Robust Preparation of Sub-20-nm-Thin Lamellae for Aberration-Corrected Electron Microscopy.

Aberration-corrected scanning transmission electron microscopy (STEM) has been advancing resolution, sensitivity, and microanalysis due to the intense demands of atomic-level microstructural investigations. Recent STEM technologies require preparing a thin lamella whose thickness is ideally below 20 nm. Although focused-ion-beam/scanning-electron-microscopy (FIB/SEM) is an established method to prepare a high-quality lamella, nanometer-level controllability of lamella thickness remains a fundamental problem. Here, the robust preparation of a sub-20-nm-thin lamella is demonstrated by FIB/SEM with real-time feedback from thickness quantification. The lamella thickness is quantified by back-scattered-electron SEM imaging in a thickness range between 0 and 100 nm without any reference to numerical simulation. Using real-time feedback from the thickness quantification, the FIB/SEM terminates thinning a lamella at a targeted thickness. The real-time feedback system eventually provides 1-nm-level controllability of the lamella thickness. As a proof-of-concept, a near-10-nm-thin lamella is prepared from a SrTiO3 crystal by our methodology. Moreover, the lamella thickness is controllable at a target heterointerface. Thus, a sub-20-nm-thin lamella is prepared from a LaAlO3 /SrTiO3 heterointerface. The methodology offers a robust and operator-independent platform to prepare a sub-20-nm-thin lamella from various materials. This platform will broadly impact aberration-corrected STEM studies in materials science and the semiconductor industry.

Epitaxy and transport properties of alkali-earth palladate thin films.

Topological insulators and semimetals are an interesting class of materials for new electronic and optical applications owing to their characteristic electromagnetic responses originating from the spin-orbit coupled band structures. However, topological electronic structures are rare in oxide materials despite their chemical stability being preferable for applications. In this study, given the theoretical prediction of Dirac bands in CaPd3O4, we investigate the fabrication and transport properties of SrPd3O4 and CaPd3O4 thin films as candidates of oxide Dirac semimetals. We have found that these materials are epitaxially grown on MgO (100) substrate under limited growth conditions by pulsed laser deposition. The transport properties show a weak temperature dependence, suggestive of narrow-gap properties, although unintentionally doped holes hinder us from revealing the presence of the Dirac band. Our study establishes the basic thermodynamics of thin-film fabrication of these materials and will lead to interesting properties characteristic of topological band structure by modulating the electronic structure by, for example, chemical substitutions or pressure.

A cascade of phase transitions in an orbitally mixed half-filled Landau level.

Half-filled Landau levels host an emergent Fermi liquid that displays instability toward pairing, culminating in a gapped even-denominator fractional quantum Hall ground state. While this pairing may be probed by tuning the polarization of carriers in competing orbital and spin degrees of freedom, sufficiently high quality platforms offering such tunability remain few. We explore the ground states at filling factor ν = 5/2 in ZnO-based two-dimensional electron systems through a forced intersection of opposing spin branches of Landau levels taking quantum numbers N = 1 and 0. We reveal a cascade of phases with distinct magnetotransport features including a gapped phase polarized in the N = 1 level and a compressible phase in N = 0, along with an unexpected Fermi liquid, a second gapped, and a strongly anisotropic nematic-like phase at intermediate polarizations when the levels are near degeneracy. The phase diagram is produced by analyzing the proximity of the intersecting levels and highlights the excellent reproducibility and controllability that ZnO offers for exploring exotic fractionalized electronic phases.

Gate-tuned quantum Hall states in Dirac semimetal (Cd1-x Zn x )3As2.

The recent discovery of topological Dirac semimetals (DSMs) has provoked intense curiosity not only regarding Weyl physics in solids but also about topological phase transitions originating from DSMs. One specific area of interest is controlling the dimensionality to realize two-dimensional quantum phases such as quantum Hall and quantum spin Hall states. For investigating these phases, the Fermi level is a key controlling parameter. From this perspective, we report the carrier density control of quantum Hall states realized in thin films of DSM Cd3As2. Chemical doping of Zn combined with electrostatic gating has enabled us to tune the carrier density both over a wide range and continuously, even across the charge neutrality point. Comprehensive analyses of gate-tuned quantum transport have revealed Landau-level formation from linearly dispersed sub-bands and its contribution to the quantum Hall states. Our findings also pave the way for investigating the low-energy physics near the Dirac points of DSMs.

Structural characterisation of high-mobility Cd3As2 films crystallised on SrTiO3.

Cd3As2 has long been known as a high-mobility semiconductor. The recent finding of a topological semimetal state in this compound has demanded growth of epitaxial films with high crystallinity and controlled thickness. Here we report the structural characterisation of Cd3As2 films grown on SrTiO3 substrates by solid-phase epitaxy at high temperatures up to 600 °C by employing optimised capping layers and substrates. The As triangular lattice is epitaxially stacked on the Ti square lattice of the (001) SrTiO3 substrate, producing (112)-oriented Cd3As2 films exhibiting high crystallinity with a rocking-curve width of 0.02° and a high electron mobility exceeding 30,000 cm2/Vs. The systematic characterisation of films annealed at various temperatures allowed us to identify two-step crystallisation processes in which out-of-plane and subsequently in-plane directions occur with increasing annealing temperature. Our findings on the high-temperature crystallisation process of Cd3As2 enable a unique approach for fabricating high-quality Cd3As2 films and elucidating quantum transport by back gating through the SrTiO3 substrate.

Electric-field control of anomalous and topological Hall effects in oxide bilayer thin films.

One of the key goals in spintronics is to tame the spin-orbit coupling (SOC) that links spin and motion of electrons, giving rise to intriguing magneto-transport properties in itinerant magnets. Prominent examples of such SOC-based phenomena are the anomalous and topological Hall effects. However, controlling them with electric fields has remained unachieved since an electric field tends to be screened in itinerant magnets. Here we demonstrate that both anomalous and topological Hall effects can be modulated by electric fields in oxide heterostructures consisting of ferromagnetic SrRuO3 and nonmagnetic SrIrO3. We observe a clear electric field effect only when SrIrO3 is inserted between SrRuO3 and a gate dielectric. Our results establish that strong SOC of nonmagnetic materials such as SrIrO3 is essential in electrical tuning of these Hall effects and possibly other SOC-related phenomena.

Quantum Hall states observed in thin films of Dirac semimetal Cd3As2.

A well known semiconductor Cd3As2 has reentered the spotlight due to its unique electronic structure and quantum transport phenomena as a topological Dirac semimetal. For elucidating and controlling its topological quantum state, high-quality Cd3As2 thin films have been highly desired. Here we report the development of an elaborate growth technique of high-crystallinity and high-mobility Cd3As2 films with controlled thicknesses and the observation of quantum Hall effect dependent on the film thickness. With decreasing the film thickness to 10 nm, the quantum Hall states exhibit variations such as a change in the spin degeneracy reflecting the Dirac dispersion with a large Fermi velocity. Details of the electronic structure including subband splitting and gap opening are identified from the quantum transport depending on the confinement thickness, suggesting the presence of a two-dimensional topological insulating phase. The demonstration of quantum Hall states in our high-quality Cd3As2 films paves a road to study quantum transport and device application in topological Dirac semimetal and its derivative phases.

Evolution of Insulator-Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating.

An interface between an oxide and an electrolyte gives rise to various processes as exemplified by electrostatic charge accumulation/depletion and electrochemical reactions such as intercalation/decalation under electric field. Here we directly compare typical device operations of those in electric double layer transistor geometry by adopting A-site vacant perovskite WO3 epitaxial thin films as a channel material and two different electrolytes as gating agent. In situ measurements of X-ray diffraction and channel resistance performed during the gating revealed that in both the cases WO3 thin film reaches a new metallic state through multiple phase transitions, accompanied by the change in out-of-plane lattice constant. Electrons are electrostatically accumulated from the interface side with an ionic liquid, while alkaline metal ions are more uniformly intercalated into the film with a polymer electrolyte. We systematically demonstrate this difference in the electrostatic and electrochemical processes, by comparing doped carrier density, lattice deformation behavior, and time constant of the phase transitions.

MgZnO/ZnO heterostructures with electron mobility exceeding 1 × 10(6) cm(2)/Vs.

The inherently complex chemical and crystallographic nature of oxide materials has suppressed the purities achievable in laboratory environments, obscuring the rich physical degrees of freedom these systems host. In this manuscript we provide a systematic approach to defect identification and management in oxide molecular beam epitaxy grown MgZnO/ZnO heterostructures which host two-dimensional electron systems. We achieve samples displaying electron mobilities in excess of 1 × 10(6) cm(2)/Vs. This data set for the MgZnO/ZnO system firmly establishes that the crystalline quality has become comparable to traditional semiconductor materials.