Language：Japanese   Publisher：Journal of the Physical Society of Japan  

An excitonic insulator, in which an optical gap is formed by exciton condensation, is expected to exhibit large photoresponses. Here, we report the results of femtosecond reflection spectroscopy covering a wide infrared region on a typical excitonic insulator, Ta2NiSe5, which gives direct information not only about a change of excitonic gap but also a metallization. With a weak excitation, the excitonic gap decreases due to the weakening of excitonic effect by photocarriers. With a strong excitation, a Drude-like high-reflection band appears, showing an excitonic-insulator-to-metal transition. Analyses of spectral and temporal changes of reflectivity revealed that the metallization is accompanied by large structural changes. This demonstrates an important role of electron–lattice interaction on the stabilization of excitonic phase.

DOI： 10.7566/JPSJ.91.023701

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Publisher：Journal of the Physical Society of Japan  

We have prepared a set of polycrystalline samples of Ba3Ca(Ru1−xNbx)2O9 consisting of the face-shared (Ru,Nb)2O9 dimer block from x = 0 to 0.12, and have measured the x-ray diffraction and magnetization. We find that all the samples of x > 0 show Curie–Weiss-like susceptibility below 30 K with the Weiss temperature θNb of 2–7 K, which suggests that the Nb-substituted NbRuO9 dimer interacts the Ru2O9 dimers magnetically. We also find the NbRuO9 dimer effectively works as S = 1 in the dilution limit, which is also suggested by the first-principle calculation. The Ru2O9 dimer lattice is nontrivial in the sense that S = 1 moment in the NbRuO9 dimer is screened for large x without any magnetic long range order. A possible origin is discussed.

DOI： 10.7566/JPSJ.90.114706

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1039/d1tc03897d

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Open Ceramics  

The aluminum-sodalite-type oxide, Ca8[AlO2]12(MoO4)2, shows an improper ferroelectricity below a ferroelectric phase transition temperature around 630 ​K. Here we present effects of a Ga-substitution for Al on ferroelectric properties of Ca8[AlO2]12(MoO4)2. Systematic investigations with synchrotron x-ray powder diffractions and dielectric/pyroelectric measurements have clarified that the Ga-substitution suppresses the ferroelectricity and markedly enhances a thermal hysteresis of the ferroelectric phase transition. In contrast to the case of Sr-substitution for Ca, with which the ground state results in a relaxor-like state, the polar ordering of macroscopic scale has been found to survive under the Ga-substitution. The present results shed light on intuitive designing of ferroelectricity and the phase transition properties in the aluminum-sodalite-type oxide.

DOI： 10.1016/j.oceram.2021.100080

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Authorship：Lead author,　Corresponding author   Language：English   Publisher：Physical Society of Japan  

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：日本物理学会  

We have measured the dielectric constant and thermopower of single crystal samples of the excitonic insulator candidate Ta2NiSe5. The dielectric constant below 50 K shows relaxor-like relaxation, implying the existence of randomly distributed electric dipoles. We have evaluated the concentration of the dipoles to be 10^19 cm^−3 at 30 K, which gives a rough estimate of the exciton concentration in this compound. A large thermopower of 600 µV/K at 100 K suddenly drops toward zero down to 50 K, and this zero thermopower evidences the neutral particles flow against the applied temperature gradient. We have semi-quantitatively explained the above two anomalies in terms of spontaneously generated excitons at low temperatures where thermally excited holes are quenched.ファイル公開：2020/11/15

Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.1.0_2550

CiNii Research

Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.1.0_2549

CiNii Research

Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.1.0_2551

CiNii Research

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.98.045139

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1107/S2052252517018334

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PubMed

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0051283

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Chemistry of Materials  

DOI： 10.1021/acs.chemmater.0c04032

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Materials Chemistry C  

DOI： 10.1039/d0tc03539d

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/JPSJ.89.093707

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Language：English   Publisher：Physical Review B  

DOI： 10.1103/PhysRevB.101.094108

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.chemmater.9b04022

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Publisher：The Japan Society of Vacuum and Surface Science  

<p>Ta₂NiSe₅ is a candidate for the excitonic insulator. Recent photoemission study revealed that the valence band minimum shifts to the low energy side at low temperature, which is consistent with the excitonic insulator. To confirm this, it is necessary to observe the conduction band. In this study, we examined the conduction band of Ta₂NiSe₅ using the low energy inverse photoelectron spectroscopy (LEIPS). The lowest unoccupied band was observed at 0.55 eV above the Fermi level. This band did not shift at the low temperature of 80 K. This result supports the recent proposed picture that the Ta₂NiSe₅ is different from the conventional excitonic insulator; the noninteracting band structure is a band-overlap semimetal and the bandgap opens by the strong electron-hole attraction.</p>

DOI： 10.14886/jvss.2020.0_135

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.chemmater.9b02373

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.99.104109

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1039/c7cc08202a

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acsami.7b14132

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PubMed