Browsing by Author "Helberg, Hans W."

Now showing 1 - 11 of 11

Open Access
α- and β-(BEDT-TTF)2+I3-: two dimensional organic metals
(1985) Hennig, Ingolf; Bender, Klaus; Schweitzer, Dieter; Dietz, Klaus; Endres, Helmut; Keller, Heimo J.; Gleitz, Arno; Helberg, Hans W.
Electronic properties of α- and β-(BEDT-TTF)2I3 crystals are reported.
Open Access
(BEDO-TTF)2ReO4·(H2O): a new organic superconductor
(1991) Kahlich, Siegfried; Schweitzer, Dieter; Heinen, Ilsabe; Song, En Lan; Nuber, Bernhard; Keller, Heimo J.; Winzer, Klaus; Helberg, Hans W.
The structure and the temperature dependence of the resistivity, thermopower and ac-susceptibility of the new organic metal (BEDO-TTF)2ReO4(H2O) was investigated. The resistivity and thermopower data indicate phase transitions at 213K, around 90K and 35K. Below 2.5K an onset to superconductivity is observed in the resistivity data. Superconductivity was suppressed in the resistivity at 1.3K by applying a magnetic field of about 0.2T. Ac-susceptibility data indicate that superconductivity is a bulk effect in (BEDO-TTF)2ReO4(H2O) but the onset for superconductivity observed in the ac-susceptibility is only at 0.9K and the transition seems to be complete only at temperatures below 50 mK. This broad transition might be due to some disorder in the structure created by the low temperature phase transitions.
Open Access
(BEDT-TTF)2+J3- : a two-dimensional organic metal
(1984) Bender, Klaus; Dietz, Klaus; Endres, Helmut; Helberg, Hans W.; Hennig, Ingolf; Keller, Heimo J.; Schäfer, Herbert; Schweitzer, Dieter
Temperature-dependent thermopower-, dc- and microwave-conductivity measurements on the two-dimensional organic metal (BEDT-TTF)2+J3- are reported.
Open Access
Frequency-dependent conductivity in pure and iodine-doped α-(BEDT-TTF)2I3
(1987) Przybylski, Michael; Helberg, Hans W.; Schweitzer, Dieter; Keller, Heimo J.
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.
Open Access
Highly conducting perylene radical salts
(1982) Koch, P.; Schweitzer, Dieter; Harms, Ralf H.; Keller, Heimo J.; Schäfer, Herbert; Helberg, Hans W.; Wilckens, Rudolf; Geserich, Hans Peter; Ruppel, Wolfgang
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 times 0,85 CH2Cl2 are described. The data prove metallic behaviour of this organic solid down to 200 K.
Open Access
Microwave conductivity in polycrystalline (BEDT-TTF)2I3 material
(1991) Müller, Gerhard; Helberg, Hans W.; Schweitzer, Dieter; Keller, Heimo J.
Polycrystalline material of the α-phase of (BEDT-TTF)2I3 was compressed to small samples (4-mm x 1mm, thickness 0.3 mm typically) at a pressure of 10 kbar. Annealing at 70°C yields the superconducting αt-phase. Microwaves (10,2 GHz) enable the measurements of the conductivity for stepwise annealing after every annealing step in always the same sample. For annealing times 10 min all conductivity versus temperature curves are intersecting in an isosbestlc point at 190 K. This behaviour can be described by a conductivity relation for a two component system, from which was determined the volume fraction of the new grown αt-phase in dependence of the annealing time. Starting annealing (annealing times < 10 min) shows another unexpected phase transformation. After 2 min annealing the conductivity at 200 K increases by more than one order of magnitude, but then decreases of further annealing (5-10 min) down to the value for the unannealed sample.
Open Access
Microwave conductivity in the pure and iodine-doped organic conductor α-(BEDT-TTF)2I3
(1987) Kremer, Werner; Helberg, Hans W.; Gogu, Emil; Schweitzer, Dieter; Keller, Heimo J.
The microwave conductivity in dependence of temperature was measured on pure and iodine doped crystals of α-(BEDT-TTF)2I3 along the stack direction [100] and perpendicular to it [010]. The exposure times to iodine were varied in many steps between 2 min and 360 min. The contactless microwave technique enables to keep tbe same crystal in each case throughout all measurements. The results show that the microwave conductivity vs. temperature is not influenced distinctly by iodine doping. Only at the onset or the plateau range immediately below the metal-insolator transition at 135 K a small enhancement of the conductivity is found for exposure times larger than 60 min. In no case a transition to a supercooducting state is observed. Therefore recently reported superconductivity may be due to surface effects.
Open Access
New radical salts of BEDO-TTF: structures and electronic properties of organic metals and superconductors
(1993) Schweitzer, Dieter; Kahlich, Siegfried; Heinen, Ilsabe; Song, En Lan; Nuber, Bernhard; Keller, Heimo J.; Winzer, Klaus; Helberg, Hans W.
Structural data as well as some measurements on the electronic properties of a few organic metals of the donor BEDO-TTF are reported.
Open Access
A novel molecular metal: (oxamide oximato)(oxamide oxime)nickel(II) tetracyanoquinodimethanide, [Ni(oaoH)(oaoH2)]tcnq, and physical properties of its semiconducting Pt analogue
(1985) Endres, Helmut; Bongart, August; Nöthe, Dietrich; Hennig, Ingolf; Schweitzer, Dieter; Schäfer, Herbert; Helberg, Hans W.; Flandrois, Serge
(C4H11N8NiO4)+(C12H4N4)-, Mr = 498.09 is triclinic, p1, a -=3.7718(6), b = 7.436(2), c =17.511(4) A, a=88.67(2), β=86.93(2), γ=85.05(2), γ= 488.51 A 3, Z = 1, d c=1.69 gcm -3, final R w= 0.035 for 1454 observed independent reflections. The crystals consist of segregated regular parallel stacks of planar metal complex cations and tcnq - counterions with intermolecular H bonds stabilizing the structure. The compound is metallic at room temperature. A metal to semiconductor transition around 230 K shows up in thermopower data, in the microwave conductivity and epr around 170 K. It is not visible in the static magnetic susceptibility.
Open Access
Optical study of the transformation of the α-phase of (BEDT-TTF)2I3 into the superconducting αt-phase (Tc = 8K)
(1988) Helberg, Hans W.; Schweitzer, Dieter; Keller, Heimo J.
The tempering of α-(BEDT-TTF)2I3 crystals above 70°C for several days leads to the transformation into the superconducting αt-phase (Tc = 8 K at 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 β-phase. But the indicatrix orientation rather indicates a new phase than the β-phase.
Open Access
TCNQ salts of planar metal complex cations: novel molecular conductors and semiconductors
(1985) Endres, Helmut; Bongart, August; Nöthe, Dietrich; Hennig, Ingolf; Schweitzer, Dieter; Helberg, Hans W.; Schäfer, Herbert
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.