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Scanning Tunneling Microscopy Studies at High Magnetic Fields: Visualizing Pnictide and Heavy Fermion Superconductivity

Final Report Summary - EXTREMEFIELDIMAGING (Scanning Tunneling Microscopy Studies at High Magnetic Fields: Visualizing Pnictide and Heavy Fermion Superconductivity)

The purpose of the EU research project ‘Scanning Tunneling Microscopy Studies at High Magnetic Fields: Visualizing Pnictide and Heavy Fermion Superconductivity’ (ExtremeFieldImaging) is to provide basic knowledge about how the properties of superconductors vary at the local level under strong magnetic fields. The goals of the project are to investigate high magnetic field superconducting phases and to understand the interplay between magnetic and electronic correlations in pnictide and heavy fermion materials.

To achieve these objectives, dilution refrigerator Scanning Tunneling Microscopy (STM) studies are performed down to the mK regime and under magnetic fields up to 17 T in the Low Temperature Laboratory at the Universidad Autónoma de Madrid (LBTUAM). Within the project, compact STM devices have been built to operate at the highest available magnetic fields and enable the possibility of future STM measurements in international high field facilities.

During the project we have designed and built a 17 T STM microscope. Also, a new compact microscope free from magnetic components to be operated under much stronger magnetic fields is under construction. New laboratory space has been designed and developed during the project consisting of isolated experimental rooms equipped with new vibration free technology to implement very high magnetic field STM. We have also grown crystals of several pnictide superconductors.

We have studied the superconducting pnictide compositions of the series Co-doped CaFe2As2 and CaKFe4As4 and the heavy fermions URu2Si2 and Rh-doped CeRu2Si2. In Co-doped CaFe2As2 we have investigated nematic and superconducting properties and have visualized the vortex lattice under magnetic fields. In CaKFe4As4 we have reported evidence for two gap superconductivity consistent with s\pm pairing symmetry, observed the vortex lattice and studied the bandstructure measuring electronic interference patterns. In the superconducting heavy fermion In URu2Si2, we have observed the superconducting gap opening and studied the bandstructure using interference patterns. In Rh-doped CeRu2Si2, we show electronic correlations influencing the bandstructure close to the Fermi level.

The integration of the candidate within the group and the university has been excellent. The candidate obtained several international recognitions (Axa Research fund project, FBBVA fund project, N. Kurti prize and recently ERC StG). Moreover, the researcher has obtained a tenure track position through the Ramon y Cajal Program and participates in the main European, national and regional wide projects of the group (COST project recently awarded). All this, in particular the ERC StG and the RyC fellowship, guarantee the development as a fully independent researcher.

The microscopic approach to high field superconductivity proposed by the researcher will bring about new challenges and opportunities through the combination of local studies and novel superconducting properties. In particular, it is proposed to directly image the spatial variations in the superconducting pair function and determine the electronic structure through Landau level spectroscopic measurements. The latter is a new local technique which we will first implement in superconductors by the researcher.