Some Properties of the (TMTSF)2X Superconductors

Measurements of the anisotropy of the critical field in (TMTSF)₂X superconductors are presented. The results show that the a axis H_c2 is Pauli limited based on a T_c near 1K. The large energy gap obtained from tunneling data is therefore thermodynamically untenable in terms of the present critical field studies and the superconductivity is singlet rather than triplet. Moreover, we find that low levels of irradiation induced defects eliminate the superconductivity in the ClO₄ and PF₆ salts and leave the thermopower of the ClO₄ salt below 100K unchanged. These results are incompatible with the proposal for 1D superconducting fluctuations below 40K. A discussion of the arguments for and against the occurance of superconducting fluctuations in the (TMTSF)₂X salts is given with emphasis on the role of interchain interactions and other possible interpretations of the tunneling data.     

Low-Temperature Electron Spin Resonance in (TMTSF)2PF6 in the High Pressure Metallic Phase

Abstract

The electron spin resonance of (TMTSF)₂PF₆ has been observed at low fields (H_o < 110 Oe) in the high pressure, metallic phase (p > 6.5 kbar) in the temperature range 1-4^?K. The anisotropy in the g value is similar to that observed at ambient pressure above the metal-insulator transition. The linewidth is very narrow and the spin susceptibility strongly decreases as the superconducting transition is approached from above. We interpret this as evidence for singlet-paired superconductivity. Superconductivity is observed at 1.1 K and the critical field has angular dependence in the be plane. These observations lead us to conclude that (TMTSF)₂PF₆ is a singlet paired superconductor.     

X-Ray Diffuse Scattering Study of Some (TMTSF)2X and (TMTTF)2X Salts

X-ray diffuse scattering experiments on the superconducting salts (TMTSF)₂X with X = PF₆, AsF₆ and ClO₄ reveal no charge density wave instability, in contrast with the poorly conducting (TMTTF)₂PF₆ salt. New structural phase transitions coinciding with conductivity anomalies are found in (TMTSF)₂NO₃, (TMTSF)₂ReO₄, and (TMTTF)₂ClO₄. It is suggested that they correspond to order-disorder transitions involving the non-centrosymmetric counter ions. Relationships between these structural instabilities and superconductivity are also discussed.     

Thermal Expansion in TMTSF-Dmtcnq & (TMTSF)2PF6

We have measured the anisotropic thermal expansion of TMTSF-DMTCNQ and (TMTSF)₂PF₆ in the temperature range 10-300 K using an X-ray diffraction technique with a relative accuracy of 3 parts in 10⁴. Between 10 K and 300 K we find expansion in the a*, b* and c* directions to be respectively 2.5%, 0.2% and 1.7% for TMTSF-DMTCNQ and 3.7%, 1.6% and 1.3% for (TMTSF)₂PF₆     

NMR Studies of (TMTSF)2PF6

Abstract

Pulsed NMR determinations of the relaxation times T₂, T₂* and T₁, as a function of temperature and field, for the methyl group protons in (TMTSF)₂PF₆ are reported. Below the metal-insulator transition (T_MI)T₂* shortens while T₂ increases, indicating a line which is inhomogeneously broadened due to the onset of a SDW. The SDW also contributes to the spin-lattice relaxation rate, T₁ ^?1, in the neighborhood of T_MI. A frequency dependent maximum in T₁ ^?1 is observed near 20K and is attributed to methyl group rotation. A frequency independent maximum at 58K suggests a structural phase transition involving rearrangement of the methyl and PF₆ groups.     

Magnetism of the (TMTSF)2X Family of Organic Conductors

Abstract

The magnetic properties of the (TMTSF)₂X series of conductors have been studied using magnetic susceptibility, electron spin resonance and both CW and pulsed NMR measurements. The results imply the existence of a spin-density-wave ground state, at ambient pressure, in at least the PF₆ ^?, AsF₆ ^? and SbF₆ ^? salts. The amplitude of the SDW, the exchange interaction, the anisotropy energy and the single particle gap are extracted from the experimental data.     

High Electric Field Transport In (TMTSF)2 Pf6 [Bis-Tetramethyltetraselenafulvalene Hexafluorophosphate] at Low Temperatures

We obtain electron and hole mobilities from the measured Hall mobility of (TMTSF)₂ PF₆. At 4K these are greater than 10⁶cm²/Vsec and agree with the mobility calculated for acoustic phonon scattering. The initial increase of conductivity in dc fields found at low temperatures in this material can be accounted for by single particle effects, specifically heating of the carriers by the field. To explain further increases we suggest that the gap is spatially nonuniform. This could be accounted for by dislocations.     

Possibility of Coexistence of Spin Density Wave and Superconductivity in Organic Conductor (TMTSF)2PF6

A theory for the coexistence problem of the spin density wave (SDW) and superconductivity (SC) in highly anisotropic materials is presented. On the basis of a simplified electronic band model a Hartree-Fock approximation is applied. It is concluded that SDW is precluded when SC developes at a higher temperature. When the SDW onset temperature is higher than that of SC, these long range orders generally coexist unless two orders interchange by a first order phase transition. Discussions on possible phase diagrams for (TMTSF)₂PF₆ under pressure are given.     

New (TMTSF)2X Derivatives: A Change in the Selenium Network Dimensionality Derived From the Molecular and Crystal Structures of (TMTSF)2(Fso3) [T=298K, 123K] and (TMTSF)2(Bro4) T=298K

Abstract

We report the first crystallographic analysis, as a function of temperature, of a TMTSF derivative. Both (TMTSF)₂(FSO₃) and (TMTSF)₂(BrO₄) are isostructural (triclinic, with space group PI) with superconducting (TMTSF)₂(ClO₄). (TMTSF)₂(FSO₃) undergoes a metal-to-insulator transition at 86-90K as observed by microwave conductivity, D.C. conductivity, and magnetic susceptibility. The crystal structure contains 2-dimensional sheets of short Se-Se contacts in the molecular stacking direction and perpendicular to the stacking direction. The temperature dependent variations in these contact distances appear to be of special importance in determining the conduction properties of these materials, and are observed to change in a surprising manner when (TMTSF)₂(FSO₃) is cooled (298 → 123K). The homoatomic Se separations within each TMTSF molecule appear to increase slightly, but not significantly. At the same time the entire 2-dimensional sheet of intermolecular (intra- and interstack) Se-Se contacts between TMTSF molecules contract quite anisotropically, which results in an increase in “dimensionality” of the Se-Se network. Hence, an increase in electrical conduction, in the absence of insulating phenomena, over the temperature range 298 → 123K is not surprising. The intermolecular Se-Se contact distances in (TMTSF)₂(BrO₄) are significantly longer than in (TMTSF)₂(FSO₃) which suggests that the room temperature electrical conductivity of the (BrO₄)^? salt may be diminished compared to the (FSO₃)^? analogue.     

One Dimensional Organic Superconductivity in (TMTSF)2PF6 and (TMTSF)2C104 Detected Via Tunnel Spectroscopy

Electron tunnelling data in (TMTSF)₂ X-GaSb Schottky barriers support the idea that strong superconducting fluctuations exist in quasi-one-dimensional (TMTSF)₂X up to temperatures which are about one order or magnitude higher than the 3-D ordering critical temperature. Other tunnelling data tend to indicate that stabilization of superconductivity is possible at high temperature via chain cross-linking.     

Resistive and Magnetic Susceptibility Transitions in Superconducting (TMTSF)2C104

We report measurements of the ac magnetic susceptibility and dc resistive superconducting transitions in the organic superconductor (TMTSF)₂C10₄. Inductive measurements show complete diamagnetic shielding below a broad transition and initial flux penetration at very low fields [H_c1(0) < 1 Oe]. The resistive transition is also broad, but occurs at a significantly higher temperature than the inductive transition, T_c = 1.0 K and 0.65 K respectively. Resistance measurements also show evidence of a phase transition in the vicinity of 24 K. Magnetic field induced transitions, measured both inductively and resistively, show marked anistropy both in magnitude and in breadth of the transition. Results suggest that (TMTSF)₂C10₄ is a quasi ID or 2D superconductor at high temperatures and high magnetic fields and an anisotropic bulk superconductor at low temperatures and fields. Associated thermoelectric power measurements suggest that spin density waves coexist with the superconducting state.     

(TMTSF)2(2,5-Tcnqbr2): Structure and Physical Properties

The 2:1 charge-transfer salt (TMTSF)₂(2,5-TCNQBr2) has been prepared and its physical properties investigated. Its crystal structure consists of segregated stacks of TMTSF donors (ring-over-bond overlap pattern; mean interplanar spacing of 3.6A) and chains of edge-on and disordered 2,5-TCNQBr2 acceptors. Infrared data are suggestive of unit charge on the 2,5-TCNQBr2 molecule and, therefore, half charge on the TMTSF donor. Resistivity data are successfully interpreted on the basis of a percolation construction. Magnetic data are also presented.     

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, P_c, where a superconducting transition is observed near 1 K. We find a correlation between P_c 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.     

Superconducting Transition of (TMTSF)2CLO4 in Magnetic Fields

Abstract

Superconducting transition of (TMTSF)₂ClO₄ was studied by conductivity measurements along the most conductive a-axis in magnetic fields applied along three different principal crystallographic axes. The GL coherence lengths at OK along the a-, b-, and c*-axes, ?_a(0)≈ 600A, ?(0) ≈ 540A, and ?_c*(0) ≈ 60A were obtained from the measurements of the temperature dependence of the upper critical field H_C2 near the transition temperature. The anisotropy is discussed in terms of the dimensionality arising from the crystal structure and of the conductivity anisotropy in the normal state.     

TMTSF2X Salts. Preparation,Structure and Effect of the Anions

The preparation of the TMTSF molecule and some of its properties are reviewed. The preparation of metallic and superconducting TMTSF X salts is described and some structural aspects are discussed, with emphasis of possible order-disorder transitions when X is a non-centrosymmetric anion. Preliminary results for TMTSF₂ TeF₅ which remain conducting to at least 5 K are presented.     

On the Structure and Properties of TMTTF and TMTSF Salts: Experimental Evidence for the Importance of Interchain Couplings

The structure and physical properties of tetramethyl-tetrathiofulvalène (TMTTF) salts are reviewed. New results are given about (TMTTF)₂ PF₆, (TMTTF)₂ ClO₄ and the selenium analog (TMTSF)₂ ClO₄. An experimental relationship is shown between the electrical and magnetic properties and the closest contacts S-S or Se-Se of neighbouring stacks.     

Magnetotransport and Nonlinear Effects In (TMTSF)2PF6

We have measured the magnetoresistance, Hall effect and the nonlinear conductivity characteristics in pure and radiation damaged (TMTSF>₂PF₆. We find that the material below the metal-insulator transition temperature is best described as quasi-two-dimensional with a very high mobility (10⁵ cm²/volt-sec). The nonlinear conductivity is suppressed by the application of a magnetic field and by radiation damage. The magnetoresistance, metal-insulator transition temperature and the superconducting transition temperature are also reduced by radiation damage of order 1000 ppm for the latter and 100 ppm for the former.     

Superconductivity at Normal Pressures of Some Organic Metals of (BEDT-TTF)-I System

The discovery by K. Bechgaard, D. Jerome et al.¹ in 1980 of the first organic superconductor, (TMTSF)₂ PF₆, was, in some sence, the final step of a long way to organic superconductivity begun about ten years earliler. Although the existence of a number of organic metals was well established already by 1975, till 1978 there was no assureance that organic superconductors may exist. Because all first organic metals were of 1d type, and their metallic properties did not conserve at low enough temperatures.     

Organic Superconductors: A Survey of Low Dimensional Phenomena

An experimental investigation of the electronic properties of the conducting cation radical salts (TMTSF)₂X, has shown how the development of superconductivity at low temperature is affected by the low dimensional character of the band structure. Electron Schottky tunnel spectroscopy has given evidence that the critical temperature is not an intrinsic property of the intrachain superconducting pairing. The pairing energy in (TMTSF)₂PF₆ (P=11 Kbar) derived from the pseudo-gap in the quasi particle density of states amounts to ≈40 K i.e about 40 times the critical temperature. Consequently, properties such as conductivity, thermal conductivity, thermopower, etc… of these Quasi-One-Dimensional Superconductors are to a large extent dominated by the onset of 1-D fluctuating superconducting pairing below 30 K or so. Moreover, the stabilization of these fluctuations into a 3-D ordered superconducting state at higher temperature, namely 12 K for (TMTSF)₂PF₆, (P=11 Kbar) seems feasible via chemically induced chain bridging. The large pairing energy as well as the presence of a spin density waves state in the (T.P) phase diagram of these organic superconductors suggest the importance of the Coulomb interactions in the mechanism of Cooper pair formation.