arXiv subjects

Hiroyo Segawa

Publications and source records attributed to Hiroyo Segawa.

3 recordsLinked to original sources

arXiv2026

Imaging and characterization of spontaneous vortices in a proximity-induced superconductor

Observation of spontaneous symmetry breaking is crucial for understanding continuous second-order phase transitions from disordered to ordered states, which often leads to the formation of topological defects. In superconductors, such topological defects manifest as quantized vortices. However, the formation and observation of spontaneous vortices in a uniform superconductor are challenging because extremely rapid cooling (>108 K/s) is generally required for that purpose. Here we conducted scanning superconducting quantum interference device microscope (SSM) measurements on an MgB2-based proximity-induced superconductor, an intrinsically inhomogeneous system. In this system, individual superconducting domains will reach internal equilibrium independently during cooling and choose their own phase before the global phase coherence is established via the long-range proximity coupling. The SSM measurements demonstrate that vortices are nucleated spontaneously even at a relatively slow cooling rate (~0.2 K/s). We also find that the vortices with different polarities, sizes, and shapes appear stochastically under near-zero-field conditions. The geometry of the spontaneous vortices is more extended than that of the field-induced Abrikosov vortices. Magnetic field profile analysis based on the London model elucidates that penetration depths of the extended vortices are anomalously large, exceeding several micrometers. This unusual morphology of the spontaneous vortices most likely imprints the information that is frozen at the moment of vortex formation. Our findings not only provide insights into the local phase differences present in the early stage of the phase transition in this proximity-induced superconducting system, but they also shed insights into the structure, formation, and stabilization of topological defects in highly disordered and inhomogeneous superconducting systems.

cond-mat.supr-con↗

arXiv2025

Establishment of global phase coherence in a highly disordered fractal MgO/MgB2 nanocomposite: Roles of interface, morphology and defect

Recently, we have reported that a highly disordered fractal MgO/MgB2 nanocomposite exhibits bulk-like superconducting properties with isotropic pinning, showing an excellent phase-coherent capability irrespective of the low volume fraction (~30 vol. %) of MgB2 [Uchino et al., Phys. Rev. B 101, 035146 (2020); Teramachi et al,, Phys. Rev. B 108, 155146 (2023)]. Hence, this nanocomposite provides a useful experimental system to investigate the relationship between the structural disorder and the establishment of the superconducting phase coherence. In this work, we show from 3D focused ion beam scanning electron microscopy (FIB-SEM) data that in the nanocomposite, a complex MgO/MgB2 microstructure spreads isotropically throughout the sample with a constant fractal dimension of ~1.67. Atomic-resolution scanning transmission electron microscopy (STEM) has revealed that the MgO/MgB2 interfaces are atomically clean and free from amorphous grain boundaries, even leading to atomically coherent interfaces. Detailed ac susceptibility measurements have demonstrated a smooth crossover from an intragranular to an intergranular superconducting regime, giving evidence of the establishment of the critical state due to strong intergranular coupling just below the superconducting transition temperature. Also, spatially-resolved cathodoluminescence measurements have demonstrated that oxygen vacancies in the MgO-rich phase tend to aggregate near the MgO/MgB2 boundary regions, forming long channels of oxygen vacancies through the nanocomposite. These channels of oxygen vacancies will contribute to the long-range carrier transfer and the related Andreev reflection via coherent tunneling of charge carriers among the oxygen vacancy sites.

cond-mat.supr-con↗

arXiv2024

Atomic-Scale Observation of Moire potential in Twisted Hexagonal Boron Nitride Layers by Electron Microscopy

Moire superlattices (MSLs) are an emerging class of two-dimensional functional materials whose electronic states can be tuned by the twist angle between two van der Waals layers and/or the relative placement of the layers. The intriguing properties of MSLs are closely correlated to the moir\'e potential, which is the electrostatic potential induced by interlayer coupling. Intensive efforts have been made to understand the nature and distribution of the moire potential by using various experimental and theoretical techniques. However, the experimental observation of the moir\'e potential is still challenging because of the possible presence of the surface and/or interlayer contaminants. In this work, we develop a method to obtain hexagonal boron nitride (hBN) nanolayers (with or without twist) using a specially designed chemical exfoliation technique. The resulting hBN nanolayers are atomically clean and strain free, hence providing ideal MSLs for the investigation of their moir\'e potential. Aberration-corrected high resolution transmission electron microscopy measurements on the twisted hBN nanolayers allow us to observe moir\'e diffraction spots in Fourier space. Then, the moire potential is reconstructed by the inverse fast Fourier transform of the moire diffraction spots. It has been revealed that the local interlayer atomic overlap plays a decisive role in determining the periodicity and distribution of the moir\'e potential, as supported by density functional theory calculations. This work not only provides a general strategy to observe the moire potential in MSLs, but it also expands the application of electron microscopy to the further study of MSLs with atomic resolution.

cond-mat.mtrl-sci↗