Stephan Gärtner

Superconductivity at 10 K and ambient pressure in the organic metal (BEDT-TTF)2Cu(SCN)2

We confirm the observation of superconductivity at ambient pressure above 10 K in the organic metal (BEDT-TTF)2Cu(SCN)2 as reported recently by Urayama et al [12]. In addition we have measured ESR, ac-susceptibility and thermopower in crystals of (BEDT-TTF)2Cu(SCN)2 and have shown that in contrast to other organic superconductors here a relatively sharp superconducting transition even in the ac-susceptibility can be observed which saturates already around 8 K. The thermopower measurements indicate a clear metal-metal phase transition at 100 K and a possible second phase transition at around 50 K, while from the temperature dependence of the resistivity and susceptibility (ESR) these phase transitions cannot be observed.

Superconductivity at ambient pressure in BEDT-TTF radical salts

Abstract Crystals of (BEDT-TTF)2Cu(NCS)2 were prepared by several methods and the super-conducting transitions investigated by resistivity and ac susceptibility measurements. Depending on the preparation of the crystals a variation of the temperature of the superconducting transition is observed. This variation is manifested in the upper critical fields Hc2 and proton NMR relaxation measurements at temperatures below TC show it as well. The upper critical fields HC2 of crystals of αt-(BEDT-TTF)2I3 were determined in dependence of the temperature and of the direction of the magnetic field with respect to the various crystal axes by measuring the mid transition of the resistivity and of the rf penetration depth. The data are analyzed with the anisotropic effective mass model in the picture of the Ginsburg Landau (GL) theory as well as in the picture of a layered superconductor. 13C Knight shifts by magic angle sample spinning and NMR cross polarisation methods support the picture of the layered superconductor.

Superconductivity in polycrystalline pressed samples of organic metals

We report bulk superconductivity at ambient pressure in polycrystalline pressed samples of αt-(BEDT-TTF)2I3. As in single crystals of αt-(BEDT-TTF)2I3 the onset for superconductivity in the polycrystalline pressed samples lies at about 9K but the transition is broader. Zero resistivity is observed at 2.2K and the middle of the transition is found at 5K. The measurement of the ac susceptibility shows that at 2K about 50% of the sample is superconducting, but at this temperature the transition is still not saturated. The observation of bulk superconductivity in this polycrystalline pressed samples is of special interest because it demonstrates that organic superconductors can be used in principle as well for the production of electronic devices such as squids and superconducting cables. In addition the finding of bulk superconductivity in large pressed samples of crystallites of the typical diameter of 1μm of an organic metal indicates that the observation of superconductivity in organic polymers should be possible as well.

BEDT-TTF salts with square platinates(II) as counterions : [BEDT-TTF]4[Pt(C2O4)2], a new organic metal

Abstract The title compound, [BEDT-TTF] 4 [Pt(C 2 O 4 ) 2 ], is obtained by electrocrystallization, which yields black platelets of stoichiometry C 44 H 32 O 8 PtS 32 , M r = 1909.87. Crystal structure determination at room temperature gives a triclinic cell P 1 , with a = 8.678(2) A, b = 11.878(5) A, c = 15.757(7) A, α = 105.49(3)°, β = 91.05(3)°, V = 1563.64 A 3 , d c = 2.03 g/cm 3 , Z = 1. The solid contains sheets of BEDT-TTF cations separated by sheets of [Pt(C 2 O 4 ) 2 ] 2− counterions. The BEDT-TTF sheets are made up of columns that interact strongly through S-S contacts. The columns are built up by pairs of BEDT-TTF molecules. There are strongly varying bond distances in the two different BEDT-TTF moieties that make up a pair. This fact is especially clearly expressed in the surprisingly short CC distances of the central CC double bond of the two BEDT-TFF ions: 1.273 A and 1.327 A respectively. These are by far the shortest distances for the central CC bonds observed so far in conductive BEDT-TTF salts. The compound behaves like a metal down to about 60 K. Temperature-dependent d.c. conductivity, e.s.r. and thermopower measurements show metallic behaviour at room temperature ( σ 300 = 20 S/cm) with a metal-to-metal phase transition around 200 K. At about 60 K a second broad phase transition occurs and the crystals become semiconducting.

Static and ESR susceptibilities of the ambient pressure superconductors β- and αt-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(SCN)2

Abstract Whereas the static spin-susceptibility χ(T) of β- and α t -(BEDT-TTF) 2 I 3 at 5.7 tesla increases in a slow monotonic manner with temperature, χ s (T) for the organic superconductor (BEDT-TTF) 2 Cu(SCN) 2 displays a markedly structured temperature dependence. This gives evidence for two or more phase transitions in the metallic phase at temperatures near 100 K and 50 K, in addition to the onset of superconductivity at 5.7 tesla below ∼ 8 K. Good agreement is found between the absolute magnitudes of static and ESR susceptibility results, thus confirming that orbital susceptibility contributions are of negligible importance here. Cooling in low magnetic fields reveals that superconductivity in (BEDT-TTF) 2 Cu(SCN) 2 sets in at a temperature near 8.5 K, with a 7.6% Meissner effect in 14 G field.

Radical cation salts of an unsymmetrical BEDT-TTF derivative: DIMET☆

Abstract Crystal structures, temperature dependent conductivity measurements and thermopower data of several DIMET radical salts are presented and discussed.

RADICAL CATION SALTS OF BEDT-TTF AND AN UNSYMMETRICAL BEDT-TTF DERIVATIVE

Investigations of several BEDT-TTF and DIMET radical salts are presented. Measurements on β-(BEDT-TTF)2IAuI crystals show that their conducting and superconducting properties might vary strongly depending on the decomposition of (IAuI)− anions during the electrochemical process.

BEDT-TTF-Salze mit quadratischen Platinaten(II) als Gegenionen: [BEDT-TTF]4[Pt(CN)4] / BEDT-TTF-Salts with Square Platinates(II) as Counter Ions: [BEDT-TTF]4[Pt(CN)4]

The compound [BEDT-TTF]4[Pt(CN)4], BEDT-TTF = Bis(4.5-ethylendithiolo)tetrathiafulvalen = C10H8S8, stoichiometry C44H32N4S32Pt, Mr = 1837.9 (g/mol), was obtained by electrocrystallization in at least three different modifications. Two of them could be identified by single crystal X-ray structure elucidation. 1 = β-modification, is isolated in black, lustrous platelets: triclinic, P 1̄, Z = 1, α = 9.7280(30) Å, b = 10.9237(27) Å, c = 16.5432(60) Å, a = 95.820(25)°, β = 98.296(26)°, γ = 115.215(24)°, V = 1547.56 A3, dc = 2.0 (g/cm3), room temperature, Rw = 0.082 for 7322 observed reflections. 2 = y-modification, is isolated in black not very regular crystals: monoclinic, C 2/c, Z = 4, a = 11.163(6) Å, b = 33.56(2) Å, c = 16.56(1) Å, β = 91.39(4)°, V = 6202.04 A3, dc = 1.97 (g/cm3), room temperature, Rw = 0.053 for 1404 observed independent reflections. Both solids contain sheets of BEDT-TTF cations separated by sheets of the counter anions. 1 contains two crystallographically independent BEDT-TTF moieties while 2 contains four independent donor units per cell. Of interest are the very different lengths of the central C=C bonds of the BEDT-TTF moieties which are assumed to reflect the charge density on these ions. In 1 these distances are 1.350(14) Å and 1.306(14) A, respectively, for the two different BEDT-TTF cations in the lattice, in 2 they are 1.372(34) A, 1.401(42) A and 1.289(54) Å respectively. The remarkable differences in these bond lengths indicate, according to published concepts, quite different charges on the different BEDT-TTF units. Two of the distances are shorter than in the neutral donor. Measurements of the specific resistance and the thermopower show that 1 is a metal with a room temperature conductivity of 10 S/cm. At around 200 K 1 becomes semiconducting. 2 turns out to be a semiconductor with a room temperature conductivity of 1—2 S/cm. Around 225 K a structural phase transition occurs. ESR measurements show an antiferromagnetic coupling between the spins in the solid. A third modification (δ) (semiconducting with a room temperature conductivity around 2 × 10-2 S/cm) was identified by ESR investigations.

Transport properties of single crystals and polycristalline pressed samples of (BEDT-TTF)2X salts and related coordination polymers

Ten years ago, in 1979, superconductivity was observed for the first time in an organic metal [1]. Today, about 30 different organic metals are known, which become superconducting under pressure or ambient pressure. The organic superconductors with the highest transition temperatures are all radical salts of the donor bis(ethylenedithiolo)-tetrathiafulvalene (BEDT-TTF), namely at ambient pressure (BEDT-TTF)2Cu(NCS)2 (Tc = 10.4 K) [2] and αt-(BEDT-TTF)2I3 (Tc = 8 K) [3] and under isotropic pressure βH - (BEDT-TTF)2I3 (0.5 kbar, Tc = 7.5 K) [4,5]. The latter βH-phase can even become superconducting at 8 K and ambient pressure, after a special pressure-temperature cycling procedure [6], i.e. pressurization up to 1 kbar at room temperature, and release of the helium gas pressure at temperatures below 125 K. Nevertheless, this superconducting state at 8 K in βH-(BEDT-TTF)2I3 is only metastable [6,7], since warming up the crystal above 125 K, and cooling down again under ambient pressure, results only in superconductivity at 1.3 K, the so-called βL - or β-phase.