General Scope: At first glance, superconductivity and ferromagnetism are antagonistic ground states. While the possibility of p-wave equal-spin-pairing superconductivity has been discussed already forty years ago, only the discovery of superconductivity in the itinerant ferromagnets UCoGe and URhGe opened the possibility to study experimentally the interplay of superconductivity and ferromagnetism in itinerant ferromagnetic systems. Both systems show a very rich superconducting phase diagram with a field enhancement (in UCoGe) or a re-entrant superconducting phase (in URhGe) as a function of a magnetic field applied along the b axis of the orthorhombic crystals.
Subject: URhGe orders ferromagnetic at 9.5 K. Under a magnetic field applied along the b axis the ferromagnetic order is suppressed near 12 T and field-induced superconductivity occurs. The collapse of the ferromagnetism coincides with a field reorientation of the magnetic moments and the reorientation occurs through a first order transition. UCoGe orders ferromagneticaly at 2.5 K, while superconductivity occurs below 0.5 K. In difference to URhGe, the suppression of the ferromagnetism under field is not related to a first order transition. In this internship we want to study the magnetic and superconducting phase diagram of URhGe and UCoGe by thermal expansion and magnetostriction measurements, a thermodynamic probe which is extremely sensitive to the order of the transitions. We intend to follow the ferromagnetic transition even inside the superconducting phase, and get information on quantum tricritical fluctuation which may drive the field enhanced superconducting phases in both systems.
Environment and collaborations: At the IMAPEC team of Pheliqs in CEA, research on topological superconductivity in strongly correlated uranium compounds is the central research activity. The coexistence of ferromagnetism and superconductivity in URhGe and UCoGe has already been intensively studied in IMAPEC and high-quality single crystals are available. IMAPEC has strong interactions with the high magnetic field laboratory LNCMI Grenoble, and the proposed experiments will be continued at the LNCMI under fields up to 30 T. The candidate will further profit from strong theoretical support from the theory group of Pheliqs, and have the possibility to interact with groups in Japan where tight collaborations are well established.
Required skills: The candidate should have sound knowledge in solid state physics, good practical skills and strong interest in the development of instrumentation. The subject can be continued by a PhD thesis.