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Atomic-scale STudies Of the Nature of and conditions for Inducing Superconductivity at High-temperatures

Mid-Term Report Summary - ASTONISH (Atomic-scale STudies Of the Nature of and conditions for Inducing Superconductivity at High-temperatures)

Our project is focussed on fundamental studies of the mechanisms leading to high-Tc superconductivity. The experimental investigations concentrate on model-type systems which are prepared and thoroughly characterized with atomic level precision. The growth of the model-type samples is controlled vertically one atomic layer at a time and laterally by making use of single-atom manipulation techniques. Atomic-scale characterization at low energy-scales is performed by low-temperature spin-resolved elastic and inelastic scanning tunnelling microscopy (STM) and spectroscopy (STS) as well as by non-contact atomic force microscopy based techniques. Transport experiments are conducted by a four-probe STM setup under well-defined ultra-high vacuum conditions.
The first type of model systems we have studied in the first reporting period consist of metal-based superconductors. We aimed at using elemental superconductors with strong spin-orbit coupling as a template for the growth of magnetic adlayers and adatoms in order to study theoretically predicted exotic electron phases. As promising candidate for such a template we focussed on lanthanum (La) thin films on a W(110) substrate. Interestingly, we could show that the Tc of La can be increased by 40% with respect to the bulk Tc value by an atomic-level control of the purity of the La material during thin film growth.
Furthermore, we studied superconductor-magnet hybrid systems where the superconductor is again an elemental material with large spin-orbit coupling, e.g. Fe on Ta(110), Fe on Ta(001), and Fe on Re(0001). Several different non-collinear spin states, including Néel states and spin spiral states, could be revealed by SP-STM in ultrathin Fe films epitaxially grown on Re(0001). This sample system appears to be highly promising for studies of exotic phases, including theoretically predicted unconventional superconducting phases in hybrids of non-collinear spin textures and superconducting substrates, and will be in the focus of our investigations in the second project period.
Another major result in the first reporting period was the realization of model-type Hund’s impurities, the basic constituents of Hund’s metals. The concept of Hund’s metals has recently been introduced in order to describe exotic phases of matter such as unconventional superconductivity in iron pnictides and chalcogenides, as well as non-Fermi liquid behaviour in ruthenates. We have explored Hund’s impurities in the interesting regime of four almost degenerate low-energy scales: Kondo temperature, magnetic anisotropy, Zeeman energy and temperature. Our study provided a sound basis for understanding the complex physics of systems ranging from unconventional superconductors to transition-metal oxides and non-Fermi liquids.
The second kind of model systems explored in the first reporting period are ultrathin Fe-chalcogenide films epitaxially grown on Bi-based topological insulator substrates such as Bi2X3 (X=Se,Te). We have achieved defect-free Fe-chalcogenide films by growing small amounts of Fe on top of the Bi2X3 substrates. To our great surprise, we found strong evidence for interfacial superconductivity in atomic layers of FeTe epitaxially grown on Bi2Te3 with coexisting superconducting and bi-collinear antiferromagnetic (AFM) order, as directly revealed by combining low-temperature STS and SP-STM experiments at the same sample location. Moreover, we found a reorientation of the spin direction in the bi-collinear AFM state at the surface of bulk and thin film FeTe samples.
For the analogous system of FeSe0.5Te0.5 epitaxially grown on Bi2(Se,Te)3 we found strong evidence for a two-fold symmetric superconducting gap in a monolayer of FeSe0.5Te0.5 whereas for the epitaxially grown FeSe on Bi2Se3 no superconducting gap was observed down to 4K. This is in strong contrast to the relatively large Tc-values reported by other groups for ultrathin FeSe films epitaxially grown on SrTiO3 substrates. Since we will continue the highly exciting studies of ultrathin Fe-chalcogenide layers on various substrates in the second project period, we have set up a new MBE growth chamber which will solely be dedicated to the preparation of ultrathin chalcogenide films.
As a third type of model systems we started to investigate oxide-based systems epitaxially grown on elemental superconductors with strong spin-orbit coupling. In the limit of ultrathin oxide films we could study the superconducting properties by low-temperature STS. However, for thicker oxide films and oxide-based heterostructures, force microscopy-based techniques are required for spatially resolved studies of the superconducting properties of such samples. Therefore, we have set up and further developed a low-temperature atomic force microscope which can operate down to 300 mK and in magnetic fields up to 10 T, while being capable of atomic-resolution imaging and simultaneous spectroscopic mapping of superconducting properties.