arXiv subjects

Takeshi Nakagawa

Publications and source records attributed to Takeshi Nakagawa.

9 recordsLinked to original sources

arXiv2026

Pressure-induced lattice instabilities and phonon softening in the orthorhombically distorted ferrimagnet Ni4Nb2O9

The ambient- and high-pressure behavior of the ferrimagnet Ni4Nb2O9 (orthorhombically distorted honeycomb structure), is investigated using NMR, Raman spectroscopy, and synchrotron XRD. Ambient-pressure NMR measurements reveal, despite its orthorhombic symmetry, the local environment of Ni4Nb2O9 closely resembles that of its trigonal analogue Mn4Nb2O9. In contrast, substantially different paramagnetic shifts observed in the two compounds reflect their distinct average crystal symmetries, governing orbital overlap and magnetic exchange pathways. Under external pressure, Ni4Nb2O9 exhibits pronounced sensitivity to lattice distortions and phonon instabilities. Three isostructural transitions are identified near 2, 6, and 10 GPa, manifested by mode splitting, frequency shifts, line broadenings, intensity anomalies, and slope changes in the evolution of lattice parameters. At higher pressure, around 13 GPa, signatures of an incipient long-range structural transition from orthorhombic Pbcn to monoclinic P2/c symmetry emerge, signaling the onset of a symmetry-lowering transformation. The anomalous softening of the 192 cm^-1 Raman mode, accompanied by multiple linewidth and spectral-weight anomalies, serve as a key fingerprint of these structural instabilities, linking local symmetry breaking at low pressures to the long-range transition into the P2/c phase. Notably, pronounced linewidth anomalies, strongly anisotropic pressure coefficients, together with a marked enhancement of the intensity of the low-frequency branch over the 2-13 GPa range, point toward a pressure-induced regime influenced by coupled spin, orbital, and lattice degrees of freedom. The close correspondence of transition pressures in Ni4Nb2O9 and those reported for Mn4Nb2O9 highlights a common mechanism rooted in their similar local structural environments, as revealed by NMR.

cond-mat.mtrl-sci↗

arXiv2026

muT2-NMR: Micro-Scale Correlation Relaxometry for in-situ High-Pressure Nuclear Magnetic Resonance

Over the last decade, frequency-domain in-situ high-pressure nuclear magnetic resonance (NMR) spectroscopy in diamond anvil cells (DACs) has been employed as a structural and electronic probe of condensed matter systems at pressures well into the megabar range. However, extensive spin interactions and sample heterogeneities under pressure often lead to significant spectral overlap, inhibiting independent observation of chemically similar spin sub-species in the same sample. In this work, we introduce a time-domain relaxometry framework specifically suited for DAC experiments, named muT2-NMR. Experimental flexibility and operational robustness are benchmarked on three hydrogen-rich molecular solids at pressures up to 72 GPa. We demonstrate that muT2-NMR can resolve individual molecular subunits in relaxation space, paving the way for novel high-pressure, high-resolution NMR applications in molecular solids.

physics.ins-det↗

arXiv2025

Structure determination of flat honeycomb Bi grown on Ag(111)

Honeycomb bismuthene structures on Ag(111) were investigated using low-energy electron diffraction (LEED) and density functional theory. LEED I(V) analysis revealed that 0.5 monolayer (ML) of Bi forms an ultraflat honeycomb lattice with negligible buckling at ~120 K, which transforms into other structures upon warming to room temperature. A similar flat bismuthene structure also forms in Mn/Bi/Ag(111), which remains stable even at room temperature. Mn deposition on $(p\times \sqrt{3})$-rect Bi/Ag(111) induces Bi surface segregation, as confirmed by X-ray photoelectron spectroscopy, resulting in a p$(2\times2)$ honeycomb bismuthene. The detailed structural investigation provides fundamental insights into the characterization of two-dimensional topological properties of bismuthene grown on Ag(111).

cond-mat.mtrl-sci↗

arXiv2024

Narrowing band gap chemically and physically: Conductive dense hydrocarbon

Band gap energy of an organic molecule can be reduced by intermolecular interaction enhancement, and thus, certain polycyclic aromatic hydrocarbons (PAHs), which are insulators with wide band gaps, are expected to undergo insulator-metal transitions by simple compression. Such a pressure-induced electronic transition can be exploited to transform non-metallic organic materials into states featuring intriguing electronic characteristics such as high-temperature superconductivity. Numerous attempts have been made to metalize various small PAHs, but so far only pressure-induced amorphization well below the megabar region was observed. The wide band gap energy of the small PAHs and low chemical stability under simple compression are the bottlenecks. We have investigated the band gap energy evolution and the crystal structural compression of the large PAH molecules, where the band gap energy is significantly reduced by increasing the number of π-electrons and improved chemical stability with fully benzenoid molecular structure. Herein, we present a pressure-induced transition in dicoronylene, C48H20, an insulator at ambient conditions that transforms into a semi-metallic state above 23.0 GPa with a three-order-of-magnitude reduction in resistivity. In-situ UV-visible absorption, transport property measurement, Raman spectroscopy, X-ray diffraction and density functional theory calculations were performed to provide tentative explanations to the alterations in its electronic structure at high pressure. The discovery of an electronic transition at pressures well below the megabar is a promising step towards realization of a single component purely hydrocarbon molecular metal in the near future.

cond-mat.mtrl-sci↗

arXiv2024

Diffusion Driven Transient Hydrogenation in Metal Superhydrides at Extreme Conditions

In recent years, metal hydride research has become one of the driving forces of the high-pressure community, as it is believed to hold the key to superconductivity close to ambient temperature. While numerous novel metal hydride compounds have been reported and extensively investigated for their superconducting properties, little attention has been focused on the atomic and electronic states of hydrogen, the main ingredient in these novel compounds. Here, we present combined $^{1}H$- and $^{139}La$-NMR data on lanthanum superhydrides, $LaH_{x}$, ($x = 10.2 - 11.1$), synthesized after laser heating at pressures above 160 GPa. Strikingly, we found hydrogen to be in a highly diffusive state at room temperature, with diffusion coefficients in the order of $10^{-6}~cm^2s^{-1}$. We found that this diffusive state of hydrogen results in a dynamic de-hydrogenation of the sample over the course of several weeks, approaching a composition similar to its precursor materials. Quantitative measurements demonstrate that the synthesized superhydrides continuously decompose over time. Transport measurements underline this conclusion as superconducting critical temperatures were found to decrease significantly over time as well. This observation sheds new light on formerly unanswered questions on the long-term stability of metal superhydrides.

cond-mat.mtrl-sci↗

arXiv2024

Hexagonal to Monoclinic Phase Transition in Dense Hydrogen Phase III Detected by High-Pressure NMR

Conclusive crystal structure determination of the high pressure phases of hydrogen remains elusive due to lack of core electrons and vanishing wave vectors, rendering standard high-pressure experimental methods moot. Ab-initio DFT calculations have shown that structural polymorphism might be solely resolvable using high-resolution nuclear magnetic resonance (NMR) spectroscopy at mega-bar pressures, however technical challenges have precluded such experiments thus far. Here, we present in-situ high-pressure high-resolution NMR experiments in hydrogen phase III between 181 GPa and 208 GPa at room temperature. Our spectra suggest that at lower pressures phase III adopts a hexagonal P6122 crystal structure, transitioning into a monoclinic C2/c phase at about 197 GPa. The high resolution spectra are in excellent agreement with earlier structural and spectral predictions and underline the possibility of a subtle P6122 to C2/c phase transition in hydrogen phase III. These experiments show the importance of a combination of ab-initio calculations and low-Z sensitive spectral probes in high-pressure science in elucidating the structural complexity of the most abundant element in our universe.

cond-mat.mtrl-sci↗

arXiv2024

Unveiling a Novel Metal-to-Metal Transition in LuH2: Critically Challenging Superconductivity Claims in Lutetium Hydrides

Following the recent report by Dasenbrock-Gammon et al. (2023) of near-ambient superconductivity in nitrogen-doped lutetium trihydride (LuH3-δNε), significant debate has emerged surrounding the composition and interpretation of the observed sharp resistance drop. Here, we meticulously revisit these claims through comprehensive characterization and investigations. We definitively identify the reported material as lutetium dihydride (LuH2), resolving the ambiguity surrounding its composition. Under similar conditions (270-295 K and 1-2 GPa), we replicate the reported sharp decrease in electrical resistance with a 30% success rate, aligning with Dasenbrock-Gammon et al.'s observations. However, our extensive investigations reveal this phenomenon to be a novel, pressure-induced metal-to-metal transition intrinsic to LuH2, distinct from superconductivity. Intriguingly, nitrogen doping exerts minimal impact on this transition. Our work not only elucidates the fundamental properties of LuH2 and LuH3 but also critically challenges the notion of superconductivity in these lutetium hydride systems. These findings pave the way for future research on lutetium hydride systems while emphasizing the crucial importance of rigorous verification in claims of ambient temperature superconductivity.

cond-mat.supr-con↗

arXiv2023

Structure responsible for the superconducting state in La3Ni2O7 at high pressure and low temperature conditions

Very recently, a new superconductor with Tc = 80 K was reported in nickelate (La3Ni2O7) at around 15 - 40 GPa conditions (Nature, 621, 493, 2023) [1], which is the second type of unconventional superconductor, beside the cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement at low temperature and high pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results was carried out at room temperature. This encouraged us to carry out in situ high pressure and low temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from orthorhombic Amam structure to orthorhombic Fmmm structure, a tetragonal phase with space group of I4/mmm was discovered when the sample was compressed to 19 GPa at 40 K where the superconductivity takes palce in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states approached to the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2-y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni-O octahedra regularity and the in-plane Ni-O-Ni bonding angles, are analyzed. This work sheds new lights to identify what is the most likely phase responsible for superconductivity in the double layered nickelate.

cond-mat.supr-con↗

arXiv2010

Mathematical Model for Hit Phenomena

The mathematical model for hit phenomena in entertainments is presented as a nonlinear, dynamical and non-equilibrium phenomena. The purchase intention for each person is introduced and direct and indirect communications are expressed as two-body and three-body interaction in our model. The mathematical model is expressed as coupled nonlinear differential equations. The important factor in the model is the decay time of rumor for the hit. The calculated results agree very well with revenues of recent 25 movies.

physics.soc-ph↗