REBa 2 Cu 3 O 7−x (REBCO, RE = rare earth) compounds with different single RE elements were grown via TFA-MOD (metal-organic deposition of trifluoroacetates) to clarify their T c values when grown by the same preparation method and their processing windows; here: the crystallisation temperatures at a constant process gas composition (pO 2 , pH 2 O). We focussed on the lanthanides (Ln) Nd, Sm, Gd, Dy, Ho, Er and Yb as substituents for Y in the REBCO phase and investigated their growth behaviour in terms of resulting physical (inductive T c and J c (77 K)) and structural properties (determined by XRD, SEM, TEM). All phases were grown as pristine films on LaAlO 3 and SrTiO 3 and compared to their respective nanocomposites with 12 mol% BaHfO 3 for in-field pinning enhancement.With regard to T c and J c (77 K), the optima of both values shift towards higher growth temperatures for increasing and decreasing RE ion size with respect to yttrium. Highest T c values achieved so far do not show a trend that can solely be related to the RE ionic size. On the contrary, T c,90 values of the LnBCO compounds from Sm to Er range between 94.0 and 95.3 K and are, therefore, significantly larger than the highest values of the average-size non-lanthanide, Y, with T c,90 = 91.5 K. J c,sf values at 77 K seem to plateau between 5 and 6 MA cm −2 from Sm to Er and are again clearly above the maximum values we ever achieved for Y with 4.2 MA cm −2 . REBCO phase formations of the very small Yb and large Nd turned out to be more difficult and require further adjustment of growth parameters. All REBCO phases investigated here show distinct dependences of T c on the lattice parameter c.

In the search for novel organic charge transfer salts with variable degrees of charge transfer we have studied the effects of two modifications of the recently synthesized donor-acceptor system [tetramethoxypyrene (TMP)]-[tetracyanoquinodimethane (TCNQ)]. One is of chemical nature by substituting the acceptor TCNQ molecules by F4TCNQ molecules. The second consists in simulating the application of uniaxial pressure along the stacking axis of the system. In order to test the chemical substitution, we have grown single crystals of the TMP-F4TCNQ complex and analyzed its electronic structure via electronic transport measurements, ab initio density functional theory (DFT) calculations and UV/VIS/IR absorption spectroscopy. This system shows an almost ideal geometrical overlap of nearly planar molecules stacked alternately (mixed stack) and this arrangement is echoed by a semiconductor-like transport behavior with an increased conductivity along the stacking direction. This is in contrast to TMP-TCNQ which shows a less pronounced anisotropy and a smaller conductivity response. Our band structure calculations confirm the one-dimensional behavior of TMP-F4TCNQ with pronounced dispersion only along the stacking axis. Infrared measurements illustrating the C≡N vibration frequency shift in F4TCNQ suggest however no improvement in the degree of charge transfer in TMP-F4TCNQ with respect to TMP-TCNQ. In both complexes about 0.1e is transferred from TMP to the acceptor. Concerning the pressure effect, our DFT calculations on the designed TMP-TCNQ and TMP-F4TCNQ structures under different pressure conditions show that application of uniaxial pressure along the stacking axis of TMP-TCNQ may be the route to follow in order to obtain a much more pronounced charge transfer.

The competition between phase formation of BaF2 and Ba(Fe1−x Co x )2As2 on CaF2 single crystals has been analysed. Ba(Fe0.92Co0.08)2As2 thin films have been deposited by pulsed laser deposition. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies have revealed that the formation of secondary phases and misorientations as well as the growth modes of the Ba(Fe0.92Co0.08)2As2 thin films strongly depend on the growth rate. At high growth rates, formation of BaF2 is suppressed. The dependency of the Ba(Fe0.92Co0.08)2As2 lattice parameters supports the idea of fluorine diffusion into the crystal structure upon suppression of BaF2 formation similar as was proposed for FeSe1−x Te x thin films on CaF2. Furthermore, a growth mode transition from a layer growth mechanism to a three-dimensional growth mode at high supersaturation has been found, suggesting similarities between the growth mechanism of iron-based superconductors and high-T c cuprate thin films.

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