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- 저자 : Joji Nasu (검색결과 4 건)
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Finite-Temperature Spin Dynamics and Transport Phenomena in Kitaev Quantum Spin Liquids
Joji Nasu 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
Quantum spin liquids (QSLs) have been the subject of great interest since Anderson’s suggestion. Recently, Kitaev proposed a canonical model of QSLs termed the Kitaev model on a honeycomb lattice (Fig. 1), which provides exact realizations of QSLs with topological order and fractional excitations. Moreover, exchange interactions in transition metal compounds with the strong spin-orbit coupling, such as iridium and ruthenium compounds, are suggested to be dominated by the Kitaev-type interaction. To discuss experimental results in the viewpoint of the Kitaev physics, we investigate the thermodynamic properties and spin dynamics of the Kitaev model. We calculate the specific heat using the quantum Monte Carlo simulations and find a double-peak structure, where half of the entropy is released at each peak. The result suggests that a quantum spin is fractionalized into two types of elementary excitations, itinerant Majorana fermions and localized Z2 fluxes, due to quantum many-body effects. We also find fingerprints of fractionalization in the spin dynamics. The fermionic temperature dependence in the Raman scattering and incoherent broad structure in the dynamical spin structure factor are the manifestations of the itinerant Majorana fermions. We also examine the thermal transport governed by the itinerant emergent Majorana fermions in the presence of the magnetic field. The thermal Hall conductivity shows nonmonotonic temperature dependence due to the Majorana chiral edge mode and the thermally fluctuating Z₂ gauge field emergent from the fractionalization of quantum spins. Finally, we discuss disorder effects on the Kitaev QSL, which inevitably exist in real materials. We investigate the impact of two types of disorder, bond randomness and site dilution, on the spin dynamics and transport in the Kitaev model. We compare the calculation results with experimental ones. The relevance to real materials will be discussed in the presentation. 〈그림 본문참조〉
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Electron Correlation and Dynamical Jahn-Teller Effect in an Orbitally Degenerate System
Joji Nasu,Sumio Ishihara 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.12
Motivated by recent experiments on Ba3CuSb2O9, we study the dynamical Jahn-Teller effect ina spin-orbital coupled system. We analyze a low-energy model Hamiltonian which includes thesuperexchange-type interaction and the Jahn-Teller interaction by using a combined method of themean-field approximation and the quantum Monte-Carlo simulation. We find that magnetic ordersdo not appear in a wide parameter region and that the spin-dimer state with an orbital order isrealized. The orbital order is strongly suppressed by the dynamical Jahn-Teller effect.
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Shinnosuke Koyama,Joji Nasu 한국물리학회 2023 새물리 Vol.73 No.12
We investigate the damping effect of magnons in the Kitaev model on a honeycomb lattice in the presence of magnetic fields by employing a spin-wave theory where the magnon-magnon interactions are taken into account at finite temperatures. We calculate the dynamical spin structure factor and find that the higher-energy part is smeared significantly by the effect of the magnon damping at zero temperature. On the other hand, the low-energy part of the dynamical spin structure is largely intact, even in the presence of magnon-magnon interactions. We demonstrate that this low-energy structure is also smeared with increasing temperature due to scattering with thermally excited magnons.
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Majorana fermions in the Kitaev quantum spin system α-RuCl<sub>3</sub>
Do, Seung-Hwan,Park, Sang-Youn,Yoshitake, Junki,Nasu, Joji,Motome, Yukitoshi,Kwon, Yong ,Seung,Adroja, D. T.,Voneshen, D. J.,Kim, Kyoo,Jang, T.-H.,Park, J.-H.,Choi, Kwang-Yong,Ji, Sungdae Nature Publishing Group 2017 Nature physics Vol.13 No.11
<P>Geometrical constraints to the electronic degrees of freedom within condensed-matter systems often give rise to topological quantum states of matter such as fractional quantum Hall states, topological insulators, and Weyl semimetals(1-3). In magnetism, theoretical studies predict an entangled magnetic quantum state with topological ordering and fractionalized spin excitations, the quantum spin liquid(4). In particular, the so-called Kitaev spin model(5), consisting of a network of spins on a honeycomb lattice, is predicted to host Majorana fermions as its excitations. By means of a combination of specific heat measurements and inelastic neutron scattering experiments, we demonstrate the emergence of Majorana fermions in single crystals of alpha-RuCl3, an experimental realization of the Kitaev spin lattice. The specific heat data unveils a two-stage release of magnetic entropy that is characteristic of localized and itinerant Majorana fermions. The neutron scattering results corroborate this picture by revealing quasielastic excitations at low energies around the Brillouin zone centre and an hour-glass-like magnetic continuum at high energies. Our results confirm the presence of Majorana fermions in the Kitaev quantum spin liquid and provide an opportunity to build a unified conceptual framework for investigating fractionalized excitations in condensed matter(1,6-8).</P>
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