J. Amiell

ESR study of κ-D4H4(BEDTTTF)2Cu(SCN)2 molecular superconductor

Abstract We have investigated the magnetic properties of the κ-phase molecular superconductor (BEDTTTF) 2 Cu(SCN) 2 partially or totally deuteriated. By ESR spectroscopy we have evidenced the linewidth and g factor temperature dependences in the metallic state above the transition temperature ( T c = 9−10 K). We have also confirmed by the Meissner effect the presence of a strong and inverse isotope effect of these SC salts. A discussion in relation to the influence of electronic localization on both the magnetic and transport properties in the conducting state and the appearance of the SC state in these lamellar type compounds has been developed.

Physical properties in the normal state of the molecular superconductor κ-(BEDT)2Cu2(CN)3

Abstract We have investigated the electrical, spectroscopic and magnetic properties of the κ-phase molecular conductor κ -(BEDT) 2 Cu 2 (CN) 3 between room temperature and the liquid He 4 temperature range. We have shown that an intermittent superconducting materials is present around or below 4 K which presents above this temperature limited spin fluctuations. The influence of disorder inside the anionic layers is discussed with respect to the X-ray crystal structure, and also the intrinsic effects due to temperature or pressure to explain this situation. Finally, the specific behaviour of the ESR linewidth at low temperature is analysed as a magnetic precursor regime of the supposed superconducting ground state.

Some new radical-cation salts based on unsymmetrical tetraheterofulvalenes : (MDTTTF)4Pt(CN)4, the first κ-phase salt, with a square planar dianion

Abstract We report the synthesis of the organic donor radical cations, methylenedithiotetrathiafulvalene (MDTTTF), ethylenedithiotetrathiafulvalene (EDTTTF) and vinylenedithiotetrathiafulvalene (VDTTTF), with the square planar dianions, Pt(CN) 4 and Ni(CN) 4 . Among these salts, the κ-(MDTTTF) 4 Pt(CN) 4 (CH 2 Cl 2 ) phase constitutes the first κ-phase, produced with a nonlinear dianion. Crystal and electronic structures, electrical and optical properties and ESR measurements have been investigated for these salts. Crystal and electronic structures of κ-(MDTTTF) 4 Pt(CN) 4 are similar to those of (MDTTTF) 2 AuI 2 , which is an ambient pressure superconductor ( T c = 4.5 K). However, d.c. conductivity measurements show a semiconducting behaviour, associated with an ESR linewidth decreasing with temperature in contrast with all other known κ salts. An explanation of the origin of these different behaviours is proposed.

Antiferromagnetic resonance in the BEDTTTF series

The antiferromagnetic ground state of beta -(BEDTTTF)2ICl2 and AuCl2 is probed by antiferromagnetic resonance. The parameters deduced (position of the magnetic axes and magnitude of the anisotropy) are discussed using a strongly localised picture of the spin ordering as suggested by the strongly dimerised structure of the ICl2 salt and the low electrical conductivity of these compounds. An excellent agreement is found between experimental and calculated results, assuming the dipole-dipole interactions are at the origin of the observed anisotropy.

Magnetic Properties of (DIMET)2SbF6: Quantitative Discussion of the Antiferromagnetic Behaviour

The low-temperature magnetic properties of (DIMET)2SbF6 have been probed by antiferromagnetic resonance experiments and the parameters which characterize the magnetic ordering are determined. Taking advantage of the structure of this compound which presents strongly dimerized dimers, we show that our results can be quantitatively explained using standard expressions for both the exchange and the anisotropy energies. Finally a brief comparison with other antiferromagnetic organic materials is presented.

Electronic structure and physical properties of unsymmetrical TTF based organic salts

Abstract The structural and electronic properties of four organic salts based on unsymmetrical molecules are discussed: (I): (MDTTTF)2AuI2; (II): (EDTTTF)2AuBr2; (III): (EDTTTF)2IBr2; (IV): (EDTDSDTF)2IBr2.(I), (III), and (IV) are metallic down to low T but only (I) exhibits a superconducting state at Tc ≈ 4.5K. (II) undergoes a metal to semiconductor transition at 125K under ambient pressure, but at 12K only under a pressure of 13kbar. The ESR results are presented and discussed: in (IV), a phase transition to an AF or a SP state is observed at 20K. The electronic dimensionnality is discussed in terms of the anisotropy of the extended Huckel transfer integrals and of the Fermi surface topology.

Metal-insulator phase transition in (BEDITTF)4M(CN)4 salts

Abstract Two isostoichiometric phases of metallocyanate salts (BEDT) 4 (M(CN) 4 ) 1 with M=Pt, Ni are isolated characterized; for each of them, phase transitions are evidenced by ESR and IR spectroscopies.

Antiferromagnetic properties of (DIMET)2SbF6

Abstract A new antiferromagnetic organic salt, (DIMET) 2 SbF 6 is presented. The peculiar, strongly dimerized structural arrangement of this salt allows its description in terms of quasi-isolated dimers. In this modelization the exchange coupling constants are deduced from transfer integrals and the anisotropy energy is calculated from dipolar interactions. The good agreement found between our description and experimental results is an asset for discussion about the origin of antiferromagnetism in organic conductors.

ANTIFERROMAGNETIC RESONANCE: A PROBE FOR VARIOUS ORGANIC CONDUCTORS

Abstract Antiferromagnetic resonance is shown to be relevant to probe the low temperature ground state of various organic conductors. A comparison of experimental results obtained for salts based on different molecules is first presented. A discussion of the characteristics of the AF ground state including the magnitude of the anisotropy and the position of the magnetic axes is then given.

New conducting radical cation salts based on BEDTTTF and tetracyanometallates square planar dianions [M(CN)42-, M=Pt(II), Ni(II)]

Abstract The BEDTTTF salts based on tetracyanometallate dianions exhibit a rich polymorphism giving rise to semiconductor, metallic and even superconductor materials. New (BEDTTTF) 4 M(CN) 4 (H 2 O) x (M=Ni, Pt; x= 0→4) phases were prepared. The stoichiometries have been determined by X ray analysis and agree well with the TGA (theromo-gravimetric analysis) results. These salts are isostructural and show a conductor-insulator transition at low temperature. A single crystal structure on the Ni (x=4) salt has been carried out. Crystals belong to the monoclinic space group P2/c with a=17.403(4), b=6.767(9), 15.030(3), β=109.26(2), R=0.05 for 2795 reflexions. The ESR line width dependence on temperature revealed a phase transition at about 180K. These phases are highly localised and present good candidates for a low temperature Peierls transition.