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論文

Spin and lattice dynamics in the van der Waals antiferromagnet MnPSe $_{3}$

Liao, J.*; Huang, Z.*; Shangguan, Y.*; Zhang, B.*; Cheng, S.*; Xu, H.*; 梶本亮一; 蒲沢和也*; Bao, S.*; Wen, J.*

Physical Review B, 109(22), p.224411_1 - 224411_10, 2024/06

https://doi.org/10.1103/PhysRevB.109.224411

被引用回数：0 パーセンタイル：0.00(Materials Science, Multidisciplinary)

Antiferromagnetic van der Waals family P $X_{3}$ ( = Fe, Mn, Co, and Ni; = S and Se) have attracted significant research attention due to the possibility of realizing long-range magnetic order down to the monolayer limit. Here, we perform inelastic neutron scattering measurements on single-crystal samples of MnPSe $_{3}$ , a member of the P $X_{3}$ family, to study the spin dynamics and determine the effective spin model. The excited magnon bands are well characterized by a spin model, which includes a Heisenberg term with three intraplane exchange parameters ( $J_{1} = -0.73$ meV, $J_{2} = -0.014$ meV, $J_{3} = -0.43$ meV) and one interplane parameter ( $J_{c} = -0.054$ meV), and an easy-plane single-ion anisotropy term ( meV). Additionally, we observe the intersection of the magnon and phonon bands but no anomalous spectral features induced by the formation of magnon-phonon hybrid excitations at the intersecting region. We discuss possible reasons for the absence of such hybrid excitations in MnPSe $_{3}$ .

論文

Microscopic origin of the spin-reorientation transition in the kagome topological magnet TbMn $_{6}$ Sn $_{6}$

Huang, Z.*; Wang, W.*; Ye, H.*; Bao, S.*; Shangguan, Y.*; Liao, J.*; Cao, S.*; 梶本亮一; 池内和彦*; Deng, G.*; et al.

Physical Review B, 109(1), p.014434_1 - 014434_9, 2024/01

https://doi.org/10.1103/PhysRevB.109.014434

被引用回数：0 パーセンタイル：0.00(Materials Science, Multidisciplinary)

TbMn $_{6}$ Sn $_{6}$ is a correlated topological magnet with a Mn-based kagome lattice, in which a Chern gap opens at the Dirac point at low temperatures. The magnetic moment direction of the ferrimagnetic order changes from in the kagome plane to the out-of-plane upon cooling, which is essential for generating the Chern gap, but the underlying mechanism for the spin reorientation remains elusive. Here, we investigate the spin-reorientation transition in TbMn $_{6}$ Sn $_{6}$ using neutron scattering. We provide direct evidence for the spin-reorientation transition and unveil the coexistence of two Tb modes at 200 K. To account for these results, we put forward a model based on SU(N) spin-wave theory, in which there is a temperature evolution of the ground state Tb orbitals, driven by the crystalline electric field, single-ion anisotropy, and exchange interactions between Tb and Mn ions. Our findings shed light on the complex magnetism of TbMn $_{6}$ Sn $_{6}$ , despite its relatively simple ground state magnetic structure, and provide insights into the mechanisms for tuning magnetic topological materials.

論文

Direct observation of topological magnon polarons in a multiferroic material

Bao, S.*; Gu, Z.-L.*; Shangguan, Y.*; Huang, Z.*; Liao, J.*; Zhao, X.*; Zhang, B.*; Dong, Z.-Y.*; Wang, W.*; 梶本亮一; et al.

Nature Communications (Internet), 14, p.6093_1 - 6093_9, 2023/09

https://doi.org/10.1038/s41467-023-41791-9

被引用回数：12 パーセンタイル：92.34(Multidisciplinary Sciences)

Magnon polarons are novel elementary excitations possessing hybrid magnonic and phononic signatures, and are responsible for many exotic spintronic and magnonic phenomena. Despite long-term sustained experimental efforts in chasing for magnon polarons, direct spectroscopic evidence of their existence is hardly observed. Here, we report the direct observation of magnon polarons using neutron spectroscopy on a multiferroic Fe $_{2}$ Mo $_{3}$ O $_{8}$ possessing strong magnon-phonon coupling. Specifically, below the magnetic ordering temperature, a gap opens at the nominal intersection of the original magnon and phonon bands, leading to two separated magnon-polaron bands. Each of the bands undergoes mixing, interconverting and reversing between its magnonic and phononic components. We attribute the formation of magnon polarons to the strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction. Intriguingly, we find that the band-inverted magnon polarons are topologically nontrivial. These results uncover exotic elementary excitations arising from the magnon-phonon coupling, and offer a new route to topological states by considering hybridizations between different types of fundamental excitations.

論文

Neutron spectroscopy evidence on the dual nature of magnetic excitations in a van der Waals metallic ferromagnet Fe $_{2.75}$ GeTe $_{2}$

Bao, S.*; Wang, W.*; Shangguan, Y.*; Cai, Z.*; Dong, Z.-Y.*; Huang, Z.*; Si, W.*; Ma, Z.*; 梶本亮一; 池内和彦*; et al.

Physical Review X, 12(1), p.011022_1 - 011022_15, 2022/02

https://doi.org/10.1103/PhysRevX.12.011022

被引用回数：25 パーセンタイル：94.45(Physics, Multidisciplinary)

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, the nature of the magnetic excitations 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 columnlike 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 electrons but are rarely clearly observed in materials with light electrons. These findings shed light on the understanding of magnetism in transition-metal compounds.

論文

Evidence for strong correlations at finite temperatures in the dimerized magnet Na $_{2}$ Cu $_{2}$ TeO $_{6}$

Shangguan, Y.*; Bao, S.*; Dong, Z.-Y.*; Cai, Z.*; Wang, W.*; Huang, Z.*; Ma, Z.*; Liao, J.*; Zhao, X.*; 梶本亮一; et al.

Physical Review B, 104(22), p.224430_1 - 224430_8, 2021/12

https://doi.org/10.1103/PhysRevB.104.224430

被引用回数：1 パーセンタイル：5.86(Materials Science, Multidisciplinary)

Dimerized magnets forming alternating Heisenberg chains exhibit quantum coherence and entanglement and thus can find potential applications in quantum information and computation. However, magnetic systems typically undergo thermal decoherence at finite temperatures. Here, we show inelastic neutron scattering results on an alternating antiferromagnetic-ferromagnetic chain compound Na $_{2}$ Cu $_{2}$ TeO $_{6}$ that the excited quasiparticles can counter thermal decoherence and maintain strong correlations at elevated temperatures. At low temperatures, we observe clear dispersive singlet-triplet excitations arising from the dimers formed along the crystalline -axis. The excitation gap is of 18 meV and the bandwidth is about half of the gap. The band top energy has a weak modulation along the [100] direction, indicative of a small interchain coupling. The gap increases while the bandwidth decreases with increasing temperature, leading to a strong reduction in the available phase space for the triplons. As a result, the Lorentzian-type energy broadening becomes highly asymmetric as the temperature is raised. These results are associated with a strongly correlated state resulting from hard-core constraint and quasiparticle interactions. We consider these results to be not only evidence for strong correlations at finite temperatures in Na $_{2}$ Cu $_{2}$ TeO $_{6}$ , but also for the universality of the strongly correlated state in a broad range of quantum magnetic systems.

論文

Evidence for magnon-phonon coupling in the topological magnet Cu $_{3}$ TeO $_{6}$

Bao, S.*; Cai, Z.*; Si, W.*; Wang, W.*; Wang, X.*; Shangguan, Y.*; Ma, Z.*; Dong, Z.-Y.*; 梶本亮一; 池内和彦*; et al.

Physical Review B, 101(21), p.214419_1 - 214419_8, 2020/06

https://doi.org/10.1103/PhysRevB.101.214419

被引用回数：20 パーセンタイル：73.24(Materials Science, Multidisciplinary)

We perform thermodynamic and inelastic neutron scattering (INS) measurements to study the lattice dynamics (phonons) of a cubic collinear antiferromagnet Cu $_{3}$ TeO $_{6}$ which hosts topological spin excitations (magnons). While the specific heat and thermal conductivity results show that the thermal transport is dominated by phonons, the deviation of the thermal conductivity from a pure phononic model indicates that there is a strong coupling between magnons and phonons. In the INS measurements, we find a mode in the excitation spectra at 4.5 K, which exhibits a slight downward dispersion around the Brillouin zone center. This mode disappears above the Nel temperature and thus cannot be a phonon. Furthermore, the dispersion is distinct from that of a magnon. Instead, it can be explained by the magnon-polaron mode, collective excitations resulting from the hybridization between magnons and phonons. We consider the suppression of the thermal conductivity and emergence of the magnon-polaron mode to be evidence for magnon-phonon coupling in Cu $_{3}$ TeO $_{6}$ .

論文

Spin dynamics of a magnetic Weyl semimetal Sr $_{1-x}$ Mn $_{1-y}$ Sb $_{2}$

Cai, Z.*; Bao, S.*; Wang, W.*; Ma, Z.*; Dong, Z.-Y.*; Shangguan, Y.*; Wang, J.*; Ran, K.*; Li, S.*; 蒲沢和也*; et al.

Physical Review B, 101(13), p.134408_1 - 134408_10, 2020/04

AA2020-0050.pdf:5.09MB

https://doi.org/10.1103/PhysRevB.101.134408

被引用回数：6 パーセンタイル：33.94(Materials Science, Multidisciplinary)

ディラック物質は量子現象と共にキャリアの役割を探索するのに良い系で、磁気ワイル半金属のSr $_{1-x}$ Mn $_{1-y}$ Sb $_{2}$ はワイルフェルミオンとマグノンの関係を研究する良い舞台である。本研究では、Sr $_{1-x}$ Mn $_{1-y}$ Sb $_{2}$ の単結晶の中性子非弾性散乱実験を行い、磁性Mn層の分散は約76meVまで広がり、層間の分散は6meVの狭いバンド幅を持つことを見出した。また、これらの実験結果はハイゼンベルグ・スピンハミルトニアンを用いた線形スピン波理論によって再現可能である。本研究では、ワイルフェルミオンとマグノンが共存しているにもかかわらず、Sr $_{1-x}$ Mn $_{1-y}$ Sb $_{2}$ 中のワイルフェルミオンが磁気ダイナミクスに影響を与えているという明確な証拠は得られなかった。これは、ワイルフェルミオンとマグノンがSb層とMn層に別々に存在し、この擬二次元性による層間結合の弱さに起因するもの考えられる。

論文

Evidence for singular-phonon-induced nematic superconductivity in a topological superconductor candidate Sr $_{0.1}$ Bi $_{2}$ Se $_{3}$

Wang, J.*; Ran, K.*; Li, S.*; Ma, Z.*; Bao, S.*; Cai, Z.*; Zhang, Y.*; 中島健次; 河村聖子; ermk, P.*; et al.

Nature Communications (Internet), 10, p.2802_1 - 2802_6, 2019/06

https://doi.org/10.1038/s41467-019-10942-2

被引用回数：22 パーセンタイル：78.96(Multidisciplinary Sciences)

Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent -wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of Sr $_{0.1}$ Bi $_{2}$ Se $_{3}$ , a prime candidate for realizing topological superconductivity by doping the topo-logical insulator Bi $_{2}$ Se $_{3}$ , we found that there exist singular phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the [001] direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic -wave nematic superconducting pairing in the MxBi $_{2}$ Se $_{3}$ (M = Cu, Sr, Nb) superconductor family. Therefore, we show that these superconductors may be the first examples where electron-phonon interaction can induce more exotic superconducting pairing than the -wave.