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

Periodic Reporting for period 4 - MARS (Electronic Order, Magnetism, and Unconventional Superconductivity probed in Real-Space)

Reporting period: 2020-03-01 to 2020-08-31

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.

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, and to achieve maximum resolution in energy and real-space. This new instrument aims at studying the most important classes of unconventional superconductors, viz. cuprate, iron-arsenide, and heavy-fermion superconductors.

Further experimental and theoretical work within this project aims to better rationalize the physics of unconventional superconductors and correlated phases. For example, we address nematicity in iron-based superconductors as well as strongly underdoped correlated phases close to the Mott transition, including the mechanism of the doping-induced insulator-metals transition. Thus we hope contributed to the still sought-after complete rationalization of this physics which indeed is the crucial milestone required to eventually achieve superconductivity at technologically better exploitable temperatures. This, eventually, can be expected to open up new technologies ranging from energy saving to quantum computing.
In this project, we are building from scratch a new type of low-temperature scanning tunneling microscope (STM) which operates at 30mK with a vector magnetic field (9 Tesla vertical and 4 Tesla horizontal). Today, the instrument has been completed to a stage where high resolution measurements at base temperature can routinely be performed. Once completed, we can already say today that this novel instrument has the potential to deliver novel insight into the physics of novel quantum materials with very high spatial and energetic resolution.

In addition to this constructive work, numerous experimental studies mainly using scanning tunneling spectroscopy but als transport experiments have been performed on several types of unconventional superconductors and closely related correlated electron systems. Here we highlight two fields which are not only representative for our research, but also bear the capability to generate further novel experimental approaches to the quantum phases that eventually constitute unconventional superconductivity: One example concerns the nematicity of iron-based superconductors. Here we have scrutinized ways to reveal nematic fluctuations, e.g. by analyzing the long-wavelength Friedel-oscillations in a particularly clean materials (LiFeAs) or by studying the nematic susceptibility by elastotransport. Another concerns the discovery of spectroscopic signatures of the internal degrees of freedom of the spin-polaron, i.e. a confined charged particle in an antiferromagnetic background by performing low-temperature scanning tunneling spectroscopy on the spin-orbit coupling induced antiferromagnetic Mott insulator Sr2IrO4. These and other results are constantly being disseminated through scientific journals, talks, and poster presentation at conferences.
The most prominent result of this project is the novel 30mK STM. Already now, i.e. prior to the completion of its final stage it represents one of few instruments in the world which are capable of scanning tunneling spectroscopy at such a low temperature. Further, since it has been designed to be spin sensitive it has the potential to become a unique instrument for studying novel quantum phases. Completion is imminent.

In addition, 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.
Furthermore, the first-time detection of the ladder spectrum of the spin polaron in an antiferromagnetic Mott insulator and the revelation of a percolative nature of the insulator-metal transition in Sr2IrO4 represents a milestone in understanding the nature of correlated electrons, in particular the motion of charges in an antiferromagnetic background.
Intertwined phases of unconventional superconductors tackled by STM