Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - PNICTEYES (Using extreme magnetic field microscopy to visualize correlated electron materials)

Teaser

In 1986, Bednorz and Müller discovered a family of materials based on copper (cuprates) that are high critical temperature superconductors (HTc). Only a few months later, they were jointly awarded the Nobel Prize in physics-- the shortest time between the discovery and the...

Summary

In 1986, Bednorz and Müller discovered a family of materials based on copper (cuprates) that are high critical temperature superconductors (HTc). Only a few months later, they were jointly awarded the Nobel Prize in physics-- the shortest time between the discovery and the prize award for any scientific Nobel Prize. Superconductors are so exciting because they lose their electrical resistance and can carry electricity without losses. About 20 percent of the electrical energy is lost between power plants and households due to the resistance of the cables, amounting to billions of Euros wasted. With zero resistance cables, we could build more effective motors, wind generators or computers. We could use underground small size power cables, instead of the huge high voltage lines which impact our landscape. Superconductors would reduce effectively the strain on the grid, decreasing risks associated with power supply failure at consumption peaks. Eliminating resistance of electrical conductors would be transformational to our society as a whole.

But superconductors still require being cooled and are expensive to utilize. Unfortunately, today, we don\'t have superconductivity at room temperature. And we nearly work in the dark when we search for materials with higher superconducting critical temperature—because it’s so difficult to understand why superconductivity occurs, it’s also nearly impossible to know what is needed to have it at higher temperatures. Determining the origin of the HTc superconductivity is one of the major problems in condensed matter physics.

The iron pnictide materials are known since relatively recently and these have stirred again the community of condensed matter physicists. In a decade, we have gone from the discovery, to the establishment of some basic understanding and to the first applications. However, most important is that we are slowly seeing that these materials might hold the key to the long-sought solution of the puzzle. They are high Tc superconductors, but they are metals, not insulators as the cuprates, and their electronic properties are highly tunable showing a high degree of correlation with lattice, magnetism and among electrons themselves.

The purpose of the ERC StG project “Using extreme magnetic field microscopy to visualize correlated electron materials (PNICTEYES)” is to contribute to solve the puzzle by directly visualizing these correlations.

To achieve this objective, we use scanning tunneling microscopes (STM) at extreme magnetic fields, designed and built in my group. These microscopes provide neat images of electronic correlations because they are operated close to zero temperature and at very high magnetic fields, directly probing the electronic ground state. In my group, we operate solenoids capable of providing magnetic fields up to 22T, i.e. approximately 500 000 times the Earth’s magnetic field and about 20 times the field produced by neodymium magnets. My group is now hosting the most intense magnetic field available outside international high magnetic field facilities, as the European Magnetic Field Laboratory (EMFL) or the National High Magnetic Field Laboratory at the US (MagLab). I am leading the construction of a new microscope for this magnet and I am also building, together with my colleagues, microscopes for high magnetic field facilities that could operate to fields higher than 30 T. I hope to help building up the capacities of these facilities with this development.

Microscopes under very high magnetic fields will allow the direct visualization of electronic correlations and give conclusive answers to key questions in Condensed Matter Physics. High field microscopes can be used in graphene, nanotechnology, superconductivity or magnetism. We are coming closer to the dream of having widely used superconducting systems.
This project aims at contributing to the basic understanding of the physics behind HTc superconductors which likely will help t

Work performed

When I started the project, we realized that the available laboratory space was not adequate to hold a 22 T magnet. Thus, I set myself to obtain new laboratory space and to set this up in an optimal way for my experiments. I seized the opportunity and have spent a lot of time to design, using simple means, new facilities that considerably improve the performance of our microscopes. I used local construction companies, and, with the help of my university, I have been able to set-up a laboratory with specifications that are quite competitive and will help me a lot to develop an independent line of research. I also installed the 22 T superconducting magnet, which is now fully operative.

During this time, I have used available microscopes to study the superconducting properties of Co-doped CaFe2As2 and of the new family of compounds CaKFe4As4. In Co-doped CaFe2As2 we have investigated nematic and superconducting properties and have visualized the vortex lattice under magnetic fields. We have directly imaged the nematic properties and shown that superconductivity is highly inhomogeneous. We are now figuring out how this can influence useful properties of superconductors, such as pinning. In CaKFe4As4 we have reported evidence for two gap superconductivity consistent with s pairing symmetry, observed the vortex lattice and studied the band-structure measuring electronic interference patterns.

The results obtained have been disseminated in international scientific conferences such as APS March Meeting 2018 and 28th International Conference on Low Temperature Physics. These have been also communicated to the general public through press releases and radio interviews.

Final results

Maybe the major advancement is to obtain, at millikelvin temperatures and high magnetic fields, atomic resolution in a number of strongly correlated electron systems, including pnictide superconductors. It’s not just that we reach the relevant phase space to address the problem of strong correlations in condensed matter. We open, with the developments of this project, a new line of research in microscopy. The new high magnetic field STM microscope will deserve the large community of condensed matter physics, providing for a new looking glass into the properties of metals, semiconductors, quantum dots, or layered materials as graphene or transition metal dichalcogenides. The microscope will go beyond the state of the art devices currently available in the world and provide an unprecedented tool, also useful in other areas within physics and material science. What is more important, probably, it’s that the microscope is a small piece of engineering made by physicists that will hopefully help the establishment of further developments in the commercially important field of scanning probe microscopies.

The most relevant scientific implication of my project arises from the nanoscale characterization of the fundamental physics behind HTc superconductivity in different model FeSC. Direct observation and vivid imaging in clean superconducting materials will provide the necessary breakthrough to obtain required experimental information about how HTc superconductivity emerges from the interaction between electrons. I have proposed a new method in pnictide materials, consisting of Landau-level spectroscopy, which will likely nurture effectively the feedback with theory, because it provides directly the electronic band-structure. The knowledge of physics of HTc superconductors with extremely large upper critical fields will be of use to both theorists and experimentalist working on designing materials with high critical fields for magnet applications. I hope to impact material science providing new fresh input in the everlasting effort of obtaining materials to solve pressing societal needs.