Special Issue "Advances in Organic Conductors and Superconductors"

A special issue of Crystals (ISSN 2073-4352).

Deadline for manuscript submissions: closed (31 March 2018)

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Special Issue Editor

Guest Editor
Prof. Dr. Martin Dressel

Universität Stuttgart, 1. Physikalisches Institut, Stuttgart, Germany
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Interests: condensed matter physics; solid state spectroscopy; correlated electron systems; low-dimensional materials; molecular conductors; superconductors; optical and magnetic properties; electrodynamics

Special Issue Information

Dear Colleagues,

Crystalline conductors and superconductors based on organic molecules are a rapidly progressing field of solid-state science, comprising chemists, and experimental and theoretical physicists from all around the world. In focus are solids with electronic properties governed by delocalized π-electrons. Although carbon-based materials of various shades have gained enormous interest in recent years; however, charge transfer salts are still paradigmatic in this field. Progress in molecular design is achieved via tiny but ingenious modifications, as well as by fundamentally different approaches. The wealth of exciting physical phenomena is unprecedented and could not be imagined when the field took off almost half a century ago. Organic low-dimensional conductors are prime examples of Luttinger liquids, exhibit a tendency toward Fermi surface instabilities, but can also be tuned across a dimension­a­lity driven phase diagram like no other system. Superconductivity comes at the border to ordered phases in the spin and charge sectors, and, at high fields, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state is well established. The interplay between charge and magnetic order is still under debate, but electronic ferroelectricity is well established. After decades of intense search, the spin liquid state was first discovered in organic conductors when the amount of geometrical frustration and electronic correlations are just right. They drive the metal and superconductor into an insulating Mott state, solely via electron–electron interactions. However, what do we know about the effect of disorder? Can we tune the electronic properties by pressure, by light, or by field? Research is still aiming at basic questions, but devices are not out of reach. These are currently open questions, as well as hot and timely topics. In 2012, Reizo Kato edited a seminal collection of 45 contributions on various aspects of “Molecular Conductors” as a Special Issue of Crystals, which, at present, has received more than 260 citations. The present Special Issue on “Advances in Organic Conductors and Superconductors” may become a status reports summarizing the progress achieved in the last five years.

Prof. Dr. Martin Dressel
Guest Editors

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Keywords

  • synthesis of novel molecules and crystal growth of new organic conductors
  • molecular superconductors
  • molecular Dirac electron systems and zero-gap conductors
  • Molecular π-d systems
  • organic FET, molecular electronics and spintronics
  • strongly correlated electron systems
  • metal-insulator transition, Mott transition
  • charge order, electronic ferroelectricity of molecular materials
  • molecular spin liquids, geometrical frustration
  • photo-induced phenomena, non-equilibrium physics
  • disorder effects in organic conductors
  • pressure effects
  • X-ray diffraction
  • vibrational spectroscopy
  • optical properties
  • non-linear phenomena
  • transport properties
  • dielectric properties
  • magnetic properties
  • NMR and ESR
  • thermodynamic properties
  • electronic state calculations
  • theoretical models

Published Papers (19 papers)

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Editorial

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Open AccessEditorial Advances in Organic Conductors and Superconductors
Crystals 2018, 8(9), 332; https://doi.org/10.3390/cryst8090332
Received: 18 August 2018 / Accepted: 20 August 2018 / Published: 21 August 2018
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Abstract
Crystalline conductors and superconductors based on organic molecules are a rapidly progressing field of solid-state science, involving chemists, and experimental and theoretical physicists from all around the world[...] Full article

Research

Jump to: Editorial, Review

Open AccessArticle Inhomogeneous Superconductivity in Organic and Related Superconductors
Crystals 2018, 8(7), 285; https://doi.org/10.3390/cryst8070285
Received: 16 May 2018 / Revised: 23 June 2018 / Accepted: 24 June 2018 / Published: 11 July 2018
Cited by 1 | PDF Full-text (2950 KB) | HTML Full-text | XML Full-text
Abstract
Evidence of inhomogeneous superconductivity, in this case superconductivity with a spatially modulated superconducting order parameter, has now been found in many materials and by many measurement methods. Although the evidence is strong, it is circumstantial in the organic superconductors, scant in the pnictides,
[...] Read more.
Evidence of inhomogeneous superconductivity, in this case superconductivity with a spatially modulated superconducting order parameter, has now been found in many materials and by many measurement methods. Although the evidence is strong, it is circumstantial in the organic superconductors, scant in the pnictides, and complex in the heavy Fermions. However, it is clear some form of exotic superconductivity exists at high fields and low temperatures in many electronically anisotropic superconductors. The evidence is reviewed in this article, and examples of similar measurements are compared across different families of superconductors. An effort is made to find a consistent way to measure the superconducting energy gap across all materials, and use this value to predict the Clogston–Chandrasakhar paramagnetic limit Hp. Methods for predicting the existence of inhomogeneous superconductivity are shown to work for the organic superconductors, and then used to suggest new materials to study. Full article
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Open AccessArticle Raman Scattering as a Probe of the Magnetic State of BEDT-TTF Based Mott Insulators
Crystals 2018, 8(6), 233; https://doi.org/10.3390/cryst8060233
Received: 14 March 2018 / Revised: 30 April 2018 / Accepted: 16 May 2018 / Published: 23 May 2018
Cited by 1 | PDF Full-text (501 KB) | HTML Full-text | XML Full-text
Abstract
Quasi-two-dimensional Mott insulators based on BEDT-TTF molecules have recently demonstrated a variety of exotic states, which originate from electron–electron correlations and geometrical frustration of the lattice. Among those states are a triangular S = 1/2 spin liquid and quantum dipole liquid. In this
[...] Read more.
Quasi-two-dimensional Mott insulators based on BEDT-TTF molecules have recently demonstrated a variety of exotic states, which originate from electron–electron correlations and geometrical frustration of the lattice. Among those states are a triangular S = 1/2 spin liquid and quantum dipole liquid. In this article, we show the power of Raman scattering technique to characterize magnetic and electronic excitations of these states. Our results demonstrate a distinction between a spectrum of magnetic excitations in a simple Mott insulator with antiferromagnetic interactions, and a spectrum of an insulator with an additional on-site charge degree of freedom. Full article
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Open AccessArticle Electrodynamics in Organic Dimer Insulators Close to Mott Critical Point
Crystals 2018, 8(5), 190; https://doi.org/10.3390/cryst8050190
Received: 26 March 2018 / Revised: 18 April 2018 / Accepted: 18 April 2018 / Published: 27 April 2018
Cited by 4 | PDF Full-text (3172 KB) | HTML Full-text | XML Full-text
Abstract
Organic layered charge-transfer salts κ -(BEDT-TTF) 2X form highly frustrated lattices of molecular dimers in which strong correlations give rise to Mott insulating states situated close to the metal-to-insulator phase boundary. The salts κ -(BEDT-TTF) 2 Cu 2 (CN) 3 and κ
[...] Read more.
Organic layered charge-transfer salts κ -(BEDT-TTF) 2 X form highly frustrated lattices of molecular dimers in which strong correlations give rise to Mott insulating states situated close to the metal-to-insulator phase boundary. The salts κ -(BEDT-TTF) 2 Cu 2 (CN) 3 and κ -(BEDT-TTF) 2 Ag 2 (CN) 3 have been considered as prime candidates for a quantum spin liquid, while κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl has been suggested as a prototypical charge-order-driven antiferromagnet. In this paper, we summarize and discuss several key results, including some not reported previously, obtained in search to clarify the competition of these two ground states. The origin of anomalous dielectric response found at low temperatures in all three salts is also discussed. We conclude by pointing out the relevant new insights into the role of frustration and random disorder in the suppression of magnetic ordering and formation of the spin liquid state. Full article
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Open AccessArticle Spatially Nonuniform Superconductivity in Quasi-Two-Dimensional Organic Charge-Transfer Salts
Crystals 2018, 8(5), 183; https://doi.org/10.3390/cryst8050183
Received: 14 February 2018 / Revised: 19 April 2018 / Accepted: 20 April 2018 / Published: 24 April 2018
Cited by 1 | PDF Full-text (1103 KB) | HTML Full-text | XML Full-text
Abstract
In the following, a brief overview on the recently found robust experimental evidence for the existence of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state in layered organic superconductors is given. These electronically quasi-two-dimensional (2D) clean-limit superconductors are ideally suited for observing FFLO states. Applying a magnetic
[...] Read more.
In the following, a brief overview on the recently found robust experimental evidence for the existence of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state in layered organic superconductors is given. These electronically quasi-two-dimensional (2D) clean-limit superconductors are ideally suited for observing FFLO states. Applying a magnetic field parallel to the layers suppresses orbital effects and superconductivity is observed beyond the Pauli paramagnetic limit. Both, thermodynamic as well as microscopic experimental data show the existence of an additional high-field low-temperature superconducting state having a one-dimensionally modulated order parameter. Full article
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Open AccessArticle (BEDT-TTF)2Cu2(CN)3 Spin Liquid: Beyond the Average Structure
Crystals 2018, 8(4), 158; https://doi.org/10.3390/cryst8040158
Received: 27 February 2018 / Revised: 29 March 2018 / Accepted: 30 March 2018 / Published: 4 April 2018
Cited by 4 | PDF Full-text (74664 KB) | HTML Full-text | XML Full-text
Abstract
We present here the first accurate determination of the exact structure of κ-(BEDT-TTF)2Cu2(CN)3. Not only did we show that the room temperature structure used over the last twenty years was incorrect, but we were also able to
[...] Read more.
We present here the first accurate determination of the exact structure of κ-(BEDT-TTF)2Cu2(CN)3. Not only did we show that the room temperature structure used over the last twenty years was incorrect, but we were also able to correctly and precisely determine it. The results of our work provide evidence that the structure presents a triclinic symmetry with two non-equivalent dimers in the unit cell, which implies a charge disproportionation between the dimers. However, structural refinement shows that the charge disproportionation is quite weak at room temperature. Full article
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Open AccessArticle β”-(CNB-EDT-TTF)4BF4; Anion Disorder Effects in Bilayer Molecular Metals
Crystals 2018, 8(4), 142; https://doi.org/10.3390/cryst8040142
Received: 3 March 2018 / Revised: 17 March 2018 / Accepted: 18 March 2018 / Published: 21 March 2018
Cited by 1 | PDF Full-text (2126 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The preparation and characterization of new salts based on the dissymmetrical TTF derivative CNB-EDT-TTF (cyanobenzene-ethylenedithio-tetrathiafulvalene) and BF4 anions, are reported. Depending on the electrocrystallization conditions salts with different stoichiometries, (CNB-EDT-TTF)BF4 and β”-(CNB-EDT-TTF)4BF4, can be obtained.
[...] Read more.
The preparation and characterization of new salts based on the dissymmetrical TTF derivative CNB-EDT-TTF (cyanobenzene-ethylenedithio-tetrathiafulvalene) and BF4 anions, are reported. Depending on the electrocrystallization conditions salts with different stoichiometries, (CNB-EDT-TTF)BF4 and β”-(CNB-EDT-TTF)4BF4, can be obtained. The 1:1 salt is an electrical insulator isostructural to the ClO4 analogue previously described. The 4:1 salt is a new member of the family of 2D metals of this donor with different small anions X, (CNB-EDT-TTF)4X, characterized by a bilayer arrangement of the donors and it was obtained in a monoclinic polymorph with a β”-type donor packing pattern. The small anions in this compound are severely disordered between the donor bilayers, which present slightly larger lattice parameters than the isostructural ClO4 analogue. Both electrical conductivity and thermoelectric power measurements in single crystals denote metallic properties as predicted by electronic band structure calculations. As a consequence of the anion disorder the metallic regime of the electrical conductivity denotes electronic localization effects with a progressive increase of resistivity below ~25 K. Because of the larger lattice parameters the intermolecular interactions and electronic bandwidth are decreased compared to other (CNB-EDT-TTF)4X salts. The large and positive thermoelectric power S of this compound (~110 μV/K in the range 100–330 K) and its electrical conductivity σ = 20 S/cm at room temperature lead to a power factor S2σ = 24 μW/K2m, quite large among molecular conductors, placing these compounds as potential candidates for thermoelectric materials. Full article
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Open AccessArticle Synthesis and Characterization of Charge Transfer Salts Based on [M(dcdmp)2] (M = Au, Cu and Ni) with TTF Type Donors
Crystals 2018, 8(3), 141; https://doi.org/10.3390/cryst8030141
Received: 6 March 2018 / Revised: 16 March 2018 / Accepted: 17 March 2018 / Published: 20 March 2018
Cited by 1 | PDF Full-text (8864 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The charge transfer salts α-DT-TTF[Au(dcdmp)2] (1), BET-TTF[Au(dcdmp)2] (2M and 2T), α-DT-TTF[Cu(dcdmp)2] (3), ET[Cu(dcdmp)2] (4), (BET-TTF)2[Cu(dcdmp)2] (5), (ET)2[Ni(dcdmp)2]
[...] Read more.
The charge transfer salts α-DT-TTF[Au(dcdmp)2] (1), BET-TTF[Au(dcdmp)2] (2M and 2T), α-DT-TTF[Cu(dcdmp)2] (3), ET[Cu(dcdmp)2] (4), (BET-TTF)2[Cu(dcdmp)2] (5), (ET)2[Ni(dcdmp)2] (6), and α-mtdt[Cu(dcdmp)2] (7) were obtained by electrocrystallization of different electron donor molecules derived from TTF (α-DT-TTF = alpha-dithiophene-tetrathiafulvalene; BET-TTF = (bis(ethylenethio)tetrathiafulvalene; ET = bis(ethylenedithio)-tetrathiafulvalene; α-mtdt = alpha-methylthiophenetetrathiafulvalene) in the presence of transition metal complex [M(dcdmp)2] (M = Au (III), Cu (III) and Ni (II)) (dcdmp = 2,3-dicyano-5,6-dimercaptopyrazine). Compounds 1 and 2 (2M and 2T) have a similar packing pattern composed of mixed stacks of alternating donor-acceptor molecules. For (BET-TTF)[Au(dcdmp)2] two different crystal structures (2M and 2T) were obtained indicating polymorphism. Compounds 3 and 4 are isostructural being composed of zigzag chains of alternating donor and acceptor molecules. The salts with a 2:1 stoichiometry, (BET-TTF)2[Cu(dcdmp)2] (5), and (ET)2[Ni(dcdmp)2] (6) present the donor molecules fully oxidized and [M(dcdmp)2] (M = Ni and Cu) in a dianionic state. The salt of the dissymmetric donor α-mtdt with [Cu(dcdmp)2], α-mtdt[Cu(dcdmp)2] (7) has a crystal structure composed of segregated donor stacks that are positioned in a head-to-head fashion and alternate with the anion stacks. All charge transfer salts (17) are modest semiconductors with conductivities in the range 10−1–10−5 S/cm, with the highest values obtained in α-DT-TTF salts, compounds 1 and 3. Full article
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Open AccessArticle Temperature Dependence of Crystal Structures and Band Parameters in Quantum Spin Liquid β′-EtMe3Sb[Pd(dmit)2]2 and Related Materials
Crystals 2018, 8(3), 138; https://doi.org/10.3390/cryst8030138
Received: 27 February 2018 / Revised: 14 March 2018 / Accepted: 15 March 2018 / Published: 17 March 2018
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Abstract
In an isostructural series of anion radical salts β′-(Me4-xEtxZ)[Pd(dmit)2]2 (Z = P, As, Sb; x = 0, 1, 2), [Pd(dmit)2]2 units form a two-dimensional Mott insulator layer with a quasi-isosceles triangular lattice.
[...] Read more.
In an isostructural series of anion radical salts β′-(Me4-xEtxZ)[Pd(dmit)2]2 (Z = P, As, Sb; x = 0, 1, 2), [Pd(dmit)2]2 units form a two-dimensional Mott insulator layer with a quasi-isosceles triangular lattice. The anisotropy of the triangular lattice is characterized by a ratio of interdimer transfer integrals, t′/t. The crystal structures of EtMe3Sb, Me4Sb, Me4As, and Et2Me2As salts were determined in the range of 5–295 K by the single crystal X-ray diffraction technique. Interdimer transfer integrals, Fermi surface, and band structures at low temperatures were calculated by the tight binding method and the first-principles density-functional theory (DFT) method based on experimentally obtained crystal structures. Interdimer transfer integrals increased with lowering temperature. At 5 K, the ratio t′/t decreased by about 15% from the room temperature value in every salt. The relationship between the transfer integrals and interdimer S⋯S distances indicated that the change of the t′/t value with temperature was due to a thermal contraction, rather than the arch-shaped molecular distortion of the Pd(dmit)2 molecule associated with the cation dependence of t′/t. Full article
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Open AccessArticle Optical Conductivity in a Two-Dimensional Extended Hubbard Model for an Organic Dirac Electron System α-(BEDT-TTF)2I3
Crystals 2018, 8(3), 137; https://doi.org/10.3390/cryst8030137
Received: 25 February 2018 / Revised: 13 March 2018 / Accepted: 14 March 2018 / Published: 16 March 2018
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Abstract
The optical conductivity in the charge order phase is calculated in the two-dimensional extended Hubbard model describing an organic Dirac electron system α-(BEDT-TTF)2I3 using the mean field theory and the Nakano-Kubo formula. Because the interband excitation is characteristic in
[...] Read more.
The optical conductivity in the charge order phase is calculated in the two-dimensional extended Hubbard model describing an organic Dirac electron system α -(BEDT-TTF) 2 I 3 using the mean field theory and the Nakano-Kubo formula. Because the interband excitation is characteristic in a two-dimensional Dirac electron system, a peak structure is found above the charge order gap. It is shown that the peak structure originates from the Van Hove singularities of the conduction and valence bands, where those singularities are located at a saddle point between two Dirac cones in momentum space. The frequency of the peak structure exhibits drastic change in the vicinity of the charge order transition. Full article
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Open AccessArticle Structural and Electronic Properties of (TMTTF)2X Salts with Tetrahedral Anions
Crystals 2018, 8(3), 121; https://doi.org/10.3390/cryst8030121
Received: 14 February 2018 / Revised: 27 February 2018 / Accepted: 28 February 2018 / Published: 4 March 2018
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Abstract
Comprehensive measurements of the pressure- and temperature-dependent dc-transport are combined with dielectric spectroscopy and structural considerations in order to elucidate the charge and anion orderings in the quasi-one-dimensional charge-transfer salts (TMTTF)2X with non-centrosymmetric anions X = BF4, ClO4
[...] Read more.
Comprehensive measurements of the pressure- and temperature-dependent dc-transport are combined with dielectric spectroscopy and structural considerations in order to elucidate the charge and anion orderings in the quasi-one-dimensional charge-transfer salts (TMTTF) 2 X with non-centrosymmetric anions X = BF 4 , ClO 4 and ReO 4 . Upon applying hydrostatic pressure, the charge-order transition is suppressed in all three compounds, whereas the influence on the anion order clearly depends on the particular compound. A review of the structural properties paves the way for understanding the effect of the anions in their methyl cavities on the ordering. By determining the complex dielectric constant ϵ ^ ( ω , T ) in different directions we obtain valuable information on the contribution of the anions to the dielectric properties. For (TMTTF) 2 ClO 4 and (TMTTF) 2 ReO 4 , ϵ b exhibits an activated behavior of the relaxation time with activation energies similar to the gap measured in transport, indicating that the relaxation dynamics are determined by free charge carriers. Full article
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Open AccessArticle Specific Structural Disorder in an Anion Layer and Its Influence on Conducting Properties of New Crystals of the (BEDT-TTF)4A+[M3+(ox)3]G Family, Where G Is 2-Halopyridine; M Is Cr, Ga; A+ Is [K0.8(H3O)0.2]+
Crystals 2018, 8(2), 92; https://doi.org/10.3390/cryst8020092
Received: 24 January 2018 / Revised: 8 February 2018 / Accepted: 8 February 2018 / Published: 10 February 2018
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Abstract
New crystals (14) of organic conductors based on the radical cation salts of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with paramagnetic and diamagnetic tris(oxalato)metallate anions {A+[M3+(ox)3]3−G}2−, where M is Cr, Ga; G
[...] Read more.
New crystals (14) of organic conductors based on the radical cation salts of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with paramagnetic and diamagnetic tris(oxalato)metallate anions {A+[M3+(ox)3]3−G}2−, where M is Cr, Ga; G is 2-chloropyridine, 2-bromopyridine; and A+ is [K0.8(H3O)0.2]+ have been prepared and their crystal structure and transport properties were studied. All crystals belong to the monoclinic group of the (BEDT-TTF)4A+[M3+(ox)3]G family with β″-packing type of conducting BEDT-TTF layers. In contrast to the known superconducting crystals with M3+ = Fe3+ and G = 2-chloro- or 2-bromopyridine (Tc = 4.0–4.3 K), crystals with Cr3+ and Ga3+ ions exhibit metallic properties down to 0.5 K without superconducting transition. Upon cooling these crystals, the incommensurate superstructure appears, which has never been observed before in the numerous β″-salts of the family. In addition, orthorhombic (sp. group Pbca) semiconducting crystals α″-(BEDT-TTF)5[Ga(ox)3]·3.4·H2O·0.6 EtOH (5) were obtained. It is a new compound in the family of BEDT-TTF crystals with tris(oxalato)metallate anions. Full article
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Open AccessArticle Low-Temperature Lattice Effects in the Spin-Liquid Candidate κ-(BEDT-TTF)2Cu2(CN)3
Crystals 2018, 8(2), 87; https://doi.org/10.3390/cryst8020087
Received: 13 December 2017 / Revised: 29 January 2018 / Accepted: 3 February 2018 / Published: 6 February 2018
Cited by 2 | PDF Full-text (1559 KB) | HTML Full-text | XML Full-text
Abstract
The quasi-two-dimensional organic charge-transfer salt κ-(BEDT-TTF)2Cu2(CN)3 is one of the prime candidates for a quantum spin-liquid due the strong spin frustration of its anisotropic triangular lattice in combination with its proximity to the Mott transition. Despite intensive
[...] Read more.
The quasi-two-dimensional organic charge-transfer salt κ -(BEDT-TTF) 2 Cu 2 (CN) 3 is one of the prime candidates for a quantum spin-liquid due the strong spin frustration of its anisotropic triangular lattice in combination with its proximity to the Mott transition. Despite intensive investigations of the material’s low-temperature properties, several important questions remain to be answered. Particularly puzzling are the 6 K anomaly and the enigmatic effects observed in magnetic fields. Here we report on low-temperature measurements of lattice effects which were shown to be particularly strongly pronounced in this material (R. S. Manna et al., Phys. Rev. Lett. 2010, 104, 016403)). A special focus of our study lies on sample-to-sample variations of these effects and their implications on the interpretation of experimental data. By investigating overall nine single crystals from two different batches, we can state that there are considerable differences in the size of the second-order phase transition anomaly around 6 K, varying within a factor of 3. In addition, we find field-induced anomalies giving rise to pronounced features in the sample length for two out of these nine crystals for temperatures T < 9 K. We tentatively assign the latter effects to B-induced magnetic clusters suspected to nucleate around crystal imperfections. These B-induced effects are absent for the crystals where the 6 K anomaly is most strongly pronounced. The large lattice effects observed at 6 K are consistent with proposed pairing instabilities of fermionic excitations breaking the lattice symmetry. The strong sample-to-sample variation in the size of the phase transition anomaly suggests that the conversion of the fermions to bosons at the instability is only partial and to some extent influenced by not yet identified sample-specific parameters. Full article
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Open AccessArticle Effects of Disorder on the Pressure-Induced Mott Transition in κ-(BEDT-TTF)2Cu[N(CN)2]Cl
Crystals 2018, 8(1), 38; https://doi.org/10.3390/cryst8010038
Received: 14 December 2017 / Revised: 9 January 2018 / Accepted: 11 January 2018 / Published: 16 January 2018
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Abstract
We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt κ-(BEDT-TTF)2Cu[N(CN)2]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals
[...] Read more.
We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt κ -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals to X-ray irradiation. The crystals were then fine-tuned across the Mott transition by the application of continuously controllable He-gas pressure at low temperatures. Measurements of the thermal expansion and resistance show that the first-order character of the Mott transition prevails for low irradiation doses achieved by irradiation times up to 100 h. For these crystals with a moderate degree of disorder, we find a first-order transition line which ends in a second-order critical endpoint, akin to the pristine crystals. Compared to the latter, however, we observe a significant reduction of both, the critical pressure p c and the critical temperature T c . This result is consistent with the theoretically-predicted formation of a soft Coulomb gap in the presence of strong correlations and small disorder. Furthermore, we demonstrate, similar to the observation for the pristine sample, that the Mott transition after 50 h of irradiation is accompanied by sizable lattice effects, the critical behavior of which can be well described by mean-field theory. Our results demonstrate that the character of the Mott transition remains essentially unchanged at a low disorder level. However, after an irradiation time of 150 h, no clear signatures of a discontinuous metal-insulator transition could be revealed anymore. These results suggest that, above a certain disorder level, the metal-insulator transition becomes a smeared first-order transition with some residual hysteresis. Full article
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Open AccessArticle Light-Induced Current Oscillations in the Charge-Ordered State of (TMTTF)2SbF6
Crystals 2017, 7(9), 278; https://doi.org/10.3390/cryst7090278
Received: 31 July 2017 / Revised: 8 September 2017 / Accepted: 13 September 2017 / Published: 15 September 2017
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Abstract
Below TCO=157 K the quasi-one-dimensional charge-transfer salt (TMTTF)2SbF6 undergoes a pronounced phase transition to a charge-ordered ground state. We have explored the non-linear and photoconductive behavior as a function of applied voltage, laser pulse energy and temperature.
[...] Read more.
Below T CO = 157 K the quasi-one-dimensional charge-transfer salt (TMTTF) 2 SbF 6 undergoes a pronounced phase transition to a charge-ordered ground state. We have explored the non-linear and photoconductive behavior as a function of applied voltage, laser pulse energy and temperature. Besides a decay of the photoconductive signal in a double exponential fashion in the millisecond range, we discover current oscillations in the kHz range induced by the application of short laser pulses. While the resonance frequencies do not depend on voltage or laser intensity and vary only slightly with temperature, the amplitude changes linearly with the laser intensity and voltage. We suggest that photo-induced fluctuations of the charge-ordered state alter the current flow of the single particles and hence, the photocurrent. The findings are discussed and compared to comparable phenomena in other low-dimensional electron systems. Full article
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Review

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Open AccessReview Low-Frequency Dynamics of Strongly Correlated Electrons in (BEDT-TTF)2X Studied by Fluctuation Spectroscopy
Crystals 2018, 8(4), 166; https://doi.org/10.3390/cryst8040166
Received: 22 March 2018 / Revised: 5 April 2018 / Accepted: 8 April 2018 / Published: 14 April 2018
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Abstract
Fluctuation spectroscopy measurements of quasi-two-dimensional organic charge-transfer salts (BEDT-TTF)2X are reviewed. In the past decade, the method has served as a new approach for studying the low-frequency dynamics of strongly correlated charge carriers in these materials. We review some basic aspects
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Fluctuation spectroscopy measurements of quasi-two-dimensional organic charge-transfer salts (BEDT-TTF) 2 X are reviewed. In the past decade, the method has served as a new approach for studying the low-frequency dynamics of strongly correlated charge carriers in these materials. We review some basic aspects of electronic fluctuations in solids, and give an overview of selected problems where the analysis of 1 / f -type fluctuations and the corresponding slow dynamics provide a better understanding of the underlying physics. These examples are related to (1) an inhomogeneous current distribution due to phase separation and/or a percolative transition; (2) slow dynamics due to a glassy freezing either of structural degrees of freedom coupling to the electronic properties or (3) of the electrons themselves, e.g., when residing on a highly-frustrated crystal lattice, where slow and heterogeneous dynamics are key experimental properties for the vitrification process of a supercooled charge-liquid. Another example is (4), the near divergence and critical slowing down of charge carrier fluctuations at the finite-temperature critical endpoint of the Mott metal-insulator transition. Here also indications for a glassy freezing and temporal and spatial correlated dynamics are found. Mapping out the region of ergodicity breaking and understanding the influence of disorder on the temporal and spatial correlated fluctuations will be an important realm of future studies, as well as the fluctuation properties deep in the Mott or charge-ordered insulating states providing a connection to relaxor or ordered ferroelectric states studied by dielectric spectroscopy. Full article
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Open AccessReview Thermodynamic Picture of Dimer-Mott Organic Superconductors Revealed by Heat Capacity Measurements with External and Chemical Pressure Control
Crystals 2018, 8(4), 143; https://doi.org/10.3390/cryst8040143
Received: 24 February 2018 / Revised: 18 March 2018 / Accepted: 19 March 2018 / Published: 21 March 2018
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Abstract
This article reviews and discusses the thermodynamic properties of dimer-Mott-type molecular superconductive compounds with (BEDT-TTF)2X composition, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes counter-anions, respectively. We focus mainly on the features occurring in the κ-type structure in which the d-wave superconductive
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This article reviews and discusses the thermodynamic properties of dimer-Mott-type molecular superconductive compounds with (BEDT-TTF)2X composition, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes counter-anions, respectively. We focus mainly on the features occurring in the κ-type structure in which the d-wave superconductive phase appears depending on the Coulomb repulsion U and the bandwidth W, which is tunable by external and chemical pressures. First, we report the high-pressure ac (alternating current) calorimetry technique and experimental system constructed to measure single-crystal samples of molecule-based compounds to derive low-temperature thermodynamic parameters. Using extremely small resistance chips as a heater and a thermometer allows four-terminal detection of an accurate temperature and its oscillation in the sample part with sufficient sensitivity. From the analyses of the temperature dependence of the ac heat capacity of κ-(BEDT-TTF)2Cu(NCS)2 under external pressures, we discuss the changes in the peak shape of the thermal anomaly at the superconductive transition temperature Tc at various external pressures p. The rather sharp peak in CpT−1 at Tc = 9.1 K with a strong coupling character at ambient pressure is gradually reduced to weaker coupling as the pressure increases to 0.45 GPa concomitant with suppression of the transition temperature. This feature is compared with the systematic argument of the chemical–pressure effect on the basis of thermal anomalies around the superconductive transition of κ-(BEDT-TTF)2X compounds and other previously studied typical dimer-Mott 2:1 compounds. Finally, the discussion is extended to the chemical pressure effect on the normal state electronic heat capacity coefficient γ obtained by applying magnetic fields higher than Hc2 and the residual γ*, which remains in the superconductive state due to the induced electron density of states around the node structure. From the overall arguments with respect to both chemical and external pressures, we suggest that a crossover of the electronic state inside the superconductive phase occurs and the coupling strength of electron pairs varies from the electron correlation region near the metal-insulator boundary to the band picture region. Full article
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Open AccessReview Effects of Carrier Doping on the Transport in the Dirac Electron System α-(BEDT-TTF)2I3 under High Pressure
Crystals 2018, 8(3), 126; https://doi.org/10.3390/cryst8030126
Received: 31 January 2018 / Revised: 6 March 2018 / Accepted: 6 March 2018 / Published: 8 March 2018
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Abstract
A zero-gap state with a Dirac cone type energy dispersion was discovered in an organic conductor α-(BEDT-TTF)2I3 under high hydrostatic pressures. This is the first two-dimensional (2D) zero-gap state discovered in bulk crystals with a layered structure. Moreover, the Dirac
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A zero-gap state with a Dirac cone type energy dispersion was discovered in an organic conductor α-(BEDT-TTF)2I3 under high hydrostatic pressures. This is the first two-dimensional (2D) zero-gap state discovered in bulk crystals with a layered structure. Moreover, the Dirac cones are highly tilted in a k-space. This system, thus, provides a testing ground for the investigation of physical phenomena in the multilayered, massless Dirac electron system with anisotropic Fermi velocity. Recently, the carrier injection into this system has been succeeded. Thus, the investigations in this system have expanded. The recent developments are remarkable. This effect exhibits peculiar (quantum) transport phenomena characteristic of electrons on the Dirac cone type energy structure. Full article
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Open AccessReview Design of Spin-Frustrated Monomer-Type C60•− Mott Insulator
Crystals 2018, 8(3), 115; https://doi.org/10.3390/cryst8030115
Received: 30 January 2018 / Revised: 23 February 2018 / Accepted: 25 February 2018 / Published: 28 February 2018
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Abstract
Spin-frustrated monomer-type Mott insulator C60•− solids are discussed in this review article. For the C60•− solids, the interfullerene center-to-center distance (r) is the key parameter that controls the competition between covalent bond-formation, itinerancy, and spin frustration. Eight
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Spin-frustrated monomer-type Mott insulator C60•− solids are discussed in this review article. For the C60•− solids, the interfullerene center-to-center distance (r) is the key parameter that controls the competition between covalent bond-formation, itinerancy, and spin frustration. Eight C60•− salts with various compositions and dimensionalities are reviewed. In all of these C60•− salts except one, neither bond-formation nor long-range magnetic ordering was observed down to low temperatures. A plot of Weiss temperature (|ΘCW|) against r shows that |ΘCW| grows rapidly below r = 10.0 Å. Full article
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