TMTSF Salts Research Articles

New results on the phase diagram of the (TMTSF) 2FSO 3 salt

We present transport measurements under pressure on the (TMTSF) 2FSO 3 salt from P=1 bar to 8kbar using a helium gas pressure cell, in order to clarify the pressure-temperature phase diagram of this compound. The interest in this TMTSF salt comes from the existence of non-centrosymmetric tetrahedral anion with a permanent dipole moment related to the presence of the fluorine atom. Two types of disorder can be involved in the metal to insulator transitions at low temperature: one due to the orientation of the tetrahedra and the second to the position of the fluorine atom inside the anion. For all studied pressures a semiconducting phase occurs at T ≈ 90K via a very sharp transition. At lower temperatures, the activation energy decreases as the pressure is increased. At last, the resistivity saturates at P=8kbar from T=80K down to 10K where the metallic state is restored. This new study shows that phase diagrams derived with the use of the clamped pressure techniques should be revised.

Vibrational spectra of two new organic semiconductors: tetrathiafulvalene (TTF) and tetramethyltetraselenafulvalene (TMTSF) salts of paranitrophenylmalononitrile (PNMA)

Two new complexes of stoichiometry 2:1 are reported for the donors tetrathiafulvalene (TTF) and tetramethyltetraselenafulvalene (TMTSF) with the acceptor paranitrophenylmalononitrile (PNPMA). Both compounds are semiconductors with a resistivity of about 4 × 10−4 Ω m for (TMTSF)2PNPMA and 0.58 Ω m for (TTF)2PNPMA. The larger conductivity of the first complex can be attributed to the disorder of the PNPMA anions. Vibrational spectra were obtained by FTIR and Raman spectroscopy, in order to determine the degree of charge transfer in these systems. Both complexes have the electron distribution (D+0.4)2A−0.8. As a result the donors D stack in tetramerized units and exhibit vibronic activation of certain symmetric monomer modes, thus indicating the presence of strong electron–vibrational interactions in the donor stacks. Key words: TTF and TMTSF salts, charge transfer complexes, IR and Raman spectra, degree of charge transfer, paranitrophenylmalononitrile (PNPMA).

Phase diagram of superconductivity and spin density wave in (DMET) 2Au(CN) 2

The pressure-temperature phase diagram of (DMET) 2Au(CN) 2 is determined through electrical resistivity measurement. At ambient pressure, (DMET) 2Au(CN) 2 is metallic at room temperature and undergoes phase transition into spin density wave (SDW) at 22 – 28 K. With the increase of pressure, SDW is suppressed, vanishing above 3.5 kbar. The superconductivity is present between 1.5 kbar and 5.5 kbar. Thus, the pressure region in which both SDW and superconductivity are observed is as wide as ∼ 4 kbar, which is much wider than the salts ever known. In the SDW state, in contrast to the phases of TMTSF salts, the divergence of electrical resistivity is incomplete, which must be due to a Fermi surface still remaining in the SDW state. Also observed is a sharp dip in the temperature dependence of electrical resistivity at 180 K at ambient pressure, suggesting a phase transition, whose temperature increases with pressure.

Transport properties of (DMDCNQI) 2Ag at normal and applied pressure

We report results of the electrical resistivity of the silver salt of 2,5-dimethyl-N,N'-dicyanoquinonediimine, (DMDCNQI) 2Ag, at normal and applied pressure as well as of the thermoelectric power at ambient pressure. At room temperature, pressure induces a transition towards a more conductive state. The phase diagram of this compound includes the temperatures of resistivity minima and those of the metal-insulator transition. The first are shifted to lower temperature by pressure while the second are less sensitive to applied pressure. The present data together with previous results on ESR and X-ray diffuse scattering support the idea that the ground state is spin-Peierls. The phase diagram is discussed within the framework of a theory of competing interactions, elaborated for TMTSF and TMTTF salts.

Superconductivity in (DMET)2AuCl2and (DMET)2AuI2

An organic conductor, (DMET) 2 AuCl 2 , was found to undergo a superconducting transition at 0.83 K at ambient pressure. (DMET) 2 AuI 2 on the other hand was found to undergo a superconducting transition at 0.55 K at a pressure of 5 kbar. At ambient pressure, however, it appeared to undergo metal–insulator transition at about 20 K. The electrical properties suggested that the superconducting phase was located next to that of a spin-density wave. The transition to field-induced SDW phases could not be detected by applying a magnetic field up to 5 kOe, in contrast to TMTSF salts.

Network of the intermolecular contacts in organic conductors and superconductors

The directions of the SeSe and Se-anion contacts in some (TMTSF) 2X salts and also of the S-S contactsin α-(BEDT-TTF) 2I 3 and β-(BEDT-TTF) 2I 3 are examined in stereographic projections. The angular dependence of the transverse magnetoresistance emphasizes that particular interstack contacts are involved with particular directions of the magnetic field. This is also effective for SdH oscillations and for the Meissner effect. Furthermore special magnetic field directions in TMTSF and BEDT-TTF salts are predicted for which magneto effects are expected.

The effect of magnetic fields on the electronic structure of Density-Wave systems

The effect of high magnetic fields on the electronic structure of density-wave systems is examined. Small electron and hole pockets, caused by the density wave, may be obliterated by the magnetic field. The phenomenon is highly anisotropic, changes drastically the energy spectrum and may considerably enhance the density-wave critical temperature in some cases. The theory applies to recent experiments in niobium triselenide and in the TMTSF salts.

States determined by photoelectron spectroscopy in the perchlorate and perrhenate of TMTSF

The photoelectron spectroscopic binding energies of Se 3d5/2, O 1s, and Cl 2p3/2 or Re 4f7/2 have been measured in tetralmethyltetraselenafulvalenium (TMTSF) perchlorate and perrhenate, tetrabutylammonium perchlorate and perrhenate, and potassium perchlorate and perrhenate. It has been found that the binding energies of Se 3d5/2 are the same in the perchlorate and the perrhenate even though the spectrum is more asymmetrically broadened in the perrhenate. The binding energy of O 1s depends on the cation and on the central atom in the anion. In the organic perrhenates it is at 530.96 eV, whereas in the perchlorates it is at 532.08 eV. In the potassium salts the O 1s binding energies differ by 1.89 eV in the perchlorate and the perrhenate. The binding energies of Cl 2p3/2 and Re 4f7/2 differ by 0.84 and zero respectively from those in the potassium salts. The energetic differences among the values for O 1s indicate the extent to which the electronic structure of the TMTSF salts are determined by the electronic structure of the anions. The difference in binding energies between the TMTSF perchlorate and perrhenate is such that the application of pressure to the perrhenate should transfer electrons to the cation and presumably to the conduction band.

Cation radical salts with magnetic anions: Preparation and characterization of FeCl 4 salts of TMTTF and TMTSF

A series of new TMTTF and TMTSF salts have been prepared with FeCl 4 -, MnCl 2- 4 and ZnCl 4 2- as anions. A large numberof different phases with different structures and different stoichiometries were characterized. A preliminary report is presented here, focusing on two compounds, TMTTF-FeCl 4 and TMTSF-FeCl 4, which have been characterized by structural, magnetic and conductivity measurements. When compared to the Bechgaard salts, the most important structural feature of both compounds is the presence of organic stacks isolated from each other by characteristic inorganic frameworks. In these new compounds, the fully oxidized stacks are insulating and the large susceptibilities are shown to be dominated by the magnetic anions which interact weakly through the organic molecules.

New Results on Two Synthetic Conductors (TMTSF)2BRO4 and (BEDT-TTF)2I3

Abstract We present experimental studies of transport properties of (TMTSF)2BrO4 and (BEDTTTF)2I3. The behaviour of the TMTSF salt suggests transport via variable-range hopping among localized states. The BEDTTTF-salt exhibits thermopower which indicates almost isotropic metallic behaviour in the ab-plane.

Neutron-diffraction evidence for structural phase transitions in the organic conductors bis-tetramethyltetraselenafulvalenium salts [(TMTSF)2X,X=ClO4 −andBF4−

Single-crystal neutron-diffraction experiments using the time-of-flight Laue technique establish that organic conductors, bis-tetramethyltetraselenafulvalenium salts [${(\mathrm{TMTSF})}_{2}X$, $X={\mathrm{ClO}}_{4}^{ \ensuremath{-}}$ and ${\mathrm{BF}}_{4}^{\ensuremath{-}}$], undergo crystallographic phase transitions at low temperatures. This is the first neutron-diffraction evidence for structural phase transitions in any TMTSF salts. The phase transition in the ${\mathrm{ClO}}_{4}^{\ensuremath{-}}$ salt leads to the creation of two crystallographically independent TMTSF columns with the possibility of different electrontransport properties for each column which can lead to independent low-temperature phase transitions.

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

Abstract The structure and physical properties of tetramethyl-tetrathiofulvalene (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.