Special Issue "Advances in Organic Conductors and Superconductors"

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

Deadline for manuscript submissions: closed (31 January 2018)

Special Issue Editor

Guest Editor
Prof. Dr. Martin Dressel

Universität Stuttgart, 1. Physikalisches Institut, Stuttgart, Germany
Website | E-Mail
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

Manuscript Submission Information

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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 (4 papers)

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Research

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; doi: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
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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
(This article belongs to the Special Issue Advances in Organic Conductors and Superconductors)
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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; doi:10.3390/cryst8020087
Received: 13 December 2017 / Revised: 29 January 2018 / Accepted: 3 February 2018 / Published: 6 February 2018
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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
(This article belongs to the Special Issue Advances in Organic Conductors and Superconductors)
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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; doi: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
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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
(This article belongs to the Special Issue Advances in Organic Conductors and Superconductors)
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Open AccessArticle Light-Induced Current Oscillations in the Charge-Ordered State of (TMTTF)2SbF6
Crystals 2017, 7(9), 278; doi: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
(This article belongs to the Special Issue Advances in Organic Conductors and Superconductors)
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