2016, Maljuk, Andrey, Lin, C.T.

The crystal growth of high-temperature oxide superconductors has been hampered by the complexities of these materials and the lack of knowledge of corresponding phase diagrams. The most common crystal growth technique adopted for these materials is the so-called “Flux” method. This method, however, suffers from several drawbacks: (i) crystals are often crucible and flux contaminated; (ii) crystals are difficult to detach from solidified melt; and (iii) crystals are rather small. In most cases, these drawbacks can be overcome by the crucible-free floating zone method. Moreover, this technique is suitable for crystal growth of incongruently melting compounds, and has been thus successfully used to make large single crystals of Bi2Sr2Ca2Cu3Oy superconductor. In this review, the authors summarize the published and their own growth efforts as well as detailed characterization of as-grown and post-growth annealed samples. The optimal growth conditions that allowed one to obtain the large-size, almost single phase and homogeneous in composition Bi2Sr2Ca2Cu3Oy single crystals are presented. The effect of long lasting post-growth heat treatment on both crystal quality and superconducting properties has also been demonstrated.

2009, Evtushinsky, D.V., Inosov, D.S., Zabolotnyy, V.B., Viazovska, M.S., Khasanov, R., Amato, A., Klauss, H.-H., Luetkens, H., Niedermayer, Ch., Sun, G.L., Hinkov, V., Lin, C.T., Varykhalov, A., Koitzsch, A., Knupfer, M., Büchner, B., Kordyuk, A.A., Borisenko, S.V.

Here we present a calculation of the temperature-dependent London penetration depth, λ(T), in Ba1-xKxFe 2As2 (BKFA) on the basis of the electronic band structure (Zabolotnyy et al 2009 Nature 457 569, Zabolotnyy et al 2009 Physica C 469 448) and momentum-dependent superconducting gap (Evtushinsky et al 2009 Phys. Rev. B 79 054517) extracted from angleresolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of λ(T) by muon spin rotation (μSR) (Khasanov et al 2009 Phys. Rev. Lett. 102 187005). The value of λ(T = 0), calculated with no adjustable parameters, equals 270 nm, while the directly measured one is 320 nm; the temperature dependence λ(T) is also easily reproduced. Such agreement between the two completely different approaches allows us to conclude that ARPES studies of BKFA are bulk-representative. Our review of the available experimental studies of the superconducting gap in the new ironbased superconductors in general allows us to state that most of them bear two nearly isotropic gaps with coupling constants 2ΔkBTc = 2.5±1.5 and 7±2.

2009, Urbanik, G., Hänke, T., Hess, C., Büchner, B., Ciszewski, A., Hinkov, V., Lin, C.T., Keimer, B.

We performed scanning tunneling microscopy and spectroscopy on untwinned crystals of underdoped YBa2Cu3O7- δ at δ = 0.4. A comprehensive statistical analysis of our topographic data indicates a doping dependent cleaving behavior of this material. We find in particular that at δ = 0.4 the material primarily cleaves in multiples of one unit cell along the c-axis with a high corrugation of the topmost layer. Our data suggest that the low temperature cleaving mainly results in a disruption of the CuO chain layers involving a redistribution of the layer atoms onto the two cleaving planes. In a few instances, fractional step heights (in terms of the c-axis lattice constant) are observed as well. Scanning tunneling spectroscopy reveals that such fractional steps connect surfaces which differ significantly in their tunneling conductance.