45 research outputs found
Broken Screw Rotational Symmetry in the Near-Surface Electronic Structure of -Stacked Crystals
We investigate the electronic structure of - and
by angle-resolved photoemission spectroscopy (ARPES) and
photoemission intensity calculations. Although in bulk form, these materials
are expected to exhibit band degeneracy in the 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. Our
results show that the near-surface electronic structure can be qualitatively
different from the bulk one due to partially broken nonsymmorphic symmetries.Comment: 6+11 pages, 4+13 figure
Semiconducting Electronic Structure of the Ferromagnetic Spinel Revealed by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy
We study the electronic structure of the ferromagnetic spinel
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. We conclude that the theoretical prediction of a Weyl
semimetal state in underestimates the
band gap, and this material is a ferromagnetic semiconductor.Comment: 6+13 pages, 4+13 figure
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:CH3NH3I) and inorganic halide (B-site: PbI2) 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 in situ quartz crystal microbalance and calibrated against ex situ stylus profilometer easurements. A computer-controlled movable mask system enabled the deposition of combinatorial thin film libraries, where each library contains a vertically homogeneous film with spatially programmable A- and B-layer thicknesses. On the composition gradient film, a hole transport Spiro-OMeTADlayer was spin-coated and dried followed by the vacuum evaporation of Ag electrodes to form the solar cell. The preliminary cell performance was evaluated by measuring I-V characteristics at seven different positions on the 12.5 mm × 12.5 mm combinatorial library sample with seven 2 mm × 4 mm slits under a solar simulator irradiation. The combinatorial solar cell library clearly demonstrated that the energy conversion efficiency sharply changes from nearly zero to 10.2% as a function of the illumination area in the library. The exploration of deposition parameters for obtaining optimum performance could thus be greatly accelerated. Since the thickness ratio of PbI2 and CH3NH3I can be freely chosen along the shadow mask movement, these experiments show the potential of this system for high-throughput screening of optimum chemical composition in the binary film library and application to halide perovskite solar cell
Thermally Stable Sr<sub>2</sub>RuO<sub>4</sub> Electrode for Oxide Heterostructures
The
use of thermally stable Sr<sub>2</sub>RuO<sub>4</sub> electrodes
in high-temperature synthesis of oxide heterostructures was investigated.
Atomically smooth Sr<sub>2</sub>RuO<sub>4</sub> thin films were grown
on SrTiO<sub>3</sub>(001) substrates by pulsed laser deposition and
used as a bottom electrode for ferroelectric BaTiO<sub>3</sub> capacitors
grown at temperatures of up to 1000 °C. The thermal stability
of Sr<sub>2</sub>RuO<sub>4</sub> electrodes was verified by structural
and electrical measurements of the ferroelectric BaTiO<sub>3</sub> films. The best growth temperature for the BaTiO<sub>3</sub> films
was found to be 900 °C, exhibiting the largest spontaneous polarization,
dielectric constant, and pyroelectric response. We conclude that Sr<sub>2</sub>RuO<sub>4</sub> films are suitable for use as thermally stable
electrodes in heterostructures synthesized at temperatures up to at
least 1000 °C and oxygen pressures from 10<sup>–6</sup> to 10<sup>–1</sup> Torr. This range of growth film conditions
is much wider than that for other common oxide electrode materials
such as SrRuO<sub>3</sub>, widening the available process window for
optimizing the performance of oxide electronic devices
Critical Role of Terminating Layer in Formation of 2DEG State at the LaInO3/BaSnO3 Interface
Based on the interface polarization model, the 2D electron gas (2DEG) at LaInO3(LIO)/BaSnO3(BSO) interfaces is understood to originate from a polarization discontinuity at the interface and the conduction band offset between LIO and BSO. In this scenario, the direction of polarization at the interface is determined by whether the first atomic LIO layer at the interface is LaO+ or InO2-. The role of the terminating layer is investigated at the LIO/BSO interface in creating the 2DEG. Based on conductance measurements of the in situ grown LIO/BSO heterostructures, it has been reported in this work that the 2DEG only forms when the BSO surface is terminated mainly with a SnO2 layer. The terminating layer is controlled by additional SnO2 deposition on the BSO surface. It has been shown that the as-grown BSO surface has a mixed terminating layer of BaO and SnO2 while the BSO surfaces prepared with additional SnO2 deposition are terminated mainly with the SnO2 layer. The terminating layer is confirmed by coaxial impact collision ion scattering spectroscopy. The finding is consistent with the interface polarization model for 2DEG formation at LIO/BSO interfaces, in which the direction of the interfacial polarization in LIO is determined by the terminating layer of the BSO surface.11Nsciescopu
Synthesis and characterization of (111)-oriented BaTiO3 thin films
The synthesis of (111)-oriented BaTiO3 thin films on Nb-doped SrTiO3 and bilayers of BaTiO3 and La0.7Sr0.3MnO3 on SrTiO3 are investigated. With increasing thickness the films are found to exhibit a decreasing out-of-plane lattice parameter and increased surface roughness. The BaTiO3 films on doped SrTiO3 are found to be relaxed with indications of increasing defect density with increasing thickness. Through piezoresponse force microscopy, pyroelectric measurements, and tunneling electroresistance measurements, indications of ferroelectric behavior are found in (111)-oriented BaTiO3 down to a thickness of 5 nm