of an information limit in PFM is established. Experimental determination of the object transfer function and the subsequent reconstruction of an 'ideal image' is demonstrated. This contrast transfer theory provides a quantitative basis for image interpretation and allows for the comparison of different instruments in PFM. It is shown that experimentally observed domain sizes can be limited by the resolution of the scanning probe microscope to the order of tens of nanometres even though smaller domains, of the order of several nanometres, can be created.

Electronic stability of magnetic Fe/Co superlattices with monatomic layer alternation.

We report a surprising observation that the growth of the [Fe(1 ML)/Co(1 ML)](n) superlattice of L1(0) structure on Cu(100) is stable only up to six atomic layers (n=3), which cannot be rationalized by stress arguments. Instead, first-principles calculations reveal a transition from the L1(0) to the B2 structure due to the effect of dimensionality on the stability of the electronic structure of the superlattice. Whereas the majority-spin electrons are energetically insensitive to the layer thickness, the minority-spin electrons induce the transition at n=3.