2X Family Research Articles

Contrasted structural properties of organic superconductors

We present structural studies of organic conductors and superconductors of the (TMTSF) 2X and (BEDT-TTF) 2X families. These materials undergo a large variety of structural phase transitions like ordering of non-centrosymmetric anions X − in the (TMTSF) 2X salts and commensurate ((BEDT-TTF) 2ReO 4) or incommensurate (β(BEDT-TTF) 2I 3, (TMTSF) 2SCN) lattice modulations. We shall pay special attention on new results concerning the pressure induced anion-ordering transformation in (TMTSF) 2ReO 4 and the two competing phase transitions of (TMTSF) 2PF 2O 2.

Tuber yield of true potato seed families from different breeding schemes

Economical and environmental conditions in the south of Italy suggest that the direct utilization of TPS, as propagation material for potatoes, could be worth exploring. Thus, it is important to identify the most appropriate breeding scheme to enhance productivity and uniformity of the progeny. For this purpose 44 TPS families deriving from different breeding schemes and emphasizing different levels of heterozygosity were tested for yield at two locations in the U.S.A. and one in Italy. The results indicate the superiority of the 4x progeny from 4x X 2x crosses as a family group, and in the frequency of high yielding individual families in all locations. The 4x X 2x family group outyielded the OP progeny group by 45%, 79% and 114% at Hancock, Rhinelander (U.S.A.) and in Italy, respectively.

Importance of one-dimensional correlations in the phase diagram of the (TMTTF) 2-(TMTSF) 2-X salts

In this work, we explain the ingredients of a model for a semi-quantitative analysis of the large variety of phase transitions observed in the two similar families of organic compounds (TMTTF) 2X and (TMTSF) 2X (X=PF 6, AsF 6, Br, TaF 6 …). We find that the sequence of magnetic transitions in the (TMTTF) 2X family up to moderate pressure are dominated by significant changes in the one-dimensional (1D) or intrachain correlations of the 1D electron and electron-phonon systems. These 1D effects are stabilized by the interchain pair tunneling processes generated by the single-particle hopping. In the (TMTSF) 2X family, however, these interchain interactions significantly decrease the range in temperature (T<T X 1) in which there is 3D nesting while contributing to the superconductivity pairing interaction below the crossover temperature (T X 1).

Meissner effect under pressure in organic superconductors of the β-(BEDT-TTF) 2X family ( X = I3, IBr2, AuI2)

We present the temperature-pressure phase diagram of the title compounds derived from d.c. magnetization data. From the ratio of Meissner- to shielding susceptibility all salts, including the high-T c-phase of β-ET 2I 3, are found to be volume superconductors.

Pressure effect on the magnetic susceptibility of the (TMTSF) 2X family

The temperature dependence of the spin-susceptibility χ S of (TMTSF) 2X for X=PF 6, C10 4 and ReO 4 was measured to hydrostatic pressures of 10 kbar using a Faraday magnetometer. For these three materials the fractional pressure dependence d≡∂ℓnχ S/∂P is negative and decreases in magnitude with increasing temperature from approximately -4.5%/kbar below 100 K to -3%/kbar at ambient temperature. This behavior contrasts with that of TTF-TCNQ, where |d| appears to increase with temperature, and with β-(BEDT-TTF) 2I 3, where d is temperature independent.

Temperature dependence of the transfer integrals in the (TMTSF)2X and (TMTTF)2X families

The authors have evaluated the temperature dependence of the tight-binding transfer integral between neighbouring cation molecules for some representative compounds of the (TMTSF)2X and (TMTTF)2X families using their recently determined low-temperature and/or high-pressure crystallographic structures. The changes in the transverse integrals in going from the new structures to the ambient temperature and pressure structures are largest for those integrals which show an antibonding character compared to the intrastack interaction. These changes lead to a low-temperature electronic picture which differs markedly from the well known high-temperature description. The transverse bonding and antibonding interactions tend to equalise for the atmospheric pressure supraconductor (TMTSF)2ClO4. The results suggest that the structure of the most metallic TMTTF salt, (TMTTF)2Br, is equivalent at a temperature T1 to the structure of (TMTSF)2X at T=T1 or (TMTTF)2PF6 at T=T1+300K with regard to the values of the transverse transfer integrals.

Quantum Cascade of Field-Induced SDW Phases in Quasi-1D Conductors

The metallic open Fermi surface of quasi-1D conductors, such as the (TMTSF)2X family, is destabilized in moderate magnetic fields inducing a semi-metallic SDW phase. The origin of this phenomenon is a quantum interference effect: Because of phase coherence between the SDW and the electron-hole wave function, the susceptibility diverges at low T for a set of quantized wavevector, leading to a metal SDW critical line made of a succession of segments, each of them labelled by a quantum number. Below this line, we expect, in increasing field, a cascade of first-order transitions between SDW phases corresponding to different quantum numbers, as observed recently in salts of the (TMTSF)2X family. Within each phase, SDW ordering and orbital quantization open an infinite series of gaps in the quasi-particle spectrum. The wave vector is quantized so that the Fermi level lies in the middle of a Landau gap. Significant departures from BCS behaviour are predicted.

Spin-orbit interaction effects in quasi-one-dimensional conductors

The author studies the effects of the spin-orbit interaction on the competing pairing states of a model quasi-one-dimensional conductor with hopping-type interchain coupling. Within a ladder approximation scheme use of the renormalisation-group method allows him to account explicitly for anisotropy effects involving the intrachain and interchain transfer integrals. Parametrising the spin-orbit interaction by a backward, g1n<0, and forward, g2n<0, scattering coupling constant, he concludes that its main effect is to stabilize the triplet-pairing states of spin density wave and triplet superconductivity over the corresponding singlet-pairing states of charge density wave and singlet superconductivity. In particular the boundary line between the density-wave-type states and the superconducting states is determined as: g1+g1n=2g2, where g1 and g2 are the usual spin-independent backward- and forward-scattering coupling constants. The boundary line between the triplet- and singlet-pairing states is determined as: 2g1=g1n+g2n- mod g1n-g2n mod . The direction of the triplet-pairing polarization axis m with respect to the spin-orbit-anisotropy axis n is shown to be dependent only upon the sign of the difference: g1n-g2n. The implications of these results of the (TMTSF)2X and (TMTTF)2X families of organic conductors is discussed. Finally, it is shown that the spin-dependent exchange interaction introduced recently by Abrikosov (1983) can be reduced to a simple definition of the spin-independent couplings g1 and g2. He concludes that a ferromagnetic type of exchange also favours the triplet-pairing states over the singlet-pairing states.

(TMTSF) 2F2PO2: An unusual member of the (TMTSF) 2X family of organic metals

Resistivity measurements were carried out as a function of temperature and pressure in (TMTSF) 2F2PO2. A strong metal-insulator transition is observed at ∼137K which is only incompletely suppressed by high pressures. The nature of the resistivity curves obtained at high pressure suggest the onset of a Fermi surface instability; however, the metallic-like transport at lower temperatures suggests that the energy gap covers only part of the Fermi surface. The surprising increase in ϱ (T) at temperatures below 20K (under pressure) may result from disorder induced localization arising from the strong electronic scattering from the randomly oriented dipole moments.

Pressure dependence of the metal-insulator transitions in (TMTTF)2SCN

The pressure-temperature phase diagram (for pressures up to 23 kbar) of bis-tetramethyltetrathiofulvalene thiocyanide, (TMTTF)2SCN, has been determined through needle axis resistivity measurements. The authors find two distinct metal-insulator (MI) transitions. At low pressures there is an MT transition at high temperature TI(P) (near 160K at 1 bar); TI falls with increasing pressure (dTI/dP approximately -2K kbar-1) and is abruptly suppressed for pressures above 15 kbar where a low-temperature (TII approximately 15K) MI transition is stabilised. In a narrow pressure domain just below 15 kbar pronounced hysteretic behaviour is observed near TI and effects associated with both of these MI transitions are seen. By comparison with the behaviour of other members of the isostructural families (TMTTF)2X and (TMTSeF)2X they associate TI with an anion-disorder transition and TII with a lattice distortion or spin density wave instability on the TMTTF stack. These results confirm a strong correspondence between the properties of the (TMTTF)2X and (TMTSeF)2X families, in particular with respect to the behaviour of those materials containing non-centrosymmetric anions.

Spin density waves in organic metals (invited)

The magnetic properties of the (TMTSF)2X family of organic conductors are reviewed. Susceptibility, ESR, and NMR measurements have led to the conclusion that the metal-insulator transition for X = PF−6, AsF−6, and SbF−6 is driven by the onset of a spin density wave. Near the transition temperature (TSDW), the ESR line shifts, broadens, and rapidly loses intensity. In the semiconducting phase of (TMTSF)2PF6 the static susceptibility is anisotropic and reveals a spin-flop transition at 6 kG. The increase in the proton NMR linewidth yields a value for the SDW amplitude of μ/μB∼ 10−1, in rough agreement with the prediction of mean field theory. Fluctuations near TSDW contribute to proton spin-lattice relaxation.

Superconductivity in the Organic Charge Transfer Salts: (TMTSF)2X and (TMTTF)2X

Abstract We report pressure dependent studies of the a-axis resistivity as a function of temperature for several members of the isostructural families of organic charge transfer salts, (TMTSF)2X and (TMTTF) 2X. For a typical (TMTSF)2X material the low temperature metal-insulator transition seen at 1 bar is suppressed above some critical pressure, Pc, where a superconducting transition is observed near 1 K. We find a correlation between Pc and the ambient pressure c lattice parameter which reflects the anion size. The (TMTTF) cX salts exhibit very different ambient pressure behaviour but we find that with the application of sufficiently high pressures (∼30 kbar) their behaviour resembles that seen in the (TMTSF)2X family but at lower pressures. In particular we find evidence of a possible superconducting transition near 4 K in (TMTTF)2Br at 25 kbar. At this pressure the conductivity near 4 K is extremely high with a value approaching 106 (Ωcm)−1 and the resistivity ratio is about 400.

Bilateral sexual polyploidization in potatoes

Bilateral sexual tetraploidization was achieved by intercrossing diploid Phureja × haploid Tuberosum hybrids. Both parents contribute gametes with the somatic number of chromosomes (diplogynoids and diplandroids). More than 100 tetraploid progeny appeared in nine 2x−2x families. The tetraploids are generally more vigorous and significantly outyield their diploid ‘full-sibs’. The superiority of the tetraploids is interpreted on the basis of the mode of diplandroid (2n pollen) formation by first meiotic division restitution (FDR). Heterozygosis, epistasis and genetic diversity, if present in the parental diploid genotype, are largely maintained in the 2n gametes formed by FDR. and syngamy brings about more opportunities for heterotic responses in conjunction with polyploidization. Differences in tuber yield among tetraploid clones within families were found to be statistically significant (P <0.05). Thus, unlike somatic chromosome doubling, which can merely increase chromosome numbers, or somatic hybridization, which could in addition bring about heterosis, sexual polyploidization is also associated with genetic variability. The degree of inbreeding and the genotypic structure in the population of newly arisen tetraploids is discussed. The relationship among asexual reproduction, preservation of favorable nonadditive genetic effects by FDR. restoration of sexuality, and polyploidization is explored. The possible evolutionary significance of these factors is emphasized.

The significance of 2N gametes in potato breeding

Phureja-haploid Tuberosum hybrids, which produce 2n gametes in addition to n gametes, were used to obtain diploid progenies in 2x - 2x matings, and tetraploid progeny in 4x - 2x matings. Seven of these diploid clones were intermated in a modified diallel design, and also crossed to seven cultivars, to obtain 21, 16-clone diploid; and 49 (35, 2x X 4x and 14, 4x X 2x) 24-clone tetraploid families, respectively. These progenies were included together with the 14 parental clones, in a tuber yield trial conducted in two locations in Wisconsin. Heterotic responses were striking in 4x X 2x progenies. Three of these entries had mean yields of 5.0, 5.0 and 5.2 lbs/hill, for 24 unselected clones, thus outyielding not only the mid-parent but also the best tetraploid cultivars in the experiment, which are highly selected; Wis 643 (4.8 lbs/hill), and Kennebec (4.8 lbs/hill). As a group, the 14, 4x X 2x families averaged 4.4 lbs/hill, while the mean of all seven cultivars was 4.0 lbs/hill, and the mid-parent was at 3.4 lbs/hill. This unusually high mean performance of 336 unselected clones represented in the 14, 4x X 2x progenies, is interpreted as a manifestation of the capacity of 2n pollen, formed by first meiotic division restitution (FDR), to pass onto the progeny the already heterotic diploid genotype in a largely intact array. Beneficial intra and inter-locus interactions are presumably compounded upon syngamy with an unrelated n egg from the tetraploid parent. -The performance of 2x X 4x progenies was at or below that of the midparent. The failure of these families to perform as well as 4x X 2x families may be a reflection of the incapacity of 2n megasporogenesis to avoid meiotic reassortment as efficiently as FDR does, which would result in inbred 2n gametes. However, the method of 2n megasporogenesis is not known.Hereditary variances were large, both within and among families, and indicated considerable nonadditivity in the action of genes affecting tuber yield at both levels of ploidy. Inbreeding was strongly depressing at both ploidy levels.