arXiv subjects

Shin-ichi Shamoto

Publications and source records attributed to Shin-ichi Shamoto.

11 recordsLinked to original sources

arXiv2026

Dynamic Magnetic Pair-Density Function of a One-Dimensional Ferromagnet

The dynamic magnetic pair-density function (DymPDF) $D_{\rm M}(r, E)$ is derived by extending the static magnetic pair distribution function (mPDF) to finite energy transfer. The analytical DymPDF of a one-dimensional Heisenberg ferromagnet is obtained from the magnon dispersion and compared with simulations performed using {\textsc SpinW}. Excellent agreement is achieved for energy dependence of the nearest-neighbor spin-pair correlation, demonstrating that the DymPDF changes sign at the magnon-mode transition occurring at one-half of the maximum magnon energy. The real-space DymPDF at low energy is also reproduced with a model including finite instrumental resolution, magnetic correlation length, and Fourier-termination effects. These results establish the theoretical foundation of DymPDF analysis for investigating local spin dynamics in magnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

Local magnon modes studied by dynamic magnetic pair-density function analysis

The dynamic magnetic pair-density function (DymPDF) $D_{\rm M}(r, E)$ is obtained via the Fourier transform of the dynamic magnetic structure factor, $S_{\rm M}(Q, E)$, which is measured using nonpolarized inelastic neutron scattering. While there is a long history of magnetic excitation studies with $S_{\rm M}(Q, E)$, there are no reports on $D_{\rm M}(r, E)$. In this study, we examine simple magnet models and representative magnet examples, such as FeTiO$_{3}$ and YBa$_{2}$Cu$_{3}$O$_{6}$, to investigate the real-space dynamics of $D_{\rm M}(r, E)$. We derive the $D_{\rm M}(r, E)$ equations for simple magnet models in a low energy limit. By comparing these equations to the simulations, we demonstrate the characteristic energy dependence of real-space local magnon modes, including the transition of the magnon mode from acoustic to optical. Our novel analysis reveals the local magnon modes accompanied by a sign change in each spin-pair correlation at a given energy in nanoscale real space even under non-periodic conditions. This method is unique for studying local magnetic dynamics.

cond-mat.str-el↗

arXiv2021

Q Dependence of Magnetic Resonance Mode on FeTe$_{0.5}$Se$_{0.5}$ Studied by Inelastic Neutron Scattering

Inelastic neutron scattering measurements have been performed on a superconducting single crystal FeTe$_{0.5}$Se$_{0.5}$ to examine the ${\bf Q}$-dependent enhancement of the dynamical structure factor, $S({\bf Q},E)$, from ${\bf Q}$ = (0, 0) to ($π$, $π$), including ($π$, 0) in the superconducting state. In most of iron-based superconductors, $S({\bf Q},E)$ is enhanced at ${\bf Q}$ = ($π$, 0), where the "magnetic resonance mode" is commonly observed in the unfolded Brillouin zone. Constant-$E$ cuts of $S({\bf Q},E)$ suggest that the enhancement is not uniform in the magnetic excitation, and limited around ${\bf Q}$ = ($π$, 0). This result is consistent with the theoretical simulation of the magnetic resonance mode due to the Bardeen$-$Cooper$-$Schrieffer coherence factor with the sign-reversing order parameter of s$_{\pm}$ wave.

cond-mat.supr-con↗

arXiv2021

Neutron Scattering Study on Yttrium Iron Garnet for Spintronics

Spin current -- a flow of the spin degree of freedom in matter -- has vital importance in spintronics. Propagation of the spin current ranges over a whole momentum space; however, generated spin currents are mainly detected in the long-wavelength limit. To facilitate practical uses of spintronics and magnonics, microscopic understanding of the spin current is necessary. We here address yttrium iron garnet, which is a well-employed ferrimagnet for spintronics, and review {\it in re} the momentum- and energy-resolved characteristics of its magnetism. Using {\it unpolarized} neutrons, we refined its detailed crystal and magnetic structure, and examined magnetic excitations through four decades (10~$μ$eV-100~meV) using chopper spectrometers in J-PARC, Japan. We also measured mode-resolved directions of the precessional motion of the magnetic moment, i.e., magnon polarization, which carries the spin current in insulators through {\it polarized} neutron scattering, using a triple-axis spectrometer in ILL, France. The magnon polarization is a hitherto untested fundamental property of magnets, affecting the thermodynamic properties of the spin current. Our momentum- and energy-resolved experimental findings provide an intuitive understanding of the spin current and demonstrate the importance of neutron scattering techniques for spintronics and magnonics.

cond-mat.mtrl-sci↗

arXiv2019

Local Structure of Functional Solids

The local structure study reveals important aspects of the physical properties, because it is closely related to the electronic structure. Standard crystallographic analysis based on a space group fails to observe disorder in the crystal structure. Many functional and industrial materials have disorder, because of the optimized modifications of crystal structures such as the substitution of an element. Doped elements inevitably induce local strains in the matrix. Some neutron total scattering results are reviewed to show the local functional properties of solids from optical recording to negative thermal expansion to Mott transition materials.

cond-mat.mtrl-sci↗

arXiv2019

High-energy spin fluctuation in low-$T_{\rm c}$ iron-based superconductor LaFePO$_{0.9}$

Spin fluctuations are widely believed to play an important role in the superconducting mechanisms of unconventional high-temperature superconductors. Spin fluctuations have been observed in iron-based superconductors as well. However, in some iron-based superconductors such as LaFePO$_{0.9}$, they have not been observed by inelastic neutron scattering (INS). LaFePO$_{0.9}$ is an iron-based superconductor with a low superconducting transition temperature ($T_{\rm c}$= 5 K), where line nodes are observed in the superconducting gap function. The line-node symmetry typically originates from sign reversal of the order parameter in spin-fluctuation-mediated superconductivity. This contradiction has been a long-standing mystery of this superconductor. Herein, spin fluctuations were found at high energies such as 30$-$50 meV with comparable intensities to an optimally doped LaFeAs(O,F). Based on this finding, the line-node symmetry can be explained naturally as spin-fluctuation-mediated superconductivity.

cond-mat.supr-con↗

arXiv2018

Neutron-scattering study of yttrium iron garnet

The nuclear and magnetic structure and full magnon dispersions of yttrium iron garnet Y$_3$Fe$_5$O$_{12}$ have been studied by neutron scattering. The refined nuclear structure is distorted to a trigonal space group of $R\bar{3}$. The highest-energy dispersion extends up to 86 meV. The observed dispersions are reproduced by a simple model with three nearest-neighbor-exchange integrals between 16$a$ (octahedral) and 24$d$ (tetrahedral) sites, $J_{aa}$, $J_{ad}$, and $J_{dd}$, which are estimated to be 0.00$\pm$0.05, $-$2.90$\pm$0.07, and $-$0.35$\pm$0.08 meV, respectively. The lowest-energy dispersion below 14 meV exhibits a quadratic dispersion as expected from ferromagnetic magnons. The imaginary part of $q$-integrated dynamical spin susceptibility $χ$"($E$) exhibits a square-root energy-dependence in the low energies. The magnon density of state is estimated from the $χ$"($E$) obtained on an absolute scale. The value is consistent with a single polarization mode for the magnon branch expected theoretically.

cond-mat.mtrl-sci↗

arXiv2013

Local Lattice Distortion Caused by Short Range Charge Ordering in LiMn$_2$O$_4$

We have performed powder neutron diffraction on $^7$Li-enriched sample of LiMn$_2$O$_4$ at 300 K. The crystal structure determined by Rietveld analysis is a cubic spinel with space group of $Fd\bar{3}m$ in which all Mn atoms are crystallograghically equivalent, consistent with many preceding studies. However, the atomic pair distrubution function (PDF) of this compound can not be fitted by the cubic structure with space group of $Fd\bar{3}m$ satisfactorily, and it can be reproduced by the orthorhombic structure with $Fddd$. It corresponds with the structure of charge ordered phase below about 260 K, indicating a short range charge ordering. In the local structure determined by PDF analysis, two types of MnO$_6$ octahedra with long and short atomic distances between Mn and O atoms exist and their Mn-O distances are almost consistent with the distances in the charge ordered phase. From these results, valence electrons are localized at Mn sites like a glass even in the cubic phase, resulting in the non-metallic electrical conductivity.

cond-mat.str-el↗

arXiv2011

Hydrogen in layered iron arsenide: indirect electron doping to induce superconductivity

Utilizing the high stability of calcium and rare earth hydrides, CaFeAsF1-xHx (x = 0.0-1.0) and SmFeAsO1-xHx (x = 0.0-0.47) have been first synthesized using high pressure to form hydrogen-substituted 1111 type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H- ions occupying F- sites in the blocking layer of CaFeAsF. The resulting CaFeAsF1-xHx is non-superconducting, whereas SmFeAsO1-xHx is a superconductor, with an optimal Tc = 55 K at x 0.2. It was found that up to 40% of the O2- ions can be replaced by H- ions, with electrons being supplied into the FeAs-layer to maintain neutrality (O2- = H-+ e-). When x exceeded 0.2, Tc was reduced corresponding to an electron over-doped region.

cond-mat.supr-con↗

arXiv2011

Structural Analysis on Iron-Based Superconductor Pr1111 System with Oxygen Deficiency and Flourine Substitution

We have performed structural analyses on iron-based superconductors, PrFeAsO$_{1-y}$ and PrFeAsO$_{1-x}$F$_x$, systematically, by means of Rietveld method on neutron powder diffraction data. The shifts of iron ion valence from +2, $δ$, are accurately determined from the occupancies of O and O$_{1-x}$F$_x$ sites obtained by the Rietveld analysis and F-concentration obtained by secondary ion-microprobe mass spectrometry. $T_\textrm{c}$-$δ$ curve of PrFeAsO$_{1-y}$ is different from the curve of PrFeAsO$_{1-x}$F$_x$, indicating that $δ$ is not a principal parameter for $T_\textrm{c}$ in so-called 1111 system. Structural parameters of the FeAs layers, for example, As-Fe-As bond angle and As-height from Fe layer, are different between both systems with similar $δ$-values. Their parent compounds are also found to have different structural parameters, possibly due to the different synthetic conditions. These results suggest that the difference of structural parameters of FeAs layer is the origin of the discrepancy of $T_\textrm{c}$-$δ$ curves of both systems and the $T_\textrm{c}$-value in the 1111 system is sensitive to the structural parameters. It may be attribute to an energy balance of the conducting bands contributing to the superconductivity.

cond-mat.supr-con↗

arXiv2008

Evidence of spin density wave in LaFeAsO, the parent material of the new Fe-based oxypnictide superconductors

The Fe-based Oxypnicide superconductors have generated a huge amount of interest; they are a high temperature superconductor, with a Tc of 55K, but do not have the two dimensional copper oxygen layer that was thought essential for superconductivity at these high temperatures. Initial studies have hinted towards the possibility of a spin density wave (SDW) in these compounds and how they could play an important role in the superconductivity. SDW's occur at low temperatures in low-dimensional materials with strong electron correlations or in metals with a high density of states at the Fermi surface. A transition to the SDW state has many similarities to the superconducting transition, driven by the condensation energy with an energy gap opening, and in many materials the SDW state occurs adjacent in the phase diagram to the superconducting state. In these materials it is believed that electron doping suppresses the SDW instability allowing superconductivity to emerge. In this paper we report on the first direct experimental evidence of a SDW in the LaFeAsO parent compound as observed with inelastic neutron scattering. We show that these excitation derive from a two-dimensional Fermi surface nesting with a nesting vector of (π,π,0).

cond-mat.supr-con↗