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.     

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.     

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.     

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.     

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.     

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.     

Magnetotransport In (TMTSF)2PF6 and (TMTSF)2C104 Under Pressure

We have shown from the magnetotransport that the Fermi surface of (TMTSF)₂PF₆ is effectively closed two-dimensionally and is compensated in the metallic state at low temperature. These results are inconsistent with a broad regime of one-dimensional superconducting fluctuations.

Independent electron theory is seen to be inadequate to explain either the Fermi surface or the threshold field for oscillations, thereby implying that correlation effects are still important in the metallic state. Finally, an anomaly has been observed in the magnetotransport of (TMTSF)₂C10₄ which is analogous to the threshold field in (TMTSF)₂PF₆.     

Raman Scattering From TMTSF-Salts

Abstract

Experimental results for Raman scattering from TSeF, TMTSF-D12, (TMTSF)₂PF₆ and (TMTSF)₂AsF₆ excited with blue-green laser light at liquid nitrogen temperature are reported. A strong resonance effect for excitation of TMTSF with 4579 Å was observed and an assignment for the a_g modes was obtained by comparison with results from TTF. Methyl group modes were identified at 328 cm^?1, 472 cm^?1 and 916 cm^?1, respectively. Charging the TMTSF molecule resulted in a strong frequency shift of -122 cm^?1 and -76 cm^?1 for the a_gu₂ mode, respectively. A study of the temperature dependence of the scattering from molecular modes between 2 K and 300 K did not reveal any correlation with the phase transitions in the conducting compounds.     

Synthesis and Study of Electrochemically Grown Salts of Organic II-Donors

Abstract

The effects of purity and crystal perfection on the properties of (TMTSF)₂ClO₄ are examined via SEM's and transition temperatures. Careful electrochemical techniques using gradient sublimed TMTSF starting material and short term oxidation result in high quality crystals. Preliminary work on HMTSF salts of PF ^? ₆ and AsF^? ₆ is also reported.     

Anion Symmetry and the Separability of Structural Parameters for Tetramethyltetraselenafulvalenium Salts, (TMTSF)2X

Empirical correlations of various structural parameters for a series of (TMTSF)₂X salts have been explored utilizing a van der Waals-like estimate for the radius of the counterion X. Included amongst the parameters studied are: (a) the lengths of the transverse cell constants c and b; (b) the interdonor Se .. Se contact distances; (c) the alternate separation between molecular centers of mass along the columnar stack of donors (a); and (d) the symmetry and strength of the anion coupling to its donor-dominated structural cavity. The notable result is that independent correlations are found for salts containing centrosymmetric and non-centrosymmetric anions.     

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.     

(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.     

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.     

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     

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.     

The Electronic Structures of (PH4)3C60 and (ClO4)3C60

Abstract

We have studied theoretically the electronic structures of a hypothetical donor-type material, (PH₄)₃C₆₀, and a hypothetical acceptor-type material, (ClO₄)₃C₆₀ from first principles by using a full-potential linear-combination-of-atomic-orbitals method based on the density-functional theory within the local-density approximation. It is found that the charge transfer from the PH₄ molecules to the C₆₀ molecules is perfect while the charge transfer from the ClO₄ molecules to the C₆₀ molecules is not perfect. We compare the latter result with the electronic structures of two typical acceptor-type organic conductors, (TMTSF)₂ClO₄ and (TMTSF)₂PF₆, and discuss the differences.     

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.     

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.     

A Spin-Charge Separation Hypothesis Based on the Structure of the Salts (TMTSF)2 × (X=PF− 6, ASF− 6)

A detailed study of the three-dimensional structure of (TMTSF)₂AsF₆ isomorphous to the PF₆ and ClO₄ salts) has led to the conclusion that these salts are pseudo two-dimensional, that there are strong interactions between certain selenium atoms in specific ways (clustering) and that there is a periodic lattice of anions. These anion sheets could stabilize partial separation of positive charges and unpaired electrons (spins). This separation can be used to formulate an hypothesis to explain the apparent absence of CDW's in these salts.