Surface magnetism in organic charge transfer salts

We have investigated the magnetic properties of the charge transfer salts Qn(TCNQ)₂, Cs₂(TCNQ)₃ and TTF—TCNQ in the form of single crystals and after strong pressing or grinding to a fine powder, which introduces lattice defects and increases the surface area. It is found that for the former two compounds pressing or grinding leads to a non-linear, saturating component in the magnetisation field curves, whereas the effect is absent for TTF—TCNQ. It is suggested that this behaviour could arise from strongly coupled localised spins at the surface, i.e. surface magnetism in these materials.     

Intersubband optical transitions in one dimensional quantum wire structures

One dimensional (1D) quantum wire structures are emerging as the new generation of semiconductor nanostructures offering exciting physical properties which have significant potential for novel device applications. These structures have been the subject of intensive investigation recently including extensive theoretical and experimental studies of their interband optical properties. In this work we present the results of our study of the intersubband optical transitions in 1D semiconductor quantum wires. The crescent shaped quantum wire structures used for this research were grown on non-planar GaAs substrates. The intersubband transition energy spectra, the selection rules, and the two dimensional envelope wavefunctions were theoretically investigated by using our new LENS (local envelope states) expansion. We present recent experimental results on modulation doped V-groove quantum wires, including PL, PLE, TEM, CL, and infrared polarization resolved spectroscopy. We have observed a very unusual absorption lineshape at the far-infrared wavelengths that we assigned to phonon assisted Fano resonance in a modulation doped quantum wire structure.     

Coupled charge density waves in nearly one dimensional systems

A mean field theory of coupling between charge density waves CDW in linear chain systems is described. It is shown that interchain coupling can stabilize CDW's and lead to a semiconducting behavior. The model is applied to semiconducting TCNQ salts.     

Photoinduced phase transitions in a Peierls system

A theory is constructed that explains photoinduced phase transitions in a Peierls system being irradiated by light with a finite width of the optical spectrum and a central frequency close to the upper van Hove singularity of the first kind in the combined density of electron states. The electron spectrum and the matrix elements of the dipole-moment operator are calculated by Bogolyubov’s method of canonical transformations. The interaction with the light is described by the Liouville equation for the density matrix of the electron subsystem in the dipole approximation. The light field is considered a quasimonochromatic time-independent random process with a Lorentzian spectrum. The derived equations are analyzed for two limits: (1) when the width of the optical spectrum tends to zero (a monochromatic light field), and (2) when the width of the optical spectrum is close to the upper limit (a bifurcation point) at which a photoinduced phase transition can still be observed. An existence criterion for such a transition is obtained, and the main parameters of the transition (the critical points and the size of the hysteresis loop) are calculated. The broadening of the optical spectrum of the incident light is shown to narrow the range of values of the central frequency of the light field and to reduce the size of the hysteresis loop. Finally, near the phase transition point, cavityless optical bistability sets in in the system, with light absorption increasing in the process. Zh. éksp. Teor. Fiz. 114, 1407–1420 (October 1998)     

Peierls transition in a quasi-one dimensional system

We study the thermodynamics of a noninteracting electron system with a half filled strongly anisotropic tight binding band in the mean field approximation. This system may exhibit a structural transition dimerizing along the high conductivity direction. The conditions for the instability to occur are given as a function of the degree of anisotropy. The relationship to the behavior of TCNQ salts is discussed.     

Optical properties of the charge transfer salts of tetrathiotetracene

Powder optical absorption spectra of various tetrathiotetracene (TTT) salts are reported together with solution spectra of neutral and ionized TTT and TTT dimers. Intramolecular transitions are identified by comparing the solid state and solution spectra. The Coulomb correlation energy corresponding to charge fluctuation to TTT is 0.7 eV, independent of stoichiometry and of solid state properties. Complex TTT salts have a low lying optical excitation due to a transition between ionized and neutral molecules.     

The possibility of a “spill-over” Peierls regime due to electron-electron interactions in one dimensional systems

Arguments are given suggesting that in the presence of repulsive interactions between electrons on one dimensional conducting chains, the possibility exists for a new phase in which the energy gap in the single particle spectrum is absent even though a Peierls lattice distortion is present.     

Thermoelectric power,Coulomb correlation and charge transfer in TCNQ salts

We show how the amount of charge transfer for a large class of TCNQ systems can be simply deduced from a measurement of the high temperature saturation value of the thermoelectric power. Of particular interest is the value - 60 μV/°K characteristic of a quarter-filled correlated band, and found in almost all 2 : 1 TCNQ salts.     

Molecular orbital theory and the assignment of charge transfer transitions

A survey of the present situation in the analysis of charge transfer transitions is given. The possibilities of the combinations of observable characteristics of charge transfer bands such as energy, intensity (including polarisation in a non-cubic system), band shape (including vibrational structure if resolved) and angular momentum properties by deriving molecular eigenvectors and by theoretical elaboration of experimental data of great number of halides and oxides are demonstrated.     

Optical properties of charge transfer salts of tetracyanoquinodimethane (TCNQ)

A unified interpretation is given for the infrared and optical spectra of many TCNQ compounds which exhibit a wide variety of physical properties. Inter- and intra-molecular transitions are distinguished by polarization measurements on single crystals of several TCNQ salts. It is suggested that there is incomplete charge transfer in both tetrathiofulvalinium (TTG)- and N-methylphenazinium (NMP)-TCNQ and that the electron-electron interactions are not appreciably screened by the excitonic polarizability of the cations.     

Structure and electronic properties of BEDT-TTF and it's charge transfer salts

We report a detailed computational study, of the organic superconducting charge transfer BEDT-TTF salts, whose electronic properties depend strongly on the packing of the donor radical cations. Electronic structure calculations have been performed at the molecular and periodic level using density functional theory (DFT). These calculations have been used to investigate the magnetic ordering in the BEDT-TTF[FeBr₄] salt, where we obtained an antiferromagnetic solution in agreement with experiment. Detailed geometry optimisations have been carried out for both BEDT-TTF[FeBr₄] salt and the neutral BEDT-TTF crystal.     

Electronic spectra of the organic charge transfer salts TTT-In

Electronic spectra of TTT—I_n both in solution and in the solid state were measured between 8000 and 40,000 cm^-1. The highly conducting phase TTT—I_1.5 is assigned as (TTT^oTTT⁺) (T₃)^-. A phase TTT—I_2.7 is obtained and its possible electronic structure discussed. Conductivity results on both phases are presented.     

Peierls and spin-Peierls phase transitions in structurally unstable quasi-one-dimensional solids

The Peierls and spin-Peierls phase transitions are studied in solids in which a structural instability is already present. It is found that the presence of this intrinsic mode can increase considerably the critical temperature. For small values of the critical temperature, the transition is of the BCS-type, like the Peierls (or spin-Peierls) phase transition, but with an effective electron (or spin)-phonon coupling constant renormalized by the anharmonicity and by the instability of the phonon. Numerical results are also presented for larger critical temperatures. Then the BCS behaviour is no longer observed.     

Intermediate dimensional character of charge transfer excitation modes in a two-leg cuprate ladder

Collective charge modes in the cuprate ladder compound Sr₁₄Cu₂₄O₄₁ are studied over the one-dimensional Brillouin zone using resonant inelastic X-ray scattering. Low energy (2-4 eV) spectral weight across the Mott gap is dominated by a broad, dispersive feature containing two distinct peaks that may be interpreted as independent modes. Details of low energy dispersion, intensity distribution across the Brillouin zone and peak composition fall between the characteristic spectra of quasi-1D (e.g. SrCuO₂) and -2D cuprates (e.g. Nd₂CuO₄). We demonstrate that dispersion and splitting between the two observed modes can be understood in a variant of the strong coupling limit (Hubbard-U?t) with a single band Hubbard model.     

Pairing and superconductivity driven by strong quasiparticle renormalization in two-dimensional organic charge transfer salts

We introduce and analyze a variational wave function for quasi-two-dimensional kappa-(ET)(2) organic salts containing strong local and nonlocal correlation effects. We find an unconventional superconducting ground state for intermediate charge carrier interaction, sandwiched between a conventional metal at weak coupling and a spin liquid at larger coupling. Most remarkably, the excitation spectrum is dramatically renormalized and is found to be the driving force for the formation of the unusual superconducting state.     

Soliton model for the dielectric permeability of some quasi one-dimensional charge transfer salts

The dielectric permeability of bound soliton antisoliton pairs is considered. The model yields and explanation for the anomanously large dielectric constant, the conductivity at microwave frequencies and their low temperature behaviour observed in some quasi one-dimensional organic charge transfer.