Prof. Morten Eskildsen

Department of Physics

University of Notre Dame

Host: Gannon

Title: Fission induced vortex lattice disordering in UPt₃

Abstract:  Subjecting a type-II superconductor to a magnetic field will cause the formations of quantized vortices.  Due to their repulsive interaction the vortices will, in an ideal situation, arrange themselves into a perfectly order vortex lattice (VL).  In reality, however, thermal effects and/or pinning to material defects are present, and the balance between these competing factors determine both the structural and dynamic properties of vortex matter.  This leads to a rich phase diagram comprised of both ordered and disordered solid phases and vortex liquids.  While transitions between the different phases are driven by changes of intensive quantities such as the magnetic field or temperature, their locations in the phase diagram are sensitive to the amount of defects in the host superconductor.  This provides an experimental handle with which one can tune the vortex matter phase diagram, e.g by introducing impurities during the material synthesis or by bombardment with heavy ions to create columnar defects post growth.

Here we report on small-angle neutron scattering (SANS) studies of vortices in the topological superconductor UPt₃, and specifically how the VL in this material undergoes a gradual disordering as it is subjected to a beam of cold neutrons.  The disordering occurs on a time scale of tens of minutes, and is attributed to local heating events caused by neutron induced fission of ²³⁵U, which temporarily heat regions of the sample above the critical temperature.  Vortices in the affected regions remain in a disordered configuration after re-cooling, which is most likely a quenched vortex glass.  Moreover, the rate of disordering is proportional to the magnetic field, suggesting a direct relation to collective VL properties such as the elastic moduli.  While the VL does not spontaneously re-order once the local heating has been dissipated it is possible to re-anneal the VL by the application of a damped field oscillation, indicating that no permanent radiation damage of the UPt₃ crystal occur within experimental time scales.  The results demonstrate a novel avenue for vortex matter studies, allowing an introduction of localized and reversible quenched disorder.