arXiv subjects

Shin-ichiro Yano

Publications and source records attributed to Shin-ichiro Yano.

6 recordsLinked to original sources

arXiv2026

Magnon Damping as a Probe of Kondo Coupling in Magnetically Ordered Systems

In $d$-electron systems, there can also be intricate interplay between Kondo coupling and magnetic interactions as that in $f$-electron systems, but the underlying mechanism remains elusive. Here, using inelastic neutron scattering, we investigate the temperature evolution of the low-energy spin waves (magnons) in a metallic van der Waals ferromagnet Fe$_{3-x}$GeTe$_{2}$, and observe that the magnon damping diverges at both low and high temperatures and exhibits a minimum at an intermediate temperature. These behaviours are described by a formula that combines logarithmic and power-law terms, representing the dominant contributions from Kondo coupling and thermal fluctuations, respectively. These findings can be explained by considering electron-magnon scattering of spin-flip type within the ferromagnetic Kondo-Heisenberg lattice model, distinct from the original Kondo effect which only considers the coupling between itinerant electrons and isolated impurity spins. These results unveil the intriguing interplay between itinerant electrons and spin waves in metallic 3$d$-electron systems with magnetic order, and provide magnon damping as a new effective probe of Kondo coupling in metallic quantum magnets, thereby opening new avenues for exploring Kondo physics from the magnon perspective.

cond-mat.str-el↗

arXiv2025

Complex magnetic phases and thermodynamics of CuB_2O_4

The copper metaborate CuB_2O_4(CBO) has been studied within the parametric space of magnetic field (B) and temperature (T), showing a series of distinct phases, including commensurate and incommensurate magnetic orders, as well as magnetic soliton phases. However, no single study has simultaneously demonstrated all these phases. moreover, the existence of additional phases below 2 K or above 2 T for B perpenducular to c has been scarcely explored. This work presents the first comprehensive construction of the complex B-T phase diagram for T = 0.1 to 25 K and B = 0 to 9 T (B perpenducular to c), utilizing thermodynamic probes, magnetic measurements, and neutron scattering on a single batch of CBO crystals. Our findings elucidate new magnetic phases and regimes. First-principles calculations help to gain a deeper understanding of this complex phase diagram. The emergence of various phases within such a narrow B and T regime is attributed to two primary mechanisms: the delicate competition between ferromagnetic and antiferromagnetic interactions, and the weak exchange and superexchange interactions within and between the Cu(A) and Cu(B) sublattices. This competition also leads to a magnetically frustrated ground state at absolute zero where the total magnetic entropy release from high to low temperatures is only 0.76 Rln2, lower than Rln2 expected for copper spin 1/2. Using CBO as a model system, the present work elucidates the intricate mechanisms underlying the formation of complex magnetic phases.

cond-mat.str-el↗

arXiv2022

Neutron spectroscopy evidence for a possible magnetic-field-induced gapless quantum-spin-liquid phase in a Kitaev material $α$-RuCl$_3$

As one of the most promising Kitaev quantum-spin-liquid (QSL) candidates, $α$-RuCl$_3$ has received a great amount of attention. However, its ground state exhibits a long-range zigzag magnetic order, which defies the QSL phase. Nevertheless, the magnetic order is fragile and can be completely suppressed by applying an external magnetic field. Here, we explore the evolution of magnetic excitations of $α$-RuCl$_3$ under an in-plane magnetic field, by carrying out inelastic neutron scattering measurements on high-quality single crystals. Under zero field, there exist spin-wave excitations near the $M$ point and a continuum near the $\mitΓ$ point, which are believed to be associated with the zigzag magnetic order and fractional excitations of the Kitaev QSL state, respectively. By increasing the magnetic field, the spin-wave excitations gradually give way to the continuous excitations. On the verge of the critical field $μ_0H_{\rm c}=7.5$ T, the former vanish and only the latter is left, indicating the emergence of a pure QSL state. By further increasing the field strength, the excitations near the $\mitΓ$ point become more intense. By following the gap evolution of the excitations near the $\mitΓ$ point, we are able to establish a phase diagram composed of three interesting phases, including a gapped zigzag order phase at low fields, possibly-gapless QSL phase near $μ_0H_{\rm c}$, and gapped partially polarized phase at high fields. These results demonstrate that an in-plane magnetic field can drive $α$-RuCl$_3$ into a long-sought QSL state near the critical field.

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arXiv2022

Neutron spectroscopy evidence on the dual nature of magnetic excitations in a van der Waals metallic ferromagnet Fe$_{2.72}$GeTe$_{2}$

In the local or itinerant extreme, magnetic excitations can be described by the Heisenberg model which treats electron spins as localized moments, or by the itinerant-electron model where the exchange interaction between electrons leads to unequal numbers of electrons with up and down spins. However, it has been elusive when both local moments and itinerant electrons are present in the intermediate range. Using inelastic neutron scattering, we provide direct spectroscopic evidence on the coexistence of and interplay between local moments and itinerant electrons in a van der Waals metallic ferromagnet Fe$_{2.72}$GeTe$_{2}$, which can sustain tunable room-temperature ferromagnetism down to the monolayer limit. We find that there exist ferromagnetic spin-wave excitations dispersing from the zone center at low energies resulting from local moments, and a column-like broad continuum at the zone boundary at high energies up to over 100 meV resulting from itinerant electrons. Unlike the two-dimensional crystal structure, the low-energy mode exhibits a three-dimensional nature, and the high-energy mode also has an out-of-plane dependence. Both modes persist well above the Curie temperature of 160 K. Our neutron spectroscopic data reveal that the low-energy spin waves at 100 K are more coherent than those at 4 K, which is evidence of the weakening of the Kondo screening at high temperatures. These results unambiguously demonstrate the coexistence of local moments and itinerant electrons, and the Kondo effect between these two components in Fe$_{2.72}$GeTe$_{2}$. Such behaviors are generally expected in heavy-fermion systems with heavy $f$ electrons but rarely clearly observed in materials with light $d$ electrons. These findings shed light on the understanding of magnetism in transition-metal compounds.

cond-mat.str-el↗

arXiv2019

Spin dynamics of edge-sharing spin chains in SrCa13Cu24O41

The low-energy magnetic excitation from the highly Ca-doped quasi-one-dimensional magnet SrCa13Cu24O41 was studied in the magnetic ordered state by using inelastic neutron scattering. We observed the gapless spin-wave excitation, dispersive along the a and c axes but nondispersive along the b axis. Such excitations are attributed to the spin wave from the spin-chain sublattice. Model fitting to the experimental data gives the nearest-neighbour interaction Jc as 5.4 meV and the interchain interaction Ja = 4.4 meV. Jc is antiferromagnetic and its value is close to the nearest-neighbour interactions of the similar edge-sharing spin-chain systems such as CuGeO3. Comparing with the hole-doped spin chains in Sr14Cu24O41, which shows a spin gap due to spin dimers formed around Zhang-Rice singlets, the chains in SrCa13Cu24O41 show a gapless excitation in this study. We ascribe such a change from gapped to gapless excitations to holes transferring away from the chain sublattice into the ladder sublattice upon Ca doping.

cond-mat.str-el↗

arXiv2018

Low-energy spin dynamics of orthoferrites AFeO$_3$ (A = Y, La, Bi)

YFeO$_3$ and LaFeO$_3$ are members of the rare-earth orthoferrites family with \textit{Pbnm} space group. Using inelastic neutron scattering, the low-energy spin excitations have been measured around magnetic Brillouin zone center. Splitting of magnon branches and non-zero magnon gap is observed for both compounds, which is similar to the behavior observed in multiferroic BiFeO$_3$. Spin wave calculations which include both Dzyaloshinsky-Moriya interactions and single-ion anisotropy in the spin-Hamiltonian comprehensively accounts for all the experimentally observed features. Our results offer insight into the unifying physics underlying the Fe$^{3+}$-based perovskites as well as their distinguishing characteristics.

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