研究者詳細 - 澤　博

論文 - 澤　博

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Orbital hybridization of donor and acceptor to enhance the conductivity of mixed-stack complexes 国際誌

Fujino T., Kameyama R., Onozuka K., Matsuo K., Dekura S., Miyamoto T., Guo Z., Okamoto H., Nakamura T., Yoshimi K., Kitou S., Arima T.H., Sato H., Yamamoto K., Takahashi A., Sawa H., Nakamura Y., Mori H.

Nature Communications 15 巻 ( 1 ) 頁： 3028 - 3028 2024年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Nature Communications

Mixed-stack complexes which comprise columns of alternating donors and acceptors are organic conductors with typically poor electrical conductivity because they are either in a neutral or highly ionic state. This indicates that conductive carriers are insufficient or are mainly localized. In this study, mixed-stack complexes that uniquely exist at the neutral–ionic boundary were synthesized by combining donors (bis(3,4-ethylenedichalcogenothiophene)) and acceptors (fluorinated tetracyanoquinodimethanes) with similar energy levels and orbital symmetry between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Surprisingly, the orbitals were highly hybridized in the single-crystal complexes, enhancing the room-temperature conductivity (10−4–0.1 S cm−1) of mixed-stack complexes. Specifically, the maximum conductivity was the highest reported for single-crystal mixed-stack complexes under ambient pressures. The unique electronic structures at the neutral–ionic boundary exhibited structural perturbations between their electron-itinerant and localized states, causing abrupt temperature-dependent changes in their electrical, optical, dielectric, and magnetic properties.

DOI： 10.1038/s41467-024-47298-1

Metastable ordered states induced by low-temperature annealing of δ- Ag2/3 V2 O5

Kubo T., Kojima K., Katayama N., Runčevski T., Dinnebier R.E., Gibbs A.S., Isobe M., Sawa H.

Physical Review B 109 巻 ( 10 ) 2024年3月

出版者・発行元：Physical Review B

In δ-Ag2/3V2O5 with charge degrees of freedom in V, it is known that the charge ordering state and physical properties of V that appear at low temperatures depend strongly on the ordering state of Ag. In this study, we focus on the Ag ions in the interlayer and study the structure using synchrotron radiation powder diffraction depending on temperature. We find that when the sample is slowly cooled from room temperature and ordering occurs at the Ag sites, V4+/V5+ charge ordering of V and subsequent V4+-V4+ structural dimers are produced. Although quenching the sample from room temperature suppresses the ordering of Ag, annealing at around 160 K promotes partial ordering of Ag and allows a metastable phase to be realized. This metastable phase is maintained even when the temperature is lowered again, producing a remarkable change in low-temperature properties. These results indicate that the ordered state of Ag, which is the key to control the charge-ordered state and physical properties, can be controlled by low-temperature annealing. The results of this study may provide a methodology for the realization of metastable states in a wide range of material groups of vanadium compounds, in which competition among various charge-ordered states underlies the physical properties.

DOI： 10.1103/PhysRevB.109.104115

Enhancement of Superconductivity and Structural Instability by Sn Substitution in LaO0.5F0.5BiS2

Kobayashi, S; Hara, T; Noguchi, K; Takeda, T; Kurihara, R; Nomura, A; Demura, S; Yaguchi, H; Sawa, H; Sakata, H

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 93 巻 ( 2 ) 2024年2月

出版者・発行元：Journal of the Physical Society of Japan

Layered BiS2-based superconductor La(O,F)BiS2 can exhibit superconductivity via carrier doping, and its superconducting properties can be changed by applying hydrostatic or chemical pressure through elemental substitution. Single-crystal LaO0.5F0.5BiS2 exhibits filamentary superconductivity with Tc of approximately 3 K. We found that the partial substitution of Bi with Sn in LaO0.5F0.5BiS2 induces a structural phase transition from the P4=nmm to P21=m structure. Bulk superconductivity with Tc of approximately 6 K emerged near this structural phase transition. We also observed filamentary superconductivity with Tc of approximately 8 K, which was approximately three times higher than that of the unsubstituted sample. This finding provides a new route for further increasing the superconducting transition temperature of this system.

DOI： 10.7566/JPSJ.93.024707

Zigzag chain order of LiVSe2 developing away from the vanadium trimer phase transition boundary

Kojima K., Katayama N., Sugimoto K., Hirao N., Ohta Y., Sawa H.

Physical Review B 108 巻 ( 9 ) 2023年9月

掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review B

The phenomenon of self-assembly of constituent elements to form molecules at low temperatures appears ubiquitously in transition metal compounds with orbital degrees of freedom. Recent progress in local structure studies using synchrotron radiation x rays is shifting the interest in structural studies in such molecule-forming systems from the low-temperature ordered phase to the short-range order that appears like a precursor at high temperatures. In this study, we discuss both experimentally and theoretically the relationship between the trimer structure that appears in the layered LiVX2(X = O, S, Se) system with a two-dimensional triangular lattice of vanadium and the zigzag chainlike local structure that appears near the phase transition boundary where molecular formation occurs. The vanadium trimerization that persistently appears in both low-temperature phases of LiVO2 and LiVS2 disappears in LiVSe2, and a regular triangular lattice is thought to be realized in LiVSe2, but this study reveals that the zigzag chain local distortion appears with a finite correlation length. These zigzag chain state local distortions are similar to the motif of local distortions in the high-temperature phase of LiVS2, indicating that the local distortions are persistent away from the trimer phase transition boundary. On the other hand, it is concluded that the zigzag chain order appearing in LiVSe2 is more stable than that in LiVS2 in terms of the temperature variation of atomic displacement and correlation length. The zigzag chain order is considered to be competitive with the trimer order appearing in the LiVX2 system. In this paper, we discuss the similarities and differences between the parameters that stabilize these electronic phases and the local distortions that appear in other molecular formation systems.

DOI： 10.1103/physrevb.108.094107

その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.108.094107/fulltext

Visualization of Formula Presented orbital electrons in a pyrochlore-type oxide

Kitou S., Kaneko Y., Nakamura Y., Sugimoto K., Nomura Y., Arita R., Tokura Y., Sawa H., Arima T.H.

Physical Review B 108 巻 ( 2 ) 2023年7月

出版者・発行元：Physical Review B

A real-space observation of the distribution state of valence electrons, which is responsible for physical properties, is a key methodology to understand the relation between the structure and functions of matter. Here, we observe Mo Formula Presented orbital electrons at subangstrom resolution in a pyrochlore-type oxide Formula Presented based on the core differential Fourier synthesis (CDFS) analysis of high-energy x-ray diffraction data. The Formula Presented orbital state is directly determined from the obtained valence electron density (VED) distribution. We also find a dip in the radial profile corresponding to a node of the Formula Presented wave function. The VED distribution around the Nd site might be ascribed to the hybridization of neighboring O Formula Presented with Nd Formula Presented orbitals as well as the anisotropic Formula Presented electrons, which cannot be reproduced by the simple j-j or Formula Presented coupling pictures. In this paper, we demonstrate the usefulness of the CDFS analysis to investigate the orbital states in crystalline materials.

DOI： 10.1103/PhysRevB.108.024103

Observation of local atomic displacements intrinsic to the double zigzag chain structure of 1T-MTe2 (M = V, Nb, Ta)

Katayama, N; Matsuda, Y; Kojima, K; Hara, T; Kitou, S; Mitsuishi, N; Takahashi, H; Ishiwata, S; Ishizaka, K; Sawa, H

PHYSICAL REVIEW B 107 巻 ( 24 ) 2023年6月

出版者・発行元：Physical Review B

We describe the existence of local distortion discovered in the synchrotron x-ray single-crystal structure analysis of layered ditelluride 1T-MTe2 (M=V, Nb, Ta). In 1T-TaTe2, the double zigzag chain structure of Ta is deformed at about 170 K, and heptamer molecules are formed periodically at low temperatures. We found that some of the Ta atoms that compose the double zigzag chain structure appearing at high temperatures are locally displaced, resulting in local dimerization. This tendency weakens when Ta is replaced by V or Nb. Our results indicate that the local distortion persistently survives in these ditellurides, where the electronic degrees of freedom, including orbitals, are weakened. We further discuss the origin of local distortion in these ditellurides, which is different from many usual material systems where molecular formation occurs at low temperatures.

DOI： 10.1103/PhysRevB.107.245113

Temporal Evolution and Fluence Dependence of Band Structure in Photoexcited Ta2Ni0.9Co0.1Se5 Probed by Time- and Angle-Resolved Photoemission Spectroscopy

Takahashi, Y; Suzuki, T; Hattori, M; Okawa, M; Takagi, H; Katayama, N; Sawa, H; Nohara, M; Zhong, YG; Liu, KC; Kanai, T; Itatani, J; Shin, S; Okazaki, K; Mizokawa, T

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 92 巻 ( 6 ) 2023年6月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Physical Society of Japan

We have investigated the temporal evolution and fluence dependence of band structure in Ta2Ni0.9Co0.1Se5 by means of time- and angle-resolved photoemission spectroscopy (tr-ARPES). With the fluence of 3.00 mJ cm−2 and the delay time of 170 fs, the conduction band crossing the Fermi level (EF) and the valence band reaching EF are observed indicating the photoinduced semimetal phase. However, at the delay time of 70 fs, the valence band top does not reach EF creating a partial gap or a pseudogap, and the transition from the excitonic insulator state to the semimetal state is not completed. The conduction band crossing EF is observed even with the shorter delay time (down to 50–70 fs) and=or the lower fluence (down to 0.32 mJ cm−2). On the other hand, the valence band top stays well below EF up to the fluence of 1.39 mJ cm−2 even at the delay time of 160–170 fs. The different optical responses of the valence and conduction bands suggest a vital role of the lattice fluctuations pointed out by Nakano et al. [Phys. Rev. B 98, 045139 (2018)] on the residual pseudogap at 70 fs after the photoexcitation.

DOI： 10.7566/JPSJ.92.064706

Room-temperature plastic deformation modes of cubic ZnS crystals

Kitou S., Oshima Y., Nakamura A., Matsunaga K., Sawa H.

Acta Materialia 247 巻頁： 118738 - 118738 2023年4月

掲載種別：研究論文（学術雑誌）出版者・発行元：Acta Materialia

Light environment drastically alters the plasticity of zinc sulfide (ZnS) single crystals: they exhibit ductility in darkness while brittleness in lights. This study investigated their microscopic deformation modes of the light-environment-dependent plasticity, by using synchrotron X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning TEM (STEM) measurements. The XRD analyses disclosed that the ZnS crystals originally have two domains with almost the same volume fraction with a twin relationship and plastically deform in darkness, accompanying changes in the domain volume ratio (namely extinction of twins). It was also observed that the domain volume ratios become constant to be 0.2 at more than 10% plastic strains. The STEM analyses indicated that the deformation mode in darkness switches at around 10% plastic strain from slip deformation by glide of isolated partial dislocations to by simultaneous glide of paired partials inside the major domains. Although ZnS single crystals exhibited only a few percent of plastic strains in lights, localized glide of isolated partials and the resultant changes in domain volume ratios similar to in darkness were confirmed. That is, the deformation modes of ZnS at the initial stage were similar regardless of light environments although the macroscopic deformation behaviors were quite different.

DOI： 10.1016/j.actamat.2023.118738

Quasi One-Dimensional Band Structure of Photoinduced Semimetal Phase of Ta2Ni1-xCoxSe5 (x=0.0 and 0.1)

Mitsuoka, T; Takahashi, Y; Suzuki, T; Okawa, M; Takagi, H; Katayama, N; Sawa, H; Nohara, M; Watanabe, M; Xu, JD; Ren, QH; Fujisawa, M; Kanai, T; Itatani, J; Okazaki, K; Shin, S; Mizokawa, T

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 92 巻 ( 2 ) 2023年2月

掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Physical Society of Japan

The photoinduced semimetal phase of Ta2Ni1−xCoxSe5 (x = 0.0 and 0.1) is studied by time-resolved angle-resolved photoemission spectroscopy. The observed band dispersions roughly agree with the generalized gradient approximation (GGA) calculation which fails to reproduce the band gap at the ambient condition. This suggests that the electron–hole interaction, which is not exactly included in GGA, is screened out by the photoexcited carriers. There is no indication of pseudogap. The conduction and valence bands provide quasi one-dimensional Fermi surfaces with similar momentum positions suggesting a quasi one-dimensional line-node semimetal.

DOI： 10.7566/JPSJ.92.023703

Pressure suppression of the excitonic insulator state in Ta2NiSe5 observed by optical conductivity

Okamura, H; Mizokawa, T; Miki, K; Matsui, Y; Noguchi, N; Katayama, N; Sawa, H; Nohara, M; Lu, Y; Takagi, H; Ikemoto, Y; Moriwaki, T

PHYSICAL REVIEW B 107 巻 ( 4 ) 2023年1月

掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review B

The layered chalcogenide Ta2NiSe5 has recently attracted much interest as a strong candidate for a long-sought excitonic insulator (EI). Since the physical properties of an EI are expected to depend sensitively on the external pressure (P), it is important to clarify the P evolution of a microscopic electronic state in Ta2NiSe5. Here we report the optical conductivity [σ(ω)] of Ta2NiSe5 measured at high P to 10 GPa and at low temperatures to 8 K. With cooling at P=0, σ(ω) develops an energy gap of about 0.17 eV and a pronounced excitonic peak at 0.38 eV as reported previously. With increasing P, the energy gap becomes narrower and the excitonic peak is diminished. Above a structural transition at Ps≃3 GPa, the energy gap becomes partially filled, indicating that Ta2NiSe5 is a semimetal after the EI state is suppressed by P. At higher P, σ(ω) exhibits metallic characteristics with no energy gap. The detailed P evolution of the energy gap and σ(ω) is presented, and discussed mainly in terms of a weakening of excitonic correlation with P.

DOI： 10.1103/PhysRevB.107.045141

その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.107.045141/fulltext

Short-range order and increased transition temperature in LiVO2 with weakened trimer frustration

Kojima, K; Kattayama, N; Matsuda, Y; Shiomi, M; Ishii, R; Sawa, H

PHYSICAL REVIEW B 107 巻 ( 2 ) 2023年1月

出版者・発行元：Physical Review B

Vanadium atoms in layered LiVO2 form in-plane periodic vanadium trimers at low temperatures, but the trimers appear randomly in the stacking direction because there are many trimer configurations with comparable lattice energy. We detailed an original modeling scheme to represent glassy states with a completely disordered trimer configuration in the stacking structure. Through pair distribution function analysis using this model, we show that the synthesis method can yield two types of low-temperature stacking structures: a completely disordered stacking structure and a short-range order in the stacking structure. The phase-transition temperature of the former sample is about 15 K lower than that of the latter. We discuss that this is due to the strong trimer frustration that appears in the sample without short-range order, which suppresses the phase-transition temperature, similar to the frustration effect in conventional spin systems.

DOI： 10.1103/PhysRevB.107.L020101

Spin-orbital liquid state and liquid-gas metamagnetic transition on a pyrochlore lattice

Tang, N; Gritsenko, Y; Kimura, K; Bhattacharjee, S; Sakai, A; Fu, MX; Takeda, H; Man, HY; Sugawara, K; Matsumoto, Y; Shimura, Y; Wen, JJ; Broholm, C; Sawa, H; Takigawa, M; Sakakibara, T; Zherlitsyn, S; Wosnitza, J; Moessner, R; Nakatsuji, S

NATURE PHYSICS 19 巻 ( 1 ) 頁： 92 - + 2023年1月

出版者・発行元：Nature Physics

Crystal structures with degenerate electronic orbitals are unstable towards lattice distortions that lift the degeneracy. Although these Jahn–Teller distortions have profound effects on magnetism, they are typically unaffected by the onset of magnetic ordering because of a separation in energy scales. Here we show the contrary case in Pr2Zr2O7, where orbital degeneracy remains down to the millikelvin range due to an interplay between spins and orbitals. Pr2Zr2O7 is a multipolar spin ice with strongly localized 4f electrons in an even-number configuration, giving rise to a non-Kramers doublet that carries transverse quadrupolar and longitudinal dipolar moments. Our study of ultrapure single crystals of Pr2Zr2O7 finds comprehensive evidence for enhanced spin–orbital quantum dynamics of the non-Kramers doublet. This dynamical Jahn–Teller effect is encapsulated by the liquid–gas metamagnetic transition that is characteristic of spin ice being accompanied by strong lattice softening. This behaviour suggests that a spin–orbital liquid state forms on the pyrochlore lattice at low temperatures and low magnetic fields.

DOI： 10.1038/s41567-022-01816-4

Do electron distributions with orbital degree of freedom exhibit anisotropy? 査読有り

Manjo, T; Kitou, S; Katayama, N; Nakamura, S; Katsufuji, T; Nii, Y; Arima, TH; Nasu, J; Hasegawa, T; Sugimoto, K; Ishikawa, D; Baron, AQR; Sawa, H

MATERIALS ADVANCES 3 巻 ( 7 ) 頁： 3192 - 3198 2022年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Materials Advances

Orbital degrees of freedom appear in partially filled degenerate orbitals and are responsible for a variety of physical properties. Although many methods for directly observing orbital states in ordered states have been proposed, it is difficult to determine degenerate orbital states directly. We found that high-precision diffraction data obtained by synchrotron radiation X-rays can be used to observe only the valence electron density distribution with minimal analytical bias. The results obtained using this method clearly showed the anisotropy of the partially filled degenerate 3d orbital states in spinel vanadium oxide FeV2O4. The 3d orbital quantum state was successfully determined from this anisotropy.

DOI： 10.1039/d1ma01113h

Charge and Lattice Dynamics in Excitonic Insulator Ta<sub>2</sub>NiSe<sub>5</sub> Investigated Using Ultrafast Reflection Spectroscopy 査読有り

Miyamoto, T; Mizui, M; Takamura, N; Hirata, J; Yamakawa, H; Morimoto, T; Terashige, T; Kida, N; Nakano, A; Sawa, H; Okamoto, H

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 91 巻 ( 2 ) 2022年2月

記述言語：日本語出版者・発行元：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

Charge-ordered state satisfying the Anderson condition in LiRh<sub>2</sub>O<sub>4</sub> arising from local dimer order

Shiomi, M; Kojima, K; Katayama, N; Maeda, S; Schneeloch, JA; Yamamoto, S; Sugimoto, K; Ohta, Y; Louca, D; Okamoto, Y; Sawa, H

PHYSICAL REVIEW B 105 巻 ( 4 ) 2022年1月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review B

We report on the charge-ordered structure of LiRh2O4 arising below the metal-insulator transition at 170 K. Structural studies using synchrotron x rays have revealed that the charge-ordered states of Rh3+ and Rh4+ with dimerization are realized in the low-temperature phase below 170 K. Although the low-temperature ground state resembles that of CuIr2S4, a charge-ordering pattern satisfying the Anderson condition is realized in LiRh2O4. Based on structural information such as the short-range order of dimers appearing above the transition temperature and the weakening of the correlation between rhodium one-dimensional chains appearing in the crystal structure, we argue that the Coulomb interaction plays an important role in determining the charge-ordering patterns.

DOI： 10.1103/PhysRevB.105.L041103

その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.105.L041103/fulltext

Hybridization-Gap Formation and Superconductivity in the Pressure-Induced Semimetallic Phase of the Excitonic Insulator Ta<sub>2</sub>NiSe<sub>5</sub>

Matsubayashi, K; Okamura, H; Mizokawa, T; Katayama, N; Nakano, A; Sawa, H; Kaneko, T; Toriyama, T; Konishi, T; Ohta, Y; Arima, H; Yamanaka, R; Hisada, A; Okada, T; Ikemoto, Y; Moriwaki, T; Munakata, K; Nakao, A; Nohara, M; Lu, YF; Takagi, H; Uwatoko, Y

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 90 巻 ( 7 ) 頁： 074706 - 074706 2021年7月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of the Physical Society of Japan

The excitonic insulator Ta2NiSe5 experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps ~ 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of ~0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Nivalence bands due to the lattice distortion, indicative of the importance of electron-lattice coupling. The transition is suppressed with increasing pressure to Pc ~ 8 GPa. Superconductivity with a maximum Tsc ~ 1.2K emerges around Pc, likely mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5.

DOI： 10.7566/JPSJ.90.074706

Complex changes in structural parameters hidden in the universal phase diagram of the quasi-one-dimensional organic conductors (TMTTF)<sub>2</sub><i>X</i> (<i>X</i> = NbF<sub>6</sub>, AsF<sub>6</sub>, PF<sub>6</sub>, and Br) 査読有り

Kitou, S; Zhang, LD; Nakamura, T; Sawa, H

PHYSICAL REVIEW B 103 巻 ( 18 ) 2021年5月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review B

Structural parameters in quasi-one-dimensional organic conductors (TMTTF)2X (X=NbF6, AsF6, PF6, and Br) are investigated by synchrotron X-ray diffraction. The temperature dependences of the crystal structures differ significantly between octahedral and monatomic anion systems, corresponding to the presence or absence of a charge-ordering transition. Changes in temperature and chemical pressure control the dimensionality of the lattice and the degree of dimerization. Furthermore, the antiferromagnetic and spin-Peierls ground states in this system depend on the spin frustration due to transfer integral paths between the one-dimensional chain molecules.

DOI： 10.1103/PhysRevB.103.184112

Tetramethyltetrathiafulvalene [(NbOF<sub>4</sub>)<SUP>-</SUP>]<sub>∞</sub>: One-Dimensional Charge Transfer Salt with an Infinite Anion Chain

Nakamura, T; Zhang, LD; Kitou, S; Sawa, H

INORGANIC CHEMISTRY 60 巻 ( 7 ) 頁： 5206 - 5211 2021年4月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Inorganic Chemistry

An unusual valence one-dimensional (1D) molecular charge transfer salt (TMTTF)(NbOF4) [TMTTF = tetramethyltetrathiafulvalene] with infinite anion chains was prepared. To understand the crystal structure and electronic states of the (TMTTF)(NbOF4) salt, we performed synchrotron X-ray diffraction, electron spin resonance, and static magnetization measurements. There is only one independent TMTTF molecule in the unit cell of (TMTTF)(NbOF4). The TMTTF1+ cation radicals stack to form 1D columns. The effective charge of the TMTTF molecule in the crystal was estimated to be +1. The electric charge of TMTTF donors is compensated by the infinite anion chains [(NbOF4)-]∞. The magnetic susceptibility of (TMTTF)(NbOF4) is 4 × 10-4 emu/mol at room temperature and shows weak temperature dependence above 60 K. However, some deviation appears below 60 K. The temperature dependence of the spin susceptibility shows a noticeable enhancement below 60 K. Below 5 K, the magnetization curve as a function of the magnetic field deviates from the straight line and shows a saturation tendency. The experimental results can be reproduced well with the S = 2 spin system at 2 K. The detailed analysis of the crystal structure and anomalous low-temperature magnetic state magnetic properties of (TMTTF)(NbOF4) are discussed.

DOI： 10.1021/acs.inorgchem.1c00208

Detecting electron-phonon coupling during photoinduced phase transition

Suzuki, T; Shinohara, Y; Lu, YF; Watanabe, M; Xu, JD; Ishikawa, KL; Takagi, H; Nohara, M; Katayama, N; Sawa, H; Fujisawa, M; Kanai, T; Itatani, J; Mizokawa, T; Shin, S; Okazaki, K

PHYSICAL REVIEW B 103 巻 ( 12 ) 2021年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review B

Photoinduced phase transitions have been intensively studied owing to the potential capability to control a material of interest in the ultrafast manner, which can induce exotic phases unable to be attained at equilibrium. However, the key mechanisms are still under debate, and it is currently a central issue as to how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photoinduced phase transitions. Here, we use a recently developed analysis method, which we call frequency-domain angle-resolved photoemission spectroscopy (FDARPES), and reveal mode- and band-selective electron-phonon couplings during the photoinduced insulator-to-metal transition for Ta2NiSe5. We find that the lattice modulation corresponding to the 2 THz phonon mode, where the Ta lattice is sheared along the a axis, is the most relevant for the emergence of photoinduced semimetallic state. Furthermore, we find that the semimetallic and semiconducting bands coexist in the transient state, and demonstrate that FDARPES spectra can selectively detect the phonon-specific couplings to the two coexistent band structures during the photoinduced phase transition by resolving them in the frequency domain.

DOI： 10.1103/PhysRevB.103.L121105

Slow dynamics of disordered zigzag chain molecules in layered LiVS<sub>2</sub> under electron irradiation

Katayama, N; Kojima, K; Yamaguchi, T; Hattori, S; Tamura, S; Ohara, K; Kobayashi, S; Sugimoto, K; Ohta, Y; Saitoh, K; Sawa, H

NPJ QUANTUM MATERIALS 6 巻 ( 1 ) 2021年2月

掲載種別：研究論文（学術雑誌）出版者・発行元：npj Quantum Materials

Electronic instabilities in transition metal compounds often spontaneously form orbital molecules, which consist of orbital-coupled metal ions at low temperature. Recent local structural studies utilizing the pair distribution function revealed that preformed orbital molecules appear disordered even in the high-temperature paramagnetic phase. However, it is unclear whether preformed orbital molecules are dynamic or static. Here, we provide clear experimental evidence of the slow dynamics of disordered orbital molecules realized in the high-temperature paramagnetic phase of LiVS2, which exhibits vanadium trimerization upon cooling below 314 K. Unexpectedly, the preformed orbital molecules appear as a disordered zigzag chain that fluctuate in both time and space under electron irradiation. Our findings should advance studies on soft matter physics realized in an inorganic material due to disordered orbital molecules.

DOI： 10.1038/s41535-021-00313-w

その他リンク： http://www.nature.com/articles/s41535-021-00313-w

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