arXiv subjects

Moeta Tsukamoto

Publications and source records attributed to Moeta Tsukamoto.

11 recordsLinked to original sources

arXiv2026

Epitaxial growth of topological insulator $β$-Ag2Te thin films

We report epitaxial growth of $β$-Ag2Te thin films by molecular beam epitaxy. $β$-Ag2Te, recently identified as a topological insulator, was grown by depositing Ag on InP substrate at room temperature followed by Te supply at elevated temperature. X-ray diffraction measurements and transmission electron microscopy analyses confirmed the (002) crystal orientation and the epitaxial atomic arrangement of $β$-Ag2Te thin films. Electrical transport measurements revealed that the $β$-Ag2Te thin film exhibits two-dimensional metallic conduction while the bulk remains insulating. The epitaxial $β$-Ag2Te thin films obtained here provide a viable platform for investigating emergent phenomena arising from surface Dirac states and for designing heterojunction-based device structures.

cond-mat.mtrl-sci↗

arXiv2025

Quantitative imaging of nonlinear spin-wave propagation using diamond quantum sensors

Spin waves propagating in magnetic materials exhibit nonlinear behavior at large amplitudes due to the competition between excitation and relaxation, providing an attractive platform for exploring nonlinear wave dynamics. In particular, spin waves with a non-zero wavenumber that carry momentum undergo nonlinear relaxation and experience wavenumber modulation in the nonlinear regime. This nonlinearity has been observed experimentally, for example in S. R. Lake et al., Phys. Rev. Appl. 17, 034010 (2022), but a quantitative comparison with theory has not yet been carried out. Here, We image nonlinear spin-wave propagation in two yttrium iron garnet thin films with distinct spin-wave decay rates using a wide-field quantum diamond microscope. We obtain quantitative distributions of spin-wave amplitude and phase as a function of the excitation microwave strength. As a result, we observe a threshold in the spin-wave amplitude beyond which nonlinear effects become evident and confirm that this threshold is consistent with theoretical predictions based on four-magnon scattering for both samples. Moreover, as the amplitude of the spin waves increases, we observe modulation of the wavenumber across the field of view. We attribute this modulation primarily to a reduction in the saturation magnetization caused by incoherent spin waves generated by multi-magnon scattering. Our quantitative measurements provide a pathway for visualizing nonlinear spin-wave dynamics and are crucial for deepening our understanding of the underlying mechanisms.

cond-mat.mes-hall↗

arXiv2025

Investigations of optical aberration on quantum diamond microscopy toward high spatial resolution and sensitivity

Quantum diamond microscopy (QDM), which employs nitrogen-vacancy (NV) center ensembles, is a promising approach to quantitatively imaging magnetic fields with both high resolution that approaches the diffraction limit and a wide field of view. The commonly adopted setups of QDM capture the photoluminescence through transparent diamonds, which inevitably entail aberrations -- optical errors that degrade the optical resolution and contrast of the obtainable image. In this study, we delve into the impact of optical aberrations, focusing on their dependence on diamond thickness. We first introduce a rigorous model [Richards et al., Braat et al.] of diffraction that incorporates aberrations, producing the NV center optical image. We confirm that this model accurately reproduces the confocal images of single NV centers obtained at various depths in diamonds. Extending this model to a wide-field microscope, we find that the model also accurately reproduces the USAF 1951 resolution test chart obtained through diamonds of various thicknesses. Based on these investigations, we quantitatively assess the consequent resolution constraints and propose thinning the diamond as a viable solution. We present a robust method to quantitatively ascertain resolution in optical systems influenced by aberrations caused by ray transmission through diamonds. For instance, for a typical microscope with an objective lens of NA = 0.7, the diffraction limit is achievable through diamonds that are 30 $μ$m thick, and a resolution of 1 $μ$m is obtained through diamonds that are 100 $μ$m thick. Those results opens up avenues for enhanced performance in QDM. The Julia package used to calculate the vectorial PSFs is available at ${\texttt VectorPSFs.jl}$.

physics.optics↗

arXiv2025

Nanodiamond quantum thermometry assisted with machine learning

Nanodiamonds (NDs) are quantum sensors that enable local temperature measurements, taking advantage of their small size. Though the model based analysis methods have been used for ND quantum thermometry, their accuracy has yet to be thoroughly investigated. Here, we apply model-free machine learning with the Gaussian process regression (GPR) to ND quantum thermometry and compare its capabilities with the existing methods. We prove that GPR provides more robust results than them, even for a small number of data points and regardless of the data acquisition methods. This study extends the range of applications of ND quantum thermometry with machine learning.

quant-ph↗

arXiv2024

Wideband wide-field imaging of spin-wave propagation using diamond quantum sensors

Imaging spin-wave propagation in magnetic materials in a wide frequency range is crucial for understanding and applying spin-wave dynamics. Recently, nitrogen-vacancy (NV) centers in diamond have attracted attention as sensors capable of quantitatively measuring the amplitude and phase of coherent spin waves. However, the conventional sensing protocol has been limited to detecting spin waves whose frequencies match the resonance frequency of the NV spins. We demonstrate that by utilizing the AC Zeeman effect, it is possible to image spin waves propagating in a yttrium iron garnet (YIG) thin film over a wide frequency range up to a maximum detuning of 567 MHz without changing the external magnetic field. Our results expand the applicability of NV centers for spin-wave sensing and pave the way for quantitative investigations of the dynamics in various magnetic materials, such as metallic ferromagnets and van der Waals magnets.

cond-mat.mes-hall↗

arXiv2024

Systematic characterization of nanoscale $h$-BN quantum sensor spots created by helium-ion microscopy

The nanosized boron vacancy ($V_\mathrm{B}^-$) defect spot in hexagonal boron nitride ($h$-BN) is promising for a local magnetic field quantum sensor. One of its advantages is that a helium-ion microscope can make a spot at any location in an $h$-BN flake with nanometer accuracy. In this study, we investigate the properties of the created nanosized $V_\mathrm{B}^-$ defect spots by systematically varying three conditions: the helium-ion dose, the thickness of the $h$-BN flakes, and the substrate on which the $h$-BN flakes are attached. The physical background of the results obtained is successfully interpreted using Monte Carlo calculations. From the findings obtained here, a guideline for their optimal creation conditions is obtained to maximize its performance as a quantum sensor concerning sensitivity and localization.

cond-mat.mes-hall↗

arXiv2024

Observation of chiral domain walls in an octupole-ordered antiferromagnet

Spin chirality in antiferromagnets offers new opportunities for spintronics. The kagome antiferromagnet Mn3Sn is a paradigmatic material in which the antiferromagnetic order parameter can be detected and controlled by electrical means. However, direct investigation of the magnetic texture of Mn3Sn has been challenging because of the tiny moment hosted in its magnetic octupole, hindering further clarification of this unique material. Here, we address this issue by observing the stray magnetic field from Mn3Sn using a diamond quantum scanning magnetometer. The spatially-resolved intrinsic domains and domain walls in a high-quality single-crystalline Mn3Sn film quantitatively reveal the polarization angle of the magnetic octupole in the kagome plane, the domain's local magnetization, the domain wall's width and chirality, and the octupole order in domain walls. Our nanoscale investigation of Mn3Sn, a powerful complement to macroscopic measurements, paves the road for developing chiral antiferromagnetism and its potential for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Optical-power-dependent splitting of magnetic resonance in nitrogen-vacancy centers in diamond

Nitrogen-vacancy (NV) centers in diamonds are a powerful tool for accurate magnetic field measurements. The key is precisely estimating the field-dependent splitting width of the optically detected magnetic resonance (ODMR) spectra of the NV centers. In this study, we investigate the optical power dependence of the ODMR spectra using NV ensemble in nanodiamonds (NDs) and a single-crystal bulk diamond. We find that the splitting width exponentially decays and is saturated as the optical power increases. Comparison between NDs and a bulk sample shows that while the decay amplitude is sample-dependent, the optical power at which the decay saturates is almost sample-independent. We propose that this unexpected phenomenon is an intrinsic property of the NV center due to non-axisymmetry deformation or impurities. Our finding indicates that diamonds with less deformation are advantageous for accurate magnetic field measurements.

cond-mat.mes-hall↗

arXiv2023

Magnetic field imaging by hBN quantum sensor nanoarray

Placing a sensor close to the target at the nano-level is a central challenge in quantum sensing. We demonstrate high-spatial-resolution magnetic field imaging with a boron vacancy (V$_\text{B}^-$) defects array in hexagonal boron nitride with a few 10 nm thickness. V$_\text{B}^-$ sensor spots with a size of (100 nm)$^2$ are arranged periodically with nanoscale precision using a helium ion microscope and attached tightly to a gold wire. The sensor array allows us to visualize the magnetic field induced by the current in the wire with a spatial resolution beyond the diffraction limit. Each sensor exhibits a practical sensitivity of $73.6~μ\text{T/Hz}^{0.5}$, suitable for quantum materials research. Our technique of arranging V$_\text{B}^-$ quantum sensors periodically and tightly on measurement targets will maximize their potential.

cond-mat.mes-hall↗

arXiv2022

Lock-in thermography using diamond quantum sensors

Precise measurement of temperature distribution and thermal behavior in microscopic regions is critical in many research fields. We demonstrate lock-in thermography using nitrogen-vacancy centers in diamond nanoparticles. We successfully visualize thermal diffusion in glass coverslip and Teflon with micrometer resolution and deduce their thermal diffusivity. By spreading diamond nanoparticles over the sample surface, temperature variation can be measured directly without any physical contact, such as lead wires, making it possible to visualize the micrometer-scale thermal behavior of various materials.

physics.app-ph↗

arXiv2022

Machine-learning-enhanced quantum sensors for accurate magnetic field imaging

Local detection of magnetic fields is crucial for characterizing nano- and micro-materials and has been implemented using various scanning techniques or even diamond quantum sensors. Diamond nanoparticles (nanodiamonds) offer an attractive opportunity to chieve high spatial resolution because they can easily be close to the target within a few 10 nm simply by attaching them to its surface. A physical model for such a randomly oriented nanodiamond ensemble (NDE) is available, but the complexity of actual experimental conditions still limits the accuracy of deducing magnetic fields. Here, we demonstrate magnetic field imaging with high accuracy of 1.8 $μ$T combining NDE and machine learning without any physical models. We also discover the field direction dependence of the NDE signal, suggesting the potential application for vector magnetometry and improvement of the existing model. Our method further enriches the performance of NDE to achieve the accuracy to visualize mesoscopic current and magnetism in atomic-layer materials and to expand the applicability in arbitrarily shaped materials, including living organisms. This achievement will bridge machine learning and quantum sensing for accurate measurements.

quant-ph↗