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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.
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〉