arXiv subjects

Koji Sato

Publications and source records attributed to Koji Sato.

At least 19 recordsLinked to original sources

arXiv2026

Systematic Investigation of Acceptor Removal in HPK LGADs with Modified Gain Layers

Low-Gain Avalanche Diodes (LGADs) are fast silicon sensors with internal charge multiplication and are key candidates for precision timing layers in future high-energy hadron colliders. Their operation in harsh radiation environments, however, is limited by acceptor removal in the gain layer, which reduces the active acceptor concentration and degrades the internal electric field required for avalanche multiplication. Improving the radiation tolerance of the gain layer is therefore essential for future 4D tracking applications. In this work, we investigated several LGAD prototypes produced in collaboration with Hamamatsu Photonics K.K. (HPK), featuring modified gain-layer designs, including oxygen-modified, carbon-implanted, and boron--phosphorus compensated structures. The sensors were studied after proton and reactor-neutron irradiation. Radiation tolerance was characterized using the acceptor-removal coefficient extracted from IV measurements and the operation voltage required to recover the timing performance after irradiation. The results show that carbon implantation is the only approach among those studied here that provides a clear improvement in radiation tolerance. In contrast, neither oxygen-related modification, including the Partially Activated Boron (PAB) approach, nor gain-layer compensation alone yields a significant improvement, and the compensated carbon-implanted structure shows no clear advantage over the carbon-only case. In addition, the acceptor-removal coefficient is found to depend on the irradiation particle type and energy.

physics.ins-det↗

arXiv2026

High-throughput, Non-Destructive, Three-Dimensional Imaging of GaN Threading Dislocations with in-Plane Burgers Vector Component via Phase-Contrast Microscopy

We demonstrate a nondestructive, high-throughput method for observing dislocations in GaN (0001) using phase-contrast microscopy (PCM). The PCM images (359x300 $μ$m$^2$) analyzed in this study were acquired with an exposure time of 3 ms per image. The one-to-one correspondence between threading dislocation (TD) contrasts in PCM images and the corresponding contrasts in multiphoton excitation photoluminescence (MPPL) images provides clear evidence that PCM can detect TDs with in-plane Burgers vector components. The contrast shape in PCM reflects the inclination of dislocations with respect to the surface normal: dot contrasts correspond to vertical dislocations, whereas line contrasts correspond to inclined dislocations. By shifting the focal plane from the top surface to the back surface, the three-dimensional propagation paths of dislocations can be visualized. The PCM image obtained represents a projection of threading dislocations within a thickness of approximately 43 $μ$m. Dislocations spaced as close as 1.3 $μ$m can be individually resolved. In addition, the capability of PCM to detect scratches, subsurface scratches, facet boundaries, and voids was demonstrated. This study establishes PCM as a versatile and laboratory-accessible technique for three-dimensional, nondestructive characterization of dislocations and other defects in wide-bandgap semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

High-speed, High-Resolution, Three-Dimensional Imaging of Threading Dislocations in beta-$Ga_{2}O_{3}$ via Phase-Contrast Microscopy

This study presents a nondestructive, high-resolution method for three-dimensional imaging of threading dislocations in beta-$Ga_{2}O_{3}$ (010) using phase-contrast microscopy (PCM). A one-to-one correspondence between dislocation contrasts in PCM images and synchrotron X-ray topography (SR-XRT) images confirms the detection capability of PCM. Compared to SR-XRT, PCM provides enhanced spatial resolution, enabling the distinction of closely spaced dislocations with sub-10-micrometer separation. PCM facilitates direct visualization of dislocation propagation paths along the depth (z) direction by systematically shifting the focal plane into the crystal. In addition, the projection of stacked PCM images enables in-plane (XY) tracing of dislocation lines, providing insight into the preferred slip systems in beta-$Ga_{2}O_{3}$. This work establishes PCM as a versatile and laboratory-accessible technique for three-dimensional, nondestructive characterization of dislocations across entire wide-bandgap semiconductor wafers within a practically acceptable time frame.

cond-mat.mtrl-sci↗

arXiv2026

Development of Pixelated Capacitive-Coupled LGAD (ACLGADpix) Detectors

The Low-Gain Avalanche Diode (LGAD) is a semiconductor detector capable of achieving excellent timing resolution (~20 ps) for minimum ionizing particles (MIPs). To realize a pixelated detector with both high timing precision and spatial resolution, we have been developing Capacitive-Coupled LGADs (ACLGADs) for future collider experiments, such as the latter phase of the High-Luminosity LHC. We have successfully fabricated a pixelated ACLGAD (ACLGADpix) with a 100 $μ$m %\times% 100 $μ$m pixel pitch, maintaining uniform timing performance across the active area. In this presentation, we will report recent measurement results from ACLGADpix prototypes using beta rays, an infrared laser, and a 3 GeV electron beam. We will also discuss potential readout electronics for future collider applications.

physics.ins-det↗

arXiv2020

Excitation and relaxation dynamics of spin-waves triggered by ultrafast photo-induced demagnetization in a ferrimagnetic insulator

Excitation and propagation dynamics of spin waves in an iron-based garnet film under out-of-plane magnetic field were investigated by time-resolved magneto-optical imaging. The experimental results and the following data analysis by phase-resolved spin-wave tomography reveal the excitation of spin waves triggered by photo-induced demagnetization (PID) along the sample depth direction. Moreover, the fast relaxation of PID accompanied by the spin transfer due to spin-wave emission was observed. Possible scenarios of PID in the garnet film are discussed. Finally, we develop a model for the spin-wave excitation triggered by PID and explain the magnetic-field dependence in the amplitude of the observed spin waves.

cond-mat.mtrl-sci↗

arXiv2019

Magnetization Dynamics in Holographic Ferromagnets: Landau-Lifshitz Equation from Yang-Mills Fields

We introduce a new approach to understand magnetization dynamics in ferromagnets based on the holographic realization of ferromagnets. A Landau-Lifshitz equation describing the magnetization dynamics is derived from a Yang-Mills equation in the dual gravitational theory, and temperature dependences of the spin-wave stiffness and spin transfer torque appearing in the holographic Landau-Lifshitz equation are investigated by the holographic approach. The results are consistent with the known properties of magnetization dynamics in ferromagnets with conduction electrons.

hep-th↗

arXiv2019

Topologically nontrivial magnonic solitons

The intrinsic non-linearities of the spin dynamics in condensed matter systems give rise to a rich phenomenology that can be strongly affected by topology. Here we study formation of magnonic solitons in the topologically nontrivial bandgap of a spin lattice realization of the Haldane model, in both static and dynamic (Floquet) regimes. We consider nonlinearities caused by magnetic crystalline anisotropy and magnon-magnon interactions. We find soliton formation power thresholds as a function of anisotropy coefficient and interaction strength. We predict different classes of topological solitons for the same topological class of the underlying lattice and explain it in terms of a transition from a topologically nontrivial mass to a trivial one. Our findings imply that a soliton can phase-separate, containing boundaries between topologically trivial and non-trivial phases, which is associated with a vanishing spin wave gap.

cond-mat.mes-hall↗

arXiv2019

The Krein-Schrödinger Formalism of Bosonic BdG and Certain Classical Systems and Their Topological Classification

To understand recent works on classical and quantum spin equations and their topological classification, we develop a unified mathematical framework for bosonic BdG systems and associated classical wave equations; it applies not just to equations that describe quantized spin excitations in magnonic crystals but more broadly to other systems that are described by a BdG hamiltonian. Because here the generator of dynamics, the analog of the hamiltonian, is para-, aka Krein-, hermitian but not hermitian, the theory of Krein spaces plays a crucial role. For systems which are thermodynamically stable, the classical equations can be expressed as a Schrödinger equation with a hermitian hamiltonian. We then proceed to apply the Cartan-Altland-Zirnbauer classification scheme: to properly understand what topological class these equations belong to, we need to conceptually distinguish between symmetries and constraints. Complex conjugation enters as a particle-hole constraint (as opposed to a symmetry), since classical waves are necessarily real-valued. Because of this distinction only commuting symmetries enter in the topological classification. Our arguments show that the equations for spin waves in magnonic crystals are a system of class A, the same topological class as quantum hamiltonians describing the Integer Quantum Hall Effect. Consequently, the magnonic edge modes first predicted by Shindou et al. are indeed analogs of the Quantum Hall Effect, and their net number is topologically protected.

cond-mat.other↗

arXiv2019

Antiferromagnetic Interorbital Spin-Exchange Interaction of $^{171}$Yb

We report on the investigation of the scattering properties between the ground state $^1S_0$ and the metastable state $^3P_0$ of the fermionic isotope of $^{171}$Yb. We successfully measure the $s$-wave scattering lengths in the two-orbital collision channels as $a_{eg}^+=225(13)a_0$ and $a_{eg}^-=355(6)a_0$, using the clock transition spectroscopy in a three-dimensional optical lattice. The result shows that the interorbital spin-exchange interaction is antiferromagnetic, indicating that $^{171}$Yb atom is a promising isotope for the quantum simulation of the Kondo effect with the two-orbital system.

cond-mat.quant-gas↗

arXiv2018

Solitons in one-dimensional mechanical linkage

It has been observed that certain classical chains admit topologically protected zero-energy modes that are localized on the boundaries. The static features of such localized modes are captured by linearized equations of motion, but the dynamical features are governed by its nonlinearity. We study quasi-periodic solutions of nonlinear equations of motion of one-dimensional classical chains. Such quasi-periodic solutions correspond to periodic trajectories in the configuration space of the discrete systems, which allows us to define solitons without relying on a continuum theory. Furthermore, we study the dynamics of solitons in inhomogeneous systems by connecting two chains with distinct parameter sets, where transmission or reflection of solitons occurs at the boundary of the two chains.

cond-mat.soft↗

arXiv2017

Fu-Kane-Mele monopoles in semimetals

In semimetals with time-reversal symmetry, the interplay between Weyl points and Fu-Kane-Mele indices results in coexisting surface Dirac cones and Fermi arcs that are transmutable without a topological phase transition. We show that Weyl points act as a new type of monopole, and that their connectivity is essential for capturing the full topology of semimetals and their role as intermediaries of topological insulator transitions. The history of Weyl point creation-annihilation provides a simple and mathematically equivalent way to classify semimetals, and directly prefigures the surface state topology. We further predict the possibility of a topological Dirac cone on the interface between two Weyl semimetals.

cond-mat.mes-hall↗

arXiv2017

Tunable sign change of spin Hall magnetoresistance in Pt/NiO/YIG structures

Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide range of temperature and NiO thickness. The SMR shows a negative sign below a temperature which increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to Pt/YIG bilayer which always predicts a positive SMR. The negative SMR is found to persist even when NiO blocks the spin transmission between Pt and YIG, indicating it is governed by the spin current response of NiO layer. We explain the negative SMR by the NiO 'spin-flop' coupled with YIG, which can be overridden at higher temperatures by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers.

cond-mat.mes-hall↗

arXiv2016

Skyrmion Hall Effect Revealed by Direct Time-Resolved X-Ray Microscopy

Magnetic skyrmions are highly promising candidates for future spintronic applications such as skyrmion racetrack memories and logic devices. They exhibit exotic and complex dynamics governed by topology and are less influenced by defects, such as edge roughness, than conventionally used domain walls. In particular, their finite topological charge leads to a predicted "skyrmion Hall effect", in which current-driven skyrmions acquire a transverse velocity component analogous to charged particles in the conventional Hall effect. Here, we present nanoscale pump-probe imaging that for the first time reveals the real-time dynamics of skyrmions driven by current-induced spin orbit torque (SOT). We find that skyrmions move at a well-defined angle Θ_{SH} that can exceed 30° with respect to the current flow, but in contrast to theoretical expectations, Θ_{SH} increases linearly with velocity up to at least 100 m/s. We explain our observation based on internal mode excitations in combination with a field-like SOT, showing that one must go beyond the usual rigid skyrmion description to unravel the dynamics.

cond-mat.mtrl-sci↗

arXiv2016

Electrically controlled pinning of Dzyaloshinskii-Moriya domain walls

We propose a method to all-electrically control a domain-wall position in a ferromagnetic nanowire with Dzyaloshinskii-Moriya interaction. The strength of this interaction can be controlled by an external electric field, which in turn allows a fine tuning of the pinning potential of a spin-spiral domain wall. It allows to create more mobile pinning sites and can also be advantageous for ultra-low power electronics.

cond-mat.mes-hall↗

arXiv2015

Anomalous Hall effect driven by dipolar spin waves in uniform ferromagnets

A new type of anomalous Hall effect is shown to arise from the interaction of conduction electrons with dipolar spin waves in ferromagnets. This effect exists even in homogeneous ferromagnets without relativistic spin-orbit coupling. The leading contribution to the Hall conductivity is proportional to the chiral spin correlation of dynamical spin textures and is physically understood in terms of the skew scattering by dipolar magnons.

cond-mat.mtrl-sci↗

arXiv2015

A Holographic Realization of Ferromagnets

A holographic realization for ferromagnetic systems has been constructed. Owing to the holographic dictionary proposed on the basis of this realization, we obtained relevant thermodynamic quantities such as magnetization, magnetic susceptibility, and free energy. This holographic model reproduces the behavior of the mean field theory near the critical temperature. At low temperatures, the results automatically incorporate the contributions from spin wave excitations and conduction electrons.

hep-th↗

arXiv2014

Detection of entanglement by helical Luttinger liquids

A Cooper-pair or electron-hole splitter is a device capable of spatially separating entangled fermionic quasiparticles into mesoscopic solid-state systems such as quantum dots or quantum wires. We theoretically study such a splitter based on a pair of helical Luttinger liquids, which arise naturally at the edges of a quantum spin Hall insulator. Equipping each helical liquid with a beam splitter, current-current cross correlations can be used to construct a Bell inequality whose violation would indicate nonlocal orbital entanglement of the injected electrons and/or holes. Due to Luttinger-liquid correlations, however, the entanglement is exponentially suppressed at finite temperatures.

cond-mat.mes-hall↗

arXiv2013

Landau-Lifshitz theory of the longitudinal spin Seebeck effect

Thermal-bias-induced spin angular momentum transfer between a paramagnetic metal and ferromagnetic insulator is studied theoretically based on the stochastic Landau-Lifshitz-Gilbert (LLG) phenomenology. Magnons in the ferromagnet establish a nonequilibrium steady state by equilibrating with phonons via bulk Gilbert damping and electrons in the paramagnet via spin pumping, according to the fluctuation-dissipation theorem. Subthermal magnons and the associated spin currents are treated classically, while the appropriate quantum crossover is imposed on high-frequency magnetic fluctuations. We identify several length scales in the ferromagnet, which govern qualitative changes in the dependence of the thermally-induced spin current on the magnetic film thickness.

cond-mat.mes-hall↗