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The organic system, $\kappa$-[(BEDT-TTF)$_{1-x}$(BEDT-STF)$_x$]$_2$Cu$_2$(CN)$_3$, showing the Mott transition between a nonmagnetic Mott insulating (NMI) state and a Fermi liquid (FL), is systematically studied by calorimetric measurements. An increase of the electronic heat capacity at the transition from the NMI state to the FL state which keeps the triangular dimer lattice demonstrates that the charge sector lost in the Mott insulating state is recovered in the FL state. We observed that the remaining low-energy spin excitations in the Mott insulating state show unique temperature dependence, and that the NMI state has a larger lattice entropy originating from the frustrated lattice, which leads to the Pomeranchuk-like effect on the electron localization. Near the Mott boundary, an unexpected enhancement and magnetic-field dependence of heat capacity are observed. This anomalous heat capacity is different from the behavior in the typical first-order Mott transition and shows similarities with quantum critical behavior. To reconcile our results with previously reported scenarios about a spin gap and the first-order Mott transition, further studies are desired., Comment: 9 pages, 4 figures

In the search for high performance n-type organic semiconductors (OSCs) a simple strategy might be substitution of aromatic CH groups for nitrogen heteroatoms. Here, we report the synthesis and characterisation of two novel N-heteroacene compounds, namely, 1,5,12-triazatetracene (TrAT1) and 2,5,12-triazatetracene (TrAT2). Their potential as n-type materials is evaluated against 5,12-diazatetracene (DAT) by UV/vis and EPR spectroscopy, cyclic voltammetry, DFT, single crystal X-ray diffraction and thin film characterisation. Increasing the number of N-heteroatoms was found to stabilise the HOMO and LUMO leading to electron affinities for TrAT1 and TrAT2 of ca. −4 eV. Both compounds were found to exhibit columns of co-facial π-stacked molecules. For TrAT1, molecules are also linked by hydrogen bonding, while the crystal structure of TrAT2 was found to be inherently disordered. Thin films of DAT, TrAT1 and TrAT2 were grown by organic molecular beam deposition (OMBD) and found to form discontinuous films, where TrAT1 exhibited a preferential orientation.

Magnetic properties of Mott–Hubbard systems are generally dominated by strong antiferromagnetic interactions produced by the Coulomb repulsion of electrons. Although theoretical possibility of a ferromagnetic ground state has been suggested by Nagaoka and Penn as single-hole doping in a Mott insulator, experimental realization has not been reported more than half century. We report the first experimental possibility of such ferromagnetism in a molecular Mott insulator with an extremely light and homogeneous hole-doping in π-electron layers induced by net polarization of counterions. A series of Ni(dmit)2 anion radical salts with organic cations, where dmit is 1,3-dithiole-2-thione-4,5-dithiolate can form bi-layer structure with polarized cation layers. Heat capacity, magnetization, and ESR measurements substantiated the formation of a bulk ferromagnetic state around 1.0 K with quite soft magnetization versus magnetic field (M–H) characteristics in (Et-4BrT)[Ni(dmit)2]2 where Et-4BrT is ethyl-4-bromothiazolium. The variation of the magnitude of net polarizations by using the difference of counter cations revealed the systematic change of the ground state from antiferromagnetic one to ferromagnetic one. We also report emergence of metallic states through further doping and applying external pressures for this doping induced ferromagnetic state. The realization of ferromagnetic state in Nagaoka–Penn mechanism can paves a way for designing new molecules-based ferromagnets in future.

Single crystal X-ray diffraction, magnetic and 57Fe Mossbauer spectroscopic studies of a new iron(II)-based spin transition complex – [Fe(3-bpp)2](CF3COO)2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) are reported. The complex exhibits an incomplete thermal high spin (HS) low spin (LS) transition at ∼226 K which is not associated with any crystallographic transition. Only one of the two crystallographic iron(II) sites undergoes the spin transition, while the other HS site remains invariant of temperature down to 10 K. The origin of the incomplete spin transition is understood by analysing the nature of crystal packing below and above the transition temperature with special attention to the degree of distortion. The present study shows the role of the degree of distortion associated with the cations in the [Fe(3-bpp)2]2+ spin transition complexes.

We report the first example of a chiral BDH-TTP radical-cation salt. Chirality is induced in the structure via the use of a chiral spiroboronate anion where three stereocentres are present, one on each chiral ligand and one on the boron centre. Despite starting from a labile racemic mixture of BS and BR enantiomers, only one enantiomer is present in the crystal lattice. The anions pack in a novel double anion layer which is the thickest anion layer found in a BDH-TTP salt. This material is chiral and shows metallic behaviour down to at least 4.2 K.

A new bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)-based salt with an organic magnetic anion, PO-CONHCH(cyclopropyl)SO3−, has been prepared (PO = 2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl free radi...

We present the synthesis, crystal structures and conducting properties of six new BEDT-TTF radical-cation salts with the tris(oxalato)gallate or -iridate anion. The use of halobenzenes gives 4 : 1 β′′ metallic or superconducting salts with tris(oxalato)gallate - fluorobenzene (PhF) or chlorobenzene (PhCl) as a guest molecule gives a metal, whilst the larger bromobenzene (PhBr) or iodobenzene (PhI) gives a superconductor. A 4 : 1 pseudo-κ semiconducting salt is obtained with benzonitrile as the guest molecule for tris(oxalato)iridate, and a novel 5 : 1 metal-insulator phase with tris(oxalato)iridate is also reported.

The construction of a thermal conductivity measurement system designed for tiny molecules-based compounds is reported. We introduce complementary usage of chip-type RuO2 thermometers and E-type thermocouples in the sample part by using thin (ϕ 13μm) constantan and chromel wires. Two pairs of the constantan and chromel wires are used as lead wires for the four-terminal measurement of the resistance of RuO2 thermometers in the low-temperature region below about 20 K. Also, in the higher temperature region above 10 K up to room temperature with the overlapping range of 10-20 K, they are used as thermocouples for detecting temperature differences from that of the heat sink. We also compare a kind of resolution parameter of several sensors as a function of temperature to discuss the rational reason to select suitable sensors depending on the temperature region. Using the constructed apparatus, we report temperature dependences of the thermal conductivity of deuterated κ-(d8:BEDT-TTF)2Cu[N(CN)2]Br in a wide temperature range between 2 and 250 K. The result provides convincing evidence for the validity of the newly developed system for the thermal measurements of molecular crystals.

We report the first examples of radical-cation salts of BEDT-TTF with spiroborate anions [B(mandelate)

We report the first examples of radical-cation salts of BEDT-TTF with spiroborate anions [B(mandelate)2]− and [B(2-chloromandelate)2]−, synthesized from either enantiopure or racemic bidentate mandelate or chloromandelate ligands. In the salts prepared using enantiopure ligands only one of two diastereoisomers of the spiroborate anion is incorporated, with the boron centre having the same stereochemistry as the enantiopure ligand. For the racemic salts one racemic pair of spiroborate anions containing an R and an S mandelate ligand is incorporated. In certain solvents helical crystals were obtained when using spiroborate anions with enantiopure ligands. Electrical and magnetic properties, and band structure calculations are reported.

Exploring new topological phenomena and functionalities induced by strong electron correlation has been a central issue in modern condensed-matter physics. One example is a topological insulator (TI) state and its functionality driven by the Coulomb repulsion rather than a spin-orbit coupling. Here, we report a "correlation-driven" TI state realized in an organic zero-gap system $\alpha$-(BETS)$_2$I$_3$. The surface metallic state that emerges at low temperatures exhibits characteristic transport properties of a gapless Dirac semimetal, evidencing the presence of a topological surface state in this compound. Moreover, we observe a topological phase switching between the TI state and non-equilibrium Dirac semimetal state by a dc current, which is a unique functionality of a correlation-driven TI state. Our findings demonstrate that correlation-driven TIs are promising candidates not only for practical electronic devices but also as a field for discovering new topological phenomena and phases., Comment: 36 pages including 10 figures

Thermodynamic investigation by calorimetric measurements of the layered organic superconductors, κ-(BEDT-TTF)2Cu[N(CN)2]Br and its partially deuterated compounds of κ-(d[2,2]-BEDT-TTF)2Cu[N(CN)2]Br and κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, performed in a wide temperature range is reported. The latter two compounds were located near the metal–insulator boundary in the dimer-Mott phase diagram. From the comparison of the temperature dependences of their heat capacities, we indicated that lattice heat capacities of the partially deuterated compounds were larger than that of the pristine compound below about 40 K. This feature probably related to the lattice softening was discussed also by the sound velocity measurement, in which the dip-like structures of the Δv/v were observed. We also discussed the variation of the electronic heat capacity under magnetic fields. From the heat capacity data at magnetic fields up to 6 T, we evaluated that the normal-state γ value of the partially deuterated compound, κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br, was about 3.1 mJ K−2 mol−1. Under the magnetic fields higher than 3.0 T, we observed that the magnetic-field insulating state was induced due to the instability of the mid-gap electronic state peculiar for the two-dimensional dimer-Mott system. Even though the volume fraction was much reduced, the heat capacity of κ-(d[3,3]-BEDT-TTF)2Cu[N(CN)2]Br showed a small hump structure probably related to the strong coupling feature of the superconductivity near the boundary.

The Fermi surface structure of a layered organic superconductor β″-(BEDT-TTF)2SF5CH2CF2SO3 was determined by angular-dependent magnetoresistance oscillations measurements and band-structure calculations. This salt was found to have two small pockets with the same area: a deformed square hole pocket and an elliptic electron pocket. Characteristic corrugations in the field dependence of the interlayer resistance in the superconducting phase were observed at any in-plane field directions. The features were ascribed to the commensurability (CM) effect between the Josephson vortex lattice and the periodic nodal structure of the superconducting gap in the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase. The CM effect was observed in a similar field region for various in-plane field directions, in spite of the anisotropic nature of the Fermi surface. The results clearly showed that the FFLO phase stability is insensitive to the in-plane field directions.

Professor Peter Day FRS was born on 20 August 1938 in Kent (UK) and attended Maidstone Grammar School [...]

The chiral and racemic salts β′′-(BEDT-TTF)2(S- and rac-PROXYL-CONHCH2SO3) (S-2 and rac-2) are almost isomorphous apart from a deviation in the C–H bond direction at the chiral centre. Both salts are metallic at room temperature, with similar broad metal-insulator transitions. Band structure calculations of the chiral and racemic salts indicate that both electronic structures are quite similar. However, at 30 K, S-2 has a resistivity that is nearly three orders of magnitude higher than that of rac-2. The results suggest a significant effect of the broken inversion symmetry, due to the positional change of only one atom.

Materials exhibiting both chirality and conductivity do not exist in nature and very few examples have been synthesised. We report here the synthesis of a chiral molecular metal which remains metallic down to at least 4.2 K. This material also exhibits room-temperature switching capabilities with a transition upon cooling below 10 °C.

The synthesis of a novel amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine-4-carboxamide ligand (bppCONH2) is described. The complex salts [Fe(bppCONH2)2](BF4)2 and [Fe(bppCONH2)2](ClO4)2 were synthesised and characterised by SQUID magnetometry, differential scanning calorimetry, variable temperature Raman spectroscopy and single crystal X-ray diffraction. DSC measurements of [Fe(bppCONH2)2](BF4)2 indicate a spin-crossover (SCO) transition with T↑ at 481 K and T↓ at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition with T↑ at 459 K and T↓ at 445 K with a 14 K hysteresis. For the BF4− salt analysis of low and high-spin state crystal structures at 101, 290 and 500 K, suggest stabilisation of the low spin state due to the formation of 1D hydrogen-bonded cationic chains. Variable temperature Raman studies of the BF4 salt support the presence of a high temperature SCO. It is speculated that the presence of hysteresis may be attributed to differences in the inter-molecular hydrogen bonding in the low spin and high spin states.

An organic anion, 2-bromoethanesulfonate (BrC2H4SO3−), provides one bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and two bis(ethylenedithio)tetraselenafulvalene (BETS) salts, the compositions of which are β’’-β’’-(BEDT-TTF)2BrC2H4SO3 (1), β’’-β’’-(BETS)2BrC2H4SO3 (2), and θ-(BETS)2BrC2H4SO3 (3), respectively. Compound 1 shows a metal–insulator transition at around 70 K. Compound 2 is isomorphous to 1, and 3 is polymorphic with 2. Compounds 2 and 3 show metallic behavior at least down to 4.2 K. The pressure dependence of the electrical resistivity of 1 is also reported.

Peter Day’s research group reported the first molecular superconductor containing paramagnetic metal ions in 1995, β″-(BEDT-TTF)4(H3O)Fe(C2O4)3·C6H5CN. Subsequent research has produced a multitude of BEDT-TTF-tris(oxalato)metallate salts with a variety of structures and properties, including 32 superconductors to date. We present here the synthesis, crystal structure, and conducting properties of the newest additions to the Day series including the first superconductor incorporating the diamagnetic tris(oxalato)aluminate anion, β″-(BEDT-TTF)4(H3O)Al(C2O4)3·C6H5Br, which has a superconducting Tc of ~2.5 K. β″-(BEDT-TTF)4(H3O)Co(C2O4)3·C6H5Br represents the first example of a β″ phase for the tris(oxalato)cobaltate anion, but this salt does not show superconductivity.

Quantum spin liquids (QSLs), in which spins are highly entangled, have been considered a groundwork for generating exotic superconductivity. Despite numerous efforts, superconductivity emerging from QSLs has been unrealized in actual materials due to the difficulties in stabilizing QSL states with metallic conductivity. Recently, an organic compound, $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}{\mathrm{Hg}}_{2.89}{\mathrm{Br}}_{8}$, with a nearly regular triangular lattice of molecular dimers was recognized as a candidate for doped QSLs. In this study, we report an unusual superconducting phase of $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}{\mathrm{Hg}}_{2.89}{\mathrm{Br}}_{8}$: unexpectedly large ratios of the upper critical field to the critical temperature ${H}_{\mathrm{c}2}/{T}_{\mathrm{c}}$ in fields not only parallel but also perpendicular to the two-dimensional conducting layers and a very wide region of fluctuating superconductivity above ${T}_{\mathrm{c}}$. Our results reveal that these peculiarities arise from strong electron correlations and possible quantum criticality unique to the doped QSL state, leading to a heavy mass of itinerant carriers and a large superconducting energy gap.

Peter Day’s research group reported the first molecular superconductor containing paramagnetic metal ions in 1995, β″-(BEDT-TTF)4(H3O)Fe(C2O4)3·C6H5CN. Subsequent research has produced a multitude of BEDT-TTF-tris(oxalato)metallate salts with a variety of structures and properties, including 32 superconductors to date. We present here the synthesis, crystal structure, and conducting properties of the newest additions to the Day series including the first superconductor incorporating the diamagnetic tris(oxalato)aluminate anion, β″-(BEDT-TTF)4(H3O)Al(C2O4)3·C6H5Br, which has a superconducting Tc of ~2.5 K. β″-(BEDT-TTF)4(H3O)Co(C2O4)3·C6H5Br represents the first example of a β″ phase for the tris(oxalato)cobaltate anion, but this salt does not show superconductivity.

Two novel salts containing the anion [Ni(mnt)2]− (mnt = maleonitriledithiolate) have been synthesized. The counter-ions, [Fe(II)(L1 or L2)2], are cationic complexes where L1 and L2 are methylated derivatives of 2,6-bis(pyazolyl)pyridine or pyrazine, which are similar to ligands found in a series of spin-crossover (SCO) complexes. Both salts are characterized by variable temperature single crystal X-ray diffraction and bulk magnetization measurements. Compound 1, [Fe(II)(L1)2][Ni(mnt)2]2 displays an incomplete and gradual SCO up to 300 K, followed by a more rapid increase in the high-spin fraction between 300 and 350 K. Compound 2, [Fe(II)(L2)2][Ni(mnt)2]2.MeNO2, shows a gradual, but more complete SCO response centered at 250 K. For compound 2, the SCO is confirmed by variable temperature Mössbauer spectroscopy. In both cases, the anionic moieties are isolated from each other and so no electrical conductivity is observed.

We report the first example of a chiral BDH-TTP radical-cation salt. Chirality is induced in the structure via the use of a chiral spiroboronate anion where three stereocentres are present, one on each chiral ligand and one on the boron centre. Despite starting from a labile racemic mixture of BS and BR enantiomers, only one enantiomer is present in the crystal lattice. The anions pack in a novel double anion layer which is the thickest anion layer found in a BDH-TTP salt. This material is chiral and shows metallic behaviour down to at least 4.2 K.

The exploration of dynamic molecular crystals is a fascinating theme for materials scientists owing to their fundamental science and potential application to molecular devices. Herein, a one-dimensional (1D) rhodium-dioxolene complex is reported that exhibits drastic changes in properties with the phase transition. X-ray photoelectron spectroscopy (XPS) revealed that the room-temperature (RT) phase is in a mixed-valence state, and therefore, the drastic changes originate from the mixed-valence state appearing in the RT phase. Another notable feature is that the mean square displacements of the rhodium atoms along the 1D chain dramatically increased in the RT phase, indicating a large-amplitude vibration of the Rh-Rh bonds. From these results, a possible mechanism for the appearance of the mixed-valence state in the RT phase was proposed based on the thermal electron transfer from the 1D d-band to the semiquinonato π* orbital coupled with the large-amplitude vibration of the Rh-Rh bonds.

New bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)-, bis(ethylenediseleno)tetrathiafulvalene (BEST)- and bis(ethylenedithio)tetraselenafulvalene (BETS)-based organic charge-transfer (CT) salts&mdash, &alpha, (BEDT-TTF)3(HOC2H4SO3)2 (1), &beta, (BEST)3(HOC2H4SO3)2&middot, H2O (2) and &alpha, (BETS)2(HOC2H4SO3)&middot, H2O (3)&mdash, have been prepared. Salts 1 and 2 show semiconducting behaviour. Salt 3, which is almost isostructural to &alpha, (BETS)2I3, shows metallic behaviour down to 70 K and then shows a broader metal&ndash, insulator transition than that of &alpha, (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&rsquo, s charge-ordered and anion&rsquo, s disordered states.

We newly constructed a thermal conductivity measurement system for single crystals of molecular compounds. This system enables us to measure the thermal conductivity of samples with the length of less than 1.0 mm. In order to evaluate the capability of detection of phase transitions occurred in molecule-based materials, we performed the thermal conductivity measurement of single crystals of organic conductors α-(BEDT-TTF)2I3 and κ-(BEDT-TTF)2Cu[N(CN)2]Br, where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene. From the temperature dependence of the thermal conductivity of α-(BEDT-TTF)2I3, we succeeded in observing the metal–insulator transition as a sudden drop of κ(T) at 135 K. In the temperature dependence of thermal conductivity of κ-(BEDT-TTF)2Cu[N(CN)2]Br, we could observe the increment of κ(T) at the metal-superconducting transition (Tc = 11.4 K). These results show that the newly developed system has high performance enough to detect various kinds of phase transitions in small crystals of molecular compounds.

A new Ni(dmit)2-based organic magnetic charge-transfer (CT) salt, (m-PO-CONH-N-methylpyridinium)[Ni(dmit)2]·CH3CN, where PO = 2,2,5,5-Tetramethyl-3-pyrrolin-1-oxyl free radical and dmit = 2-Thioxo-1,3-dithiole-4,5-dithiolate, was obtained, the crystal structure and magnetic properties of which are reported. Magnetic susceptibility of the CT salt obeys a combination of 1D ferromagnetic (1DF) Heisenberg (J1DF = +0.26 K) and Singlet-Triplet (ST) models (JST = -51.2 K) with mean field (MF) approximation (JMF = -6.7 K), suggesting that spins on the PO radicals form 1D ferromagnetic chains and spins on the Ni(dmit)2 monoanions forms spin dimers at low temperature, the latter of which was also confirmed by band calculations.

A new molecular superconductor, β”-(BEDTTTF)2[(H2O)(NH4)2Cr(C2O4)3].18-crown-6, has been synthesized from the organic donor molecule BEDT-TTF with the anion Cr(C2O4)3 3- . The crystal structure consists of conducting organic layers of BEDT-TTF molecules which adopt the β” packing motif (layer A), layers of NH4 + and Λ-Cr(C2O4)3 3- (layer B), layers of (H2O)(NH4)18- crown-6 (layer C), and layers of NH4 + and Δ-Cr(C2O4)3 3- (layer D) which produce a superstructure with a repeating pattern of ABCDABCDA. As a result of this packing arrangement this is the 2D superconductor with the widest gap between conducting layers where only a single donor packing motif is present (β”). Superconducting critical temperatures at ambient pressure observed by electrical transport and magnetic measurements are 4.0- 4.9 and 2.5 K, respectively. The strong 2D nature of this system, the broad transition to Tzero at 1.8K, and the transition of α of V ∝ Iα from 1 to 3 on I-V curves strongly suggests that the superconducting transition is very close to a Kosterlitz-Thouless transition. The magnetic field dependence of the superconducting critical temperature parallel to the conducting plane gives an upper critical field μ0Hc2// > 8 T, which is over the calculated Pauli-Clogston limit for this material.

A new BEDT-TTF-based salt with an organic free radical, α′-α′-(BEDT-TTF)2(PO-CONH-m-C6H4SO3)·H2O, has been prepared. The crystal structure consists of alternating donor and anion/water layers propagated along the c axis. The anisotropic radical anions form a head-to-head arrangement, which provides a permanent dipole moment. The effect on the nearest donor layer has been estimated to be 4.0 V. The salt has two crystallographically independent donor layers (A and B), one of which (A) is surrounded by the negative ends of the anion layer dipole whereas the B layers are bordered by the positive ends of the dipole. This structural feature suggests that layers A and B have different Fermi levels. To eliminate the imbalance, self-doping occurs whereby electron density is transferred from layer A to B. The temperature dependence of the magnetic susceptibility has 11.1% of extra Curie spins, from which a doping ratio of 12.7% has been estimated.

Magnetization measurements under pressure reveal that the external hydrostatic pressure significantly increases in the ferrimagnetic transition temperature, Tc, for A2Mn[Mn(CN)6] (A = K, Rb, Cs). In the case of monoclinic A = K and Rb, dTc/dp values are 21.2 and 14.6 K GPa–1, respectively, and Tc increases by 53 and 39%, respectively, from ambient pressure to 1.0 GPa. The cubic A = Cs compound also shows a monotonous increase with an initial rate of 4.22 K GPa–1 and about 11.4 K GPa–1 above 0.6 GPa, and an overall Tc increase by 26% at 1.0 GPa. The increase in Tc is attributed to deformation of the structure such that the MnII—N≡C angle decreases with increasing pressure. The smaller the alkali cation, the greater the decrease in the MnII—N≡C angle induced by pressure and the larger the increase of dTc/dp. This is in accordance with the ambient-pressure structures for A2Mn[Mn(CN)6] (A = K, Rb, Cs), which have decreasing MnII—N≡C angles that correlate to the observed increasing Tcs as K > Rb > Cs. The larg...

We investigate low-temperature electronic properties of the nondimeric organic superconductor $\beta^{\prime\prime}$-(BEDT-TTF)$_4$[(H$_3$O)Ga(C$_2$O$_4$)$_3$]PhNO$_2$. By examining ultrasonic properties, charge disproportionation (CD) without magnetic field dependence is detected below $T_{\rm CD}$$\sim$8~K just above the superconducting critical temperature $T_{\rm c}$$\sim$6~K. From quantum oscillations in high fields, we find variation in the Fermi surface and mass enhancement induced by the CD. Heat capacity studies elucidate that the superconducting gap function is fully gapped in the Fermi surface, but anisotropic with fourfold symmetry. We point out that the pairing mechanism of the superconductivity is possibly dominated by charge fluctuations., Comment: 6 figures

We investigate low-temperature electronic states of the series of organic conductors β'' - [bis(ethylenedithio)tetrathiafulvalene] 4[(H3O)M(C2O4)3] G, where M and G represent trivalent metalions and guest organic molecules, respectively. Our structural analyses reveal that the replacement of M and G give rise to systematic change in the cell parameters, especially in the b-axis length, which has a positive correlation with the superconducting transition temperature Tc. Analysis of temperature and magnetic field dependences of the electrical resistance including the Shubnikov–de Haas oscillations elucidates that the variation of charge disproportionation, the effective mass, and the number of itinerant carriers can be systematically explained by the change of the b-axis length. The changes of the transfer integrals induced by stretching/compressing the b axis are confirmed by the band calculation. We discuss that electron correlations in quarter-filled electronic bands lead to charge disproportionation and the possibility of a novel pairing mechanism of superconductivity mediated by charge degrees of freedom.

This work describes the synthesis of two novel functionalised 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligands, namely 2,6-bis(pyrazol-1-yl)pyridine-4-carbothioamide (bppCSNH2) and N-methyl-2,6-bis(pyrazol-1-yl)pyridine-4-carbothioamide (bppCSNHMe). The corresponding solvated or non-solvated Fe(II) salts, [Fe(bppCSNH2)2]X2 and [Fe(bppCSNHMe)2]X2 (X = BF4− or ClO4−) were synthesised and their properties measured by SQUID magnetometry, Evans NMR, differential scanning calorimetry and single crystal X-ray diffraction. In the solid state [Fe(bppCSNH2)2]2+ salts persist in the low spin state below 350 K. The structure of [Fe(bppCSNH2)2](BF4)2·2MeNO2 shows a network of intermolecular interactions responsible for the low spin state stabilisation, relative to the prototypical [Fe(bpp)2]2+ spin crossover (SCO) salts. By contrast the complexes of bppCSNHMe both display abrupt SCO above 300 K. [Fe(bppCSNHMe)2](BF4)2·MeNO2 requires solvent loss before SCO can be observed centred at 332 K. The non-solvated [Fe(bppCSNHMe)2](ClO4)2 shows SCO centred at 325 K. Analysis of solvated and non-solvated crystal structures suggests that cooperativity is facilitated by thioamide-group interactions with neighbouring pyrazolyl and pyridyl moieties.

The series of salts β''-(BEDT-TTF)

We report IR and Raman spectra for a two-dimensional superconductor, β′′-(BEDT-TTF)4[Ga(C2O4)3(H3O)]C6H5NO2, below the superconducting transition temperature. The behavior of the lattice-sensitive ...

The chiral TTF-based donor molecule bis(2′-hydroxylpropylthio)(ethylenedithio)tetrathiafulvalene (BHPT-EDT-TTF) has produced enantiopure R,R and S,S radical-cation salts with polyiodide anions I3− and I82−. Enantiomorphic 6 : 6 donor : I3 phases grown from either the R,R or S,S donor are semiconducting with similar activation energies of 0.24–0.30 eV and 0.22–0.23 eV, respectively, and contain three unique face-to-face donor pairs whose relative orientation is determined by side chain conformations and hydrogen bonding. Racemic material under the same conditions gave an insulating centrosymmetric phase with R,R and S,S donor cations in a face-to-face pair partnered with an octaiodide dianion, and with a ca. 3 : 1 disorder between the enantiomers. Enantiopure BHPT-EDT-TTF yielded two further insulating crystalline phases of composition 2 : 2 with triiodide and 2 : 1 with octaiodide.

Low temperature thermodynamic properties of an organic superconductor κ-[(BEDT-TTF)0.95(BEDSe-TTF)0.05]2Cu[N(CN)2]Br where 5% of donor molecules of κ-(BEDT-TTF)2Cu[N(CN)2]Br are substituted by BEDS...

The appearance of electron correlations–induced glassy state of low-energy phonons in the non-dimeric organic charge transfer salt in [Formula: see text]-(BEDT-TTF)2CsZn(SCN)4 is considered as a strong evidence of charge lattice coupling in molecular charge transfer salts. We discuss the temperature and the magnetic field dependences of the heat capacity of this salt in terms of the soft potential model to describe the thermodynamic properties of enhanced phonons that occur in molecular glasses. The evaluated [Formula: see text] term of [Formula: see text]-(BEDT-TTF)2CsZn(SCN)4 is about 30 mJ K[Formula: see text] mol[Formula: see text], which is much larger than other charge transfer salts described by the low-temperature approximation of the Debye model. The magnetic fields dependence of the boson peak is almost negligible, but the low-energy term, for example, in the temperature-linear term of heat capacity shows a slight change, probably due to the small amount of localized spin moments. The comparison with other systems is also performed.

The automatic frequency tuning method in the high-pressure ac calorimetry system constructed to measure heat capacity for molecules-based compounds with CuBe[Formula: see text]+[Formula: see text]NiCrAl cramp-type pressure cell is reported. This development is performed for increasing resolution and temperature ranges of the heat capacity measurements under external pressure up to 2.0 GPa. The system can check the appropriate conditions by tracing frequency dependence of [Formula: see text] to determine the oscillation frequency at the center of the plateau region of this value. The experiments using the powder samples of metal complexes clarified that the appropriate frequency changes sensitively depending on the difference of temperature and that of external pressures, especially at low temperature region. It decreases with increasing temperature and this relation was found to be almost linear with temperature in ambient pressure and under pressure conditions. The change of thermal diffusion from the sample part to the heat bath should be treated carefully in order to get enough resolution in high pressure AC heat capacity measurements of molecule-based compounds.

Thermodynamic picture induced by π-d interaction in a molecular magnetic superconductor κ-(BETS)2FeX4 (X = Cl, Br), where BETS is bis(ethylenedithio)tetraselenafulvalene, studied by single crystal calorimetry is reviewed. Although the S = 5/2 spins of Fe3+ in the anion layers form a three-dimensional long-range ordering with nearly full entropy of Rln6, a broad hump structure appears in the temperature dependence of the magnetic heat capacity only when the magnetic field is applied parallel to the a axis, which is considered as the magnetic easy axis. The scaling of the temperature dependence of the magnetic heat capacity of the two salts is possible using the parameter of |Jdd|/kB and therefore the origin of the hump structure is related to the direct magnetic interaction, Jdd, that is dominant in the system. Quite unusual crossover from a three-dimensional ordering to a one-dimensional magnet occurs when magnetic fields are applied parallel to the a axis. A notable anisotropic field-direction dependence against the in-plane magnetic field was also observed in the transition temperature of the bulk superconductivity by the angle-resolved heat capacity measurements. We discuss the origin of this in-plane anisotropy in terms of the 3d electron spin configuration change induced by magnetic fields.

The series of salts β′′-(BEDT-TTF)2[(H2O)(NH4)2M(C2O4)3]·18-crown-6 show ambient-pressure superconductivity when M = Cr, Rh. Evidence indicates that the previously reported Cr and Rh salts show a bulk Berezinski–Kosterlitz–Thouless superconducting transition. The isostructural ruthenium and iridium salts are reported here. The Ir salt represents the first radical-cation salt to contain a 5d tris(oxalato)metalate anion. The Ru and Ir salts do not show superconductivity but instead undergo a broad chemically induced metal to insulator transition at 155 K for ruthenium and at 100 K for iridium. The c axes of the Ru and Ir salts are much shorter than those of the Rh and Cr salts. Thus, the more stable metallic state of the Cr and Rh salts is associated with the more strongly 2D electronic systems. The different low-temperature behavior of the Ru and Ir salts, which exhibit a smaller interlayer spacing, could originate from a structural change in the anionic layer which thus can be easily transmitted to the donor layers and generate a localized state. However, another possibility is that it originates from Berezinski–Kosterlitz–Thouless effects., This work has been supported by a Royal Society Leverhulme Trust Senior Research Fellowship, and the JSPS (KAKENHI Grant Number JP17H01144) and Royal Society (Research Grants (RG100853 and RG081209), International Exchange Scheme (IE130367 and IE150152), and International Joint Project (JP0869972)). L.M. thanks the Royal Society and Leverhulme Trust for a Senior Research Fellowship. J.R.L. and L.M. thank Nottingham Trent University for Ph.D. funding. E.C. acknowledges support by the MICIU (Spain) through Grant PGC2018-096955-BC44 and the Severo Ochoa Centers of Excellence Program (Grant SEV-2015-0496) as well as by the Generalitat de Catalunya (2017SGR1506).

The structure and properties of a new semiconducting BEDT-TTF-based charge-transfer salt containing the magnetic monoanion octamethylferrocenedisulfonate (–O3SC5( CH3)4Fe+C5(CH3)4-SO3 – (ofds)), -(BEDT-TTF)5(ofds)2 is described (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene). The salt is a semiconductor with RT = 5.6  103 ·cm and Ea = 0.11 eV. There are two types of spins not only on the ferrocenyl parts but also the 2D donor layers. The former and latter spins obey Curie-Weiss (C = 0.721 emu K mol-1and  = -5.5 K) and 2D Heisenberg (J2DH = - 80.3 K) models, respectively.

Resistance and magnetic torque measurements are reported in a layered organic superconductor, $\ensuremath{\beta}''\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{4}[({\mathrm{H}}_{3}\mathrm{O})\mathrm{Ga}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}]{\mathrm{C}}_{6}{\mathrm{H}}_{5}{\mathrm{NO}}_{2}$ with ${T}_{c}=4.8$ K, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. Because of the large anion between the BEDT-TTF conducting layers, the superconductivity of this salt is highly anisotropic. In magnetic fields parallel to the conducting layers for $T=0.4$ K, the magnetic torque shows a large diamagnetic signal associated with hysteresis up to $\ensuremath{\sim}21$ T, suggesting the upper critical field ${H}_{c2}\ensuremath{\gtrsim}21$ T at 0.4 K. The large reduction of the diamagnetic signal is observed above 16 T, which shows a Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) phase transition. For $T=0.5$ K, the interlayer resistance has nonzero value in a wide field region up to ${H}_{c2}$, arising from the Josephson vortex dynamics. Successive dips in the second derivative curves of the resistance are observed between 16 T and ${H}_{c2}$, which are ascribed to the commensurability effect between the Josephson vortex lattice and the order parameter oscillation of the FFLO phase. The commensurability effect is observed only in nearly parallel fields, showing that the FFLO phase is stable in a very limited field angle region. The temperature-field phase diagram is determined.

A new family of six paramagnetic metals, namely, κ-(BDH-TTP)4CoCl4·(H2O)5 (I), κ-(BDH-TTP)4Co0.54Mn0.46Cl4·(H2O)5 (II), κ-(BDH-TTP)4MnCl4·(H2O)5 (III), κ⊥-(BDH-TTP)4CoBr4·(CH2Cl2) (IV), κ⊥-(BDH-TTP)4MnBr4·(CH2Cl2) (V), and κ⊥-(BDH-TTP)4MnBr4·(CH2Br2) (VI), has been synthesized and characterized by X-ray crystallography, four-probe conductivity measurements, SQUID magnetometry, and calculations of electronic structure. The newly discovered κ⊥-type packing motif of organic layers differs from the parent κ-type by a series of longitudinal shifts of BDH-TTP radical cations in the crystal structure. Salts I–VI form two isostructural groups: I–III (κ) and IV–VI (κ⊥). Salts I–III are isostructural to the previously discovered κ-(BDH-TTP)2FeIIIX4 (X = Cl, Br) even though the charge of FeX4– anions is half that of the MX42– (M = Co, Mn) anions. The tetrahedral anions are disordered in I–III but completely ordered in IV–VI. The type of included solvent molecule is solely determined by the anion size. The paramagnet...