arXiv subjects

Izumi Umegaki

Publications and source records attributed to Izumi Umegaki.

10 recordsLinked to original sources

arXiv2026

Development of a Neutron Imager using CMOS Pixelated Sensor

We acquired an image of the Siemens Star Chart using the 10B-INTPIX4 CMOS pixelated imager at J-PARC MLF BL21 (NOVA). The image blurriness, characterized by a root mean square (rms) value of 17 um in the line spread function, corresponds to an rms beam divergence of 2.7 mrad. for the neutron beam. This is equivalent to the effective geometrical ratio (L/D ratio) of 370. The contrast of the image was analyzed as a function of structural scale of the chart, and based on this contrast, the modulation transfer function (MTF) of the imaging system was evaluated. The results indicate that the structure corresponding to the 50% cutoff is 12 line-space pairs per mm.

physics.ins-det↗

arXiv2026

Recent application studies of an INTPIX4NA SOIPIX detector-based X-ray camera using an SiTCP-XG 10GbE-based high-speed readout system at KEK facilities

The Silicon-On-Insulator PIXel (SOIPIX) detector is a unique monolithic structure imaging device currently being developed by the SOIPIX group, led by the High Energy Accelerator Research Organization (KEK). Our detector team at the KEK Photon Factory (PF) has developed an X-ray camera based on the INTPIX4NA SOIPIX detector. This detector provides a sensitive area of 14.1 $\times$ 8.7 $\mathrm{mm^2}$, with 425,984 pixels arranged in an 832-column $\times$ 512-row matrix and a pixel size of 17 $\times$ 17 $\mathrm{μm^{2}}$, and offers high spatial resolution and excellent sensitivity under low-intensity X-ray conditions. The readout system used in the X-ray camera is developed at the PF. It is equipped with SiTCP-XG, a 10 Gb Ethernet network controller implemented on a field-programmable gate array, enabling high-frame-rate imaging at several hundred hertz. We are currently investigating the applicability of this X-ray camera in several experiments at KEK. Herein, we report three recent application studies: (1) X-ray zooming microscope optics using two Fresnel zone plates at PF AR-NE1A; (2) phase-contrast X-ray imaging system using a two-crystal X-ray interferometer at PF BL-14C; and (3) nondestructive lithium detection in Li-ion battery electrode materials using muonic X-rays at J-PARC MLF Muon D2.

physics.ins-det↗

arXiv2026

Magnetism and spin dynamics of Na$_5$Yb(MoO$_4$)$_4$: A weakly interacting rare-earth stretched diamond lattice

We report a comprehensive investigation of the structural and magnetic properties of Na$_5$Yb(MoO$_4$)$_4$, a member of the stretched diamond magnetic lattice family. Neutron powder diffraction at 3.3~K confirms that the compound crystallizes in the tetragonal \textit{I4$_1$/a} space group, with a large interatomic separation of 6.33~Å between magnetic Yb ions forming a three-dimensional stretched diamond framework. Magnetic susceptibility and specific heat measurements reveal no evidence of long-range magnetic order down to 60~mK. The low-temperature magnetic behavior is governed by an effective $J_{\mathrm{eff}} = 1/2$ Kramers doublet ground state, well separated from excited crystal-field levels, arising from the distorted dodecahedral oxygen coordination of Yb$^{3+}$. Density functional theory calculations within the DFT+$U$ framework indicate that exchange interactions between Yb ions are negligibly small, consistent with the long O--Mo--O super-superexchange pathways. The temperature dependence of the specific heat exhibits signatures of gapped spin excitations, most likely originating from long-range dipolar correlations and further shaped by weak exchange interactions together with the strong single-ion anisotropy of the Yb moments. Muon spin relaxation measurements reveal persistent low-energy spin dynamics, indicating that dipolar correlations remain dynamic and are insufficient to stabilize static magnetic order down to 50~mK. These results identify Na$_5$Yb(MoO$_4$)$_4$ as a rare example of a dipolar quantum paramagnet in which single-ion physics and long-range dipolar interactions dominate, while exchange interactions are suppressed to the millikelvin energy scale.

cond-mat.str-el↗

arXiv2022

Revised Magnetic Structure and Tricritical Behavior of the CMR Compound NaCr$_2$O$_4$ Investigated with High Resolution Neutron Diffraction and $μ^+$SR

The mixed valence Cr compound NaCr$_2$O$_4$, synthesized using a high-pressure technique, offers a unique playground for investigating unconventional physical properties in condensed matter. In the present study, muon spin rotation/relaxation ($μ^+$SR) and high-resolution neutron powder diffraction (NPD) measurements were carried out to clarify the true magnetic ground state of this interesting compound. Our detailed study brings new insight, allowing us to confirm the existence of a commensurate antiferromagnetic order (C-AFM) and to extract its ordered Cr moment $μ^{\rm C}_{\rm Cr}=(4.30\pm0.01)μ_B$. Such a value of the ordered moment is in fact compatible with the existence of high-spin Cr sites. Further, the value of the canting angle of the Cr spin axial vector is refined as $θ_{\rm c}=(8.8\pm0.5)^{\circ}$. Employing high-quality samples in combination with time-of-flight NPD, a novel magnetic supercell was also revealed. Such supercell display an incommensurate (IC)-AFM propagation vector (0~0~${\textstyle \frac{1}{2}-}δ$), having an ordered moment $μ^{\rm IC}_{\rm Cr}=(2.20\pm0.03)μ_B$. It is suggested that the C-AFM and IC-AFM modulations are due to itinerant and localized contributions to the magnetic moment, respectively. Finally, the direct measurement of the magnetic order parameter provided a value of the critical exponent $β= 0.245 \approx \frac{1}{4}$, suggesting a non conventional critical behavior for the magnetic phase transition in NaCr$_2$O$_4$.

cond-mat.str-el↗

arXiv2020

Investigation of Ionic and Anomalous Magnetic Behavior in CrSe$_2$ Using $^8$Li $β$-NMR

We have studied a mosaic of 1T-CrSe$_2$ single crystals using $β$-detected nuclear magnetic resonance of $^{8}$Li from 4 to 300 K. We identify two broad resonances that show no evidence of quadrupolar splitting, indicating two magnetically distinct environments for the implanted ion. We observe stretched exponential spin lattice relaxation and a corresponding rate ($1/T_1$) that increases monotonically above 200 K, consistent with the onset of ionic diffusion. A pronounced maximum in $1/T_1$ is observed at the low temperature magnetic transition near 20 K. Between these limits, $1/T_1$ instead exhibits a broad minimum with a remarkable absence of strong features in the vicinity of structural and magnetic transitions between 150 and 200 K. Together, the results suggest $^{8}$Li$^{+}$ site occupation within the van der Waals gap between CrSe$_2$ trilayers. Possible origins of the two environments are discussed.

cond-mat.mtrl-sci↗

arXiv2015

Spinon, soliton and breather in the spin-1/2 antiferromagnetic chain KCuGaF$_6$

Elementary excitations of the S=1/2 one-dimensional antiferromagnet KCuGaF_6 were investigated by inelastic neutron scattering in zero and finite magnetic fields perpendicular to the (1, 1, 0) plane combined with specific heat measurements. KCuGaF$_6$ exhibits no long-range magnetic ordering down to 50 mK despite the large exchange interaction J/k_B=103 K. At zero magnetic field, well-defined spinon excitations were observed. The energy of the des Cloizeaux and Pearson mode of the spinon excitations is somewhat larger than that calculated with the above exchange constant. This discrepancy is mostly ascribed to the effective XY anisotropy arising from the large Dzyaloshinsky-Moriya interaction with an alternating D vector. KCuGaF_6 in a magnetic field is represented by the quantum sine-Gordon model, for which low-energy elementary excitations are composed of solitons, antisolitons and their bound states called breathers. Unlike the theoretical prediction, it was found that the energy of a soliton is smaller than that of the first breather, although the energy of the first breather coincides with that observed in a previous ESR measurement.

cond-mat.str-el↗

arXiv2014

Na-ion dynamics in Quasi-1D compound NaV2O4

We have used the pulsed muon source at ISIS to study high-temperature Na-ion dynamics in the quasi-one-dimensional (Q1D) metallic antiferromagnet NaV2O4. By performing systematic zero-field and longitudinal-field measurements as a function of temperature we clearly distinguish that the hopping rate increases exponentially above Tdiff=250 K. The data is well fitted to an Arrhenius type equation typical for a diffusion process, showing that the Na-ions starts to be mobile above Tdiff . Such results makes this compound very interesting for the tuning of Q1D magnetism using atomic-scale ion-texturing through the periodic potential from ordered Na-vacancies. Further, it also opens the door to possible use of NaV2O4 and related compounds in energy related applications.

cond-mat.mtrl-sci↗

arXiv2012

Ferromagnetism induced in anisotropic stacked kagome-lattice antiferromagnet Cs$_2$Cu$_3$CeF$_{12}$

The magnetic properties of Cs$_2$Cu$_3$CeF$_{12}$ were investigated through magnetization and specific heat measurements. Cs$_2$Cu$_3$CeF$_{12}$ is composed of a buckled kagome lattice of Cu$^{2+}$, which is stacked along the b axis. The exchange network in the buckled kagome lattice is strongly anisotropic. Consequently, Cs$_2$Cu$_3$CeF$_{12}$ can be divided into two subsystems: alternating Heisenberg chains with strong antiferromagnetic exchange interactions and dangling spins. The dangling spins couple with one another via effective exchange interactions, which are mediated by chain spins. The dangling spins are further divided into two subsystems, DS1 and DS2. The dangling spins in DS1 undergo three-dimensional ferromagnetic ordering at 3.14 K, while those in DS2 remain paramagnetic down to 0.35 K. The effective interaction between the DS1 spins is approximately expressed by the ferromagnetic $XXZ$ model with the $z$ direction parallel to the crystallographic c axis. A magnetic phase diagram for $H {\parallel} c$ was obtained and was analyzed within the framework of the molecular field approximation. With increasing magnetic field, the dangling spins are polarized and the magnetization curve exhibits a wide plateau at one-third of the saturation magnetization.

cond-mat.str-el↗

arXiv2011

Thermodynamic properties of quantum sine-Gordon spin chain system KCuGaF6

We investigated the thermodynamic properties of the spin-1/2 one-dimensional Heisenberg antiferromagnet KCuGaF6 by measuring the specific heat in magnetic fields. When this compound is subjected to a uniform magnetic field H a transverse staggered magnetic field h is induced in this compound owing to the staggered component of the g tensor and the Dzyaloshinskii-Moriya interaction with an alternating D vector. Consequently, the quantum sine-Gordon (SG) model is an effective model of this compound in a uniform magnetic field. In three different field directions, we observed a magnetic-field-induced gap, which increases with H. We analyzed experimental results using specific heat theory based on quantum SG theory. The thermodynamic property for H // c is very well described in terms of the elementary excitations characteristic of the quantum SG model, while for the other field directions, significant contributions from other excitation modes beyond the framework of the quantum SG model were observed. For H // b, a quantum phase transition between gapless and gapped ground states was observed.

cond-mat.str-el↗

arXiv2009

Elementary excitations of S=1/2 one-dimensional antiferromagnet KCuGaF6 in magnetic field and quantum sine-Gordon model

Elementary excitations of the S=1/2 one-dimensional Heisenberg antiferromagnet KCuGaF$_6$ with exchange constant $J/k_B$=103 K were investigated by high-frequency ESR measurements combined with a pulsed high magnetic field. When an external magnetic field H is applied in KCuGaF$_6$, a staggered magnetic field h is induced perpendicular to H owing to the staggered g tensor and the Dzyaloshinsky-Moriya (DM) interaction with an alternating D vector. Consequently, KCuGaF$_6$ in a magnetic field is represented by the quantum sine-Gordon (SG) model. We observed many resonance modes including a soliton resonance, breathers, interbreather transitions and two-breather excitation. Their resonance conditions are beautifully described by the quantum SG field theory with one adjustable parameter $c_ s=h/H$. To investigate the relationship between the Curie term due to the DM interaction and the proportional coefficient $c_s$, magnetic susceptibility measurements were also performed varying the external field direction.

cond-mat.str-el↗