arXiv subjects

Y. Tomita

Publications and source records attributed to Y. Tomita.

10 recordsLinked to original sources

arXiv2020

Tackling the phase problem of diffraction for retrieval of photonic structures formed in nanocomposite materials

We experimentally demonstrate how to solve the phase problem of diffraction using multi-wave interference with standard diffraction experimental setups without the need for taking any auxiliary data. In particular, we show that the phases of the Fourier components of a periodic structure can be fully recovered by deliberately choosing a probe wavelength of the diffracting radiation much smaller than the lattice constant. In the course of the demonstration, we accurately determine the refractive index profile of nanocomposite phase gratings by light and neutron diffraction measurements.

physics.optics↗

arXiv2020

Light and Slow-Neutron Diffraction by Nanodiamond-Dispersed Nanocomposite Holographic Gratings

We demonstrate the use of nanodiamond in constructing holographic nanoparticle-polymer composite transmission gratings with large saturated refractive index modulation amplitudes at both optical and slow-neutron wavelengths, resulting in efficient control of light and slow-neutron beams. Nanodiamond possesses a high refractive index at optical wavelengths and large coherent and small incoherent scattering cross sections with low absorption at slow-neutron wavelengths. We describe the synthesis of nanodiamond, the preparation of photopolymerizable nanodiamond-polymer composite films, the construction of transmission gratings in nanodiamond-polymer composite films and light optical diffraction experiments. Results of slow-neutron diffraction from such gratings are also presented.

physics.optics↗

arXiv2018

Monte-Carlo simulation of neutron transmission through nanocomposite materials for neutron-optics applications

Nanocomposites enable us to tune parameters that are crucial for use of such materials for neutron-optics applications such as diffraction gratings by careful choice of properties such as species (isotope) and concentration of contained nanoparticles. Nanocomposites for neutron optics have so far successfully been deployed in protonated form, containing high amounts of $^1$H atoms, which exhibit rather strong neutron absorption and incoherent scattering. At a future stage of development, chemicals containing $^1$H could be replaced by components with more favourable isotopes, such as $^2$H or $^{19}$F. In this note, we present results of Monte-Carlo simulations of the transmissivity of various nanocomposite materials for thermal and very-cold neutron spectra. The results are compared to experimental transmission data. Our simulation results for deuterated and fluorinated nanocomposite materials predict a decrease of absorption- and scattering-losses down to about 2 % for very-cold neutrons.

physics.ins-det↗

arXiv2012

NMR Investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2As2O8-y and Ca5(Sc,Ti)4Fe2As2O11-y

$^{75}$As and $^{45}$Sc NMR measurements unravel the electronic state for Fe-based superconductors with perovskite-type blocking layers Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ ($T_c^{onset}=47$ K) and Ca$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$ ($T_c^{onset}=41$ K). In Ca$_5$(Sc,Ti)$_4$Fe$_2$As$_2$O$_{11-y}$, the nuclear spin relaxation rate $1/T_1$ shows pseudogap behavior below $\sim80$ K, suggesting that the electronic state is similar to that of LaFeAs(O,F) system with moderate electron doping. The presence of the pseudogap behavior gives an interpretation that the hole-like band (so-called $γ$ pocket) is located just below the Fermi level from the analogy to LaFeAs(O,F) system and the disappearance of the $γ$ pocket yields the suppression of the low-energy spin fluctuations. On the other hand, in Ca$_4$(Mg,Ti)$_3$Fe$_2$As$_2$O$_{8-y}$ satisfying the structural optimal condition for higher $T_c$ among the perovskite systems, the extrinsic contribution, which presumably originates in the Ti moment, is observed in $1/T_1T$; however, the moderate temperature dependence of $1/T_1T$ appears by its suppression under high magnetic field. In both systems, the high $T_c$ of $\sim40$ K is realized in the absence of the strong development of the low-energy spin fluctuations. The present results reveal that the structural optimization does not induce the strong development of the low-energy spin fluctuations. If we consider that superconductivity is mediated by spin fluctuations, the structural optimization is conjectured to provide a benefit to the development of the high-energy spin fluctuations irrespective to the low-energy part.

cond-mat.supr-con↗

arXiv2012

Three-port beam splitter for slow neutrons using holographic nanoparticle-polymer composite diffraction gratings

Diffraction of slow neutrons by nanoparticle-polymer composite gratings has been observed. By carefully choosing grating parameters such as grating thickness and spacing, a three-port beam splitter operation for cold neutrons - splitting the incident neutron intensity equally into the plus-minus first and zeroth diffraction orders - was realized. As a possible application, a Zernike three-path interferometer is briefly discussed.

quant-ph↗

arXiv2012

Mirrors for slow neutrons from holographic nanoparticle-polymer free-standing film-gratings

We report on successful tests of holographically arranged grating-structures in nanoparticle-polymer composites in the form of 100 microns thin free-standing films, i.e. without sample containers or covers that could cause unwanted absorption/incoherent scattering of very-cold neutrons. Despite their large diameter of 2 cm, the flexible materials are of high optical quality and yield mirror-like reflectivity of about 90% for neutrons of 4.1 nm wavelength.

cond-mat.mtrl-sci↗

arXiv2011

Diffraction of slow neutrons by holographic SiO_2 nanoparticle-polymer composite gratings

Diffraction experiments with holographic gratings recorded in SiO$_2$ nanoparticle-polymer composites have been carried out with slow neutrons. The influence of parameters such as nanoparticle concentration, grating thickness and grating spacing on the neutron-optical properties of such materials has been tested. Decay of the grating structure along the sample depth due to disturbance of the recording process becomes an issue at grating thicknesses of about 100 microns and larger. This limits the achievable diffraction efficiency for neutrons. As a solution to this problem, the Pendellösung interference effect in holographic gratings has been exploited to reach a diffraction efficiency of 83% for very cold neutrons.

cond-mat.mtrl-sci↗

arXiv2010

Finite-temperature phase transition to $m=1/2$ plateau phase in a S=1/2 XXZ model on Shastry-Sutherland Lattices

We study the finite-temperature transition to the $m=1/2$ magnetization plateau in a model of interacting $S=1/2$ spins with longer range interactions and strong exchange anisotropy on the geometrically frustrated Shastry-Sutherland lattice. This model was shown to capture the qualitative features of the field-induced magnetization plateaus in the rare-earth tetraboride, ${\rm TmB_4}$. Our results show that the transition to the plateau state occurs via two successive transitions with the two-dimensional Ising universality class, when the quantum exchange interactions are finite, whereas a single phase transition takes place in the purely Ising limit. To better understand these behaviors, we perform Monte Carlo simulations of the classical generalized four-state chiral clock model and compare the phase diagrams of the two models. Finally, we estimate a parameter set that can explain the magnetization curves observed in ${\rm TmB_4}$. The magnetic properties and critical behavior of the finite-temperature transition to the $m=1/2$ plateau state are also discussed.

cond-mat.str-el↗

arXiv2010

Neutron optical beam splitter from holographically structured nanoparticle-polymer composites

We report a breakthrough in the search for versatile diffractive elements for cold neutrons. Nanoparticles are spatially arranged by holographical means in a photopolymer. These grating structures show remarkably efficient diffraction of cold neutrons up to about 50% for effective thicknesses of only 200 micron. They open up a profound perspective for next generation neutron-optical devices with the capability to tune or modulate the neutron diffraction efficiency.

cond-mat.mtrl-sci↗

arXiv2010

Neutron-optical gratings from nanoparticle-polymer composites

The preparation of neutron-optical phase gratings with light-optical holography is reviewed. We compare the relevant concepts of i) Kogelnik's theory for Bragg diffraction of light by thick volume gratings, which can be used to analyze holographic gratings with both light and neutrons, and ii) the dynamical theory of neutron diffraction. Without going into mathematical detail, we intend to illuminate their correspondence. The findings are illustrated by analyzing data obtained from reconstruction of nanoparticle holographic gratings with both light and neutrons.

physics.ins-det↗