R. M. Vlasova

Optical study of the electronic structure and electron-molecular vibrational coupling in a novel organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Cl0.5Br0.5

Abstract Reflectance spectra of a new ambient-pressure superconductor, κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl 0.5 Br 0.5 ( T c = 11.3 K), where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, are measured in the range from 750 to 5500 cm −1 and from 9000 to 40 000 cm −1 for different polarizations. The detailed analysis of the infrared conductivity, obtained after a Kramers-Kronig transformation, is performed in the framework of the approach, based on the ‘phase phonons’ theory of M.J. Rice, with the help of special transformation of the experimental data. Parameters of the electronic subsystem and electron-molecular vibration coupling are determined. The determined ‘total’ dimensionless coupling constant (λ) is compared with those of the organic superconductors κ-(BEDT-TTF) 2 Cu(NCS) 2 ( T c = 10.4 K) and (BEDT-TTF) 4 Hg 2.89 Br 8 ( T c = 4.3 K). It is found that a larger λ produces a larger T c in accordance with the known Bardeen-Cooper-Schriffer formula for T c .

Electronic structure and electron-molecular vibrational coupling in organic superconductors ET4Hg2.89Br8 and ET2Cu(NCS)2: comparative studies by optical spectroscopy

Abstract Reflectance spectra of organic superconductors ET 4 Hg 2.89 Br 8 and ET 2 Cu(NCS) 2 are measured in the range from 650 to 40 000 cm −1 for different polarizations. The reflectance is analysed in the framework of the ‘phase phonons’ theory of M. J. Rice with the help of special transformation of the experimental data. Parameters of the electronic subsystem and of electron-molecular vibration coupling are determined. The role of anion-cation interaction is shown to be important for the onset of superconductivity.

Optical properties of a new organic metal (BEDO-TTF)5[CsHg(SCN)4]2

Abstract Polarized reflectance and optical conductivity spectra of single crystals of a new organic conductor based on the BEDO molecule, BEDO 5 [CsHg(SCN) 4 ] 2 , were investigated in the 600–6500 cm −1 and 9000–40 000 cm −1 range for the three principal axis at 300 K. Quasi-two-dimensional character of the conducting electronic system was established. The parameters of electronic system were obtained using the Drude model. The conducting bands of the investigated conductor appeared to be narrower than those of the recently investigated superconductor with the analogous structure, (BEDO-TTF) 2 ReO 4 (H 2 O). An assignment of the vibrational features for all the investigated polarizations was proposed.

Electronic and electron-phonon phenomena in low-dimensional BEDT-TTF-based organic conductors and superconductors

Two related groups of k-phase ion-radical salts (BEDT-TTF) with different electrical properties, namely, superconductors with different transition temperatures and conductors, which transfer to insulating state with decreasing temperature, have been studied by micro-optic spectroscopy. Polarized reflectance spectra of microcrystals have been measured for the three principal crystallographic directions within the 700–40000 cm−1 region, and the corresponding spectra of the optical functions obtained. The anisotropy of the electronic system in the crystals has been established as two-dimensional. The spectra obtained were quantitatively analyzed, the key parameters of the electronic structure and the vibronic coupling constants determined. It is concluded that the conductors have smaller vibronic coupling constants, more narrow allowed electronic bands, and stronger electron-electron interaction compared to those of the superconductors, and that vibronic coupling is the necessary condition for the onset of superconductivity in the superconductors studied.

Optical Properties of Deuterated Organic Conductor (BEDT-TTF)2[Hg(SCN)2Br]

Reflectance spectra of 2:1 charge-transfer salt single crystals of deuterated bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) cation with Hg(SCN)2Br are reported for two polarizations and compared with the data for the hydrogenated compound. The Hubbard-type model for two orthogonal dimers in the case of infinite on-site Coulomb repulsion U is developed and emlpoyed for the interpretation of the observed charge-transfer bands and vibrational peaks resulting from electron-molecular vibration coupling.

Electron and electron-phonon effects in the quasi-two-dimensional molecular conductor θ-(BETS)4HgBr4(C6H5Cl): Optical studies at 300–15 K

This paper reports on a study of the polarized reflectance and optical conductivity spectra of the quasi-two-dimensional molecular conductor θ-(BETS)4HgBr4(C6H5Cl) within the 700–6500-cm−1 region at 300–15 K and within the 9000–40 000 cm−1 region at 300 K performed along two principal directions in the crystal plane parallel to the conducting layers of the BETS molecules. The IR spectra obtained at 300 K follow a close-to-Drude behavior, with strong broad features (1200–1400 cm−1) due to electron-vibrational (vibronic) coupling (VC) superposed on the high Drude background. As the temperature is lowered in the range 180–80 K, in the spectra there appears a Lorentz term with ωt=2900 cm−1, as well as three additional VC-induced bands in the 800–1180-cm−1 region, which disappear as the temperature is decreased further. The results obtained indicate the existence of unstable structural distortions along the two principal directions in the crystal, which are accompanied by the formation of a commensurate charge-density wave.

Optical properties of new organic conductors based on the BEDT-TSeF molecule (the κ-(BETS)4Hg2.84Br8 superconductor and κ-(BETS)4Hg3Cl8 Metal) in the range 300–15 K

Polarized reflectance and optical conductivity spectra of single crystals of two new isostructural organic conductors based on the BEDT-TSeF molecule, namely, the κ-(BETS)4Hg2.84Br8 superconductor (Tc=2 K) and the κ-(BETS)4Hg3Cl8 metal, which undergoes a smooth transition to the dielectric state near 35 K, have been obtained in the spectral region 700–6500 cm−1 at temperatures of 300–15 K. At 300 K, the spectra of both compounds are nearly identical and differ from the Drude spectrum characteristic of metals. The nature of the observed difference is discussed, and the spectra are described in terms of a cluster approach with inclusion of electron-electron correlations in the Hubbard approximation combined with the Drude model. The parameters of the theory were determined, including the electron transfer integrals between molecules in a cluster. The spectra in the conducting plane of the crystals were found to be essentially anisotropic, which should be assigned to specific features of in-plane interaction between molecules. The spectra of the superconductor and the metal become increasingly different as the temperature is lowered. The spectra of the metal obtained for T<150 K exhibit splitting of the broad electronic maximum in the mid-IR region into two bands, which is accompanied by a splitting of a vibronic feature deriving from electron interaction with intramolecular BETS vibrations of ν3(Ag) symmetry. No such splitting is observed in the superconductor spectra with decreasing temperature.

Temperature dependence of infrared reflectance spectra of BEDO5[CsHg(SCN)4]2

Polarized reflectivity spectra of a new organic conductor BEDO 5 [CsHg(SCN) 4 ] 2 were measured in the spectral range of 700-6500cm -1 at 300-10 K. An anisotropy of temperature behavior of optical conductivity in the conducting plane was observed, it shows metallic character for the direction perpendicular to BEDO stacks and non-metallic behavior for the direction parallel to BEDO stacks, where a growth of a broad band at near 4000 cm -1 testifies for the appearance of an energy gap. Fitting by the Drude-Lorentz model points on an existence of a low-frequency energy gap. The gaps may appear due to dimerisation of BEDO molecules in the stacks at low temperatures.

Electronic and electronic-vibrational phenomena in the new organic conductor ϰ-(ET)2[Hg(SCN)2Cl] with a metal-insulator transition

The reflection spectra and optical conductivity spectra of the new organic conductor ϰ-(ET)2[Hg(SCN)2Cl] with a metal-insulator transition in the spectral regions 700–5500 and 9000–40 000 cm−1 have been studied in polarized light at 300 K. A comparisonis made between the spectra obtained and the corresponding spectra of related isostructural conductors based on the ET molecule, and also the properties of the crystal structure of the investigated compounds. An electronic transition between the ET molecules of the dimer (ET)2+ in the spectral region 700–5500 cm−1 has been identified, as have the features of the electronic-vibrational structure arising as a consequence of the interaction of this transition with the completely symmetric intramolecular vibrations of the ET molecule. It is found that the conductor with the stronger dimer interaction between the ET molecules has the higher the transition temperature.

Transport, Magnetic and Optical Properties of a Quasi-Two-Dimensional Organic Metal Based on BEDO-TTF (Bis-(Ethylenedioxy) Tetrathiafulvalene)

Polarized reflectivity spectra, tight binding band structure calculations as well as magnetoresistance and magnetization measurements have been carried out for the new quasi-two-dimensional organic conductor β″-(BEDO-TTF)5[CsHg(SCN)4]2. Polarized reflectivity and optical conductivity spectra have shown that at low temperatures this salt exhibits metallic behaviour in the direction perpendicular to the BEDO-TTF stacks, but non-metallic behaviour in the direction parallel to the stacks. It is supposed that the non-metallic behaviour is due to the dimerization of BEDO-TTF molecules in the stacks at low temperatures. Tight binding calculations suggest that the Fermi surface can be described as containing two contributions: a closed two-dimensional portion associated with holes and a open, pseudo-one-dimensional portion associated with electrons. Shubnikov-de Haas and de Haas-van Alphen oscillations have shown that some frequencies of their complicated Furie spectra are in good agreement with the calculated Fermi surface. The other frequencies of the spectra can be well understood in the frame of the quantum interference effect.