Petra Bele

A stable superconducting state at 8 K and ambient pressure in\(\alpha _t - (BEDT - TTF^ \star )_2 I_3 ^1 \)

We report bulk superconductivity at 8 K and ambient pressure in crystals of αt(BEDT-TTF)2I3. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)2I3 here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)2I3 above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields Hc2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting αt-crystals after tempering.

Weak ferromagnetism and magnetically modulated microwave absorption at low magnetic fields in 1,3,5-triphenyl-6-oxoverdazyl

Abstract The magnetic susceptibility and microwave properties of powder samples of 1,3,5-triphenyl-6-oxoverdazyl were investigated between room temperature and 1.8 K. The high-temperature susceptibility follows a Curie-Weiss law with a Curie constant of 0.38(1) emu K mol −1 and a Weiss constant of −12(2) K. At low temperature we find 1D magnetic behaviour evidenced by a broad maximum in the susceptibility. Below 4.9 K weak ferromagnetism is detected which is attributed to a canting of the antiferromagnetic sublattices of about 0.13°. Strong nonresonant microwave absorption appears below 4 K which exhibits a hysteresis when measured in a small scanned external field. Possible origins of the weak ferromagnetism and the nonresonant microwave absorption are discussed.

Thin films of the organic superconductor α1-(BEDT-TTF)2I3

Abstract α-(BEDT-TTF) 2 I 3 is a quasi-two-dimensional organic metal with a metal-insulator phase transition at 135 K. Thermal treatment at about 80° C leads to the metallic system α t -(BEDT-TTF) 2 I 3 , which becomes superconducting below 8 K. Thin films of the α-phase (between 500 and 3000 A thick) have been evaporated in high vacuum onto several substrates and characterised by means of X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and a low field microwave absorption technique. Depending on the temperature of the substrate and the evaporation rate, the films exhibit different degrees of microcristallinity, which under certain conditions can be strongly reduced and a completely covering film can be obtained. X-ray diffraction spectra reveal a high orientation with the c-axis perpendicular to the substrate and as well the successful conversion into the α t -phase by tempering. SEM and AFM investigations prove that the conversion takes place without reducing the mechanical quality of the films. Low-field microwave-absorption experiments show that the α t -films become superconducting with an onset at 9 K.

Characterisation of the Fermi surface and phase transitions of (BEDO-TTF)2 ReO4·(H2O) by physical property measurements and electronic band structure calculations

The electronic properties of the organic superconductor (BEDO-TTF)2 ReO4·(H2O) were investigated by temperature dependent resistivity, ESR, Hall effect and magnetoresistance measurements. Shubnikov-de Haas (SdH) oscillations were observed in magnetic fields up to 24 T in the temperature range 0.5 K to 4.2 K. The electronic band structure of (BEDO-TTF)2 ReO4·(H2O) was calculated by employing the extended Hückel tight binding method on the basis of its room temperature crystal structure. The two observed SdH frequencies of 75 T and 37 T correspond very well with two cross-sectional areas of the hole and electron Fermi surface pockets obtained from the tight binding calculation. From the temperature dependence of the SdH oscillation amplitudes, the cyclotron effective mass (mc) belonging to the larger and smaller pockets were found to be 0.9 m0 and mc=1.15 m0 respectively. Measurements of the angular dependence of the SdH frequencies show no deviation from that expected for a cylindrical Fermi surface. In terms of our tight binding calculations and experimental measurements, probable causes for the 213 K and ∼35 K phase transitions are discussed. The calculations show that (BEDO-TTF)2 ReO4·(H2O) is a two dimensional semimetal but possesses a hidden nesting. The latter is likely to cause an SDW instability leading to the ∼35 K transition. The resistivity drop associated with the 213 K transition is likely to be induced by an abrupt increase in the relaxation time. The excellent agreement between the calculated and experimentally observed Fermi surface implies that, with decreasing temperature below 35 K, (BEDO-TTF)2 ReO4·(H2O) gradually gets out of the SDW state and re-enters the “original” metallic state, in which it becomes superconducting below 2.4 K.

Magnetically modulated microwave absorption (MMMA) at low magnetic fields in (BEDT-TTF)-superconductors

Abstract We report magnetically modulated microwave absorption ( MMMA ) at low magnetic fields in crystals of the organic superconductors α t -(BEDT-TTF) 2 l 3 and κ-(BEDT-TTF) 2 Cu(NCS) 2 . In both cases a strong MMMA - signal can easily be found below the critical temperature T c . In some selected crystals series of narrow, periodically spaced lines are observed comparable to corresponding spectra of the CuO - type high T c superconductors. A model developed for the HTSC s single crystals is used to interpret the results of the organic superconductors.

Thin Films of BEDT-TTF and BEDO-TTF Radical Cation Salts

Abstract In this communication we describe the fabrication and characterization of thin films of the radical cation salts α-, αt-(BEDT-TTF)2I3 and (BEDO-TTF)2.4I3. Thin films of these compounds were made by evaporation of the salts in high vacuum onto several substrates. They were characterized by X-ray diffraction, scanning electron microscopy, infrared absorption, Raman scattering, de-conductivity measurements and magnetically modulated microwave absorption. Depending on the temperature of the substrate and the evaporation rate, the films of α-(BEDT-TTF)2I3 exhibit different degrees of microcrystallinity which under the right conditions can be strongly reduced to obtain relatively smooth, completely covering films. Additionally such films have a high degree of orientation with respect to the substrate plane. The conversion into films of the superconducting αt-phase is possible and the high orientation is maintained. Due to grain boundaries, the dcconductivity is thermally activated, but the single grain...

New high-DQE imaging plate scanner using the reflected readout laser signal for noise corrections.

Imaging Plates (IPs) are in principle ideal electron detectors combining a large active layer area with a high sensitivity, linear dynamic range detection over 5 orders of magnitude. A moderate resolution and a decreasing detection quantum efficiency (DQE) for higher electron doses limit their use so far. The decrease of the DQE results from linear noise contributed by readout laser instabilities and inhomogeneities of the IP active layer. Here we present data on a new IP drum scanner prototype. This scanner combines twin channel amplification electronics with a new type of readout laser which allows a smaller readout focus and increased stability. The current nominal pixel size is 25 μm, and the measured modulation transfer function (MTF) indicates that further reduction of the scanning step size down to pixel sizes in the range of 12–15 μm should be possible. A unique feature of the new scanner is the simultaneous recording of the reflected readout laser light. The reflected light signal can be used for a posteriori alignment of repeated scans of one individual IP and for a correction of one part of the high spatial frequency noise contribution (reflected light correction). The posteriori alignment now allows an easy conventional gain normalization of the luminescence signal without using special markers on the IP. Both corrections lead to an increase of the DQE for high electron doses. Microsc. Res. Tech. 49:281–291, 2000.

Electron paramagnetic resonance and magnetically modulated microwave absorption experiments on κ-(BEDT—TTF)2Cu[N(CN)2]Cl

Abstract Electron paramagnetic resonance (EPR) and magnetically modulated microwave absorption (MMMA) measurements were taken on differently prepared samples of the quasi two-dimensional organic conductor κ-(BEDT—TTF) 2 Cu[N(CN) 2 ]Cl to study the interesting pressure dependent phase transition of this charge-transfer compound. The MMMA results undoubtedly confirm that the substance becomes a superconductor below 12.5 K, if a moderate pressure, e.g. just by embedding the substance in silicon grease during cooling, is applied. At an onset temperature of 25 K an additional microwave absorption is observed, which is about a factor of 100 weaker than the superconducting MMMA signal. We relate this microwave absorption to an antiferromagnetic ordering resulting in weak ferromagnetism by spin-canting occurring at this temperature. The result corresponds to the EPR measurements, where considerable changes in the main EPR parameters, i.e. linewidth, signal amplitude and susceptibility, are observed, giving evidence for an antiferromagnetic ordering below 50 K and a change to weak ferromagnetism below 25 K.

Electron spin resonance measurements on single crystals of the organic superconductor κ-(BEDT-TTF)2I3

Angularly and temperature-dependent electron spin resonance (ESR) measurements on single crystals of the organic superconductor κ-(bis(ethylenedithiolo)-tetrathiafulvalene)2I3 are reported. The results on the anisotropy of theg-value, the temperature dependence of the susceptibility as well as of the ESR linewidth are discussed.