2X Family Research Articles

Electron Spin Resonance of Defects in Spin Chains—$$\hbox {o}-(\hbox {DMTTF})_2\hbox {X}$$: A Versatile System Behaving Like Molecular Magnet

The paper presents the Electron Paramagnetic Resonance study of defects in the spin chain o- (DMTTF)2X family using continuous wave and pulsed techniques. The defects in spin chains are strongly correlated and present similar microscopic structure as a molecular magnet. By means of 2D-HYSCORE and DFT calculations we show a strong reduction of hyperfine coupling between the defects and the nuclear spin bath. We assume that the reduction is due to the Heisenberg exchange interaction which screens the effect of the nuclei.

Charge-Order Phase Transition in the Quasi One-Dimensional Organic Conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3

Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the {hbox {(TMTTF)}}_2X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3. We found that the {hbox {(TMTTF)}}_2 {hbox {NO}}_3 compound develops a peculiar anisotropy with a doubled periodicity (ab'-plane) of the ESR linewidth below about T_{text {CO}}= ({250pm 10})~hbox {K}. This behavior is similar to observations in the related compounds {hbox {(TMTTF)}}_2X (X = {hbox {PF}}_6, {hbox {SbF}}_6 and {hbox {AsF}}_6), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous {hbox {(TMTTF)}}_2 {hbox {ClO}}_4, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below T_{text {CO}} approx 250~hbox {K}.

Phase transitions in a model of BEDT-TTF compound electron subsystem

Model of electronic subsystem of (BEDT-TTF)2X family is studied theoretically within Green function equation of motion approach. The configurational representation of the model Hamiltonian which allows analytical calculation of the energy spectrum is developed. Effective Hamiltonian of the model takes into account the strong intra-site Coulomb, intersite exchange interaction of electrons and correlated hopping of electrons both within the conducting plane and in transversal direction. With rise of intraatomic Coulomb repulsion in the energy spectrum a gap opens and the transition to antiferromagnetic state is realized. At the external pressure and decreasing temperature the transition from insulator to metallic phase occurs. The experimental phase diagrams for quasi-twodimensional organic conductors with strong electron correlations and correlated hopping are discussed.

Peierls and Spin-Peierls Instabilities in the Per2[M(mnt)2] Series of One-Dimensional Organic Conductors; Experimental Realization of a 1D Kondo Lattice for M = Pd, Ni and Pt

We consider structural instabilities exhibited by the one-dimensional (1D) (arene)2X family of organic conductors in relation with their electronic and magnetic properties. With a charge transfer of one electron to each anion X, these salts exhibit a quarter-filled (hole) conduction band located on donor stacks. Compounds built with donors such as fluorenthene, perylene derivatives and anions X such as PF6 or AsF6 exhibit a high temperature (TP ~ 170 K) conventional Peierls transition that is preceded by a sizeable regime of 1D 2kF charge density wave fluctuations (kF is the Fermi wave vector of the 1D electron gas located on Per stack). Surprisingly, and probably because of the presence of a multi-sheet warped Fermi surface, the critical temperature of the Peierls transition is considerably reduced in the perylene series α-(Per)2[M(mnt)2] where X is the dithiolate molecule with M = Au, Cu, Co and Fe. Special attention will be devoted to physical properties of α-(Per)2[M(mnt)2] salts with M = Pt, Pd and Ni which incorporate segregated S = 1/2 1D antiferromagnetic (AF) dithiolate stacks coexisting with 1D metallic Per stacks. We analyze conjointly the structural and magnetic properties of these salts in relation with the 1D spin-Peierls (SP) instability located on the dithiolate stacks. We show that the SP instability of Pd and Ni derivatives occurs in the classical (adiabatic) limit while the SP instability of the Pt derivative occurs in the quantum (anti-adiabatic) limit. Furthermore, we show that in Pd and Ni derivatives 1st neighbor direct and frustrated 2nd neighbor indirect (through a fine tuning with the mediated 2kF RKKY coupling interaction on Per stacks) AF interactions add their contribution to the SP instability to stabilize a singlet-triplet gap. Our analysis of the data show unambiguously that magnetic α-(Per)2[M(mnt)2] salts exhibit the physics expected for a two chain Kondo lattice.

Superconducting fluctuations in organic molecular metals enhanced by Mott criticality

Unconventional superconductivity typically occurs in materials in which a small change of a parameter such as bandwidth or doping leads to antiferromagnetic or Mott insulating phases. As such competing phases are approached, the properties of the superconductor often become increasingly exotic. For example, in organic superconductors and underdoped high-Tc cuprate superconductors a fluctuating superconducting state persists to temperatures significantly above Tc. By studying alloys of quasi-two-dimensional organic molecular metals in the κ-(BEDT-TTF)2X family, we reveal how the Nernst effect, a sensitive probe of superconducting phase fluctuations, evolves in the regime of extreme Mott criticality. We find strong evidence that, as the phase diagram is traversed through superconductivity towards the Mott state, the temperature scale for superconducting fluctuations increases dramatically, eventually approaching the temperature at which quasiparticles become identifiable at all.

Selection for verticillium wilt resistance in potato breeding populations derived from four cross types

AbstractVerticillium wilt (VW) is one of the important yield‐limiting diseases for potato production. To develop resistant clones, the potential for early generation selection was studied using three basic selection methods, individual, family, and within family selection, for two clonal generations. A total of 152 clones were derived from four cross types (2x × 2x, 2x × 4x, 4x × 2x and 4x × 4x). Clones were evaluated for maturity, symptom expression, yield and stem colonization in replicated trials. Heritability and selection response for the traits were estimated for each selection method. Direct selection in the second clonal generation and individual selection showed more gain than that from other methods. Both 2x × 2x and 4x × 2x families were higher yielding and had lower stem colonization scores than 2x × 4x and 4x × 4x crosses. Therefore, 2x × 2x or 4x × 2x crosses between carefully chosen parents with high yield and VW resistance may produce offspring with superior performance.

Field performance and resistance to tuber soft rot of potato clones derived from sexual polyploidization

Unilateral sexual polyploidization through 4x x 2x crosses is widely used to introgress genes from the diploid gene pool to the tetraploid cultivated potato Solanum tuberosum. Clones of eight 4x x 2x families, from crosses between four tetraploid cultivated potatoes and two diploid S. phureja - S. tuberosum hybrids, produc- ing 2n pollen, were evaluated for yield performances and resistance to tuber soft rot. Significant differences in terms of tuber yield and number were found between families and clones, with several clones outyielding their tetraploid parents. The source of variation due to females (4x females GCA) was significant for both tuber yield and number. A number of clones displayed inheritance of resistance to tuber soft rot. No relationship was found between agronomic traits and resistance to soft rot. Results suggest that the genotypes produced represent genet- ic material with good potentiality in the constitution of new genotypes with interesting trait combinations.

Antiferromagnetic spin fluctuations in the metallic phase of quasi-two-dimensional organic superconductors

We give a quantitative analysis of the published results of nuclear magnetic resonance (NMR) experiments for k-(ET)2X family of organic charge transfer salts by using the phenomenological spin fluctuation model of Moriya, and Millis, Monien and Pines (M-MMP). For temperatures above T_NMR ~ 50 K, the model gives a good quantitative description of the data for the paramagnetic metallic phase of several k-(ET)2X materials, with an antiferromagnetic correlation length which increases with decreasing temperature; growing to several lattice constants by T_NMR. It is shown that the fact that the dimensionless Korringa ratio is much larger than unity is inconsistent with a broad class of theoretical models (such as dynamical mean-field theory) which neglect spatial correlations and/or vertex corrections. For materials close to the Mott insulating phase, 1/T1, Ks, and K all decrease significantly with decreasing temperature below T_NMR. This cannot be described by the M-MMP model and the most natural explanation is that a pseudogap opens up in the density of states below T_NMR, as in, for example, the cuprates. We show that the NMR measurements reported for k-(ET)2Cu2(CN)3 are qualitatively inconsistent with this material having a ground state with long range magnetic order. A pseudogap in the metallic state of organic superconductors is an important prediction of the resonating valence bond theory of superconductivity. We propose specific new experiments on organic superconductors to elucidate the nature, origin, and momentum dependence of the pseudogap and its relationship to superconductivity.

Two-dimensional organic superconductors studied by NMR under pressure

The (BEDT)2X family of layered superconductors is one of the largest among nearly 50 organic superconductors currently known. One of the advantages of the organic compounds as prototype materials for studying the mechanisms of superconductivity is their relatively high sensitivity to hydrostatic pressure. We review recent NMR studies of these compounds using NMR under liquid and gas pressure. We focus on the low temperature part of the phase diagram where the physics is controlled by electronic correlations leading to a competition between magnetism and superconductivity. This interplay between different ground states is shown by the observation of a pseudo-gap and antiferromagnetic fluctuations and can be finely tuned by the application of pressure. Using a gas pressure system gives the unique possibility of sweeping the pressure at low temperature. Recently we used this technique to study the AF-SC boundary and established the existence of a first order transition line between the superconducting and antiferromagnetic states.

Anomalous temperature dependence of resistivity in quasi-one-dimensional conductors in a strong magnetic field

We present a heuristic, semiphenomenological model of the anomalous temperature (T) dependence of resistivity ρ xx recently observed experimentally in the quasi-one-dimensional (Q1D) organic conductors of the (TMTSF)2X family in moderately strong magnetic fields. We suggest that a Q1D conductor behaves like an insulator (dρ xx /dT < 0), when its effective dimensionality is one, and like a metal (dρ xx /dT > 0), when its effective dimensionality is greater than one. Applying a magnetic field reduces the effective dimensionality of the system and switches the temperature dependence of resistivity between the insulating and metallic laws depending on the magnitude and orientation of the magnetic field. We critically analyze whether various microscopic models suggested in literature can produce such a behavior and find that none of the models is fully satisfactory. In particular, we perform detailed analytical and numerical calculations within the scenario of magnetic-field-induced spin-density-wave precursor effect suggested by Gor’kov and find that the theoretical results do not agree with the experimental observations.

Magnetic-field-induced Luttinger insulator state in quasi-one-dimensional conductors

We present a heuristic, semiphenomenological model of the anomalous temperature ( T) dependence of resistivity ρ xx recently observed experimentally in the quasi-one-dimensional (Q1D) organic conductors of the (TMTSF) 2X family in moderately strong magnetic fields. We suggest that a Q1D conductor behaves like an insulator ( dρ xx/dT < 0), when its effective dimensionality is one, and like a metal dρ xx/dT > 0), when its effective dimensionality is greater than one. Applying a magnetic field reduces the effective dimensionality of the system and switches the temperature dependence of resistivity between the insulating and metallic laws depending on the magnitude of the magnetic field and its orientation along the “magic” and “nonmagic” angles.

The κ-(BEDT-TTF) 2Cu[N(CN)] 2X family: A mean field view

The phase diagram of κ-(BEDT-TTF) 2Cu[N(CN)] 2X (with X=Cl, Br, I) crystals is investigated in a mean-field approach that fully accounts for the crystal symmetry.

Anomalous temperature dependence of resistivity in quasi-one-dimensional conductors in a strong magnetic field

We present a heuristic, semiphenomenological model of the anomalous temperature (T) dependence of resistivity Rxx recently observed experimentally in the quasi-one-dimensional (Q1D) organic conductors of the (TMTSF)2X family in moderately strong magnetic fields. We suggest that a Q1D conductor behaves like an insulator (dRxx/dT<0), when its effective dimensionality is one, and like a metal (dRxx/dT>0), when its effective dimensionality is greater than one. Applying a magnetic field reduces the effective dimensionality of the system and switches the temperature dependence of resistivity between the insulating and metallic laws depending on the magnitude and orientation of the magnetic field. We critically analyze whether various microscopic models suggested in literature can produce such a behavior and find that none of the models is fully satisfactory. In particular, we perform detailed analytical and numerical calculations within the scenario of magnetic-field-induced spin-density-wave precursor effect suggested by Gor'kov and find that the theoretical results do not agree with the experimental observations.

Comparison of haploid Tuberosum- Solanum chacoense versus Solanum phureja -haploid Tuberosum hybrids as staminate parents of 4x–2x progenies evaluated under distinct crop management systems

The relative value of haploid Tuberosum-Solanum chacoense (TCH) vs. Solanum phureja-haploid Tuberosum (PTH) hybrids as male parents in 4x–2x crosses was estimated under two distinct crop management systems. The first experiment (E#1) was carried out with supplementary irrigation (three times a week) at Hancock-Wisconsin (WI) with 27 families derived from a sub-set of crosses involving 11 tetraploid (4x) cultivars and 10 diploid (2x) clones (3 TCH; 6 PTH; and one TCH-PTH hybrid clone). A second experiment (E#2) was conducted at Rhinelander-WI with the same group of families but it was only rainfed with no supplementary irrigation being provided during the entire course of the assay. For comparison, a complete set of 4x and 2x clones (used as parents) was also planted in each location. Three traits were evaluated in both locations: total tuber yield (TTY), haulm maturity (HM), and plant vigor (PV). Altogether, 18 out of 27 and 10 out of 27 families outyielded the corresponding 4x parents at E#1 and E#2, respectively. Significant differences among all 4x–2x families and among families of the TCH group were observed for TTY at both locations. No difference was found for TTY among families of PTH and [TCH–PTH] groups at each location. A total of 75% and 50% of the TCH families outyielded their 4x parents at E#1 and E#2, respectively. The percentage of families from the PTH group that outyielded their 4x parents at E#1 and E#2 was 83.3% and 25%, respectively. The TCH group had on average lower HM scores than PTH (at both locations) indicating some segregation for earliness. However, the HM values of the 4x–2x families were, in general, higher than those observed for the 4x parents. For PV, the PTH group mean was significantly higher than TCH group only in E#1. The PV values of the 4x–2x families were higher than the 4x-parent group at both locations. Our results indicated that TCH hybrids might be as good parents as PTH to increase the TTY of 4x–2x progenies. In addition, the TCH families displayed a slight higher level of performance under more stressful growing conditions than PTH hybrids as indicated by the assay at Rhinelander. Therefore, selection of genetic materials with potential broad range of adaptation seems to be feasible with both hybrids via the 4x–2x breeding scheme.

Cerium Containing Chalcogenides with Chromatic Properties

ABSTRACTλ–Ce2S3, like many other cerium containing chalcogenides, presents a strong absorption in the visible spectra in relation with the CeIII-4fl → CeIII-5d1 electronic transition taking place at around 1.9 eV. This transition energy, that induces a red hue for the phase, can be increased by changing the nature of the Ce-S bonding, in particular through an enhancement of its ionicity. This can be made by inserting/substituting some alkali metal ions in the structure of λ-Ce2S3. The sodium-doped λ-Na3/8Ce15/2S3 phase for instance undergoes a CeIII-4f1 → Cemi-5d1 transition energy increase of 0.12 eV. This shift is sufficient to modify substantially the pigment properties of the material and it has been mainly related to the narrowing of the electronic bands. In CePS4, Ce2SiS5, Ce4Si3S12and Ce6Si4S17, strong covalent bondings enhance the ionic Ce-S bond through inductive effect, leading to a CeIII-4f1 → CeIII-5d1 electronic transition in the blue (Eg - 2.5 eV) hence the yellow hue of these materials. Within the Ce-Si-S family, small differences may be attributed to difference in the [CeSx] and [SiS4] polyhedra bonding. The Ce3(TS4 )2X family (X = Cl, Br, I, T = Si, Ge) does not show pigment properties because of the Ce-X bond inducing a more Ce-S ionic character (in particular when X = Br, I) and thus a higher gap that is found in the near ultraviolet. On the other hand, the phases present an important room temperature blue fluorescence.

13C-NMR Study of Mott Insulating Phase in κ-(BEDT-TTF) 2X Family

The κ-(BEDT-TTF) 2X family are quite attractive in the study of metal-insulator transitions and competition between superconductivity and magnetic ordering. We report 13C-NMR study of X= Cu[N(CN) 2]Cl salt and discuss on the electronic structure, compared with the results of superconducting salts. In addition, we found that the deuterated Cu[N(CN) 2]Br salt, when cooled rapidly, showes AF ordering like the Cu[N(CN) 2]Cl salt with only minor fraction of superconducting phase. We also report 13C-NMR study of this salt.

NMR in the (BEDT) 2X organic superconductors

We review recent NMR studies of a layered organic superconductor of the (BEDT) 2X family. First the normal state prop erties are discussed. The central issue is the importance of the interplay between antiferromagnetism and superconductivity in these materials demonstrated by the observation of a pseudogap developing prior to the superconducting transition. The use of high pressure to tune the amplitude of AF fluctuations enables us to draw a general phase diagram of the family. In the second part the NMR experiments in the superconducting state are presented. One of the fundamental questions addressed by these experiments is the symmetry of the order parameter related to the pairing mechanism. The most spectacular NMR measurements are those performed in the lock-in state, which give access to the density of superconducting excitations at low energy. These experiments suggest that the pairing state may be unconventional featuring an anisotropic superconducting gap, possibly with nodes.

Fast oscillations in (TMTSF) 2X

We report on the temperature dependence of the “fast oscillation” phenomenon in quasi-one dimensional molecular conductors. Magnetoresistance and magnetization measurements to 30T in the ambient pressure spin density wave state have been taken for the X = NO 3, PF 6 and AsF 6 members of the (TMTSF) 2X family. Complementary data on X=AsF 6 are presented by Ulmet et al. in these proceedings. All three compounds exhibit a peak in the transverse magnetoresistances ρ xx (B//c *, I//a) and ρ yy (B//c *, I//b) and thenoscillatory components around 3K, with the oscillations vanishing at higher and lower temperatures. No magnetic moment oscillations are detected in PF 6 or AsF 6, while only extremely weak oscillations (δm~10 −10 emu) are seen in the NO 3 salt above 25T. Also, no oscillations were observed in longitudinal magnetoresistance ρ zz (B//I//c *) measurements performed on AsF 6 and PF 6. Possible scenarios which have been used to explain the data include imperfect nesting at T SDW leaving behind quantizable closed orbits, and quantum interference effects arising from magnetic breakdown orbits. Another may involve a new phase transition inside the SDW state.

Magnetic properties of DCNQI salts studied using μSR

Examples from the (R 1,R 2-DCNQI) 2X family of molecular conductors have been studied using μSR in order to provide information about the microscopic magnetic properties of the various phases. For the fully deuterated dimethyl Cu salt (d 8-DMe-Cu) there is a metalinsulator (MI) transition around 80 K and a magnetic transition around 7 K. The muon spin relaxation rate becomes enhanced in the region of the MI transition and below, reflecting the quenching of valence fluctuations and the appearance of localised spins on the Cu sites. A zero field precession signal develops below 7 K as a result of the 3D magnetic ordering of the Cu spins; the field distribution derived from the precession frequency is consistent with the proposed magnetic structure. In addition to the zero field studies, nuclear quadrupolar level crossing resonance between the muon and the imine nitrogen of the DCNQI has been used to study the temperature dependence of the electronic state of the molecular conductor.

Strong electron correlations in κ-(BEDT-TTF) 2X salts

The κ-ET 2X compounds are classified as strongly correlated electron systems. The tight-binding approach for correlated electrons is applied to the κ-ET 2X family. The electronic spectrum and density of states are calculated. The singlet superconducting pairing of different symmetries is studied based on the symmetry of the latice model of ET 2-layer, and the superconducting coupling constants are calculated. The comparison of the theoretical speculations with available experimental data is made for normal and superconducting phases in κ-ET 2X.