Events

Institute of Low Temperature and Structure Research PAS Seminar

Pulsed-field techniques applied to UTe2: high-field magnetism and the “Lazarus” superconducting state

prof. dr John Singleton

z National High Magnetic Field Laboratory Pulsed-Field Facility, Los Alamos National Laboratory w Los Alamos w USA

Abstract

High magnetic fields usually destroy superconductivity. However, this talk will describe a variety of measurements carried out in pulsed magnetic fields of up to 74 T, including transport, magnetometry, the magnetocaloric effect and MHz penetration depth, that show that UTe2 possesses multiple bulk superconducting phases. These include the “Lazarus” phase, which exhibits the highest magnetic field range of any re-entrant superconducting state; zero resistance persists from around 45 T to beyond 70 T. Despite the huge ranges of magnetic field involved, in relatively pure UTe2 all of the superconducting phases vanish at a similar temperature. High-field measurements have also revealed details of the Fermi surface. The two largest sheets are quasi-two-dimensional, being tube-like with almost rectangular cross-sections. Though these sheets dominate the electronic thermal properties, they cannot explain the rather isotropic resistivity of UTe2. Subsequent high-frequency magnetoconductivity experiments have detected additional small ellipsoidal Fermi pockets that may help to account for the resistivity data.
Superconductivity in very high magnetic fields presents a considerable challenge for current theoretical approaches. Whilst models such as the Jaccarino-Peter compensation effect can be eliminated as an explanation, the magnetic-fluctuation-mediated superconductivity mechanism thought to occur in heavy-fermion compounds such as URhGe may provide a qualitative understanding of UTe2. However, the magnetic-field-orientation dependence of the high-field superconducting phase is challenging for any interpretation; the Lazarus state wraps around the b-axis in a “halo”-like fashion.
Finally, I discuss the apparent “homogenizing” role of high magnetic fields in UTe2; whereas its zero-field properties are highly sensitive to details of the synthesis method and crystal quality, the phase diagrams are much more unified in fields above 30 T. The most extreme form of this behaviour occurs when the Lazarus phase appears at high magnetic fields in crystals that are not at all superconducting at zero field (so-called “Orphan Superconductivity”). All of these observations will form very tight constraints for any theory of UTe2.
*In UTe2 the superconductivity initially appears to be ”killed” by the magnetic field. But then, like Lazarus, it “rises from the dead” at a higher field. Thanks are due to the NHMFL publicity department for this (possibly useful) analogy which has now entered popular physics culture.
#Work carried out in collaboration with University of Maryland, NIST, Los Alamos National Laboratory, University of California, San Diego, Washington University St. Louis, and University of St. Louis.