Roland Schäfer

Two-dimensional superconductivity between SrTiO3 and amorphous Al2O3

Two-dimensional (2D) superconductivity is observed between insulating amorphous Al2O3 and TiO2-terminated (001)-oriented SrTiO3. The heterostructure displays interfacial metallic conduction from room temperature down to ≈200 mK where superconductivity sets in. The voltage-current V(I) characteristics indicate a power-law scaling behavior V∝Iα with a temperature dependent exponent α, indicative for a Berezinskii-Kosterlitz-Thouless transition at TBKT≈160 mK and a 2D superconducting state. The superconducting transition was found to be very sensitive to a magnetic field B.

Two-dimensional simulations of temperature and current-density distribution in electromigrated structures

We report on the application of a feedback-controlled electromigration technique for the formation of nanometre-sized gaps in mesoscopic gold wires and rings. The effect of current density and temperature, linked via Joule heating, on the resulting gap size is investigated. Our experiments include in situ measurements of the evolution of the electrical resistance and of the structure of the device during electromigration. Experimentally, a good thermal coupling to the substrate turned out to be crucial to reach electrode spacings below 10 nm and to avoid overall melting of the nanostructures. This finding is supported by numerical calculations of the current-density and temperature profiles for structure layouts subjected to electromigration. The numerical method can be used for optimizing the layout so as to predetermine the location where electromigration leads to the formation of a gap.

Patterning of two-dimensional electron systems in SrTiO3 based heterostructures using a CeO2 template

Two-dimensional electron systems found at the interface of SrTiO3-based oxide heterostructures often display anisotropic electric transport whose origin is currently under debate. To characterize transport along specific crystallographic directions, we developed a hard-mask patterning routine based on an amorphous CeO2 template layer. The technique allows preparing well-defined microbridges by conventional ultraviolet photolithography which, in comparison to standard techniques such as ion- or wet-chemical etching, does not induce any degradation of interfacial conductance. The patterning scheme is described in detail and the successful production of microbridges based on amorphous Al2O3-SrTiO3 heterostructures is demonstrated. Significant anisotropic transport is observed for T < 30 K which is mainly related to impurity/defect scattering of charge carriers in these heterostructures.

Anisotropic electronic transport of the two-dimensional electron system in Al₂O₃/SrTiO₃ heterostructures

Transport measurements on the two dimensional electron system in Al2O3 SrTiO3 heterostructures indicate significant noncrystalline anisotropic behavior below T = 30 K. Lattice dislocations in SrTiO3 and interfacial steps are suggested to be the main sources for electronic anisotropy. Anisotropic defect scattering likewise alters magnetoresistance at low temperature remarkably and influences spin-orbit coupling significantly by the Elliot Yafet mechanism of spin relaxation resulting in anisotropic weak localization. Applying a magnetic field parallel to the interface results in an additional field induced anisotropy of the conductance, which can be attributed to Rashba spin orbit interaction. Compared to LaAlO3 SrTiO3, Rashba coupling seems to be reduced indicating a weaker polarity in Al2O3 SrTiO3 heterostructures.