arXiv subjects

K. Iida

Publications and source records attributed to K. Iida.

At least 19 recordsLinked to original sources

arXiv2026

High-charge collimated and energy-selected laser-driven MeV electron beams produced by magnetic selection

We have developed a compact passive energy-selector for MeV-range electrons produced by irradiating solid targets by ultra-intense short-pulse lasers. The device allows for generating electron beams with a variable energy spread over a broad range of energies, from tens of keV to tens of MeV. Here we have demonstrated its use by producing electrons from solid targets in the MeV range and with a ~10% bandwidth, thereby compensating the intrinsic broadband nature of the electrons produced from such source. Coupled with a pulsed magnetic field to further compensate the intrinsic large divergence of this source, it allows to produce a highly-collimated beam of narrow-band and ultra-fast electrons, suitable for a wide range of applications, e.g. radiation therapy or time-resolved electron probing.

physics.acc-ph↗

arXiv2026

Zr-based bulk metallic glass clamp cell for high-pressure inelastic neutron scattering

We report the fabrication and characterization of a Zr-based bulk metallic glass (Zr-BMG) clamp cell designed for high-pressure inelastic neutron scattering (INS) measurements. The INS spectra of the empty cell exhibit broad and featureless backgrounds, reflecting the amorphous structure of the Zr-BMG. Test measurements using a reference sample, CsFeCl$_{3}$, confirm that the neutron transmission of the Zr-BMG cell is significantly higher than that of a conventional monobloc CuBe clamp cell. These results demonstrate that the Zr-BMG clamp cell provides both enhanced neutron transparency and a clean background profile, thereby advancing high-pressure INS studies.

cond-mat.mtrl-sci↗

arXiv2025

Phonon spectrum in the spin-Peierls phase of CuGeO$_3$

CuGeO$_3$ has long been studied as a prototypical example of the spin-Peierls transition in a $S = 1/2$ Heisenberg chain. Despite intensive investigation of this quasi-one-dimensional material, systematic measurements and calculations of the phonon excitations in the dimerized phase have not to date been possible, leaving certain aspects of the spin-Peierls phenomenon unresolved. We perform state-of-the-art density functional theory (DFT) calculations to compute the electronic structure and phonon dynamics in the low-temperature dimerized phase. We also perform high-resolution neutron spectroscopy to measure the full phonon spectrum over multiple Brillouin zones. We find excellent agreement between our numerical and experimental results that extend to all measurement temperatures. Notable features of our phonon spectra include a number of steeply dispersive modes, nonmonotonic dispersion features, and specific phonon anticrossings, which we relate to the mode eigenvectors. By calculating the magnetic interactions within DFT and studying the effects of different phonon modes on the superexchange paths, we discuss the possibility of observing spin-phonon hybridization effects in experiments performed both in and out of equilibrium.

cond-mat.str-el↗

arXiv2025

Reexamination of the charge-ordered dimer pattern in the spinel compound CuIr2S4 using single-crystal synchrotron x-ray diffraction

We have re-investigated the crystal structure of a spinel type CuIr2S4 at low temperatures using a single-crystal in a synchrotron radiation x-ray diffraction experiment. The crystal structure of the low-temperature phase of CuIr2S4 has been already studied by diffraction experiments using a powder sample, and it has been reported that the formation of dimer molecules accompanied by charge ordering of Ir has been achieved. The crystal structure of the low-temperature phase obtained in our reanalysis was the same as the previously reported structure in that it showed the formation of Ir dimers accompanied by charge ordering, but the charge ordering pattern and arrangement of the dimers in the unit cell were different. We will discuss the validity of the structure obtained in this study and provide the structural parameters revealed in the reanalysis. The results of this study should provide a basis for further studies of the physical properties of CuIr2S4, which are still being actively investigated.

cond-mat.str-el↗

arXiv2022

Inter- to Intra-Layer Resistivity Anisotropy of NdFeAs(O,H) with Various Hydrogen Concentrations

With molecular beam epitaxy and topotactic chemical reaction, we prepared NdFeAs(O,H) epitaxial thin films with various hydrogen concentrations on 5{\deg} vicinal cut MgO substrates. By measuring the resistivities along the longitudinal and transversal directions, the ab plane and the c axis resistivities (\{rho}_ab and \{rho}_c) were obtained. The resistivity anisotropy {\gamma}_\{rho}=\{rho}_c \ \{rho}_ab of NdFeAs(O,H) with various hydrogen concentrations was compared with that of NdFeAs(O,F). At the H concentrations which led to superconducting transition temperatures Tc over 40 K, {\gamma}_\r{ho} recorded ~100-150 at 50 K. On the other hand, a low {\gamma}_\{rho} value of 9 was observed with the mostly doped sample. The exponent \{beta} of the ab plane resistivity obtained by fitting a power law expression \{rho}_{ab}(T)=\{rho}_0+AT^\{beta} to the data was close to unity down to low temperature in the vicinity where the second antiferromagnetic phase locates, which may be related to the quantum critical point discussed at the over-doped side of the phase diagram.

cond-mat.supr-con↗

arXiv2021

Structure of two- and three-alpha systems in cold neutron matter

We present stability and structure of two- and three-alpha systems embedded in dilute cold neutron matter. By solving a few-alpha Schr\"{o}dinger equation with quasiparticle properties, i.e., effective mass and induced two- and three-alpha interactions, which are evaluated in terms of Fermi polarons, it is shown that $^8 \mathrm{Be}$ and the Hoyle state become bound at densities of about $10^{-4}$ and $10^{-3}$ of the saturation density, respectively. It is also seen that, under cold neutron matter environment, both systems become smaller than the corresponding systems in vacuum. Our results would affect astrophysical models for stellar collapse and neutron star mergers, as well as relevant reaction rates for nucleosynthesis.

nucl-th↗

arXiv2021

Binding two and three $\alpha$ particles in cold neutron matter

We elucidate the fate of neighboring two and three-$\alpha$ particles in cold neutron matter by focusing on an analogy between such $\alpha$ systems and Fermi polarons realized in ultracold atoms. We describe in-medium excitation properties of an $\alpha$ particle and neutron-mediated two- and three-$\alpha$ interactions using theoretical approaches developed for studies of cold atomic systems. We numerically solve the few-body Schr\"odinger equation of $\alpha$ particles within standard $\alpha$ cluster models combined with in-medium properties of $\alpha$ particles. We point out that the resultant two-$\alpha$ ground state and three-$\alpha$ first excited state, which correspond to $^8$Be and the Hoyle state, respectively, known as main components in the triple-$\alpha$ reaction, can become bound states in such a many-neutron background although these states are unstable in vacuum. Our results suggest a significance of these in-medium cluster states not only in astrophysical environments such as core-collapsed supernova explosions and neutron star mergers but also in neutron-rich nuclei.

nucl-th↗

arXiv2021

Coulomb screening effect on the Hoyle state energy in thermal plasmas

The first excited $J^\pi=0^+$ state of $^{12}$C, the so-called Hoyle state, plays an essential role in a triple-$\alpha$ ($^4$He) reaction, which is a main contributor to the synthesis of $^{12}$C in a burning star. We investigate the Coulomb screening effects on the energy shift of the Hoyle state in a thermal plasma environment using precise three-$\alpha$ model calculations. The Coulomb screening effect between $\alpha$ clusters are taken into account within the Debye-H\"uckel approximation. To generalize our study, we utilize two standard $\alpha$-cluster models, which treat the Pauli principle between the $\alpha$ particles differently. We find that the energy shifts do not depend on these models and follow a simple estimation in the zero-size limit of the Hoyle state when the Coulomb screening length is as large as a value typical of such a plasma consisting of electrons and $\alpha$ particles.

nucl-th↗

arXiv2020

Robust atomic orbital in the cluster magnet LiMoO2

In this study, we present a rutile-related material, LiMoO2, that becomes a cluster magnet and exhibits a spin singlet formation on a preformed molybdenum dimer upon cooling. Unlike ordinary cluster magnets, the atomic dyz orbital robustly survives despite the formation of molecular orbitals, thereby affecting the magnetic properties of the selected material. Such hybrid cluster magnets with the characters of molecular and atomic orbitals realize multiple independent spins on an isolated cluster, leading to an ideal platform to study the isolated spin dimers physics.

cond-mat.str-el↗

arXiv2020

$\mathbf{q}=\mathbf{0}$ long-range magnetic order in centennialite CaCu$_3$(OD)$_6$Cl$_2$$\cdot$0.6D$_2$O: A spin-1/2 perfect kagome antiferromagnet with $J_1$-$J_2$-$J_d$

Crystal and magnetic structures of the mineral centennialite CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O are investigated by means of synchrotron x-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O crystallizes in the $P\bar{3}m1$ space group and Cu$^{2+}$ ions form a geometrically perfect kagome network with antiferromagnetic $J_1$. No intersite disorder between Cu$^{2+}$ and Ca$^{2+}$ ions is detected. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O enters a magnetic long-range ordered state below $T_\text{N}=7.2$~K, and the $\mathbf{q}=\mathbf{0}$ magnetic structure with negative vector spin chirality is obtained. The ordered moment at 0.3~K is suppressed to $0.58(2)\mu_\text{B}$. Our DFT calculations indicate the presence of antiferromagnetic $J_2$ and ferromagnetic $J_d$ superexchange couplings of a strength which places the system at the crossroads of three magnetic orders (at the classical level) and a spin-$\frac{1}{2}$ PFFRG analysis shows a dominance of $\mathbf{q}=\mathbf{0}$ type magnetic correlations, consistent with and indicating proximity to the observed $\mathbf{q}=\mathbf{0}$ spin structure. The results suggest that this material is located close to a quantum critical point and is a good realization of a $J_1$-$J_2$-$J_d$ kagome antiferromagnet.

cond-mat.str-el↗

arXiv2020

Coulomb screening correction to the $Q$ value of the triple alpha process in thermal plasmas

The triple alpha reaction is a key to $^{12}$C production and is expected to occur in weakly-coupled, thermal plasmas as encountered in normal stars. We investigate how Coulomb screening affects the structure of a system of three alpha particles in such a plasma environment by precise three-body calculations within the Debye-H\"uckel approximation. A three-alpha model that has the Coulomb interaction modified in the Yukawa form is employed. Precise three-body wave functions are obtained by a superposition of correlated Gaussian bases with the aid of the stochastic variational method. The energy shifts of the Hoyle state due to the Coulomb screening are obtained as a function of the Debye screening length. The results, which automatically incorporate the finite size effect of the Hoyle state, are consistent with the conventional result based on the Coulomb correction to the chemical potentials of ions that are regarded as point charges in a weakly-coupled, thermal plasma. We have given a theoretical basis to the conventional point-charge approach to the Coulomb screening problem relevant for nuclear reactions in normal stars by providing the first evaluation of the Coulomb corrections to the $Q$ value of the triple alpha process that produces a finite size Hoyle state.

nucl-th↗

arXiv2019

Coexisting spin resonance and long-range magnetic order of Eu in EuRbFe$_4$As$_4$

Magnetic excitations and magnetic structure of EuRbFe$_4$As$_4$ were investigated by inelastic neutron scattering (INS), neutron diffraction, and random phase approximation (RPA) calculations. Below the superconducting transition temperature $T_\text{c}=36.5$~K, the INS spectra exhibit the neutron spin resonances at $Q_\text{res}=1.27(2)$~$\text{\AA}^{-1}$ and $1.79(3)$~$\text{\AA}^{-1}$. They correspond to the $\mathbf{Q}=(0.5,0.5,1)$ and $(0.5,0.5,3)$ nesting wave vectors, showing three dimensional nature of the band structure. The characteristic energy of the neutron spin resonance is $E_\text{res}=17.7(3)$~meV corresponding to $5.7(1)k_\text{B}T_\text{c}$. Observation of the neutron spin resonance mode and our RPA calculations in conjunction with the recent optical conductivity measurements are indicative of the $s_\pm$ superconducting pairing symmetry in EuRbFe$_4$As$_4$. In addition to the neutron spin resonance mode, upon decreasing temperature below the magnetic transition temperature $T_\text{N}=15$~K, the spin wave excitation originating in the long-range magnetic order of the Eu sublattice was observed in the low-energy inelastic channel. Single-crystal neutron diffraction measurements demonstrate that the magnetic propagation vector of the Eu sublattice is $\mathbf{k}=(0, 0, 0.25)$, representing the three-dimensional antiferromagnetic order. Linear spin wave calculations assuming the obtained magnetic structure with the intra- and inter-plane nearest neighbor exchange couplings of $J_1/k_\text{B}=-1.31$~K and $J_c/k_\text{B}=0.08$~K can reproduce quantitatively the observed spin wave excitation. Our results show that superconductivity and long-range magnetic order of Eu coexist in EuRbFe$_4$As$_4$ whereas the coupling between them is rather weak.

cond-mat.supr-con↗

arXiv2019

p-Wave Superconductivity in Iron-based Superconductors

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters strongly indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further investigation revealed that single atomic layer FeSe also shows p-wave pairing. In an attempt to generalize these findings, we re-examined the whole inventory of superfluid density measurements in iron-based superconductors show quite generally that most of the iron-based superconductors are p-wave superconductors.

cond-mat.supr-con↗

arXiv2019

Spin gap and L modulated intensity at the low-energy incommensurate magnetic fluctuations in the superconducting state of Sr2RuO4

Low-energy incommensurate (IC) magnetic fluctuations in the multiband superconductor Sr$_2$RuO$_4$ is investigated by high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. Below $T_\text{c}$, the substantial spin gap is observed at $\mathbf{Q}_\text{IC}=(0.3, 0.3, L)$ where the quasi-one-dimensional $\alpha$ and $\beta$ sheets consisting of the Fermi surfaces are in good nesting conditions. $L$ modulated intensity of the low-energy IC magnetic fluctuations and our RPA calculations indicate that the superconducting gaps regarding the $\alpha$ and $\beta$ sheets have the horizontal line nodes.

cond-mat.supr-con↗

arXiv2019

Quantum magnetisms in uniform triangular lattices Li2AMo3O8 (A = In, Sc)

Molecular based spin-1/2 triangular lattice systems such as LiZn$_2$Mo$_3$O$_8$ have attracted research interest. Distortions, defects, and intersite disorder are suppressed in such molecular-based magnets, and intrinsic geometrical frustration gives rise to unconventional and unexpected ground states. Li$_2$$A$Mo$_3$O$_8$ ($A$ = In or Sc) is such a compound where spin-1/2 Mo$_3$O$_{13}$ clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the $A$ site. Li$_2$InMo$_3$O$_8$ undergoes conventional $120^\circ$ long-range magnetic order below $T_\text{N}=12$~K whereas isomorphic Li$_2$ScMo$_3$O$_8$ exhibits no long-range magnetic order down to 0.5~K. Here, we report exotic magnetisms in Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ investigated by muon spin rotation ($\mu$SR) and inelastic neutron scattering (INS) spectroscopies using polycrystalline samples. Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ show completely different behaviors observed in both $\mu$SR and INS measurements, representing their different ground states. Li$_2$InMo$_3$O$_8$ exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the $120^\circ$ spin structure. In contrast, Li$_2$ScMo$_3$O$_8$ undergoes short-range magnetic order below 4~K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions.

cond-mat.str-el↗

arXiv2019

Vortex glass-liquid transition and activated flux motion in an epitaxial, super-conducting NdFeAs(O,F) thin film

An epitaxial NdFeAs(O,F) thin film of 90 nm thickness grown by molecular beam epitaxy on MgO single crystal with Tc = 44.2 K has been investigated regarding a possible vortex glass-liquid tran-sition. The voltage-current characteristics show excellent scalability according to the vortex-glass model with a static critical exponent {\nu} of around 1.35 and a temperature-dependent dynamic exponent z increasing from 7.8 to 9.0 for the investigated temperature range. The large and non-constant z values are discussed in the frame of 3D vortex glass, thermally activated flux motion, and inhomogeneity broadening.

cond-mat.supr-con↗

arXiv2018

Large entropy change derived from the orbitally assisted three-centered two-electron {\sigma} bond formation in a metallic Li0.33VS2

We discuss herein the emergence of a large entropy change in metallic Li0.33VS2 derived from the orbitally assisted loose {\sigma} bond formation. Comprehensive structural studies based on synchrotron x-ray and neutron diffraction analyses clarify the fabrication of ribbon chains at 375 K, consisting of multiple three-centered two-electron {\sigma} bonds based on the viewpoint of local chemical bonding. Although the metallic conductivity persists down to the lowest temperature measured, exceptionally large entropy change as a metal, as much as {\Delta}S = 6.6 J/mol K, appears at the transition. Emergence of a large entropy change in a metallic state expects us the possible novel functional materials, such as a heat-storage material with rapid thermal response.

cond-mat.str-el↗

arXiv2018

Effect of $\alpha$-particle irradiation on a NdFeAs(O,F) thin film

The effect of $\alpha$-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature $T_c$ and the upper critical field $H_{c2}$, and properties more of interest for applications, like the critical current density $J_c$ and the related pinning landscape. The irradiation-induced suppression of the film $T_c$ is significantly smaller than on a similarly damaged single crystal. Moreover $H_{c2}$ behaves differently, depending on the field orientation: for H//c the $H_{c2}$ slope monotonically increases with increasing disorder, whereas for H//ab it remains constant at low dose and it increases only when the sample is highly disordered. This suggests that a much higher damage level is necessary to drive the NdFeAs(O,F) thin film into the dirty limit. Despite the increase in the low temperature $H_{c2}$, the effects on the $J_c$(H//c) performances are moderate in the measured temperature and field ranges, with a shifting of the pinning force maximum from 4.5 T to 6 T after an irradiation of $2\times10^{15} cm^{-2}$. On the contrary, $J_c$(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects acting as pinning centres proportionally to the irradiation fluence but also suppresses the effectiveness of c-axis correlated pinning present in the pristine sample. We estimate that significant performance improvements may be possible at high field or at temperatures below 10 K. The suppression of the $J_c$(H//ab) performance is not related to a decrease of the $J_c$ anisotropy as found in other superconductors. Instead it is due to the presence of point defects that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure.

cond-mat.supr-con↗