ORCID Profile
0000-0002-5168-1024
Current Organisation
University of Tokyo
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Publisher: Springer Science and Business Media LLC
Date: 19-06-2020
DOI: 10.1038/S41467-020-16913-2
Abstract: Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. With ZrNiSn-based half-Heusler materials as an ex le, we use high-quality single and polycrystalline crystals, various probes, including electrical transport measurements, inelastic neutron scattering measurement, and first-principles calculations, to investigate the underlying electron-phonon interaction. We find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering, but has negligible influence on lattice thermal conductivity. Furthermore, it is possible to establish a carrier scattering phase diagram, which can be used to select reasonable strategies for optimization of the thermoelectric performance.
Publisher: American Physical Society (APS)
Date: 24-11-2020
Publisher: Springer Science and Business Media LLC
Date: 09-07-2020
DOI: 10.1038/S41467-020-17235-Z
Abstract: 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, KCu 6 AlBiO 4 (SO 4 ) 5 Cl. Comprehensive experimental studies via magnetic susceptibility, magnetisation, heat capacity, muon spin relaxation ( μ SR), 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.
Publisher: Springer Science and Business Media LLC
Date: 23-04-2019
DOI: 10.1038/S41535-019-0152-5
Abstract: When a theoretical model is realized in nature, small perturbation terms play important roles in the selection of the ground state in geometrically frustrated magnets. In case of a triangular spin tube, the two-dimensional network of the inter-tube interaction forms characteristic lattices. Among them Kagome-Triangular (KT) lattice is known to exhibit an enriched phase diagram including various types of non-trivial structures: non-coplanar cuboc structure, coplanar 120° structure with the two-dimensional propagation vector of $${\\boldsymbol{k}}_{2\\mathrm{D}}$$ k 2 D = (0, 0), $$\\sqrt 3 \\times \\sqrt 3$$ 3 × 3 structure with $${\\boldsymbol{k}}_{2{\\mathrm{D}}}$$ k 2 D = (1/3, 1/3), and incommensurate structure. We investigate the magnetic state in the model material CsCrF 4 by using neutron diffraction technique. Combination of representation analysis and Rietveld refinement reveals that a very rare structure, i.e., a quasi-120° structure with $${\\boldsymbol{k}}_{2{\\mathrm{D}}}$$ k 2 D = (1/2, 0), is realized at the base temperature. The classical calculation of the phase diagram elucidates that CsCrF 4 is the first experimental realization of the KT lattice having ferromagnetic Kagome bond. A single-ion anisotropy and Dzyaloshinskii-Moriya interaction play key roles in the selection of the ground state. Furthermore, a successive phase transition having an intermediate state represented by $${\\boldsymbol{k}}_{2{\\mathrm{D}}}$$ k 2 D = (1/3, 1/3) is observed. The intermediate state is a partially ordered 120° structure which is induced by thermal fluctuation.
Publisher: Springer Science and Business Media LLC
Date: 19-07-2021
DOI: 10.1038/S41467-021-24636-1
Abstract: 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 Fe 3+ 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.
Publisher: American Physical Society (APS)
Date: 27-11-2000
Publisher: Springer Science and Business Media LLC
Date: 21-09-2021
DOI: 10.1038/S41467-021-25567-7
Abstract: Spin-orbit coupled honeycomb magnets with the Kitaev interaction have received a lot of attention due to their potential of hosting exotic quantum states including quantum spin liquids. Thus far, the most studied Kitaev systems are 4 d /5 d -based honeycomb magnets. Recent theoretical studies predicted that 3 d -based honeycomb magnets, including Na 2 Co 2 TeO 6 (NCTO), could also be a potential Kitaev system. Here, we have used a combination of heat capacity, magnetization, electron spin resonance measurements alongside inelastic neutron scattering (INS) to study NCTO’s quantum magnetism, and we have found a field-induced spin disordered state in an applied magnetic field range of 7.5 T B (⊥ b -axis) 10.5 T. The INS spectra were also simulated to tentatively extract the exchange interactions. As a 3 d -magnet with a field-induced disordered state on an effective spin-1/2 honeycomb lattice, NCTO expands the Kitaev model to 3 d compounds, promoting further interests on the spin-orbital effect in quantum magnets.
Publisher: Physical Society of Japan
Date: 15-06-2017
Publisher: American Physical Society (APS)
Date: 12-02-2018
Publisher: American Physical Society (APS)
Date: 07-10-2019
Publisher: American Physical Society (APS)
Date: 19-01-2016
Publisher: American Physical Society (APS)
Date: 04-12-2018
No related grants have been discovered for Takatsugu Masuda.