arXiv subjects

Fumikazu Murakami

Publications and source records attributed to Fumikazu Murakami.

2 recordsLinked to original sources

arXiv2026

Optical Signature of Moiré Superlattices Formed by Twisted SrTiO$_3$ Membranes

Moiré superlattices formed at the interfaces of mismatched lattices have attracted significant interest over the past decade due to their large tunability of band parameters and interactions among electrons, spins, and lattices. Superlattices made from twisted perovskite oxides may have strong structure and potential modulation, but evidence of such modulation over macroscopic areas, particularly at large twisting angles, has not been clearly demonstrated so far. Here, we fabricated millimeter-scale twisted oxide bilayers at $36^\circ$ angle, close to the simple coincidence site lattice condition $\Sigma5$, from freestanding SrTiO$_3$ membranes. We discovered new low-frequency vibrational modes whose Raman activity, according to molecular dynamics simulations, is greatly enhanced by an asymmetric, twisted interface between the SrO and TiO$_2$ layers. Such an interface is energetically favorable from first-principles calculations and is corroborated by the observation of strong second harmonic generation from the interface comparable to that from the SrTiO$_3$ surface throughout the bilayer region. The results are consistent with interlayer coupling enhanced by high-temperature annealing and confirmed by cross-sectional scanning transmission electron microscopy imaging. Our work sheds light on the structural behavior of twisted oxides and provides directions for tuning their phononic and nonlinear optical properties in future studies.

cond-mat.mtrl-sci↗

arXiv2025

Site-selective enhancement of Eu emission in delta-doped GaN

Europium-doped gallium nitride (GaN:Eu) is a promising platform for classical and quantum optoelectronic applications. When grown using organometallic vapor-phase epitaxy, the dominant red emission from Eu exhibits an inhomogeneous photoluminescence (PL) spectrum due to contributions from several non-equivalent incorporation sites that can be distinguished with combined excitation emission spectroscopy. Energy transfer from the GaN bandgap to the majority site is inefficient, limiting the performance of GaN:Eu LEDs and resulting in an inhomogeneous emission spectrum dominated by disproportionate contributions from minority sites. In this work, we use site-selective spectroscopy to characterize the photoluminescence properties of delta-doped structures with alternating doped and undoped layers of varying thicknesses and demonstrate that they selectively enhance emission from the majority site when compared to uniformly-doped samples. Samples with 2-nm and 10-nm doped layers show much greater PL intensity per Eu concentration as well as more efficient energy transfer to the majority site, which are both highly desirable for creating power-efficient LEDs. Meanwhile, a sample with 1-nm doped layers shows emission only from the majority site, resulting in a narrow, homogeneous emission spectrum that is desirable for quantum technologies. This utilization of delta-doping has the potential to be broadly applicable for engineering desirable defect properties in rare-earth doped semiconductors.

cond-mat.mtrl-sci↗