EICMSC · Electronic Instabilities in Clean Materials with Strong Correlations

Electronic Instabilities in Clean Materials with Strong Correlations

Executive Summary: The project “Electronic Instabilities in Clean Materials with Strong Correlations” focused on studying many body behaviour of electrons in solids. In the last century physicists developed a standard theory of metals. This so-called Landau Fermi liquid theory is utterly successful to describe most materials including metals like copper and gold as well as semiconductors like silicon. In fact, the Landau Fermi-liquid theory is underlying the electronic engineering of the semiconductor industry. Yet, an increasing number of materials show deviations from the standard Landau Fermi-liquid theory. Landau Fermi-liquid theory is based on independent particles. In the presence of strong interactions new concepts are required to understand the complex behaviour. Interestingly such cases include materials which see huge international research effort like the cuprate high temperature superconductors. In this project we study selected high purity materials and tune the interaction strength. We use external pressure to tune the materials. External pressure pushes the electrons closer together and thus favours strong interactions. In addition, pressure preserves the pristine state of our samples. Our studies allow us to observe the emergence of the new structures arising from the strong interactions. In this project, we have developed novel pressure techniques in order to access higher pressures more reliably. A major difficulty of high pressure research is to feed electrical contacts from samples in a pressure cell to the outside world and measurement devices. In the first year of the project we established methods to produce new patterned anvil cells with integrated tracks for electrical measurements. In addition to the goals set out in the proposal we pioneered focused ion beam techniques together with colleagues at the ETH, Zurich to cut and contact samples of below 100µm size. In this second part of the project we use these techniques to perform cutting edge research. We perform measurements on selected materials under pressure. These include NiS2 which can be turned from an insulator into a metal at high pressures. Comprehensive x-ray scattering measurements as well as high-pressure electrical resistivity measurements demonstrate the high quality of our crystals. We have prepared a high pressure cell for further electronic structure studies which are currently being performed at the University of Cambridge. We have also studied CePt2In7 at high pressures. This material orders magnetically at low temperatures. Under pressure this order is suppressed and gives way to a superconducting state. We have prepared a pressure cell using the focused ion beam techniques for cutting and contacting the sample, and have conducted measurements of the electric resistivity under pressure as well as electronic structure measurements using the quantum oscillation technique. In addition, we have studied LaAgSb2 which features a charge ordered state very much analogous to the magnetically ordered state of CePt2In7. Again, under pressure this ordered state is suppressed and we search for emergent superconductivity. We perform electrical resistivity, Hall effect, and quantum oscillation measurements under pressure and follow the electronic structure. All three parts of the project show great progress and will yield important insight into the reorganisation of electrons in the presence of strong interactions.

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