Hans W. Helberg

New radical salts of BEDO-TTF: Structures and electronic properties of organic metals and superconductors

Structural data as well as some measurements on the electronic properties of a few organic metals of the donor BEDO-TTF are reported.

HIGHLY CONDUCTING PERYLENE RADICAL SALTS

Abstract Temperature dependent dc and microwave conductivity data together with EPR and optical reflectance measurements on the “mixed” system (pe)2(ASF6)0,75(PF6)0,35 × 0,85 CH2Cl2 are described. The data prove metallic behaviour of this organic solid down to 200 K.

Temperature and frequency dependences of microwave conductivity of isotropic reticulate doped polymers

Frequency dependence of conductivity for reticulate doped systems is observed at room temperature only around 1 GHz and even so it is relatively weak. The temperature dependence of conductivity is characteristic of the CT complex used and not of the polymer matrix. For the system containing TTF-TCNQ, for which the d.c. conductivity has a maximum at c. 230 K (i.e. metal-like behaviour at higher temperatures), this maximum becomes more pronounced and shifts towards lower temperatures with increasing frequency in the GHz range. The temperature dependence of the microwave conductivity is weaker than that of the d.c. conductivity. Such behaviour can be described by a modified Maxwell-Wagner model if an appropriate shape factor for the conducting inclusions is introduced, and if a relatively high conductivity of the continuous phase is assumed. We conclude that charge-carrier transport in reticulate doped polymers is not controlled by insulating barriers. The disorder within microcrystals plays a fundamental role, while the CT complex crystalline network is continuous in spite of very low concentration.

Conductivity of the ET Polyiodides Crystalline Networks Transformed into Superconducting Phase

The realization of new generation of devices taking advantages of the unusual electronic properties of organic molecular compounds will require the development of new processing methods and fabrication procedures. Since introduction of the concept of organic semiconductors by Akamatu and Inokuchi in 1950 [1], a large number of various conducting organic molecular solids has been elaborated. Superconductivity is the most fascinating phenomena discovered in organic crystals. However, while the supercon-ductivity transition critical temperature (Tc) is quickly growing with the discovery of new classes of organic superconductors [2], it is rather clear, that single crystals of such materials may have only limited application. It is therefore necessary to develop new materials comprising organic superconductors (composites, hybrid structures, multi-layers etc.) which can be used for fabrication of electronic circuits and devices.

TCNQ SALTS OF PLANAR METAL COMPLEX CATIONS: NOVEL MOLECULAR CONDUCTORS AND SEMICONDUCTORS.

Abstract The facile variation of positive charge of oxamide oxime metal complexes, caused by acid-base equilibrium, allows the growth of single crystals of their TCNQ salts. 1:1 salts consist of reqular segregated stacks of the components, with metallic room temperature behaviour of the Ni compound, the Pt compound being a semiconductor. Room temperature conductivities are of the order of 10 Siemens per cm. A 2:3 Pt complex TCNQ salt contains segregated acceptor stacks with half a negative charge per molecule. These stacks run perpendicular to mixed stacks -D-D-A-D-D-A-, with integral charges on donors D and acceptors A.

Dispersion of the Polarizability Tensor by Inter- and Intrastack Excitations in Organic Conductors

Abstract In (TMTSF)2FSO3 a rotation of the polarizability tensor on the stack axis by about 30° is observed in the VIS. The polarizability is governed by the polarizability parallel to the long axis of the TMTSF molecules. The interstack Se-Se contacts are orientated symmetrically referring to the TMTSF molecules. Nevertheless intramolecular excitations may lead to the observed rotation due to the simultaneous excitation along the shortest Se-anion contact. This contact is perpendicular to the stack but inclined to the long axis of the TMTSF molecules. – In (BEDT-TTF)2I3 there are more interstack contacts yielding to two-dimensional properties. The interstack S-S contacts are also arranged symmetrically. No dispersion of the polarizability tensor is observed.

Optical study of the transformation of the α-phase of (BEDT-TTF)2I3 into the superconducting αt-PHASE (Tcc = 8 K)

Abstract The tempering of α-(BEDT-TTF) 2 I 3 crystals above 70°C for several days leads to the transformation into the superconducting α t -phase (T c = 8K ambient pressure). This process was observed in very small crystals by polarizing microscope techniques in the visible and near infrared spectral range. Premature termination of the tempering yields to crystals containing both phases in the same crystal. This allows direct comparison of the optical properties of the both phases. The orientation of the polarizability tensor and the transmission spectra parallel to the principal axes were measured. The spectra are similar to the spectra of the s-phase. But the indicatrix orientation rather indicates a new phase than the s-phase.

Frequency-dependent conductivity in pure and iodine-doped α- (BEDT-TTF)2I3

Abstract Measurements of the microwave conductivity at 10 GHz show a plateau below the metal-insulator transition at 135 K in contrast to the further dropping dc conductivity. In pure material the plateau is independent of frequency (4 GHz, 10 GHz, and 22 GHz), but in the temperature range above 135 K strong frequency dependence is observed. Iodine doping causes frequency dependence also in the plateau range. Particular doping rates give rise to several conductivity behaviors for different crystal directions. The results are discussed referring to CDW depinning and relaxation processes.

Microwave conductivity of the organic charge density wave conductor (fluoranthene)2X

Abstract We have investigated the temperature dependence of the microwave conductivity of the organic CDW-conductor (FA) 2 X at frequencies of 4.2, 10.2 and 23.5 GHz and compared our data with the d.c. conductivity of these crystals [1]. Down to temperatures of about 80 to 60 K an excellent agreement has been found. In the quasi-metallic high temperature range no frequency dependence of the conductivity is observed. Upon cooling a sharp phase transition already known from d.c. measurements occurs at temperatures T p of 182 to 175 K depending on the counterion X − = PF 6 − , SbF 6 − . At temperatures far below this Peierls transition, however, deviations from d.c. results are observed; while the d.c. conductivity displays thermally activated behaviour the microwave conductivity leads into a plateau and remains almost constant down to 4 K. The onset of this plateau lies at temperatures from 80 to 60 K. At 20 K the microwave conductivity at 10.2 GHz is about 10 orders of magnitude higher than the d.c. values. We interpret these results as a clear signature of the Frohlich mode resulting from the CDW ground state in (FA) 2 X. The agreement of microwave and d.c. data even at temperatures below the Peierls transition can be ascribed to screening of the CDW by thermally excited charge carriers.

Microwave conductivity investigations of plastically deformed silicon

Abstract We present new results on microwave continuous-wave (c.w.) conductivity investigations of plastically deformed floating-zone (FZ) silicon single crystals. N-type samples with various doping and deformation levels were investigated by the cavity perturbation technique. The anisotropy and absolute values turn out to be very sensitive to experimental conditions but on the whole the one-dimensional nature of dislocation conduction is confirmed. The C.W. anistropy follows the respective dislocation structure anisotropy closely. For clear results, care has to be taken of the number of deep point-defect centres also introduced by deformation. Furthermore the existence of a low-temperature conducting state is shown for a Czchrochalski (CZ) sample annealed at 650°C containing rod-like defects.