arXiv subjects

Y. Kawamura

Publications and source records attributed to Y. Kawamura.

12 recordsLinked to original sources

arXiv2024

Improvement in the Removal Efficiency of the Ultraviolet Laser Ablation by an Additional Simultaneous Irradiation of a Weak Infrared Laser

Remarkable improvement in the removal efficiency of the ultraviolet laser (fourth harmonic wave of Nd:YAG laser) ablation was observed by irradiating a weak infrared laser (fundamental wave of Nd:YAG laser) simultaneously and additionally to various kinds of materials, such as copper, acrylic resin, alumina, silicon and amorphous carbon. The improvement ratio ranged from 50% to 280%, while the laser fluency of the infrared laser was as small as about 1.6% of that of the ultraviolet laser. Although, we have not succeed in explaining the reasons why this effect occurs, it will have the potentiality to decrease the photon cost in the wide range of ultraviolet laser machining, because the method and the phenomena are simple and clear. The improvement in the morphology was also observed, when it is applied to the laser lathe micromachining.

physics.optics↗

arXiv2024

Spontaneous rotation of a nanosatellite FITSAT-1

Spontaneous rotation of an ultra-small satellite was observed and its driving torque was explained by the thermal interaction between the air molecules and the surfaces of the satellite heated by the radiation from the earth. This mechanism has the similarity with a usual radiometer, except the point that the velocity of the satellite is sufficiently faster than that of the thermal velocity of the air molecules, and that the mean free path of the air molecules is sufficiently longer than the characteristic length of the satellite. Using dimension analysis, the torque was found to be significant to a small size of satellite. This rotation mechanism can be applied to any small objects, which are revolting around a planet radiating a black body radiation and has the atmospheric gas.

astro-ph.EP↗

arXiv2023

Presence of a chiral soliton lattice in the chiral helimagnet MnTa$_{3}$S$_{6}$

Chiral helimagnetism was investigated in transition-metal intercalated dichalcogenide single crystals of MnTa$_3$S$_6$. Small-angle neutron scattering (SANS) experiments revealed the presence of harmonic chiral helimagnetic order, which was successfully detected as a pair of satellite peaks in the SANS pattern. The magnetization data are also supportive of the presence of chiral soliton lattice (CSL) phase in MnTa$_3$S$_6$. The observed features are summarized in the phase diagram of MnTa$_3$S$_6$, which is in strong contrast with that observed in other dichalcogenides such as CrNb$_3$S$_6$ and CrTa$_3$S$_6$. The presence of the remanent state provides tunable capability of the number of chiral solitons at zero magnetic field in the CSL system, which may be useful for memory device applications.

cond-mat.mtrl-sci↗

arXiv2022

High-power laser experiment forming a supercritical collisionless shock in a magnetized uniform plasma at rest

We present a new experimental method to generate quasi-perpendicular supercritical magnetized collisionless shocks. In our experiment, ambient nitrogen (N) plasma is at rest and well-magnetized, and it has uniform mass density. The plasma is pushed by laser-driven ablation aluminum (Al) plasma. Streaked optical pyrometry and spatially resolved laser collective Thomson scattering clarify structures of plasma density and temperatures, which are compared with one-dimensional particle-in-cell simulations. It is indicated that just after the laser irradiation, the Al plasma is magnetized by a self-generated Biermann battery field, and the plasma slaps the incident N plasma. The compressed external field in the N plasma reflects N ions, leading to counter-streaming magnetized N flows. Namely we identify the edge of the reflected N ions. Such interacting plasmas form a magnetized collisionless shock.

physics.plasm-ph↗

arXiv2020

Design and Performance of a Silicon Tungsten Calorimeter Prototype Module and the Associated Readout

We describe the details of a silicon-tungsten prototype electromagnetic calorimeter module and associated readout electronics. Detector performance for this prototype has been measured in test beam experiments at the CERN PS and SPS accelerator facilities in 2015/16. The results are compared to those in Monte Carlo Geant4 simulations. This is the first real-world demonstration of the performance of a custom ASIC designed for fast, lower-power, high-granularity applications.

physics.ins-det↗

arXiv2020

Ferrimagnetism in EuFe4As12 revealed by 153Eu NMR and 75As NQR measurements

Filled skutterudite compound EuFe$_4$As$_{12}$ shows the highest magnetic ordering temperature of $T_{\rm C}$ = 154 K among Eu-based skutterudite compounds, but its magnetic ground state has not been determined yet. Here, we performed $^{153}$Eu nuclear magnetic resonance (NMR) and $^{75}$As nuclear quadrupole resonance (NQR) measurements on EuFe$_4$As$_{12}$ to reveal its magnetic ground state as well as the physical properties from a microscopic point of view. From the temperature and magnetic field dependence of $^{153}$Eu NMR spectrum in the magnetically ordered state, we found that the Eu ions are in Eu$^{2+}$ state with a nearly 7 $μ_{\rm B}$ corresponding to $S$ = 7/2 spins. Combined with the magnetization measurements which show the reduced saturation moments of 4.5 $μ_{\rm B}$/f.u., we determined the ground magnetic structure in EuFe$_4$As$_{12}$ to be ferrimagnetic where the Eu$^{2+}$ 4$f$ and the Fe 3$d$ ordered moments are ferromagnetically aligned in each sublattice but the moments between the sublattices are antiferromagnetically aligned. We also found the local distortion at the Eu site from the cubic symmetry in the magnetically ordered state. The relationship between the rattling motion of Eu atoms and the local symmetry of the Eu ions is discussed. From the $^{75}$As NQR nuclear spin-lattice relaxation time measurements as well as $^{153}$Eu NMR measurements, we found that the 4$f$ electrons of the Eu ions are well described by the local moment picture in both the magnetic and paramagnetic metallic states.

cond-mat.str-el↗

arXiv2019

Suppression of ferromagnetic spin fluctuations in the filled skutterudite superconductor SrOs4As12 revealed by 75As NMR-NQR measurements

Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe$_4$As$_{12}$ [Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in a newly discovered isostructural superconductor SrOs$_4$As$_{12}$ with a superconducting transition temperature of $T_{\rm c}$ $\sim$ 4.8 K. Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($\le$ 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/$T_1T$, is nearly independent of temperature above $\sim$ 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe$_4$As$_{12}$, we found that the values of $|K|$ and $1/T_1T$ in SrOs$_4$As$_{12}$ are smaller than those in SrFe$_4$As$_{12}$, indicating no obvious ferromagnetic spin correlations in SrOs$_4$As$_{12}$. From the temperature dependence of 1/$T_1$ in the superconducting state, an $s$-wave superconductivity is realized.

cond-mat.str-el↗

arXiv2018

Ferromagnetic spin fluctuations in the filled skutterudite SrFe4As12 revealed by 75As NMR-NQR measurements

$^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been carried out to investigate the magnetic and electronic properties of the filled skutterudite metallic compound SrFe$_4$As$_{12}$. The temperature dependence of Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($\le$ 0.5 T) shows the similar temperature dependence of the magnetic susceptibility $χ$ which exhibits a broad maximum at $T^\ast$ $\sim$ 50 K. The nuclear spin-lattice relaxation rate divided by temperature, 1/$T_1T$, increases with decreasing temperature and exhibits a broad maximum at $T$ $\sim$ 70 K, similar to the case of $χ$. The temperature dependence of $K$ and $1/T_1T$ is reasonably explained by a simple model where we assume a concave-shaped band structure near the Fermi energy. Based on a Korringa ratio analysis using the $T_1$ and $K$ data, ferromagnetic spin fluctuations are found to exist in SrFe$_4$As$_{12}$. These results indicate that SrFe$_4$As$_{12}$ can be characterized to be a metal with ferromagnetic correlations and also the peculiar band structure responsible for the suppression of $1/T_1T$ and $K$ at low temperatures.

cond-mat.str-el↗

arXiv2017

Time-of-flight elastic and inelastic neutron scattering studies on the localized $4d$ electron layered perovskite La$_5$Mo$_4$O$_{16}$

The magnetic structure and spin-wave excitations in the quasi-square-lattice layered perovskite compound La$_5$Mo$_4$O$_{16}$ were studied by a combination of neutron diffraction and inelastic neutron scattering techniques using polycrystalline sample. Neutron powder diffraction refinement revealed that the magnetic structure is ferrimagnetic in the $ab$ plane with antiferromagnetic stacking along the $c$ axis where the magnetic propagation vector is $\mathbf{k}=\left(0,0,\frac{1}{2}\right)$. The ordered magnetic moments are estimated to be $0.54(2)μ_\text{B}$ for Mo$^{5+}$ ($4d^1$) ions and $1.07(3)μ_\text{B}$ for Mo$^{4+}$ ($4d^2$) ions at 4 K, which are about half of the expected values. The inelastic neutron scattering results display strong easy-axis magnetic anisotropy along the $c$ axis due to the spin-orbit interaction in Mo ions evidenced by the spin gap at the magnetic zone center. The model Hamiltonian consisting of in-plane anisotropic exchange interactions, the interlayer exchange interaction, and easy-axis single-ion anisotropy can explain our inelastic neutron scattering data well. Strong Ising-like anisotropy and weak interlayer coupling compared with the intralayer exchange interaction can explain both the high-temperature magnetoresistance and long-time magnetization decay recently observed in La$_5$Mo$_4$O$_{16}$.

cond-mat.str-el↗

arXiv2011

Polytypes of long-period stacking structures synchronized with chemical order in a dilute Mg-Zn-Y alloy

A series of structural polytypes formed in an Mg-1at.%Zn-2at.%Y alloy has been identified, which are reasonably viewed as long-period stacking derivatives of the hcp Mg structure with alternate AB stacking of the close-packed atomic layers. Atomic-resolution Z-contrast imaging clearly revealed that the structures are long-period chemical-ordered as well as stacking-ordered; unique chemical order along the stacking direction occurs as being synchronized with a local faulted stacking of AB'C'A, where B' and C' layers are commonly enriched by Zn/Y atoms.

cond-mat.mtrl-sci↗

arXiv2009

Novel phase transition and the pressure effect in YbFe2Al10-type CeT2Al10 (T=Fe, Ru, Os)

We have succeeded in growing single crystals of orthorhombic CeT2Al10 (T=Fe, Ru, Os) by Al self-flux method for the first time, and measured the electrical resistivity at pressures up to 8 GPa, the magnetic susceptibility and specific heat at ambient pressure. These results indicate that CeT2Al10 belongs to the heavy fermion compounds. CeRu2Al10 and CeOs2Al10 show a similar phase transition at T0 = 27.3 and 28.7 K, respectively. The temperature dependences in the ordered phases are well described by the thermally activated form, suggesting that partial gap opens over the Fermi surfaces below T0. When pressure is applied to CeRu2Al10, T0 disappears suddenly between 3 and 4 GPa, and CeRu2Al10 turns into a Kondo insulator, followed by a metal. The similarity of CeT2Al10 under respective pressures suggests a scaling relation by some parameter controlling the unusual physics in these compounds.

cond-mat.str-el↗

arXiv1998

Relic Abundance of Neutralinos in Heterotic String Theory: Weak Coupling vs. Strong Coupling

The relic abundance of stable neutralinos is investigated in $E_8 \times E_8'$ heterotic string theory when supersymmetry is spontaneously broken by hidden-sector gaugino condensates. In the weak coupling regime, very large scalar masses (compared to gaugino masses) are shown to lead to a too large relic abundance of the neutralinos, incompatible with cosmological observations in most of parameter space. The problem does not arise in the strong coupling regime (heterotic M-theory) because there scalar and gaugino masses are generically of the same order of magnitude.

hep-ph↗