Using extreme magnetic field microscopy to visualize correlated electron materials

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. With zero resistance cables, we could build more effective motors, wind generators or computers. Superconductors would reduce effectively the strain on the grid, decreasing risks associated with power supply failure at consumption peaks. We would help reducing our dependence from fossil fuels. 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. 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. 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. These correlations might hold the key to unlock the mystery of HTc superconductivity.

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 of HTc superconductivity by directly visualizing these correlations.

To achieve this objective, we use scanning tunneling microscopes (STM) at extreme magnetic fields, designed and built by me and the people who work with me. 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 5000 times the Earth’s magnetic field. 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 have led the construction of a new microscope for this magnet and I am also building further microscopes. For example, within a recently awarded ERC PoC, I will considerably reduce the size of the microscope so that it fits in many new environments. With this effort, I also hope to help building up the capacities of large facilities.

The results obtained in this project have helped us to better understand how the competition between magnetism and superconductivity favors the emergence of superconductivity at high temperatures. We have seen that the magnetic order shapes the interaction between electrons producing anisotropic superconducting properties. We have also learned that quantum fluctuations change the properties of the superconductor under magnetic field, producing new spatial distribution of the paired electrons that can be related with emergence of HTc superconductivity.

I have set up set-up a new laboratory, optimized for measurements of low mechanical noise, where we operate the largest magnetic field STMs currently available.

During this time, I have used available microscopes to study the superconducting properties in iron-based superconductors. have chosen a few interesting systems on which I can answer concrete and relevant questions. One of the most important features of correlations is the appearance of new kinds of electronic behavior, giving unidirectional anisotropies that are thought to be just of electronic origin. Electrons are ordered similar as elongated molecular buildings in a nematic liquid crystal. I have directly visualized these nematic anisotropies and shown how these modify the superconducting vortex lattice, leading to completely new superconducting phases at very high magnetic fields. Another key feature of electronic correlations is what we call quantum criticality. The highest critical temperatures are often obtained when a magnetic order is driven to zero by the action of some parameter, as doping or pressure. At this point, there are huge quantum fluctuations, which provide a macroscopic quantum entangled state that might be very beneficial for superconductivity. I have visualized this quantum critical state and found that it considerably modifies superconductivity, leading to unexpected vortex properties. The pnictide family of materials is rapidly increasing, and I have stayed up to date, addressing new interesting problems, as the discovery of a new material where high critical temperature appears in a stoichiometric compound (most pnictide materials require doping to show high critical temperatures).

These results have provide unique inside on how superconductivity is affected in the proximity of an antiferromagnetic order and/or in presence of quantum critical fluctuations.

Maybe the major advancement has been 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 have reached the relevant phase space to address the problem of strong correlations in condensed matter. We have opened, with the developments of this project, a new line of research in microscopy. The microscope goes beyond the state of the art devices currently available in the world and provides an unprecedented tool, also useful in other areas within physics and material science.

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 have provided key experimental insight about how HTc superconductivity emerges from the interaction between electrons.

Periodic Reporting for period 4 - PNICTEYES (Using extreme magnetic field microscopy to visualize correlated electron materials)

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