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4 pages, 195 KiB

Open AccessEditorial

Organic Conductors

by Toshio Naito

Crystals 2022, 12(4), 523; https://doi.org/10.3390/cryst12040523 - 9 Apr 2022

Cited by 2 | Viewed by 1738

Abstract

Since the pioneering work concerning organic semiconductors in the middle of the 1900s, organic conductor research has experienced a series of milestones, from metallic to superconducting charge-transfer complexes (1980s–1990s) [...] Full article

(This article belongs to the Special Issue Organic Conductors)

Research

Jump to: Editorial, Review

14 pages, 3664 KiB

Open AccessEditor’s ChoiceArticle

Theoretical Model for a Novel Electronic State in a Dirac Electron System Close to Merging: An Imaginary Element between Sulphur and Selenium

by Toshio Naito and Yoshikazu Suzumura

Crystals 2022, 12(3), 346; https://doi.org/10.3390/cryst12030346 - 2 Mar 2022

Cited by 7 | Viewed by 10573

Abstract

Topological materials with Dirac electron systems have been extensively studied. Organic crystalline materials form a unique group of such compounds with well-defined crystal structures. While most organic compounds require high pressures to exhibit Dirac-cone-type band structures, the title compound, α-STF₂I₃ [...] Read more.

Topological materials with Dirac electron systems have been extensively studied. Organic crystalline materials form a unique group of such compounds with well-defined crystal structures. While most organic compounds require high pressures to exhibit Dirac-cone-type band structures, the title compound, α-STF₂I₃, has garnered increasing interest due to its Dirac-cone-type band structure under ambient pressure. Various experiments have been conducted under ambient pressure; their results can be compared with those of theoretical calculations to obtain insights into Dirac electron systems. However, structural disorder peculiar to the STF molecules in the solid-state has prevented any type of theoretical calculation of the states. In this study, we report a new method for calculating intermolecular interactions in disordered systems based on the extended Hückel approximation. This method enables band calculations, suggesting that this material is a rare example of a system close to merging. The obtained band structure indicates that the characteristic disorder in the STF solids distributed electrons equally on the sulphur and selenium atoms as if they belong to an imaginary element between sulphur and selenium and are arranged without disorder. Full article

(This article belongs to the Special Issue Organic Conductors)

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27 pages, 7500 KiB

Open AccessArticle

Donor-Type Nickel–Dithiolene Complexes Fused with Bulky Cycloalkane Substituents and Their Application in Molecular Conductors

by Kazuya Kubo, Mamoru Sadahiro, Sonomi Arata, Norihisa Hoshino, Tomofumi Kadoya, Tomoyuki Akutagawa, Reizo Kato and Jun-ichi Yamada

Crystals 2021, 11(10), 1154; https://doi.org/10.3390/cryst11101154 - 23 Sep 2021

Cited by 1 | Viewed by 2884

Abstract

The effects of substituents on the arrangement of metal–dithiolene complexes based on π-conjugated systems, which are extensively used to synthesize various functional materials, have not been studied adequately. New donor-type nickel–dithiolene complexes fused with bulky cycloalkane substituents [Ni(C_n-dddt)₂] (C [...] Read more.

The effects of substituents on the arrangement of metal–dithiolene complexes based on π-conjugated systems, which are extensively used to synthesize various functional materials, have not been studied adequately. New donor-type nickel–dithiolene complexes fused with bulky cycloalkane substituents [Ni(C_n-dddt)₂] (C₅-dddt = 4a,5,6,6a-pentahydro-1,4-benzodithiin-2,3-dithiolate; C₆-dddt = 4a,5,6,7,8,8a-hexahydro-1,4-benzodithiin-2,3-dithiolate; C₇-dddt = 4a,5,6,7,8,9,9a-heptahydro-1,4-benzodithiin-2,3-dithiolate; and C₈-dddt = 4a,5,6,7,8,9,10,10a-octahydro-1,4-benzodithiin-2,3-dithiolate) were synthesized in this study. All the complexes were crystallized in cis-[Ni(cis-C_n-dddt)₂] conformations with cis-oriented (R,S) conformations around the cycloalkylene groups in the neutral state. Unique molecular arrangements with a three-dimensional network, a one-dimensional column, and a helical molecular arrangement were formed in the crystals owing to the flexible cycloalkane moieties. New 2:1 cation radical crystals of [Ni(C₅-dddt)₂]₂(X) (X = ClO₄⁻ or PF₆⁻), obtained by electrochemical crystallization, exhibited semiconducting behaviors (ρ_rt = 0.8 Ω cm, E_a = 0.09 eV for the ClO₄⁻ crystal; 4.0 Ω cm, 0.13 eV for the PF₆⁻ crystal) under ambient pressure due to spin-singlet states between the dimers of the donor, which were in accordance with the conducting behaviors under hydrostatic pressure (ρ_rt = 0.2 Ω cm, E_a = 0.07 eV for the ClO₄⁻ crystal; 1.0 Ω cm, 0.12 eV for the PF₆⁻ crystal at 2.0 GPa). Full article

(This article belongs to the Special Issue Organic Conductors)

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9 pages, 1732 KiB

Open AccessArticle

First-Principles Study on the Stability and Electronic Structure of the Charge-Ordered Phase in α-(BEDT-TTF)₂I₃

by Takao Tsumuraya, Hitoshi Seo and Tsuyoshi Miyazaki

Crystals 2021, 11(9), 1109; https://doi.org/10.3390/cryst11091109 - 12 Sep 2021

Cited by 5 | Viewed by 2955

Abstract

We theoretically study the structural and electronic properties of a molecular conductor,

α

-(BEDT-TTF)

_{2}

I

_{3}

, using first-principles density-functional theory calculations, especially in its low-temperature charge-ordered state at ambient pressure. We apply a hybrid functional approach and compare the results with [...] Read more.

We theoretically study the structural and electronic properties of a molecular conductor,

α

-(BEDT-TTF)

_{2}

I

_{3}

, using first-principles density-functional theory calculations, especially in its low-temperature charge-ordered state at ambient pressure. We apply a hybrid functional approach and compare the results with a conventional exchange-correlation functional within the generalized gradient approximation. By performing structural optimization, we found a stable charge-ordered solution for the former, in contrast to the latter approach where the magnitude of the charge imbalance becomes considerably small compared to that when the experimental structure is adopted. The electronic band structure near the Fermi level, with and without structural optimization, as well as the molecule-dependent local density of states of the charge-ordered state are discussed. Full article

(This article belongs to the Special Issue Organic Conductors)

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13 pages, 2807 KiB

Open AccessArticle

Temperature-Modulated Pyroelectricity Measurements of a Thin Ferroelectric Crystal with In-Plane Polarization and the Thermal Analysis Based on One-Dimensional Layer Models

by Kaoru Yamamoto, Ayumi Kawasaki, Takumi Chinen and Kayo Ryugo

Crystals 2021, 11(8), 880; https://doi.org/10.3390/cryst11080880 - 28 Jul 2021

Cited by 7 | Viewed by 2865

Abstract

A temperature-modulated pyroelectricity measurement system for a small single crystal is developed and applied to standard sample measurements performed on a thin single crystal of lithium niobate. The modulation measurement is based on the AC technique, in which the temperature of the sample [...] Read more.

A temperature-modulated pyroelectricity measurement system for a small single crystal is developed and applied to standard sample measurements performed on a thin single crystal of lithium niobate. The modulation measurement is based on the AC technique, in which the temperature of the sample is periodically oscillated, and the synchronized pyroelectric signal is extracted using a lock-in amplifier. Temperature modulation is applied by irradiating periodic light on the sample placed in the heat exchange gas. To apply this technique to the transparent reference sample, a commercially available black resin is coated on the sample’s surface to absorb the light energy and transmits it to the specimen. The experimental results are analyzed using a two-layer heat transfer model to verify the effect of the light-absorbing layer as well as the radiative temperature modulation system. Full article

(This article belongs to the Special Issue Organic Conductors)

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9 pages, 3096 KiB

Open AccessArticle

Dihedral-Angle Dependence of Intermolecular Transfer Integrals in BEDT-BDT-Based Radical-Cation Salts with θ-Type Molecular Arrangements

by Tomofumi Kadoya, Shiori Sugiura, Toshiki Higashino, Keishiro Tahara, Kazuya Kubo, Takahiko Sasaki, Kazuo Takimiya and Jun-ichi Yamada

Crystals 2021, 11(8), 868; https://doi.org/10.3390/cryst11080868 - 27 Jul 2021

Cited by 2 | Viewed by 2368

Abstract

We report the structural and physical properties of a new organic Mott insulator (BEDT-BDT)AsF₆ (BEDT-BDT: benzo[1,2-g:4,5-g′]bis(thieno[2,3-b][1,4dithiin). This AsF₆ salt has the same structure as the PF₆ salt. Although the anions are disordered, the donor molecules form a θ-type arrangement. [...] Read more.

We report the structural and physical properties of a new organic Mott insulator (BEDT-BDT)AsF₆ (BEDT-BDT: benzo[1,2-g:4,5-g′]bis(thieno[2,3-b][1,4dithiin). This AsF₆ salt has the same structure as the PF₆ salt. Although the anions are disordered, the donor molecules form a θ-type arrangement. The temperature dependence of the resistivity exhibits semiconducting behavior. The static magnetic susceptibility follows Curie–Weiss law over a wide temperature range; however, below 25 K, the magnetic susceptibility is in agreement with a one-dimensional chain model with the exchange coupling J = 7.4 K. No structural phase transition was observed down to 93 K. At 270 K, the Fermi surface calculated by the tight-binding approximation is a two-dimensional cylinder; however, it is significantly distorted at 93 K. This is because the dihedral angles between the BEDT-BDT molecules become larger due to lattice shrinkage at low temperatures, which results in a smaller transfer integral (t₁) along the stack direction. This slight change in the dihedral angle gives rise to a significant change in the electronic structure of the AsF₆ salt. Radical-cation salts using BEDT-BDT, in which the highest occupied molecular orbital does not have a dominant sign throughout the molecule, are sensitive to slight differences in the overlap between the molecules, and their electronic structures are more variable than those of conventional θ-type conductors. Full article

(This article belongs to the Special Issue Organic Conductors)

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10 pages, 3180 KiB

Open AccessArticle

Electric Double Layer Doping of Charge-Ordered Insulators α-(BEDT-TTF)₂I₃ and α-(BETS)₂I₃

by Yoshitaka Kawasugi, Hikaru Masuda, Jiang Pu, Taishi Takenobu, Hiroshi M. Yamamoto, Reizo Kato and Naoya Tajima

Crystals 2021, 11(7), 791; https://doi.org/10.3390/cryst11070791 - 7 Jul 2021

Cited by 3 | Viewed by 3437

Abstract

Field-effect transistors based on strongly correlated insulators are an excellent platform for studying the electronic phase transition and simultaneously developing phase transition transistors. Molecular conductors are suitable for phase transition transistors owing to the high tunability of the electronic states. Molecular Mott transistors [...] Read more.

Field-effect transistors based on strongly correlated insulators are an excellent platform for studying the electronic phase transition and simultaneously developing phase transition transistors. Molecular conductors are suitable for phase transition transistors owing to the high tunability of the electronic states. Molecular Mott transistors show field-induced phase transitions including superconducting transitions. However, their application to charge-ordered insulators is limited. In this study, we fabricated electric double layer transistors based on quarter-filled charge-ordered insulators α-(BEDT-TTF)₂I₃ and α-(BETS)₂I₃. We observed ambipolar field effects in both compounds where both electron and hole doping (up to the order of

10^{13} {cm}^{- 2}

) reduces the resistance by the band filling shift from the commensurate value. The maximum field-effect mobilities are approximately 10 and 55

{cm}^{2} / Vs

, and the gate-induced conductivities are 0.96 and

3.6 e^{2} / h

in α-(BEDT-TTF)₂I₃ and α-(BETS)₂I₃, respectively. However, gate-induced metallic conduction does not emerge. The gate voltage dependence of the activation energy in α-(BEDT-TTF)₂I₃ and the Hall resistance in α-(BETS)₂I₃ imply that the electric double layer doping in the present experimental setup induces hopping transport rather than band-like two-dimensional transport. Full article

(This article belongs to the Special Issue Organic Conductors)

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11 pages, 744 KiB

Open AccessArticle

Absence of Superconductivity in the Hubbard Dimer Model for κ-(BEDT-TTF)₂X

by Dipayan Roy, R. Torsten Clay and Sumit Mazumdar

Crystals 2021, 11(6), 580; https://doi.org/10.3390/cryst11060580 - 22 May 2021

Cited by 1 | Viewed by 2376

Abstract

In the most studied family of organic superconductors

κ

-(BEDT-TTF)

_{2}

X, the BEDT-TTF molecules that make up the conducting planes are coupled as dimers. For some anions X, an antiferromagnetic insulator is found at low temperatures adjacent to superconductivity. With an average [...] Read more.

In the most studied family of organic superconductors

κ

-(BEDT-TTF)

_{2}

X, the BEDT-TTF molecules that make up the conducting planes are coupled as dimers. For some anions X, an antiferromagnetic insulator is found at low temperatures adjacent to superconductivity. With an average of one hole carrier per dimer, the BEDT-TTF band is effectively

\frac{1}{2}

-filled. Numerous theories have suggested that fluctuations of the magnetic order can drive superconducting pairing in these models, even as direct calculations of superconducting pairing in monomer

\frac{1}{2}

-filled band models find no superconductivity. Here, we present accurate zero-temperature Density Matrix Renormalization Group (DMRG) calculations of a dimerized lattice with one hole per dimer. While we do find an antiferromagnetic state in our results, we find no evidence for superconducting pairing. This further demonstrates that magnetic fluctuations in the effective

\frac{1}{2}

-filled band approach do not drive superconductivity in these and related materials. Full article

(This article belongs to the Special Issue Organic Conductors)

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19 pages, 7667 KiB

Open AccessArticle

High-Pressure Crystal Structure and Unusual Magnetoresistance of a Single-Component Molecular Conductor [Pd(dddt)₂] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate)

by Hengbo Cui, Hamish H.-M. Yeung, Yoshitaka Kawasugi, Takaaki Minamidate, Lucy K. Saunders and Reizo Kato

Crystals 2021, 11(5), 534; https://doi.org/10.3390/cryst11050534 - 11 May 2021

Cited by 4 | Viewed by 2695

Abstract

A single-component molecular crystal [Pd(dddt)₂] has been shown to exhibit almost temperature-independent resistivity under high pressure, leading theoretical studies to propose it as a three-dimensional (3D) Dirac electron system. To obtain more experimental information about the high-pressure electronic states, detailed resistivity [...] Read more.

A single-component molecular crystal [Pd(dddt)₂] has been shown to exhibit almost temperature-independent resistivity under high pressure, leading theoretical studies to propose it as a three-dimensional (3D) Dirac electron system. To obtain more experimental information about the high-pressure electronic states, detailed resistivity measurements were performed, which show temperature-independent behavior at 13 GPa and then an upturn in the low temperature region at higher pressures. High-pressure single-crystal structure analysis was also performed for the first time, revealing the presence of pressure-induced structural disorder, which is possibly related to the changes in resistivity in the higher-pressure region. Calculations based on the disordered structure reveal that the Dirac cone state and semiconducting state coexist, indicating that the electronic state at high pressure is not a simple Dirac electron system as previously believed. Finally, the first measurements of magnetoresistance on [Pd(dddt)₂] under high pressure are reported, revealing unusual behavior that seems to originate from the Dirac electron state. Full article

(This article belongs to the Special Issue Organic Conductors)

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17 pages, 460 KiB

Open AccessArticle

Evolution of Shape and Volume Fraction of Superconducting Domains with Temperature and Anion Disorder in (TMTSF)₂ClO₄

by Kaushal K. Kesharpu, Vladislav D. Kochev and Pavel D. Grigoriev

Crystals 2021, 11(1), 72; https://doi.org/10.3390/cryst11010072 - 17 Jan 2021

Cited by 4 | Viewed by 2589

Abstract

In highly anisotropic organic superconductor (TMTSF)

_{2}

ClO

_{4}

, superconducting (SC) phase coexists with metallic and spin-density wave phases in the form of domains. Using the Maxwell-Garnett approximation (MGA), we calculate the volume ratio and estimate the shape of these embedded SC [...] Read more.

In highly anisotropic organic superconductor (TMTSF)

_{2}

ClO

_{4}

, superconducting (SC) phase coexists with metallic and spin-density wave phases in the form of domains. Using the Maxwell-Garnett approximation (MGA), we calculate the volume ratio and estimate the shape of these embedded SC domains from resistivity data at various temperature and anion disorder, controlled by the cooling rate or annealing time of (TMTSF)

_{2}

ClO

_{4}

samples. We found that the variation of cooling rate and of annealing time affect differently the shape of SC domains. In all cases the SC domains have oblate shape, being the shortest along the interlayer z-axis. This contradicts the widely assumed filamentary superconductivity along the z-axis, used to explain the anisotropic superconductivity onset. We show that anisotropic resistivity drop at the SC onset can be described by the analytical MGA theory with anisotropic background resistance, while the anisotropic

T_{c}

can be explained by considering a finite size and flat shape of the samples. Due to a flat/needle sample shape, the probability of percolation via SC domains is the highest along the shortest sample dimension (z-axis), and the lowest along the sample length (x-axis). Our theory can be applied to other heterogeneous superconductors, where the size d of SC domains is much larger than the SC coherence length

ξ

, e.g., cuprates, iron-based or organic superconductors. It is also applicable when the spin/charge-density wave domains are embedded inside a metallic background, or vice versa. Full article

(This article belongs to the Special Issue Organic Conductors)

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26 pages, 3743 KiB

Open AccessArticle

Crystal Structures and Electrical Resistivity of Three Exotic TMTSF Salts with I 3 − : Determination of Valence by DFT and MP2 Calculations

by Harukazu Yoshino, Yoshiki Iwasaki, Rika Tanaka, Yuka Tsujimoto and Chiaki Matsuoka

Crystals 2020, 10(12), 1119; https://doi.org/10.3390/cryst10121119 - 8 Dec 2020

Cited by 3 | Viewed by 2930

Abstract

Three novel organic conductors (TMTSF)

_{8}

(I

_{3}

)

_{5}

, (TMTSF)

_{5}

(I

_{3}

)

_{2}

, and (TMTSF)

_{4}

(I

_{3}

)

_{4}

·THF (THF = tetrahydrofuran) were synthesized and their crystal structures were characterized by X-ray diffraction analyses, where TMTSF [...] Read more.

Three novel organic conductors (TMTSF)

_{8}

(I

_{3}

)

_{5}

, (TMTSF)

_{5}

(I

_{3}

)

_{2}

, and (TMTSF)

_{4}

(I

_{3}

)

_{4}

·THF (THF = tetrahydrofuran) were synthesized and their crystal structures were characterized by X-ray diffraction analyses, where TMTSF denotes tetramethyltetraselenafulvalene. The crystals of both the (TMTSF)

_{8}

(I

_{3}

)

_{5}

and (TMTSF)

_{5}

(I

_{3}

)

_{2}

are composed of one-dimensional stacks of TMTSF trimers separated by TMTSF monomers. The crystal of the (TMTSF)

_{4}

(I

_{3}

)

_{4}

·THF is composed of the TMTSF tetramers and I

_{3}^{-}

tetramers; and regarded as the elongated rock-salt structure. The electrical conductivity of the (TMTSF)

_{8}

(I

_{3}

)

_{5}

and (TMTSF)

_{5}

(I

_{3}

)

_{2}

is about 60 and 50 S·cm

^{- 1}

at room temperature, respectively. The electrical resistivity of (TMTSF)

_{8}

(I

_{3}

)

_{5}

is weakly metallic below room temperature and rapidly increases below 88 and 53 K on cooling suggesting two possible phase transitions. The electrical resistivity of (TMTSF)

_{5}

(I

_{3}

)

_{2}

is semiconducting below room temperature but shows an anomaly around 190 K, below which the activation energy becomes small. The application of hydrostatic pressure up to 1.7 GPa do not change these behaviors of (TMTSF)

_{8}

(I

_{3}

)

_{5}

and (TMTSF)

_{5}

(I

_{3}

)

_{2}

very much. A method to evaluate the non-integer valence of crystallographically independent TMTSF molecules is developed by using the DFT (density-functional-theory) and MP2 (Hartree–Fock calculations followed by Møller–Plesset correlation energy calculations truncated at second order) calculations. It is shown that the method gives the valence of the TMTSF molecules of the I

_{3}

salts consistent with their electrical properties. Full article

(This article belongs to the Special Issue Organic Conductors)

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10 pages, 2461 KiB

Open AccessArticle

Experimental and Modeling Study on the High-Performance p⁺⁺-GaAs/n⁺⁺-GaAs Tunnel Junctions with Silicon and Tellurium Co-Doped InGaAs Quantum Well Inserted

by Yudan Gou, Jun Wang, Yang Cheng, Yintao Guo, Xiao Xiao, Heng Liu, Shaoyang Tan, Li Zhou, Huomu Yang, Guoliang Deng and Shouhuan Zhou

Crystals 2020, 10(12), 1092; https://doi.org/10.3390/cryst10121092 - 28 Nov 2020

Cited by 10 | Viewed by 2844

Abstract

The development of high-performance tunnel junctions is critical for achieving high efficiency in multi-junction solar cells (MJSC) that can operate at high concentrations. We investigate silicon and tellurium co-doping of InGaAs quantum well inserts in p⁺⁺-GaAs/n⁺⁺-GaAs tunnel junctions and [...] Read more.

The development of high-performance tunnel junctions is critical for achieving high efficiency in multi-junction solar cells (MJSC) that can operate at high concentrations. We investigate silicon and tellurium co-doping of InGaAs quantum well inserts in p⁺⁺-GaAs/n⁺⁺-GaAs tunnel junctions and report a peak current density as high as 5839 A cm⁻² with a series resistance of 5.86 × 10⁻⁵ Ω cm². In addition, we discuss how device performance is affected by the growth temperature, thickness, and V/III ratio in the InGaAs layer. A simulation model indicates that the contribution of trap-assisted tunneling enhances carrier tunneling. Full article

(This article belongs to the Special Issue Organic Conductors)

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18 pages, 1165 KiB

Open AccessArticle

Deuteration Effects on the Transport Properties of (TMTTF)₂X Salts

by Andrea Rohwer, Martin Dressel and Toshikazu Nakamura

Crystals 2020, 10(12), 1085; https://doi.org/10.3390/cryst10121085 - 27 Nov 2020

Cited by 4 | Viewed by 2455

Abstract

The electronic properties in the quasi-one-dimensional Fabre salts are strongly affected by electronic correlations along the molecular stacks, but also by the interactions with the anions located in a cage that is formed by the methyl end groups. We systematically compare the charge [...] Read more.

The electronic properties in the quasi-one-dimensional Fabre salts are strongly affected by electronic correlations along the molecular stacks, but also by the interactions with the anions located in a cage that is formed by the methyl end groups. We systematically compare the charge transport in deuterated and protonated (TMTTF)

_{2}

X salts with the anions X = Br, PF

_{6}

, SbF

_{6}

, and ClO

_{4}

, ranging from Mott and Efros–Shklovskii variable-range hopping to activated band transport with a temperature dependent energy gap. The strong dependence of charge localization and ordering on the anion size and deuteration confirms the subtle structural involvement of the anions in the charge transport along the TMTTF stack. Full article

(This article belongs to the Special Issue Organic Conductors)

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11 pages, 885 KiB

Open AccessArticle

Synthesis, Structure and Physical Properties of (trans-TTF-py₂)_1.5(PF₆)·EtOH: A Molecular Conductor with Weak CH∙∙∙N Hydrogen Bondings

by Shohei Koyama, Morio Kawai, Shinya Takaishi, Masahiro Yamashita, Norihisa Hoshino, Tomoyuki Akutagawa, Manabu Kanno and Hiroaki Iguchi

Crystals 2020, 10(12), 1081; https://doi.org/10.3390/cryst10121081 - 26 Nov 2020

Cited by 3 | Viewed by 3179

Abstract

The studies of crystal structures with hydrogen bonds have been actively pursued because of their moderate stabilization energy for constructing unique structures. In this study, we synthesized a molecular conductor based on 2,6-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene (trans-TTF-py₂). Two pyridyl groups were introduced into the [...] Read more.

The studies of crystal structures with hydrogen bonds have been actively pursued because of their moderate stabilization energy for constructing unique structures. In this study, we synthesized a molecular conductor based on 2,6-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene (trans-TTF-py₂). Two pyridyl groups were introduced into the TTF skeleton toward the structural exploration in TTF-based molecular conductors involved by hydrogen bonds. In the obtained molecular conductor, (trans-TTF-py₂)_1.5(PF₆)·EtOH, short contacts between the pyridyl group and the hydrogen atom of the TTF skeleton were observed, indicating that hydrogen bonding interactions were introduced in the crystal structure. Spectroscopic measurements and conductivity measurement revealed semiconducting behavior derived from π-stacked trans-TTF-py₂ radical in the crystal structure. Finally, these results are discussed with the quantified hydrogen bonding stabilization energy, and the band calculation of the crystal obtained from density functional theory calculation. Full article

(This article belongs to the Special Issue Organic Conductors)

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14 pages, 2956 KiB

Open AccessArticle

Chiral Conducting Me-EDT-TTF and Et-EDT-TTF-Based Radical Cation Salts with the Perchlorate Anion

by Nabil Mroweh, Pascale Auban-Senzier, Nicolas Vanthuyne, Elsa B. Lopes, Manuel Almeida, Enric Canadell and Narcis Avarvari

Crystals 2020, 10(11), 1069; https://doi.org/10.3390/cryst10111069 - 23 Nov 2020

Cited by 8 | Viewed by 3007

Abstract

Introduction of chirality in the field of molecular conductors has received increasing interest in recent years in the frame of modulation of the crystal packing, and hence conducting properties, with the number of stereogenic centers and absolute configuration, e.g., racemic or enantiopure forms. [...] Read more.

Introduction of chirality in the field of molecular conductors has received increasing interest in recent years in the frame of modulation of the crystal packing, and hence conducting properties, with the number of stereogenic centers and absolute configuration, e.g., racemic or enantiopure forms. Here, we describe the preparation by electrocrystallization of chiral radical cation salts, based on the donors methyl-ethylenedithio-tetrathiafulvalene (Me-EDT-TTF) 1 and ethyl-ethylenedithio-tetrathiafulvalene (Et-EDT-TTF) 2 containing one stereogenic center, with the perchlorate anion. Donor 1 provided the series of crystalline materials [(rac)-1]ClO₄, [(S)-1]₂ClO₄ and [(R)-1]₂ClO₄, while for donor 2 only the 1:1 salts [(rac)-2]ClO₄ and [(R)-2]ClO₄ could be prepared as suitable single crystals for X-ray analysis. The enantiopure salts of 1 show β-type packing and metallic conductivity in the high temperature regime, with room temperature conductivity values of 5–10 S cm⁻¹, whereas compound [(rac)-2]ClO₄ is a very poor semiconductor. Tight-binding extended Hückel band structure calculations support the metallic conductivity of the enantiopure salts of 1 and suggest that small structural changes, possibly induced by thermal contraction or pressure, could lead to a pseudo-elliptic closed Fermi surface, typical for a 2D metal. Full article

(This article belongs to the Special Issue Organic Conductors)

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9 pages, 1572 KiB

Open AccessArticle

Single Crystal Heat Capacity Measurement of Charge Glass Compound θ-(BEDT-TTF)₂CsZn(SCN)₄ Performed under Current and Voltage Application

by Kosei Hino, Tetsuya Nomoto, Satoshi Yamashita and Yasuhiro Nakazawa

Crystals 2020, 10(11), 1060; https://doi.org/10.3390/cryst10111060 - 21 Nov 2020

Cited by 1 | Viewed by 2625

Abstract

Heat capacity measurements of θ-(BEDT-TTF)₂CsZn(SCN)₄ in its non-equilibrium electronic states induced by applying electric currents and voltages were performed by a modified relaxation calorimetry technique. We developed a single crystal heat capacity measurements system by which the Joule heating produced [...] Read more.

Heat capacity measurements of θ-(BEDT-TTF)₂CsZn(SCN)₄ in its non-equilibrium electronic states induced by applying electric currents and voltages were performed by a modified relaxation calorimetry technique. We developed a single crystal heat capacity measurements system by which the Joule heating produced in samples by external currents and voltages can be balanced with the cooling power to make a non-equilibrium steady state. Although temperature versus time profiles in the relaxation process in calorimetry can be obtained as exponential curves as in the usual relaxation technique, we found that the change of resistivity that occurs during the heating and relaxation process should be taken into account in analyzing the data. By correcting this factor in the analyses, we succeeded in evaluating absolute values of C_p(I) and C_p(V) in these non-equilibrium states. The experiments up to 150 μA and the constant voltage of 20 mV do not induce visible change in the structure of the Boson peak in C_pT⁻³ vs. T suggestive of the glassy ground state of phonons. Although the suppression of the short-range fluctuations of the charge density has been reported, it does not seriously affect the glassy phonons in this current range. Full article

(This article belongs to the Special Issue Organic Conductors)

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7 pages, 1031 KiB

Open AccessArticle

Microscopic Observation of π Spin Polarization by d Localized Spin in λ Type BETS Based Organic Superconductors

by Ko-ichi Hiraki, Toshihiro Takahashi, Hiroshi Akiba, Yutaka Nishio and Biao Zhou

Crystals 2020, 10(11), 1055; https://doi.org/10.3390/cryst10111055 - 20 Nov 2020

Cited by 2 | Viewed by 2000

Abstract

In this study, ⁷⁷Se NMR measurements were carried out to detect the π spin polarization of the organic BETS (BETS = Bis(ethylenedithio)tetraselenafulvalene) molecule of the field induced superconductor, λ-(BETS)₂Fe_1−xGa_xCl₄, which shows a superconducting transition [...] Read more.

In this study, ⁷⁷Se NMR measurements were carried out to detect the π spin polarization of the organic BETS (BETS = Bis(ethylenedithio)tetraselenafulvalene) molecule of the field induced superconductor, λ-(BETS)₂Fe_1−xGa_xCl₄, which shows a superconducting transition at relatively low magnetic field compared to the non-doped λ-(BETS)₂FeCl₄. From the analysis of the NMR spectrum at low temperature, it was clarified that the exchange interaction between π and 3d spins in the Ga doping system is smaller than that in the Fe salt. It is also clarified that the conduction π spins feel the “averaged” exchange field from the localized 3d spins at the dilute Fe sites. Full article

(This article belongs to the Special Issue Organic Conductors)

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12 pages, 3096 KiB

Open AccessArticle

Syntheses, Structures, and Physical Properties of Neutral Gold Dithiolate Complex, [Au(etdt)₂]·THF

by Kazuha Sakaguchi, Biao Zhou, Yuki Idobata, Hajime Kamebuchi and Akiko Kobayashi

Crystals 2020, 10(11), 1001; https://doi.org/10.3390/cryst10111001 - 4 Nov 2020

Cited by 2 | Viewed by 2417

Abstract

In order to develop new types of single-component molecular conductors with novel electronic structures and physical properties, the neutral gold dithiolate complex with an etdt (= ethylenedithiotetrathiafulvalenedithiolate) ligand, [Au(etdt)₂] was prepared. However, unlike the reported single-component molecular metals, the neutral gold [...] Read more.

In order to develop new types of single-component molecular conductors with novel electronic structures and physical properties, the neutral gold dithiolate complex with an etdt (= ethylenedithiotetrathiafulvalenedithiolate) ligand, [Au(etdt)₂] was prepared. However, unlike the reported single-component molecular metals, the neutral gold complex [Au(etdt)₂]·THF (2) contains a solvent molecule of tetrahydrofuran (THF). The crystals of 2 form a two-dimensional conducting layer structure, which are separated by the terminal ethylene groups and THF molecules. The fairly high room-temperature conductivity of 0.2 S/cm and semiconducting behavior with a low activation energy of 0.1 eV of 2, is consistent with the result of the density functional theory band structure calculations. The observed non-magnetic behavior of 2 is caused from the dimeric structure of [Au(etdt)₂] molecules. Full article

(This article belongs to the Special Issue Organic Conductors)

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14 pages, 4813 KiB

Open AccessEditor’s ChoiceArticle

Direct Observation of Molecular Orbitals Using Synchrotron X-ray Diffraction

by Shunsuke Kitou, Yuto Hosogi, Ryo Kitaura, Toshio Naito, Toshikazu Nakamura and Hiroshi Sawa

Crystals 2020, 10(11), 998; https://doi.org/10.3390/cryst10110998 - 3 Nov 2020

Cited by 9 | Viewed by 5025

Abstract

The physical properties of molecular crystals are governed by the frontier orbitals of molecules. A molecular orbital, which is formed by superposing the atomic orbitals of constituent elements, has complicated degrees of freedom in the crystal because of the influence of electron correlation [...] Read more.

The physical properties of molecular crystals are governed by the frontier orbitals of molecules. A molecular orbital, which is formed by superposing the atomic orbitals of constituent elements, has complicated degrees of freedom in the crystal because of the influence of electron correlation and crystal field. Therefore, in general, it is difficult to experimentally observe the whole picture of a frontier orbital. Here, we introduce a new method called “core differential Fourier synthesis” (CDFS) using synchrotron X-ray diffraction to observe the valence electron density in materials. By observing the valence electrons occupied in molecular orbitals, the orbital state can be directly determined in a real space. In this study, we applied the CDFS method to molecular materials such as diamond, C₆₀ fullerene, (MV)I₂, and (TMTTF)₂X. Our results not only demonstrate the typical orbital states in some materials, but also provide a new method for studying intramolecular degrees of freedom. Full article

(This article belongs to the Special Issue Organic Conductors)

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24 pages, 1256 KiB

Open AccessArticle

One-Dimensional Alternating Extended Hubbard Model at Quarter-Filling and Its Applications to Structural Instabilities of Organic Conductors

by M. Ménard and C. Bourbonnais

Crystals 2020, 10(10), 942; https://doi.org/10.3390/cryst10100942 - 16 Oct 2020

Cited by 5 | Viewed by 2770

Abstract

The one-dimensional extended Hubbard model with lattice dimerization and alternated site potentials is analyzed using the renormalization group method. The coupling of electrons to structural degrees of freedom such as the anion lattice and acoustic phonons is investigated to obtain the possible instabilities [...] Read more.

The one-dimensional extended Hubbard model with lattice dimerization and alternated site potentials is analyzed using the renormalization group method. The coupling of electrons to structural degrees of freedom such as the anion lattice and acoustic phonons is investigated to obtain the possible instabilities against the formation of lattice superstructures. Applications of the theory to anionic and spin-Peierls instabilities in the Fabre and Bechgaard salts series of organic conductors and ordered alloys are presented and discussed. Full article

(This article belongs to the Special Issue Organic Conductors)

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17 pages, 1373 KiB

Open AccessEditor’s ChoiceArticle

Electric Transport of Nodal Line Semimetals in Single-Component Molecular Conductors

by Yoshikazu Suzumura, Reizo Kato and Masao Ogata

Crystals 2020, 10(10), 862; https://doi.org/10.3390/cryst10100862 - 24 Sep 2020

Cited by 5 | Viewed by 2736

Abstract

We examine an effect of acoustic phonon scattering on the electric conductivity of a single-component molecular conductor [Pd(dddt)

_{2}

] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) with a half-filled band by applying the previous calculation in a two-dimensional model with Dirac cone [Phys. Rev. B. 98, [...] Read more.

We examine an effect of acoustic phonon scattering on the electric conductivity of a single-component molecular conductor [Pd(dddt)

_{2}

] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) with a half-filled band by applying the previous calculation in a two-dimensional model with Dirac cone [Phys. Rev. B. 98, 161205 (2018)], wherethe electric transport by the impurity scattering exhibits a noticeable interplay of the Dirac cone and the phonon scattering, resulting in maximum of the conductivity with increasing temperature. The conductor shows a nodal line semimetal, where the band crossing of HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) provides a loop of Dirac points located close to the Fermi energy followed by the density of states (DOS) similar to that of a two-dimensional Dirac cone. Using a tight-binding (TB) model [arXiv:2008.09277], which was obtained using the crystal structure observed from a recent X ray diffraction experiment under pressure, it is shown that the obtained conductivity explains reasonably the anomalous behavior in [Pd(dddt)

_{2}

] exhibiting temperature-independent resistivity at finite temperatures. This paper demonstrates a crucial role of the acoustic phonon scattering at finite temperatures in the electric conductivity of Dirac electrons. The present theoretical results of conductivity are compared with those of the experiments. Full article

(This article belongs to the Special Issue Organic Conductors)

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9 pages, 2492 KiB

Open AccessArticle

Gas-Dependent Reversible Structural and Magnetic Transformation between Two Ladder Compounds

by Jun Manabe, Kazuki Nishida, Xiao Zhang, Yuki Nakano, Masaru Fujibayashi, Goulven Cosquer, Katsuya Inoue, Seiya Shimono, Hiroki Ishibashi, Yoshiki Kubota, Misaki Shiga, Ryo Tsunashima, Yoko Tatewaki and Sadafumi Nishihara

Crystals 2020, 10(9), 841; https://doi.org/10.3390/cryst10090841 - 19 Sep 2020

Cited by 2 | Viewed by 3084

Abstract

We report reversible structural transformation that occurs in two ladder compounds: Cu₂CO₃(ClO₄)₂(NH₃)₆ (1) and Cu₂CO₃(ClO₄)₂(NH₃)₅(H₂O) ( [...] Read more.

We report reversible structural transformation that occurs in two ladder compounds: Cu₂CO₃(ClO₄)₂(NH₃)₆ (1) and Cu₂CO₃(ClO₄)₂(NH₃)₅(H₂O) (2), when they are exposed to gaseous vapors. The ladder structures of both 1 and 2 consist of two Cu²⁺ ions and one CO₃²⁻ ion. In 1, the Cu²⁺ ions are coordinated by three NH₃ molecules on each side, while those in 2 are coordinated by three NH₃ molecules on one side, and two NH₃ molecules and one H₂O molecule on the other side. We demonstrated reversible transformation of 1 and 2 via the exposure of 1 to H₂O vapor and the exposure of 2 to NH₃ vapor using a simple bench-scale method. The minor structural change observed led to a significant difference in physical properties, which we observed using several methods. Full article

(This article belongs to the Special Issue Organic Conductors)

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13 pages, 4446 KiB

Open AccessArticle

Structures and Properties of New Organic Conductors: BEDT-TTF, BEST and BETS Salts of the HOC₂H₄SO₃⁻ Anion

by Hiroki Akutsu, Yuta Koyama, Scott S. Turner, Keigo Furuta and Yasuhiro Nakazawa

Crystals 2020, 10(9), 775; https://doi.org/10.3390/cryst10090775 - 1 Sep 2020

Cited by 7 | Viewed by 3172

Abstract

New bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)-, bis(ethylenediseleno)tetrathiafulvalene (BEST)- and bis(ethylenedithio)tetraselenafulvalene (BETS)-based organic charge-transfer (CT) salts—α-(BEDT-TTF)₃(HOC₂H₄SO₃)₂ (1), β-(BEST)₃(HOC₂H₄SO₃)₂·H₂O (2) and α-(BETS)₂ [...] Read more.

New bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)-, bis(ethylenediseleno)tetrathiafulvalene (BEST)- and bis(ethylenedithio)tetraselenafulvalene (BETS)-based organic charge-transfer (CT) salts—α-(BEDT-TTF)₃(HOC₂H₄SO₃)₂ (1), β-(BEST)₃(HOC₂H₄SO₃)₂·H₂O (2) and α-(BETS)₂(HOC₂H₄SO₃)·H₂O (3)—have been prepared. Salts 1 and 2 show semiconducting behaviour. Salt 3, which is almost isostructural to α-(BETS)₂I₃, shows metallic behaviour down to 70 K and then shows a broader metal–insulator transition than that of α-(BETS)₂I₃. The reason for the difference in behaviour is estimated by the comparison of the Madelung energies of the full set of patterns of possible donor’s charge-ordered and anion’s disordered states. Full article

(This article belongs to the Special Issue Organic Conductors)

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8 pages, 11922 KiB

Open AccessArticle

Formation of Three-Dimensional Electronic Networks Using Axially Ligated Metal Phthalocyanines as Stable Neutral Radicals

by Ryoya Sato and Masaki Matsuda

Crystals 2020, 10(9), 747; https://doi.org/10.3390/cryst10090747 - 24 Aug 2020

Cited by 3 | Viewed by 2157

Abstract

Organic π-radical crystals are potential single-component molecular conductors, as they involve charge carriers. We fabricated new organic π-radical crystals using axially ligated metal phthalocyanine anions ([M^III(Pc)L₂]⁻) as starting materials. Electrochemical oxidation of [M^III(Pc)L₂] [...] Read more.

Organic π-radical crystals are potential single-component molecular conductors, as they involve charge carriers. We fabricated new organic π-radical crystals using axially ligated metal phthalocyanine anions ([M^III(Pc)L₂]⁻) as starting materials. Electrochemical oxidation of [M^III(Pc)L₂]⁻ afforded single crystals of organic π-radicals of the type M^III(Pc)Cl₂·THF (M = Co or Fe, THF = tetrahydrofuran), where the π-conjugated macrocyclic phthalocyanine ligand is one-electron oxidized. The X-ray crystal structure analysis revealed that M^III(Pc)Cl₂ formed three-dimensional networks with π-π overlaps. The electrical resistivities of Co^III(Pc)Cl₂·THF and Fe^III(Pc)Cl₂·THF at room temperature along the a-axis were 6 × 10² and 6 × 10³ Ω cm, respectively, and were almost isotropic, meaning that M^III(Pc)Cl₂·THF had three-dimensional electronic systems. Full article

(This article belongs to the Special Issue Organic Conductors)

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Review

Jump to: Editorial, Research

53 pages, 4735 KiB

Open AccessReview

Modern History of Organic Conductors: An Overview

by Toshio Naito

Crystals 2021, 11(7), 838; https://doi.org/10.3390/cryst11070838 - 20 Jul 2021

Cited by 26 | Viewed by 6397

Abstract

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have [...] Read more.

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field. Full article

(This article belongs to the Special Issue Organic Conductors)

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16 pages, 2044 KiB

Open AccessEditor’s ChoiceReview

Interplay between Vortex Dynamics and Superconducting Gap Structure in Layered Organic Superconductors

by Syuma Yasuzuka

Crystals 2021, 11(6), 600; https://doi.org/10.3390/cryst11060600 - 26 May 2021

Cited by 2 | Viewed by 4418

Abstract

Layered organic superconductors motivate intense investigations because they provide various unexpected issues associated with their low dimensionality and the strong electron correlation. Since layered organic superconductors possess simple Fermi surface geometry and they often share similarities to the high temperature oxide superconductors and [...] Read more.

Layered organic superconductors motivate intense investigations because they provide various unexpected issues associated with their low dimensionality and the strong electron correlation. Since layered organic superconductors possess simple Fermi surface geometry and they often share similarities to the high temperature oxide superconductors and heavy fermion compounds, research on layered organic superconductors is suitable for understanding the essence and nature of strongly correlated electron systems. In strongly correlated electron systems, one of the central problems concerning the superconducting (SC) state is the symmetry of the SC gap, which is closely related to the paring mechanism. Thus, experimental determination of the SC gap structure is of essential importance. In this review, we present the experimental results for the in-plane angular variation of the flux-flow resistance in layered organic superconductors k-(ET)₂Cu(NCS)₂, β″-(ET)₂SF₅CH₂CF₂SO₃, and λ-(BETS)₂GaCl₄. The interplay between the vortex dynamics and nodal structures is discussed for these superconductors. Full article

(This article belongs to the Special Issue Organic Conductors)

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