Berezinskii-Kosterlitz-Thouless transition in rhenium nitride films

The quest to manipulate and understand superconductivity demands exploring diverse materials and unconventional behaviors. Here, we investigate the BKT transition in synthesized ReN$_x$ thin films, demonstrating their emergence as a compelling platform for studying this pivotal phenomenon. By systematically varying synthesis parameters, we achieve ReN$_x$ films exhibiting a BKT transition comparable or even surpassing the archetypal NbN$_x$ system. Detailed current-voltage measurements unlock the intrinsic parameters of the BKT transition, revealing the critical role of suppressed superconducting volume in pushing ReN$_x$ towards the two-dimensional limit. Utilizing this two-dimensional electron system, we employ Beasley-Mooij-Orlando (BMO) theory to extract the vortex unbinding transition temperature and superelectron density at the critical point. Further confirmation of the BKT transition is obtained through temperature-dependent resistivity, current-voltage, and magnetoresistance measurements. Our findings suggest that native disorder and inhomogeneity within ReN$_x$ thin films act to suppress long-range coherence, ultimately driving the system towards the BKT regime. This work establishes ReN$_x$ as a promising material for exploring BKT physics and paves the way for tailoring its properties for potential applications in superconducting devices

Suppression of nucleation density in twisted graphene domains grown on graphene/SiC template by sequential thermal process

We investigated the growth of twisted graphene on graphene/silicon carbide (SiC-G) templates by metal-free chemical vapor deposition (CVD) through a sequential thermal (ST) process, which exploits the ultraclean surface of SiC-G without exposing the surface to air before CVD. By conducting control experiments with SiC-G templates exposed to air (AirE process), structural analysis by atomic force microscopy revealed that the nucleation density of CVD graphene (CVD-G) was significantly suppressed in the ST process under the same growth condition. The nucleation behavior on SiC-G surfaces is observed to be very sensitive to carbon source concentration and process temperature. The nucleation on the ultraclean surface of SiC-G prepared by the ST process requires higher partial pressure of carbon source compared with that on the surface by the AirE process. Moreover, analysis of CVD-G growth over a wide temperature range indicates that nucleation phenomena change dramatically with a threshold temperature of 1300{\deg}C, possibly due to arising of etching effects. The successful synthesis of twisted few-layer graphene (tFLG) was affirmed by Raman spectroscopy, in which analysis of the G' band proves a high ratio of twisted structure in CVD-G. These results demonstrate that metal-free CVD utilizing ultraclean templates is an effective approach for the scalable production of large-domain tFLG that is valuable for electronic applications.Comment: Authors' original version submitted to Crystal Growth & Design. Main manuscript: 23 pages, 6 figures. Supporting information: 1 page

Impedance-matched High-overtone Bulk Acoustic Resonator

A high-overtone bulk acoustic resonator (HBAR), in which a piezoelectric transducer is set on an acoustic cavity, has been attracting attention in both fundamental research and RF applications due to its scalability, high frequency, and high quality factor. The acoustic impedance matching in HBARs is crucial for efficient acoustic power transfer from the piezoelectric transducer to the cavity. However, impedance mismatch remains in most HBARs due to the metal layer insertion between the piezoelectric layer and cavity substrate. In this study, we fabricated a nearly impedance-matched high-quality HBAR using an epitaxial AlN piezoelectric layer directly grown on a conductive SiC cavity substrate with no metal layer insertion. The small impedance mismatch was verified from the variation in the free spectral range (FSR), which is comparable to the best value in previously reported HBARs. The experimentally obtained FSR spectra was greatly reproduced by using the Mason model. Broadband phonon cavity modes up to the K-band (26.5 GHz) were achieved by reducing the thickness of the AlN layer from 800 to 200 nm. The high figure of merit of $f\times\text{Q} \sim 1.3\times 10^{13}\ \textrm{Hz}$ at 10 GHz was also obtained. Our nearly impedance-matched high-quality HBAR will enable the development of RF applications, such as low-phase noise oscillators and acoustic filters, as well as research on high-frequency acoustic systems hybridized with electric, optical, and magnetic systems

Isotropic orbital magnetic moments in magnetically anisotropic SrRuO3 films

Epitaxially strained SrRuO3 films have been a model system for understanding the magnetic anisotropy in metallic oxides. In this paper, we investigate the anisotropy of the Ru 4d and O 2p electronic structure and magnetic properties using high-quality epitaxially strained (compressive and tensile) SrRuO3 films grown by machine-learning-assisted molecular beam epitaxy. The element-specific magnetic properties and the hybridization between the Ru 4d and O 2p orbitals were characterized by Ru M2,3-edge and O K-edge soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements. The magnetization curves for the Ru 4d and O 2p magnetic moments are identical, irrespective of the strain type, indicating the strong magnetic coupling between the Ru and O ions. The electronic structure and the orbital magnetic moment relative to the spin magnetic moment are isotropic despite the perpendicular and in-plane magnetic anisotropy in the compressive-strained and tensile-strained SrRuO3 films; i.e., the orbital magnetic moments have a negligibly small contribution to the magnetic anisotropy. This result contradicts Bruno model, where magnetic anisotropy arises from the difference in the orbital magnetic moment between the perpendicular and in-plane directions. Contributions of strain-induced electric quadrupole moments to the magnetic anisotropy are discussed, too

Magnetic anisotropy driven by ligand in 4d transition metal oxide SrRuO3

The origin of magnetic anisotropy in magnetic compounds is a longstanding issue in solid state physics and nonmagnetic ligand ions are considered to contribute little to magnetic anisotropy. Here, we introduce the concept of ligand driven magnetic anisotropy in a complex transition-metal oxide. We conducted X ray absorption and X ray magnetic circular dichroism spectroscopies at the Ru and O edges in the 4d ferromagnetic metal SrRuO3. Systematic variation of the sample thickness in the range below 10 nm allowed us to control the localization of Ru 4d t2g states, which affects the magnetic coupling between the Ru and O ions. We found that the orbital magnetization of the ligand induced via hybridization with the Ru 4d orbital determines the magnetic anisotropy in SrRuO3