2017, Wang, Ruining, Zhang, Wei, Momand, Jamo, Ronneberger, Ider, Boschker, Jos E., Mazzarello, Riccardo, Kooi, Bart J., Riechert, Henning, Wuttig, Matthias, Calarco, Raffaella

A highly unconventional growth scenario is reported upon deposition of GeTe films on the hydrogen passivated Si(111) surface. Initially, an amorphous film forms for growth parameters that should yield a crystalline material. The entire amorphous film then crystallizes once a critical thickness of four GeTe bilayers is reached, subsequently following the GeTe(111) 

2016, Opherden, Lars, Sieger, Max, Pahlke, Patrick, Hühne, Ruben, Schultz, Ludwig, Meledin, Alexander, Van Tendeloo, Gustaaf, Nast, Rainer, Holzapfel, Bernhard, Bianchetti, Marco, MacManus-Driscoll, Judith L., Hänisch, Jens

The addition of mixed double perovskite Ba2Y(Nb/Ta)O6 (BYNTO) to YBa2Cu3O7−δ (YBCO) thin films leads to a large improvement of the in-field current carrying capability. For low deposition rates, BYNTO grows as well-oriented, densely distributed nanocolumns. We achieved a pinning force density of 25 GN/m3 at 77 K at a matching field of 2.3 T, which is among the highest values reported for YBCO. The anisotropy of the critical current density shows a complex behavior whereby additional maxima are developed at field dependent angles. This is caused by a matching effect of the magnetic fields c-axis component. The exponent N of the current-voltage characteristics (inversely proportional to the creep rate S) allows the depinning mechanism to be determined. It changes from a double-kink excitation below the matching field to pinning-potential-determined creep above it.

2016, Dabrowski, J., Lippert, G., Avila, J., Baringhaus, J., Colambo, I., Dedkov, Yu S., Herziger, F., Lupina, G., Maultzsch, J., Schaffus, T., Schroeder, T., Kot, M., Tegenkamp, C., Vignaud, D., Asensio, M.-C.

The practical difficulties to use graphene in microelectronics and optoelectronics is that the available methods to grow graphene are not easily integrated in the mainstream technologies. A growth method that could overcome at least some of these problems is chemical vapour deposition (CVD) of graphene directly on semiconducting (Si or Ge) substrates. Here we report on the comparison of the CVD and molecular beam epitaxy (MBE) growth of graphene on the technologically relevant Ge(001)/Si(001) substrate from ethene (C2H4) precursor and describe the physical properties of the films as well as we discuss the surface reaction and diffusion processes that may be responsible for the observed behavior. Using nano angle resolved photoemission (nanoARPES) complemented by transport studies and Raman spectroscopy as well as density functional theory (DFT) calculations, we report the direct observation of massless Dirac particles in monolayer graphene, providing a comprehensive mapping of their low-hole doped Dirac electron bands. The micrometric graphene flakes are oriented along two predominant directions rotated by 30° with respect to each other. The growth mode is attributed to the mechanism when small graphene “molecules” nucleate on the Ge(001) surface and it is found that hydrogen plays a significant role in this process.

2016, Tarantini, C., Iida, K., Hänisch, J., Kurth, F., Jaroszynski, J., Sumiya, N., Chihara, M., Hatano, T., Ikuta, H., Schmidt, S., Seidel, P., Holzapfel, B., Larbalestier, D.C.

Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the REFeAs(O,F) (RE1111, RE being a rare-earth element) is the family with the highest critical temperature Tc but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 (Tc ∼ 55 K). Here we focus on the pinning properties of the lower-Tc Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density Jc at high temperatures is dominated by point-defect pinning centres, whereas at low temperatures surface pinning by planar defects parallel to the c-axis and vortex shearing prevail. When the field approaches the ab-planes, two different regimes are observed at low temperatures as a consequence of the transition between 3D Abrikosov and 2D Josephson vortices: one is determined by the formation of a vortex-staircase structure and one by lock-in of vortices parallel to the layers. This is the first study on FBS showing this behaviour in the full temperature, field, and angular range and demonstrating that, despite the lower Tc and anisotropy of Nd1111 with respect to Sm1111, this compound is substantially affected by intrinsic pinning generating a strong ab-peak in Jc.

2017, Grinenko, V., Iida, K., Kurth, F., Efremov, D.V., Drechsler, S.-L., Cherniavskii, I., Morozov, I., Hänisch, J., Förster, T., Tarantini, C., Jaroszynski, J., Maiorov, B., Jaime, M., Yamamoto, A., Nakamura, I., Fujimoto, R., Hatano, T., Ikuta, H., Hühne, R.

A quantum critical point (QCP) is currently being conjectured for the BaFe2(As1−x P x )2 system at the critical value x c  ≈ 0.3. In the proximity of a QCP, all thermodynamic and transport properties are expected to scale with a single characteristic energy, given by the quantum fluctuations. Such a universal behavior has not, however, been found in the superconducting upper critical field Hc2. Here we report Hc2 data for epitaxial thin films extracted from the electrical resistance measured in very high magnetic fields up to 67 Tesla. Using a multi-band analysis we find that Hc2 is sensitive to the QCP, implying a significant charge carrier effective mass enhancement at the doping-induced QCP that is essentially band-dependent. Our results point to two qualitatively different groups of electrons in BaFe2(As1−x P x )2. The first one (possibly associated to hot spots or whole Fermi sheets) has a strong mass enhancement at the QCP, and the second one is insensitive to the QCP. The observed duality could also be present in many other quantum critical systems.