Arcs and pockets in a model high-temperature superconductor

The phenomenon of high-temperature (high-Tc) superconductivity is one of the most exciting, thoroughly investigated yet still unresolved problems in physics. A major difficulty in understanding high-Tc systems lies in the complexity of the phase diagram where a number of phases are in delicate balance. Material specific properties and disorder strongly make it far from trivial to disentangle the reasons why a material becomes superconductive (SC) at elevated temperatures. Consequently, the number of different theoretical models describing the high-Tc phenomenon is large and further experimental effort is required to understand the mechanism of high-temperature superconductivity. Due to a series of recent important experimental results, in which my contribution was instrumental, a new picture of this phenomenon starts to emerge (News and Views: Nature 468, 184 (2010) and Nat. Phys 9, 757, (2013)). Here, I propose to build upon the well posed experimental foundations (PNAS 110, 12235 (2013), PNAS 110, 5774 (2013), Nat. Phys. 9, 761 (2013), PRL 113, 177005 (2014)) from which the following working hypotheses emerged: Effective Fermi energy of the normal (zero- magnetic field, high temperature) Fermi liquid state is large (~1 eV) and cuprates are nodal Fermi liquids. Effective Fermi energy of the high-field (low-temperature) state observed around "1/8" doping is small (~10 meV) corresponding to a small electron like pocket in the Brillouin zone. Fermi-liquid carriers (observed in pseudogaped state of cuprates) are those which become superconducting. If these hypotheses are robustly confirmed for the optimally doped samples, an important part of the high- temperature riddle would be solved clarifying the relations between neighboring phases. In particular we will study the presumed field-induced phase transition at "1/8" doping by use of resonant X-rays and by measuring transport properties in high-magnetic fields, while the Fermi- liquid to insulator transition at low dopings (below 1%) will be studied by optical/microwave conductivity and transport. Furthermore, we aim to fully characterize the normal state above the superconducting dome. Special attention will be given to temperature and doping dependence of cot Hall angle, which seems to exhibit FL behavior across the phase diagram. The important new insights into the physics of the cuprates will be primarily based on measurements on the sizable, high quality crystals of the model compound Hg1201 to study. This material features a simple crystalline structure, minimal disorder effects, and the highest Tc in the class of single-layer compound.

Every material, every standard cable, every electronic device has, in principle, some electric resistance. There are, however, superconducting materials with the ability to conduct electrical current with a resistance of exactly zero - at least at very low temperatures. Finding a material that behaves as a superconductor at room temperature would be a scientific breakthrough of incredible conceptual and technological importance. It could lead to a wide range of new applications, from levitating trains to new imaging technologies for medicine. The search for high-temperature superconductors is extremely difficult, because many of the quantum effects related to this phenomenon are not yet understood. With the support of the FWF, through the project "Arcs and pockets in a model high-temperature superconductor" we have performed series of experiments in cuprates, a class of materials which behave as a superconductor at record temperatures as high as 140K at ambient pressure. As a result of this endeavour we have obtained a remarkable set of new insights that profoundly change the way we think about these complex materials and high-temperature superconductivity in general. Perhaps the most important results is that there are two fundamentally different kinds of charge carriers in cuprates, and superconductivity crucially depends on the subtle interplay between them. Part of the charge is localized - sitting at particular set of atoms and can only move if the material is heated or doped, while the other charge carriers can move, jumping from one atom to another. It is the mobile charge that ultimately becomes superconductive, but superconductivity can only be explained by taking the immobile charge carriers into account too. Similarly, we have demonstrated that other unusual properties of this fascinating materials can be understood within the proposed picture of two electronic subsystems. (This text was largely inspired by the TU Wien's press release from 28.01.2019 - https://www.physik.tuwien.ac.at/aktuelles/news_detail/article/10467/)