Browsing by Author "Nakajima, K"

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    Crystal-liquid duality driven ultralow two-channel thermal conductivity in α-MgAgSb
    (AIP Publishing, 2024-03) Li, JY; Li, XY; Zhang, YS; Zhu, J; Zhao, E; Kofu, M; Nakajima, K; Avdeev, M; Liu, PF; Sui, Jiehe; Zhao, HSZ; Wang, FW; Zhang, JR
    The desire for intrinsically low lattice thermal conductivity (κL) in thermoelectrics motivates numerous efforts on understanding the microscopic mechanisms of heat transport in solids. Here, based on theoretical calculations, we demonstrate that α-MgAgSb hosts low-energy localized phonon bands and avoided crossing of the rattler modes, which coincides with the inelastic neutron scattering result. Using the two-channel lattice dynamical approach, we find, besides the conventional contribution (∼70% at 300 K) from particlelike phonons propagating, the coherence contribution dominated by the wavelike tunneling of phonons accounts for ∼30% of the total κL at 300 K. By considering dual contributions, our calculated room-temperature κL of 0.64 W m−1 K−1 well agrees with the experimental value of 0.63 W m−1 K−1. More importantly, our computations give a nonstandard κL ∝ T−0.61 dependence, perfectly explaining the abnormal temperature-trend of ∼T−0.57 in experiment for α-MgAgSb. By molecular dynamics simulation, we reveal that the structure simultaneously has soft crystalline sublattices with the metavalent bonding and fluctuating liquid-like sublattices with thermally induced large amplitude vibrations. These diverse forms of chemical bonding arouse mixed part-crystal part-liquid state, scatter strongly heat-carrying phonons, and finally produce extremely low κL. The fundamental research from this study will accelerate the design of ultralow-κL materials for energy-conversion applications. © 2024 AIP Publishing LLC
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    Dimensional reduction by geometrical frustration in a cubic antiferromagnet composed of tetrahedral clusters
    (Springer Nature, 2021-07-19) Okuma, R; Kofu, M; Asai, S; Avdeev, M; Koda, A; Okabe, H; Hiraishi, M; Takeshita, S; Kojima, KM; Kadono, R; Masuda, T; Nakajima, K; Hiroi, Z
    Dimensionality is a critical factor in determining the properties of solids and is an apparent built-in character of the crystal structure. However, it can be an emergent and tunable property in geometrically frustrated spin systems. Here, we study the spin dynamics of the tetrahedral cluster antiferromagnet, pharmacosiderite, via muon spin resonance and neutron scattering. We find that the spin correlation exhibits a two-dimensional characteristic despite the isotropic connectivity of tetrahedral clusters made of spin 5/2 Fe3+ ions in the three-dimensional cubic crystal, which we ascribe to two-dimensionalisation by geometrical frustration based on spin wave calculations. Moreover, we suggest that even one-dimensionalisation occurs in the decoupled layers, generating low-energy and one-dimensional excitation modes, causing large spin fluctuation in the classical spin system. Pharmacosiderite facilitates studying the emergence of low-dimensionality and manipulating anisotropic responses arising from the dimensionality using an external magnetic field. © 2021, The Author(s)
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    Gapless spin liquid in a square-kagome lattice antiferromagnet
    (Springer Nature Limited, 2020-06-09) Fujihala, M; Morita, K; Mole, RA; Mitsuda, S; Tohyama, T; Yano, SI; Yu, DH; Sota, S; Kuwai, T; Koda, A; Okabe, H; Lee, H; Itoh, H; Hawai, T; Masuda, T; Sagayama, H; Matsuo, A; Kindo, K; Ohira-Kawamura, S; Nakajima, K
    Observation of a quantum spin liquid (QSL) state is one of the most important goals in condensed-matter physics, as well as the development of new spintronic devices that support next-generation industries. The QSL in two dimensional quantum spin systems is expected to be due to geometrical magnetic frustration, and thus a kagome-based lattice is the most probable playground for QSL. Here, we report the first experimental results of the QSL state on a square-kagome quantum antiferromagnet, KCu6AlBiO4(SO4)5Cl. Comprehensive experimental studies via magnetic susceptibility, magnetisation, heat capacity, muon spin relaxation (μSR), and inelastic neutron scattering (INS) measurements reveal the formation of a gapless QSL at very low temperatures close to the ground state. The QSL behavior cannot be explained fully by a frustrated Heisenberg model with nearest-neighbor exchange interactions, providing a theoretical challenge to unveil the nature of the QSL state. © 2020 Springer Nature Limited
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    Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb
    (Springer Nature, 2020-02-18) Li, XY; Liu, PF; Zhao, EY; Zhang, ZG; Guidi, T; Le, MD; Avdeev, M; Ikeda, K; Otomo, T; Kofu, M; Nakajima, K; Chen, J; He, LH; Ren, Y; Wang, XL; Wang, BT; Ren, ZF; Zhao, HZ; Wang, FW
    Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound’s intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials. © The Author(s) 2020.