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Electronic Order, Magnetism, and Unconventional Superconductivity probed in Real-Space

Periodic Reporting for period 3 - MARS (Electronic Order, Magnetism, and Unconventional Superconductivity probed in Real-Space)

Reporting period: 2018-09-01 to 2020-02-29

The interrelation of electronic order with antiferromagnetism and superconductivity has recently emerged as a vital question for rationalizing the physics of all classes of unconventional superconductors. Typically, such electronic ordering phases, which recently have been dubbed intertwined phases, are ubiquitously found at the crossover between ostensibly competing antiferromagnetic and superconducting states. Only rarely the electronic order is sufficiently long-range correlated to render it susceptible for diffraction techniques. Instead, it usually requires a local probe to detect it experimentally. It is clear, however, that such a probe must provide sensitivity at the same time to electronic order, superconductivity, and static magnetism for a full characterization of the intertwined phases, aiming at clarifying the interrelation between these ordering phenomena. The only experimental technique which is capable of fulfilling these requirements simultaneously is spin-polarized scanning tunnelling microscopy (SP-STM), which to the best of our knowledge, has never been applied to this intriguing problem, despite the apparent mandatory necessity.

The MARS project builds on our experience in the field of unconventional superconductivity and scanning tunnelling microscopy. Recently, we were able to establish SP-STM in our microscopes, and thus the time has now come to apply this technique for the first time on unconventional superconductors. Exactly this is the goal of this project. Highest-resolution SP-STM will be systematically applied to prototype representatives of the most important classes of unconventional superconductors, viz. cuprate, iron-arsenide, and heavy-fermion superconductors. For this purpose, a unique milli-Kelvin scanning tunnelling microscope (STM) is being built which will make possible to access the ground states of all these systems with SP-STM, and to achieve maximum resolution in energy and real-space.

It can be expected, that a successful outcome of the project will lead to a better rationalization of the physics of unconventional superconductors. The complete rationalization of this physics indeed is the crucial milestone required to eventually achieve superconductivity at technologically better exploitable temperatures, which can be expected to open up new technologies ranging from energy saving to quantum computing.
In the period covered, we have worked on constructing a new type of lowest-temperature scanning tunneling microscope (STM) which operates at 30mK. More specifically, the CAD model is ready, and all the commercial parts have been ordered. The assembly of the system is currently going on. Furthermore, we have performed scanning tunneling microscopy and spectroscopy (STM/STS) on various correlated materials. On the one hand, we have scrutinized the spin-orbit coupling induced antiferromagnetic Mott insulator Sr2IrO4 at low temperature. This compound is considered an analogue to the cuprate-based antiferromagnetic Mott-insulating ground state in parent compounds. Here our spectroscopy data reveal distinct shoulder-like features for occupied and unoccupied states beyond the Mott gap. We attributed these anomalies to the spectroscopic signature of the motion of a single hole or electron in the antiferromagnetic background of this compound. More specifically, upon invoking the self-consistent Born approximation (SCBA) we find in the unoccupied states excellent agreement with the spin-polaronic ladder spectrum in this material. Whereas, for the occupied states we find separate contributions for two different values of the total quantum momentum of the spin polaron with J=0 and J=1. We believe that this fundamental insight is directly transferable to the physics of cuprate based high-temperature superconductors. A publication on this matter has been prepared and posted on the publicly available arXiv server (Guevara et al., arXiv:1802.10028).

On the other hand, the superconducting state and impurity states of LiFeAs have been investigated. We observed various types of impurities with different impact on the superconducting state. The results have been published (Schlegel et al., Phys. Status Solidi B 254, 1600159 (2017)). Further, we have studied the development of the superconducting gap as a function of temperature, where an unconventional evolution of the superconducting state with multiple transitions has been observed (Nag et al., Scientific Reports 6, 27926 (2016)). During these investigations we discovered an interesting signature of the electron-boson interaction beyond the superconducting gap energy. Another study of the quasiparticle interference (QPI) as a function of temperature, focusing on these energies revealed a resonant enhancement of the QPI amplitude at these particular energies from which the energy and the momentum of the involved boson could be extracted. Our analysis of these data reveals a novel, quite general approach to probe the electron boson interaction in complex systems.
The most prominent expected result of this project is the completion of the novel 30mK STM and it application to unconventional superconductors with spin resolution. With respect to experiments performed within the first period of the project we have obtained new insights into the physics of iron based superconductors. In particular, our work on the iron-pnictides revealed a very unconventional change of the superconducting state in LiFeAs. This finding underpins the special role of this material among the iron pnictides. Furthermore, the observed signature of the electron-boson interaction in the quasiparticle interference constitutes a new general approach towards exploring the electron-boson interaction in complex materials. We expect that its application to unconventional superconductors will promote the rationalization of the physics of such systems. Finally, the first-time detection of the ladder spectrum of the spin polaron in an antiferromagnetic Mott insulator represents a milestone in understanding the nature of correlated electrons, in particular the motion of charges in an antiferromagnetic background.