Th. Wolf

Growth of thick YBa2Cu3O7−x single crystals from Al2O3 crucibles

Abstract The growth of large, isometric and free-standing single crystals of YBa 2 Cu 3 O 7− x using a CuOBaO self-flux is reported. Within an optimum concentration range in the phase diagram centered around 4 at% Y, 30 at% Ba and 66 at% Cu, isometric crystals with aspect ratios between 1–10 and sizes of 5×5×2 mm 3 were grown. Outside this range of favourable flux compositions thin platelets up to 8×8 mm 2 with aspect ratios ≫ 10 were obtained. The thickness of the isometric crystals is essentially determined by the cooling rate: 500μm, 1 mm and 2 mm thick crystals were obtained for cooling rates of 0.5, 0.3 and 0.1° C/h, respectively. For cooling rates 30° C/h a dendritic non equilibrium morphology with a zigzag-like habit of the edge of the (001) face can be observed. Aluminum impurities arising from the Al 2 O 3 crucible have a strong influence on the crystal growth: they considerably reduce the soak time and the viscosity of the melt. The presence of Al also favours the formation of yellow-greenish needles of a new quaternary phase, YBa 3 Al 2 O 7.5 .

Investigation of inter- and intragrain critical currents in high Tc ceramic superconductors

Abstract Polycrystalline sintered bulk samples of REBa 2 Cu 3 O 7 with RE = Y , Gd were studied by a.c. susceptibility and an inductive measurement technique which allows the separation of intergrain (transport) from intragrain critical current density. Field and temperature dependence of the intergrain current are compared with theoretical predictions for a weakly coupled grain structure. Up to fields 0.3 T the weak link character of the intergrain current predominates. At higher fields it changes and points to a percolation current via superconducting grain boundaries. A quantitative correlation between intragrain current and twin spacing is hidden, probably by oxygen deficiency in the grain interior. No variation of the intragrain critical current density with temperature is observed up to 85 K in a textured specimen.

Emergence of the nematic electronic state in FeSe

We present a comprehensive study of the evolution of the nematic electronic structure of FeSe using high-resolution angle-resolved photoemission spectroscopy (ARPES), quantum oscillations in the normal state, and elastoresistance measurements. Our high-resolution ARPES allows us to track the Fermi surface deformation from fourfold to twofold symmetry across the structural transition at ∼87K, which is stabilized as a result of the dramatic splitting of bands associated with dxz and dyz character in the presence of strong electronic interactions. The low-temperature Fermi surface is that of a compensated metal consisting of one hole and two electron bands and is fully determined by combining the knowledge from ARPES and quantum oscillations. A manifestation of the nematic state is the significant increase in the nematic susceptibility approaching the structural transition that we detect from our elastoresistance measurements on FeSe. The dramatic changes in electronic structure cannot be explained by the small lattice distortion and, in the absence of magnetic fluctuations above the structural transition, point clearly towards an electronically driven transition in FeSe, stabilized by orbital-charge ordering.

Intragrain junctions in YBa2Cu3O7−x ceramics and single crystals

Abstract Intragrain critical current densities of polycrystalline bulk samples and single crystals were investigated by inductive flux profile measurements. In addition to the well known granularity in a polycrystal, granularity within a single crystal was detected. In analogy to the polycrystalline samples increasing values of j c with decreasing size of the powder particles ground from single crystal gives strong evidence for the existence of intragrain junctions. The investigation of the field dependence of j c reveals as a common feature in ceramics and single crystals a maximum at fields far below the upper critical field. This observation and the behaviour of j c in the field region below this maximum are not in accordance with a critical current as expected from a pinning interaction, but rather reveal junction character. This is supported by data of fast neutron irradiation showing a lowering of the junction current, whereas above the critical current maximum j c increases by radiation induced pinning centres. The fields at which the maxima appear are in rough quantitative agreement with the irreversibility line from the glassy state or flux creep model. Once the phase coherence of the intragrain junctions is destroyed by the neutron damage, j c shows the expected continuous decrease with increasing field.

Unusual band renormalization in the simplest iron-based superconductor FeSe 1 − x

The electronic structure of the iron chalcogenide superconductor FeSe_{1-x} was investigated by high- resolution angle-resolved photoemission spectroscopy (ARPES). The results were compared to DFT calculations showing some significant differences between the experimental electronic structure of FeSe_{1-x}, DFT calculations and existing data on FeSe_{x}Te_{1-x}. The bands undergo a pronounced orbital dependent renormalization, different from what was observed for FeSe_{x}Te_{1-x} and any other pnictides.

Anisotropy of the reflectance spectrum and of the dielectric function of YBa2Cu3O7 within the (001) plane

Abstract We have performed polarized reflection measurements on nontwinned domains of the (001) surface of YBa 2 Cu 3 O 7 single crystals in the energy range between 0.05 and 6 eV. From the spectra, obtained for the polarization direction parallel to the a- and to the b-axis, respectively, we could determine for the first time the tensor components of the dielectric function of YBa 2 Cu 3 O 7 parallel to both axis. The evaluation was done by fitting Lorentz-Drude models and by Kramers-Kronig analysis. Using the experimental data, the contributions of the CuO planes and of the CuO chains to the electronic transport properties of YBa 2 Cu 3 O 7 could be separated.

Lack of coupling between superconductivity and orthorhombic distortion in stoichiometric single-crystalline FeSe

The coupling between superconductivity and orthorhombic distortion is studied in vapor-grown FeSe single crystals using high-resolution thermal-expansion measurements. In contrast to the Ba122-based (Ba122) superconductors, we find that superconductivity does not reduce the orthorhombicity below

Nematic susceptibility of hole-doped and electron-doped BaFe2As2 iron-based superconductors from shear modulus measurements.

{T}_{c}

Evidence of Strong Correlations and Coherence-Incoherence Crossover in the Iron Pnictide Superconductor KFe2As2

. Instead we find that superconductivity couples strongly to the in-plane area, which explains the large hydrostatic pressure effects. We discuss our results in light of the spin-nematic scenario and argue that FeSe has many features that are quite different from typical Fe-based superconductors.

Evidence of a Precursor Superconducting Phase at Temperatures as High as 180 K in RBa2Cu3O7-delta (R = Y, Gd, Eu) Superconducting Crystals from Infrared Spectroscopy

The nematic susceptibility, χφ, of hole-doped Ba(1-x)K(x)Fe2As2 and electron-doped Ba(Fe(1-x)Co(x))2As2 iron-based superconductors is obtained from measurements of the elastic shear modulus using a three-point bending setup in a capacitance dilatometer. Nematic fluctuations, although weakened by doping, extend over the whole superconducting dome in both systems, suggesting their close tie to superconductivity. Evidence for quantum critical behavior of χφ is, surprisingly, only found for Ba(Fe(1-x)Co(x))2As2 and not for Ba(1-x)K(x)Fe2As2--the system with the higher maximal Tc value.