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Condensation of exciton creates Skyrmion lattice 励起子の凝縮によるスキルミオン格子
Condensation of exciton creates Skyrmion lattice 励起子の凝縮によるスキルミオン格子
Electron and hole and hole forms a bosonic quasiparticle exciton in semiconductors and semimetals. Localized magnetism renders excitons spin-polarized, leading to the triplet exciton codensation with large topological anomalous Hall effect.
Realization of the quantum compass model on the hyperhoneycomb lattice ハイパーハニカム格子における量子コンパスモデルの実現
Realization of the quantum compass model on the hyperhoneycomb lattice ハイパーハニカム格子における量子コンパスモデルの実現
See the following commentary in ISSP newsletter and Oxford Physics website.
https://www.issp.u-tokyo.ac.jp/maincontents/news2.html?pid=25412 (日本語)
Ordering of the "Negative" Chirality 負のカイラリティの秩序
Ordering of the "Negative" Chirality 負のカイラリティの秩序
The kagomé motif, an infinite array of corner-shared triangle, is one of the most studied systems because an exotic ground state such as a spin liquid is expected. CdK, what I have developed in a large single crystalline form, is shown to be a good realization of the spin 1/2 kagome antiferromagnet without chemical disorder or structural distortion. This new member of kagome mineral shows a magnetically ordered ground state called the negative chirality order, which is due to a sizable Dzyaloshinskii Moriya interaction. This study was published in Physical Review B in 2017 (R. Okuma et al., Phys. Rev. B 95, 094427, 2017). Recently we have pinned down the magnetic structure by a complementary use of neutron powder diffraction and nuclear magnetic resonance in collaboration with Dr. Gøran Nilsen in ISIS and Dr. Yoshihiko Ihara in Hokkaido University, of which results are published in Physical Review B and selected as Editor's suggestion (Y. Ihara et al., Phys. Rev. B 106, 024401 2022). The major challenges was the need of sample replaced with 114Cd and deuterium for neutron diffraction because natural Cd absorbs neutron heavily and hydrogen produces huge background.
Condensation of a Flat Band: Hexagonal Magnon Crystal 平坦なバンドの凝縮: 六角形のマグノン結晶
Condensation of a Flat Band: Hexagonal Magnon Crystal 平坦なバンドの凝縮: 六角形のマグノン結晶
Frustration makes band structures flat. Flatness means localisation in the real space. The complete flat band throughout the momentum space (not necessarily but) typically means a localised eigenstate. In the case of kagomé antiferromagnet, sufficiently high magnetic a mode localised inside the hexagon is responsible for the mode. Our observation of multiple magnetization plateau in the kagomé antiferromagnet CdK is possible first realization of a bosonic analogue of this phenomenon. Generally speaking, quantum spin system is equivalent to lattice hard core boson system; magnetization corresponds to boson density, magnetic field to chemical potential, magnetic order to superfluid, gapped phase to insulating phase, and so on. Theory has predicted that in the kagome antiferromagnet under high magnetic field self-assembly of six spins yields a molecular object similar to benzene (we call it hexagonal magnon) and at certain density it forms insulating crystalline state. Relatively small interaction in CdK allows us to measure the whole magnetization process of spin-1/2 kagomé antiferromagnet for the first time with the help of high magnetic field laboratory in ISSP. Below the saturation field of 160 T, several magnetization plateaux were observed, which indicates the metal insulator transition of hexagonal magnon. This study was published in Nature Communications in 2019 (R. Okuma et al., Nat. Comun. 10, 1299, 2019).
Dimensional Reduction by Frustration フラストレーションによる次元性の低下
Dimensional Reduction by Frustration フラストレーションによる次元性の低下
Dimensionality is the most fundamental parameter of materials but assumed to be static and tied to that of the underlying lattice as in the case of carbon atoms; carbon nanotube, graphene and diamond show one, two and three dimensional electronic properties. Geometrically-frustrated magnets, however, are dynamically low-dimensional and symmetry breaking manifests the dimensional reduction. I have experimentally shown that synthetic pharmacosiderite, comprising purely three-dimensional cubic arrays of atoms, has two-dimensionally correlated spins because of the degrees of freedom in frustrated clusters (R. Okuma et al. Nat. Commun. 12, 4382 (2021). The two-dimensionality is spontaneous and emergent, opening a new avenue for heat control by magnetic fields.
Frustration in Momentum Space 運動量空間におけるフラストレーション
Frustration in Momentum Space 運動量空間におけるフラストレーション
Metallic systems are inherently frustrated because of the complex spin interactions mdediated by conduction electron of Fermi surface. We focused on a new metallic triangular system CeSiI, which has a conductive honeycomb net of Si2- and triangular layer of magnetic Ce3+ sandwiched by van der Waals stacked I-. An unusual magnetisation step in the magnetisation curve was observed along the easy axis. Neutron diffraction confirmed an incommensurate order at zero field, likely to be a cycloidal structure caused by itinerant frustration, i.e. competition of nearest neighbor ferromagnetic and next nearest neighbor antiferromagnetic interaction. The van der Waals structure allows for investigation by surface sensitive probes such as angle resolved photoemission spectroscopy and scanning tunneling microscope experiments, which are currently under way to reveal the origin of the frustrated magnetism of CeSiI.
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