Superconductivity In Novel Uranium Systems / Supraconductivité des nouveaux composés d’uranium – SINUS

This project is dedicated to the superconductivity of uranium systems, and the exploration of new uranium compounds. It relies on the leading position of the Grenoble teams (from CEA-INAC/SPSMS and CNRS Institut Néel) on the synthesis and experimental study of new uranium systems, with strong effective feedback with the complementary theoretical approach from the Grenoble (CEA-INAC/SPSMS) and Bordeaux (CNRS-Université de Talence) Groups. The project is focused on three intimately related objectives, all of them experimental as well as theoretical: - To explore and discover new states of matter in uranium systems, in new materials but also in those presently known in extreme conditions of very low temperature, high magnetic fields or high pressure. - To determine the symmetry of the superconducting order parameter - To uncover the pairing mechanism responsible for these superconducting states. The reason for devoting an ambitious project specifically to the superconductivity of uranium systems is the incredible wealth of surprise and fundamental insights brought the study of superconductivity in these compounds. Indeed uranium superconductors provide paradigms of unconventional superconductors and display counter-intuitive behaviours which have and continue to trigger fundamental advances in our general knowledge of superconductivity, and its coexistence with other electronic instabilities. The general target of this project is to boost the research on new uranium superconductors and strengthen the leading position of Grenoble by: - Growing new materials and explore their properties, as well as producing single crystals of the highest quality of materials of central interest for our studies. - Determining the superconducting ground state of these systems, notably through the determination of the gap nodes (using 'thermal spectroscopy'), using the most relevant fundamental experiments notably very low temperature thermal conductivity. Not only for its own interest, this step is also a prerequisite for further research on the pairing mechanism. URhGe, URu2Si2, UBe13 are among the known samples we want to investigate, or reinvestigate due to the strong sample quality improvement. - Probing the pairing mechanism on some well-defined systems, by inelastic neutron studies, notably by detecting changes in the magnetic excitation spectrum across the superconducting critical temperature. URhGe and URu2Si2 are the first two obvious candidates. Interpretation will need a precise knowledge of the symmetry of the superconducting order parameter and strong feedback with theory. - Exploring the physics of these superconductors under high magnetic field at very low temperatures: a large variety of phenomenon has appeared or is predicted to appear under field. The ambition is both to understand the mechanism for the appearance of these field induced phases (for example in URhGe) and to discover new ones (may be in URu2Si2 for particular field orientations), thanks to the new materials that we will synthesize and to the extended temperature and field range available for the studies. In order to fulfil this scientific program, new means should be developed: - For the crystal growth of high purity systems and new materials, most equipment is already acquired and working. But in order to boost the efficiency of the research on new materials, we need to remove some bottlenecks, like the time of acquisition for X-ray Laue orientation needed for characterization, and sample preparation for transport measurements and neutron experiments. - To extend down to low temperatures the present measurements, we need a new equipment to realize state of the art thermal contacts on the sample. - To extend up to high fields, where the most interesting physics is to be understood, we need to acquire a stronger magnet (from 8 to 16T) on the dilution refrigerator reaching the lowest temperatures (6mK). - To explore the new high field phase, or have proper characterization of the samples for transport or neutron studies, we want to rebuild a high performance specific heat set-up, reaching 50mK in 16T. o The main experimental challeng will be to push the limits of measurements under extreme conditions of low temperature-high field: in order to explore the physics of this uranium superconductors, with critical temperatures below 1K, we want to give us the means of performing thermal conductivity down to 10mK up to 16T, specific heat down to 50mK but also up to 16T, and magnetization measurements down to 30mK. Eventually, at the end of the project, we expect to have successfully solved most technical problems, and significantly progressed on the knowledge of the superconducting ground state and the pairing mechanism of these system. However, owing to the past history of the field, the strongest expectation is that the most interesting results will have been completely unforeseen at the beginning of the project !