Linear-T scattering and pairing from antiferromagnetic fluctuations in the (TMTSF)_2X organic superconductors

An exhaustive investigation of metallic electronic transport and superconductivity of organic superconductors (TMTSF)_2PF_6 and (TMTSF)_2ClO_4 in the Pressure-Temperature phase diagram between T=0 and 20 K and a theoretical description based on the weak coupling renormalization group method are reported. The analysis of the data reveals a high temperature domain (T\approx 20 K) in which a regular T^2 electron-electron Umklapp scattering obeys a Kadowaki-Woods law and a low temperature regime (T< 8 K) where the resistivity is dominated by a linear-in temperature component. In both compounds a correlated behavior exists between the linear transport and the extra nuclear spin-lattice relaxation due to antiferromagnetic fluctuations. In addition, a tight connection is clearly established between linear transport and T_c. We propose a theoretical description of the anomalous resistivity based on a weak coupling renormalization group determination of electron-electron scattering rate. A linear resistivity is found and its origin lies in antiferromagnetic correlations sustained by Cooper pairing via constructive interference. The decay of the linear resistivity term under pressure is correlated with the strength of antiferromagnetic spin correlations and T_c, along with an unusual build-up of the Fermi liquid scattering. The results capture the key features of the low temperature electrical transport in the Bechgaard salts

Interaction between cation and anion sublattices in molecular charge transfer salts: Structural conditions for ferrimagnetism

Many molecular charge transfer salts with organo-chalcogen donors and transition-metal complex anions have been synthesised in recent years in an effort to find lattices that combine conductivity with long range magnetic order, but in most cases interaction between the donor and anion sublattices is very weak. We have approached this issue by selecting anions that contain S or Se atoms capable of forming close non-bonding contacts with the donor molecules, and which also contain aromatic rings to form π-π contacts. In this way new ferrimagnets have been synthesised and characterised, with general formula D[M(NCX)B] where D is TTF or BEDT-TTF; M = Cr, Fe; X = S, Se; B = 1,10-phenantholine, isoquinoline. We have demonstrated that cation-anion S⋯S and π⋯π contacts are necessary. © 2002 Elsevier Science B.V. All rights reserved

Interaction between cation and anion sublattices in molecular charge transfer salts: Structural conditions for ferrimagnetism

Many molecular charge transfer salts with organo-chalcogen donors and transition-metal complex anions have been synthesised in recent years in an effort to find lattices that combine conductivity with long range magnetic order, but in most cases interaction between the donor and anion sublattices is very weak. We have approached this issue by selecting anions that contain S or Se atoms capable of forming close non-bonding contacts with the donor molecules, and which also contain aromatic rings to form π-π contacts. In this way new ferrimagnets have been synthesised and characterised, with general formula D[M(NCX)B] where D is TTF or BEDT-TTF; M = Cr, Fe; X = S, Se; B = 1,10-phenantholine, isoquinoline. We have demonstrated that cation-anion S⋯S and π⋯π contacts are necessary. © 2002 Elsevier Science B.V. All rights reserved

Synthesis, crystal structures and physical properties of TTF-based conducting charge transfer salts with anions containing selenocyanate ligands

Three new charge transfer salts of tetrathiafulvalene (TTF)-based donors with selenocyanate-metal complex anions have been synthesized. The salts have been characterized as BEDT-TTF[Cr(NCSe)]·CHCl , II, TTF[Cr(NCSe)phen], III and BEDT-TTF[Cr(NCSe)phen]·CHCl , IV, where phen = 1,10′-phenanthroline and BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene. At 120 K, II crystallizes in the P2/m space group with a = 10.454(1), b = 31.990(1), c = 12.339(1) Å, β = 113.163(2)°, V = 3793.8 Å, Z = 4 and at 240 K in the same space group with a = 10.530(1), b = 32.122(1), c = 12.396(1) Å, β = 113.186(3)°, V = 3854.2 Å, Z = 4. At 240 K III crystallizes in the C2/c space group, a = 38.9272(5), b = 11.2992(1), c = 15.2540(3) Å, β = 106.8877(6)°, V = 6420.1 Å, Z = 8. The structure of IV has been solved in the P1̄ space group with a = 8.7629(3), b = 11.7977(4), c = 26.6031(9), α = 81.697(2), β = 87.858(3), γ = 74.8471(14), V = 2626.8 Å, Z = 2. All of the salts have numerous S⋯Se close atomic contacts between donors and acceptors but there is no magnetic exchange between ions, as previously seen in closely related salts such as TTF[Cr(NCS)phen] and (donor)[M(NCS)(isoquinoline)], where M = Cr, Fe and donor = TTF, BEDT-TTF or TMTTF (tetramethyltetrathiafulvalene). Indeed, II and IV are paramagnetic semiconductors whereas IlI is a paramagnetic insulator. The absence of long-range magnetic order is discussed in terms of structure-function relations since there are no π-stacking interactions between donor and acceptor, which are seen in all of the bulk magnets of this type where the donor spin is magnetically coupled to the anion. The synthesis and crystal structure of the starting material, [(CH)N][Cr(NCSe) ], I, is also reported; it crystallizes in the P1̄ space group with a = 12.220(1), b = 12.814(1), c = 13.008(1) Å, α = 99.608(6), β = 114.028(5), γ = 92.637(6)°, V = 1819.5 Å, Z = 2. © 2002 Elsevier Science (USA)

Synthesis, crystal structures and physical properties of TTF-based conducting charge transfer salts with anions containing selenocyanate ligands

Three new charge transfer salts of tetrathiafulvalene (TTF)-based donors with selenocyanate-metal complex anions have been synthesized. The salts have been characterized as BEDT-TTF[Cr(NCSe)]·CHCl , II, TTF[Cr(NCSe)phen], III and BEDT-TTF[Cr(NCSe)phen]·CHCl , IV, where phen = 1,10′-phenanthroline and BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene. At 120 K, II crystallizes in the P2/m space group with a = 10.454(1), b = 31.990(1), c = 12.339(1) Å, β = 113.163(2)°, V = 3793.8 Å, Z = 4 and at 240 K in the same space group with a = 10.530(1), b = 32.122(1), c = 12.396(1) Å, β = 113.186(3)°, V = 3854.2 Å, Z = 4. At 240 K III crystallizes in the C2/c space group, a = 38.9272(5), b = 11.2992(1), c = 15.2540(3) Å, β = 106.8877(6)°, V = 6420.1 Å, Z = 8. The structure of IV has been solved in the P1̄ space group with a = 8.7629(3), b = 11.7977(4), c = 26.6031(9), α = 81.697(2), β = 87.858(3), γ = 74.8471(14), V = 2626.8 Å, Z = 2. All of the salts have numerous S⋯Se close atomic contacts between donors and acceptors but there is no magnetic exchange between ions, as previously seen in closely related salts such as TTF[Cr(NCS)phen] and (donor)[M(NCS)(isoquinoline)], where M = Cr, Fe and donor = TTF, BEDT-TTF or TMTTF (tetramethyltetrathiafulvalene). Indeed, II and IV are paramagnetic semiconductors whereas IlI is a paramagnetic insulator. The absence of long-range magnetic order is discussed in terms of structure-function relations since there are no π-stacking interactions between donor and acceptor, which are seen in all of the bulk magnets of this type where the donor spin is magnetically coupled to the anion. The synthesis and crystal structure of the starting material, [(CH)N][Cr(NCSe) ], I, is also reported; it crystallizes in the P1̄ space group with a = 12.220(1), b = 12.814(1), c = 13.008(1) Å, α = 99.608(6), β = 114.028(5), γ = 92.637(6)°, V = 1819.5 Å, Z = 2. © 2002 Elsevier Science (USA)

An unusual phase transition in the crystal structure and physical properties of (TTF)[MO(CN)]·4HO, where TTF = tetrathiafulvalene

At 270 K, the charge transfer salt(TTF)[Mo(CN)]·4H O,I, crystallizes in the triclinic space group P1̄ with a = 9.9094(2), b = 10.6781(2), c = 23.6086(7) Å, a = 75.7910(8), β = 88.6010(9), γ = 78.5250(8)°, V = 2372.5(1) Å and Z = 2. At 120 K, the space group is unchanged with a = 9.7990(7), b = 10.6630(5), c = 22.9940(2) Å, a = 79.981(4), β = 89.798(4), γ = 79.013(4)°, V = 2321.5 Å and Z = 2. On comparing the two sets of data, we see significant changes in the cell parameters, most notably in the angle a. Variable temperature crystallographic studies indicate a first order phase transition accompanied by hysteresis, which corresponds to a change in the transport properties.I is a semiconductor and the high temperature activation energy of 0.06 eV changes sharply to 0.15 eV below 236 K. Bulk magnetic susceptibility and ESR measurements indicate that the TTF molecules are antiferromagnetically coupled. The temperature dependence of the EPR spectrum changes from 300-200 K, in approximate agreement with the transport and structural results. The optical spectrum of (TTF)[Mo(CN)]·4HO consists of several broad bands assigned to TTF charged molecules, to [Mo(CN)] and to charge transfer from the donors to the acceptor in the near infra-red range. Preliminary magnetic susceptibility measurements under light irradiation with a multi-line (752.5-799.3 nm) laser were also performed, but no photomagnetic effect was noted

Temperature and pressure dependencies of the crystal structure of the organic superconductor (TMTSF)

The crystal structure of (TMTSF)2ClO4 has been determined at (7 K, 1 bar) and at (7 K, 5 kbar) with a high accuracy. For the latter, low temperature and pressure were applied simultaneously using a X-ray diffraction instrumentation designed in our laboratory, these results are the first for molecular compounds. The effects of lowering the temperature are not the same as those produced by increasing the pressure. At (7 K, 1 bar) the anion ordering which occurs in this compound, and which is characterised by the appearance of $b^*/2$ superlattice reflections, is well observed. This anion ordering leads to the presence of two independent stacks of TMTSF cations which is the only case found in the Bechgaard salts family. The comparison of the low temperature crystal structures under atmospheric pressure and at 5 kbar shows that the centres of mass are nearly the same, independent of the pressure: the interchain interactions do not depend on the doubling of the unit cell. Under pressure, the ordering (0, 1/2, 0) does not occur at any temperature. These structural data are confirmed by the quantum chemical calculations which show that the difference in the site energy of the two independent cations is 100 meV

An unusual phase transition in the crystal structure and physical properties of (TTF)[MO(CN)]·4HO, where TTF = tetrathiafulvalene

At 270 K, the charge transfer salt(TTF)[Mo(CN)]·4H O,I, crystallizes in the triclinic space group P1̄ with a = 9.9094(2), b = 10.6781(2), c = 23.6086(7) Å, a = 75.7910(8), β = 88.6010(9), γ = 78.5250(8)°, V = 2372.5(1) Å and Z = 2. At 120 K, the space group is unchanged with a = 9.7990(7), b = 10.6630(5), c = 22.9940(2) Å, a = 79.981(4), β = 89.798(4), γ = 79.013(4)°, V = 2321.5 Å and Z = 2. On comparing the two sets of data, we see significant changes in the cell parameters, most notably in the angle a. Variable temperature crystallographic studies indicate a first order phase transition accompanied by hysteresis, which corresponds to a change in the transport properties.I is a semiconductor and the high temperature activation energy of 0.06 eV changes sharply to 0.15 eV below 236 K. Bulk magnetic susceptibility and ESR measurements indicate that the TTF molecules are antiferromagnetically coupled. The temperature dependence of the EPR spectrum changes from 300-200 K, in approximate agreement with the transport and structural results. The optical spectrum of (TTF)[Mo(CN)]·4HO consists of several broad bands assigned to TTF charged molecules, to [Mo(CN)] and to charge transfer from the donors to the acceptor in the near infra-red range. Preliminary magnetic susceptibility measurements under light irradiation with a multi-line (752.5-799.3 nm) laser were also performed, but no photomagnetic effect was noted

Laboratory high-pressure single-crystal x-ray diffraction—recent improvements and examples of studies

Some recent improvements made to the high-pressure single-crystal x-ray diffraction (XRD) experiments in use in our laboratory are reviewed..