Broken Screw Rotational Symmetry in the Near-Surface Electronic Structure of AB-Stacked Crystals.

We investigate the electronic structure of 2H-NbS_{2} and h-BN by angle-resolved photoemission spectroscopy (ARPES) and photoemission intensity calculations. Although in bulk form, these materials are expected to exhibit band degeneracy in the k_{z}=π/c plane due to screw rotation and time-reversal symmetries, we observe gapped band dispersion near the surface. We extract from first-principles calculations the near-surface electronic structure probed by ARPES and find that the calculated photoemission spectra from the near-surface region reproduce the gapped ARPES spectra.

Semiconducting Electronic Structure of the Ferromagnetic Spinel HgCr_{2}Se_{4} Revealed by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy.

We study the electronic structure of the ferromagnetic spinel HgCr_{2}Se_{4} by soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) and first-principles calculations. While a theoretical study has predicted that this material is a magnetic Weyl semimetal, SX-ARPES measurements give direct evidence for a semiconducting state in the ferromagnetic phase. Band calculations based on the density functional theory with hybrid functionals reproduce the experimentally determined band gap value, and the calculated band dispersion matches well with ARPES experiments.

Observation and control of the weak topological insulator state in ZrTe.

A quantum spin Hall (QSH) insulator hosts topological states at the one-dimensional (1D) edge, along which backscattering by nonmagnetic impurities is strictly prohibited. Its 3D analogue, a weak topological insulator (WTI), possesses similar quasi-1D topological states confined at side surfaces. The enhanced confinement could provide a route for dissipationless current and better advantages for applications relative to strong topological insulators (STIs).

Sacrificial Water-Soluble BaO Layer for Fabricating Free-Standing Piezoelectric Membranes.

The use of a sacrificial water-soluble BaO layer was investigated for the purpose of fabricating free-standing single-crystalline BaTiO membranes for vibrational energy harvesting applications. An epitaxial BaTiO/SrTiO/BaO heterostructure was deposited on a SrTiO(001) substrate by pulsed laser deposition. The sacrificial BaO layer was dissolved by immersing the heterostructure in water.

Strain-induced spontaneous Hall effect in an epitaxial thin film of a Luttinger semimetal.

Pyrochlore iridates have provided a plethora of novel phenomena owing to the combination of topology and correlation. Among them, much attention has been paid to [Formula: see text], as it is known as a Luttinger semimetal characterized by quadratic band touching at the Brillouin zone center, suggesting that the topology of its electronic states can be tuned by a moderate lattice strain and external magnetic field. Here, we report that our epitaxial [Formula: see text] thin films grown by solid-state epitaxy exhibit a spontaneous Hall effect that persists up to 50 K without having spontaneous magnetization within our experimental accuracy.

Noble Metal Nanocluster Formation in Epitaxial Perovskite Thin Films.

We studied the synthesis of nanocomposite materials consisting of noble metal clusters embedded in an oxide semiconductor matrix. The embedded nanostructures form in a simple self-organized single-step growth process. The primary interest is in developing materials for photo-electrochemical energy conversion where spatially inhomogeneous band structures can enhance photogenerated charge separation and carrier extraction from a semiconductor.

Dielectric anomalies and interactions in the three-dimensional quadratic band touching Luttinger semimetal PrIrO.

Dirac and Weyl semimetals with linearly crossing bands are the focus of much recent interest in condensed matter physics. Although they host fascinating phenomena, their physics can be understood in terms of weakly interacting electrons. In contrast, more than 40 years ago, Abrikosov pointed out that quadratic band touchings are generically strongly interacting.

Thermally Stable SrRuO Electrode for Oxide Heterostructures.

The use of thermally stable SrRuO electrodes in high-temperature synthesis of oxide heterostructures was investigated. Atomically smooth SrRuO thin films were grown on SrTiO(001) substrates by pulsed laser deposition and used as a bottom electrode for ferroelectric BaTiO capacitors grown at temperatures of up to 1000 °C. The thermal stability of SrRuO electrodes was verified by structural and electrical measurements of the ferroelectric BaTiO films.

Combinatorial screening of halide perovskite thin films and solar cells by mask-defined IR laser molecular beam epitaxy.

As an extension of combinatorial molecular layer epitaxy via ablation of perovskite oxides by a pulsed excimer laser, we have developed a laser molecular beam epitaxy (MBE) system for parallel integration of nano-scaled thin films of organic-inorganic hybrid materials. A pulsed infrared (IR) semiconductor laser was adopted for thermal evaporation of organic halide (A-site: CHNHI) and inorganic halide (B-site: PbI) powder targets to deposit repeated A/B bilayer films where the thickness of each layer was controlled on molecular layer scale by programming the evaporation IR laser pulse number, length, or power. The layer thickness was monitored with an quartz crystal microbalance and calibrated against stylus profilometer measurements.

Photoelectrochemical water splitting enhanced by self-assembled metal nanopillars embedded in an oxide semiconductor photoelectrode.

Production of chemical fuels by direct solar energy conversion in a photoelectrochemical cell is of great practical interest for developing a sustainable energy system. Various nanoscale designs such as nanowires, nanotubes, heterostructures and nanocomposites have been explored to increase the energy conversion efficiency of photoelectrochemical water splitting. Here we demonstrate a self-organized nanocomposite material concept for enhancing the efficiency of photocarrier separation and electrochemical energy conversion.

Epitaxial Bi5Ti3FeO15-CoFe2O4 pillar-matrix multiferroic nanostructures.

Epitaxial self-assembled ferro(i)magnetic spinel (CoFe2O4 (CFO)) and ferroelectric bismuth layered perovskite (Bi5Ti3FeO15 (BTFO)) pillar-matrix nanostructures are demonstrated on (001) single-crystalline strontium titanate substrates. The CFO remains embedded in the BTFO matrix as vertical pillars (∼50 nm in diameter) up to a volume fraction of 50%. Piezoresponse force microscopy experiments evidence a weak out-of-plane and a strong in-plane ferroelectricity in the BTFO phase, despite previously reported paraelectricity along the c-axis in a pure BTFO film.

Development of a new laser heating system for thin film growth by chemical vapor deposition.

We have developed a new laser heating system for thin film growth by chemical vapor deposition (CVD). A collimated beam from a high-power continuous-wave 808 nm semiconductor laser was directly introduced into a CVD growth chamber without an optical fiber. The light path of the heating laser inside the chamber was isolated mechanically from the growth area by bellows to protect the optics from film coating.