arXiv subjects

Daisuke Ishikawa

Publications and source records attributed to Daisuke Ishikawa.

At least 19 recordsLinked to original sources

arXiv2026

Observation of coherently modulated phonon band and lifetime in superlattice

Similar to the behavior of elementary particles, such as photons and electrons, the interference of phonon waves in artificial periodic nanostructures coherently modulates phonon band structures, serving as the foundation for phonon band engineering. However, direct observation of such coherently modulated phonon band structures remains challenging despite substantial insights from existing literature. Here, utilizing high-resolution inelastic X-ray scattering, we observed coherently modulated phonon band structures with phononic band gaps in a short-period GaAs/AlAs superlattice at 300 K and 500 K. Our findings provide the first direct evidence of phonon coherence at and above room temperatures, signifying a major advancement in the artificial engineering of phonon band structures. Furthermore, our experimental observations and ab initio lattice dynamics revealed that the coherently modulated phonon band structure enhances three-phonon scattering channels, strengthening high-order anharmonic effects such as three-phonon scattering and optical phonon softening. Our observations demonstrate the robustness of phonon coherence at high temperatures, and opens new routes for engineering phonon band structure and high-order phonon-phonon scattering by employing a flexible, bottom-up nanostructuring approach, with extensive applications in phononic metamaterials, microelectronics, and thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2026

Antiferromagnetic Dimers in the Parent Phase of a Correlated Kagome Superconductor

Kagome metals are prone to charge-density wave (CDW), magnetic, and superconducting phases, with their flat electronic band conducive for correlated physics. In contrast to the weakly correlated $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) kagome metals with a $2\times2$ CDW, CsCr$_3$Sb$_5$ is a correlated metal with a flat band close to the Fermi level, and exhibits a $4\times1$ CDW intertwined with magnetic order. Under pressure, the intertwined orders are suppressed and give way to a dome of superconductivity that emerges from a non-Fermi liquid normal state. Here, we solve the crystal structure of the $4\times 1$ CDW state in CsCr$_3$Sb$_5$, and show it consists of Cr dimers separated by Cr chains. First-principles calculations show the dominant exchange interaction is antiferromagnetic within the dimers, while the intra-chain and dimer-chain couplings are much weaker. The CDW transition of CsCr$_3$Sb$_5$ is found to be more strongly first-order than those in $A$V$_3$Sb$_5$, without significant soft phonons or diffuse scattering above the CDW transition temperature. These findings suggest that fluctuating antiferromagnetic dimers may play a major role in the electron pairing of superconducting CsCr$_3$Sb$_5$.

cond-mat.str-el↗

arXiv2026

Phonons reflect dynamic spin-state order in LaCoO$_3$

We investigate lattice dynamics in LaCoO$_3$ using inelastic neutron and x-ray scattering over $T = 2\mbox{-}650\,\mathrm{K}$, spanning the spin-state crossover at $T_{1} \approx 100\,\mathrm{K}$ and the insulator--metal transition at $T_{2} \approx 550\,\mathrm{K}$. Comparison with quasi-harmonic $ab-initio$ lattice-dynamical calculations helps reveal anomalous softening of a $\approx 10\,\mathrm{meV}$ oxygen phonon, confined to the temperature interval $T_{1} \leq T \leq T_{2}$ and localized in momentum space at $\boldsymbol{q}_{\mathrm{SSO}} = \left( \frac{1}{2},\frac{1}{2},\frac{1}{2} \right)_{c}$. This wave vector corresponds to the spin-state ordering originally proposed by Goodenough [J. Phys. Chem. Solids 6, 287-297 (1958)]. Our results therefore provide momentum-resolved evidence for dynamic correlations of high-spin and low-spin Co$^{3+}$ states in LaCoO$_{3}$, linking spin-state fluctuations to anomalous phonon renormalization.

cond-mat.str-el↗

arXiv2025

Acoustic phonon softening and lattice instability driven by on-site $f$-$d$ hybridization in CeCoSi

Soft phonon modes in tetragonal CeCoSi, which undergoes a structural transition at $T_0=12$ K followed by antiferromagnetic order at $T_{\text{N}}=9.5$ K, have been investigated using high-resolution inelastic x-ray scattering. Pronounced softening was detected in the transverse acoustic modes corresponding to the $(yz+zx)$-type monoclinic distortion, consistent with the experimentally determined triclinic structure. Remarkably, the softening persists up to the zone boundary along (0, 0, $q$), indicating a short correlation length of the lattice instability. This instability, characterized by a Curie-type strain susceptibility, is interpreted as a consequence of the on-site $4f$-$5d$ hybridization, which is intrinsic to this crystal structure due to the lack of inversion symmetry at the two Ce sites.

cond-mat.str-el↗

arXiv2025

Soft Mode Origin of Charge Ordering in Superconducting Kagome CsV$_3$Sb$_5$

Charge-density-wave (CDW) order and superconductivity coexist in the kagome metals AV$_3$Sb$_5$ (A=K, Cs, Rb), raising fundamental questions about the mechanisms driving their intertwined phases. Here we combine high-resolution inelastic X-ray scattering with first-principles calculations to uncover the origin of CDW formation in CsV$_3$Sb$_5$. Guided by structure factor analysis, we identify a soft phonon mode along the reciprocal M-L direction, with the strongest effect at the L point, where the elastic scattering intensity also grows most rapidly upon cooling. First-principles calculations incorporating lattice anharmonicity and electron-phonon coupling reproduce these observations and establish a soft-mode instability at the L point as the driving mechanism of CDW formation. Despite the weakly first-order character of the transition, our results unambiguously demonstrate that the CDW in CsV$_3$Sb$_5$ originates from a softened phonon, clarifying its microscopic origin and highlighting the central role of lattice dynamics in kagome metals.

cond-mat.supr-con↗

arXiv2025

Incipient ionic conductors: Ion-constrained lattices achieving superionic-like thermal conductivity by extreme anharmonicity

Phonon liquid-like thermal conduction in the solid state enables superionic conductors to serve as efficient thermoelectric device candidates. While liquid-like motion of ions effectively suppresses thermal conductivity (\kappa), their high mobility concurrently triggers material degradation due to undesirable ion migration and consequent metal deposition, making it still a challenge to balancing low \kappa and high stability. Here, we report a superionic-like thermal transport alongside restricted long-range ion migration in CsCu_2I_3 with incipient ionic conduction, using synchrotron X-ray diffraction, inelastic X-ray scattering, and machine-learning potential-based simulations. We reveal that the Cu ions exhibit confined migration between CuI_4 tetrahedra at high temperatures, displaying extreme anharmonicity of dominated phonons beyond conventional rattling and comparable to that in superionic conductorsl. Consequently, a glass-like \kappa (~0.3 W m^{-1} K^{-1} at 300 K) following the relationship of \kappa ~ T^{0.17}, was achieved along the x-direction, where Cu ion migration is three oders of magnitude lower than in superionic conductors. These results highlight the advantage of incipient ionic conductors in simultaneously maintaining both low \kappa and high stability, elucidate the thermal transport mechanism via ion migration constraints, and pave an effective pathway toward ultralow thermal conductivity in ionic conductors.

cond-mat.mtrl-sci↗

arXiv2024

Absence of Acoustic Phonon Anomaly in a Kagome Metal with Short-ranged Structural Modulation

Kagome lattice $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) superconductors without magnetism from vanadium $d$-electrons are intriguing because they have a novel charge density wave (CDW) order around 90 K and display superconductivity at $\sim$3 K that competes with the CDW order. Recently, CsCr$_3$Sb$_5$, isostructural to $A$V$_3$Sb$_5$, was found to have concurrent structural and magnetic phase transition at $T^{\ast}\approx$ 55 K that can be suppressed by pressure to induce superconductivity [Liu \textit{et al.}, \href{https://doi.org/10.1038/s41586-024-07761-x}{Nature \textbf{632}, 1032 (2024)}]. Here, we use elastic and inelastic X-ray scattering to study the microscopic origin of the structural transition in CsCr$_3$Sb$_5$. Although our elastic measurements confirm the 4$\times$1$\times$1 superlattice order below $T^{\ast}$, its underlying correlation is rather short-ranged. Moreover, our inelastic measurements at the superlattice wavevectors around (3, 0, 0) find no evidence of a significant acoustic phonon anomaly below $T^{\ast}$, similar to the case of $A$V$_3$Sb$_5$. The absence of acoustic phonon anomaly indicates a weak electron-phonon coupling in CsCr$_3$Sb$_5$, suggesting that the structural transition is likely associated with an unconventional CDW order.

cond-mat.str-el↗

arXiv2024

Charge Density Fluctuations with Enhanced Superconductivity at the Proposed Nematic Quantum Critical Point

A quantum critical point (QCP) represents a continuous phase transition at absolute zero. At the QCP of an unconventional superconductor, enhanced superconducting transition temperature and magnetic fluctuations strength are often observed together, indicating magnetism-mediated superconductivity. This raises the question of whether quantum fluctuations in other degrees of freedom, such as charge, could similarly boost superconductivity. However, because charge is frequently intertwined with magnetism, isolating and understanding its specific role in Cooper pair formation poses a significant challenge. Here, we report persistent charge density fluctuations (CDF) down to 15 K in the non-magnetic superconductor Sr$_{0.77}$Ba$_{0.23}$Ni$_{2}$As$_{2}$, which lies near a proposed nematic QCP associated with a six-fold enhancement of superconductivity. Our results show that the quasi-elastic CDF does not condense into resolution-limited Bragg peaks and displays non-saturated strength. The CDF completely softens at 25 K, with its critical behavior described by the same mathematical framework as the antiferromagnetic Fermi liquid model, yielding a fitted Curie-Weiss temperature of $\theta \approx 0$ K. Additionally, we find that the nematic fluctuations are not lattice-driven, as evidenced by the absence of softening in nematic-coupled in-plane transverse acoustic phonons. Our discovery positions Sr$_{x}$Ba$_{1-x}$Ni$_{2}$As$_{2}$ as a promising candidate for charge-fluctuation-driven nematicity and superconductivity.

cond-mat.supr-con↗

arXiv2024

Density deficit of the Earth's core revealed by a multi-megabar primary pressure scale

An accurate pressure scale is a fundamental requirement to understand planetary interiors. Here, we establish a primary pressure scale extending to the multi-megabar pressures of the Earth's core, by combined measurement of the acoustic velocities and the density from a rhenium sample in a diamond anvil cell using inelastic x-ray scattering and x-ray diffraction. Our scale agrees well with previous primary scales and shock Hugoniots in each experimental pressure range, and reveals that previous scales have overestimated laboratory pressures by at least 20% at 230 gigapascals. It suggests that the light element content in the Earth's inner core (the density deficit relative to iron) is likely to be double what was previously estimated, or the Earth's inner core temperature is much higher than expected, or some combination thereof.

physics.geo-ph↗

arXiv2023

Experimental Observation of Mesoscopic Fluctuations to Identify Origin of Thermodynamic Anomalies of Ambient Liquid Water

We report a new experimental approach for observing mesoscopic fluctuations underlying the thermodynamic anomalies of ambient liquid water. In this approach, two sound velocity measurements with different frequencies, namely inelastic X-ray scattering (IXS) in THz band and ultrasonic (US) in MHz band, are required to investigate the relaxation phenomenon with the characteristic frequency between the two aforementioned frequencies. We performed IXS measurements to obtain the IXS sound velocity of liquid water from the ambient conditions to the supercritical region of liquid-gas phase transition (LGT) and compared the results with the US sound velocity in the literature. We found that the ratio of the two sound velocities, Sf, which corresponds to the relaxation intensity, exhibits a simple but significant change. Two distinct rises were observed in the high-temperature and low-temperature regions, implying that two relaxation phenomena exist: in the high-temperature region, a peak was observed near the LGT critical ridge line, which was linked with changes in the density fluctuation and isochoric and isobaric specific heat capacities; in the low-temperature region, Sf increased toward the low-temperature region, which was linked with the change in the isochoric heat capacity. We concluded that these two relaxation phenomena are originated from critical fluctuations of liquid-gas phase transition (LGT) and liquid-liquid phase transition, respectively. The linkage between Sf and isochoric heat capacity in the low-temperature region proves that the relaxation is the cause of the well-known heat capacity anomaly of ambient liquid water. In this study, both LGT and LLT critical fluctuations were observed, and the relationship between thermodynamics and the critical fluctuations was comprehensively discussed.

cond-mat.soft↗

arXiv2022

Background Rates in Several Hard X-Ray Photon Counting Pixel Array Detectors

Background rates in several pixel array detectors are investigated with an eye toward using them with hard, >6 keV, x-rays in very low-rate experiments - e.g. at signal rates <0.1/s/cm^2. Measured background event rates for a detector with an unshielded 0.75 mm thick CdTe sensor on the experimental floor at SPring-8 varied from 0.4/s/cm^2 with a 6 keV lower-level discriminator (LLD) threshold to 0.2/s/cm^2 with a 75 keV LLD threshold. The background for a detector with a 1 mm thick silicon sensor was smaller, ~0.08/s/cm^2 for a 3keV threshold dropping to ~0.07/s/cm^2 at a 17 keV threshold. These rates are dominated by terrestrial sources, such as gamma rays emitted from trace impurities in concrete, with only a small contribution, <0.01/s/cm2, from direct detection of cosmic ray muons (CRMs). 15 mm of Pb shielding reduces the measured rates to < 0.05/s/cm2 in CdTe and to <0.02/s/cm^2 in 1 mm silicon. Additional processing, "time slicing", as may be used in low-rate experiments where backgrounds may be most problematic, is shown to reduce the background rates further, to < 0.004/s/cm^2 with the silicon sensor, and to between 0.002 and 0.02/s/cm^2 for CdTe, where the exact value for CdTe depends sensitively on the detector threshold, and the use of the retriggering and/or the use of dual discriminator thresholds. Background rates are presented as functions of discriminator threshold, shielding, and processing. We also discuss the magnitude of, and the correction for, the limitation of the detector dynamic range that can be introduced by the time slicing. Finally, we present one example where time slicing was used.

physics.ins-det↗

arXiv2021

Universal two-component dynamics in supercritical fluids

Despite the technological importance of supercritical fluids, controversy remains about the details of their microscopic dynamics. In this work, we study four supercritical fluid systems -- water, Si, Te, and Lennard-Jones fluid -- \emph{via} classical molecular dynamics simulations. A universal two-component behavior is observed in the intermolecular dynamics of these systems, and the changing ratio between the two components leads to a crossover from liquidlike to gaslike dynamics, most rapidly around the Widom line. We find evidence to connect the liquidlike component dominating at lower temperatures with intermolecular bonding, and the component prominent at higher temperatures with free-particle, gaslike dynamics. The ratio between the components can be used to describe important properties of the fluid, such as its self-diffusion coefficient, in the transition region. Our results provide insight into the fundamental mechanism controlling the dynamics of supercritical fluids, and highlight the role of spatiotemporally inhomogenous dynamics even in thermodynamic states where no large-scale fluctuations exist in the fluid.

cond-mat.soft↗

arXiv2021

Interaction of Acoustic and Quasi-Elastic Modes in Liquid Water on Nanometer Length Scales

We investigate the presence of an acoustic-quasi-elastic interaction contribution in the IXS spectra of liquid water at 301K using inelastic x-ray scattering with sub-meV energy resolution at momentum transfers 0.77<=Q<=4.20 /nm. The contribution appears due to the overlap the acoustic mode with the tail the quasi-elastic mode and is fully consistent with hydrodynamic theory. Incorporating this interaction allows us to describe the dynamic structure factor, S(Q,w), without introducing an extra mode, and may help explain earlier contradictory interpretations. The sound velocity, and relative intensity of the quasi-elastic and acoustic mode, plateau for Q>2/nm at values consistent with a viscoelastic generalization of the Landau-Placzek relation.

cond-mat.soft↗

arXiv2021

Practical Measurement of the Energy Resolution for meV-Resolved Inelastic X-ray Scattering

We compare several different ways of measuring the energy resolution for meV-resolved inelastic x-ray scattering (IXS): using scattering from poly(methyl methacrylate), PMMA, using scattering from borosilicate glass (Tempax), and using powder diffraction from aluminum. All of these methods provide a reasonable first approximation to the energy resolution, but, also, in all cases, inelastic contributions appear over some range of energy transfers. Over a range of +-15 meV energy transfer there is good agreement between the measurements of PMMA and Tempax at low temperature, and room temperature powder diffraction from aluminum so we consider this to be a good indication of the true resolution of our ~1.3 meV spectrometer. We self-consistently determine the resolution over a wider energy range using the temperature, momentum and sample dependence of the measured response. We then quantitatively investigate the inelastic contributions from the PMMA and Tempax, and their dependence on momentum transfer and temperature. The resulting data allows us to determine the resolution of our multi-analyzer array efficiently using a single scan. We demonstrate the importance of this procedure by showing that the results of the analysis of a spectrum from a glass are changed by using the properly deconvolved resolution function. We also discuss the impact of radiation damage on the scattering from PMMA and Tempax.

cond-mat.mtrl-sci↗

arXiv2020

Two-component dynamics and the liquid-like to gas-like crossover in supercritical water

Molecular-scale dynamics in sub- to super-critical water is studied with inelastic X-ray scattering and molecular dynamics simulations. The obtained longitudinal current correlation spectra can be decomposed into two main components: a low-frequency (LF), gas-like component and a high-frequency (HF) component arising from the O--O stretching mode between hydrogen-bonded molecules, reminiscent of the longitudinal acoustic mode in ambient water. With increasing temperature, the hydrogen-bond network diminishes and the spectral weight shifts from HF to LF, leading to a transition from liquid-like to gas-like dynamics with rapid changes around the Widom line.

cond-mat.soft↗

arXiv2020

Spin-orbit coupling effects on spin-phonon coupling in Cd2Os2O7

Spin-orbit coupling (SOC) is essential in understanding the properties of 5d transition metal compounds, whose SOC value is large and almost comparable to other key parameters. Over the past few years, there have been numerous studies on the SOC-driven effects of the electronic bands, magnetism, and spin-orbit entanglement for those materials with a large SOC. However, it is less studied and remains an unsolved problem in how the SOC affects the lattice dynamics. We, therefore, measured the phonon spectra of 5d pyrochlore Cd2Os2O7 over the full Brillouin zone to address the question by using inelastic x-ray scattering (IXS). Our main finding is a visible mode-dependence in the phonon spectra, measured across the metal-insulator transition at 227 K. We examined the SOC strength dependence of the lattice dynamics and its spin-phonon (SP) coupling, with first-principle calculations. Our experimental data taken at 100 K are in good agreement with the theoretical results obtained with the optimized U = 2.0 eV with SOC. By scaling the SOC strength and the U value in the DFT calculations, we demonstrate that SOC is more relevant than U to explaining the observed mode-dependent phonon energy shifts with temperature. Furthermore, the temperature dependence of the phonon energy can be effectively described by scaling SOC. Our work provides clear evidence of SOC producing a non-negligible and essential effect on the lattice dynamics of Cd2Os2O7 and its SP coupling.

cond-mat.mtrl-sci↗

arXiv2020

Furnace for Inelastic X-Ray Scattering from Liquids to 1600C

The design and implementation of a furnace for inelastic x-ray scattering from liquids with sample temperatures up to ~1600C is described. Carbon composite heaters operating in vacuum provide robust heating elements: one pair of heaters has been used for >18 days of operational time above 1500C, including 8 cycles to room temperature. High quality data has been obtained to scattering angles as low as 7 mrad in two-theta (Q<1 nm-1 at 25.7 keV) from a sample at 1560C.

physics.ins-det↗

arXiv2019

Lattice dynamics in FeSe via inelastic x-ray scattering and first-principles calculations

We report an inelastic x-ray scattering investigation of phonons in FeSe superconductor. Comparing the experimental phonon dispersion with density functional theory (DFT) calculations in the non-magnetic state, we found a significant disagreement between them. Improved overall agreement was obtained by allowing for spin-polarization in the DFT calculations, despite the absence of magnetic order in the experiment. This calculation gives a realistic approximation, at DFT level, of the disordered paramagnetic state of FeSe, in which strong spin fluctuations are present.

cond-mat.supr-con↗