Order Parameter Structure in Thin Films and Disordered Superfluid 3He

Northwestern University, Evanston IL

Investigators

Abstract

This project will investigate the influence of disorder on superfluid 3He, one of an important class of quantum fluids that includes ruthenate superconductors, heavy fermion superconductors, and high temperature superconductors. These unconventional systems are formed from pairs of fermions in BCS states. They all have the common important characteristic of spontaneously broken symmetries beyond gauge symmetry. As such they are particularly sensitive to impurity effects, which must be investigated if the underlying behavior of these systems is to be understood and if the potential for their application is to be realized. The proposed research will bring precise experimental tools, transverse high frequency acoustics, heat capacity, and thin film methods, that are available for 3He in aerogel glass, to determine order parameter suppression and the density of low-lying excitations and to compare with recent theoretical predictions for gapless superconductivity. The results will have broad implications for unconventional superconductors, Bose-Einstein condensation, and exotic vortex matter. The participation of graduate students in this work will provide them with technical training relevant to basic research and industrial interests. Undergraduate projects on acoustics will give opportunities for learning complementary to, but outside, the classroom. This project will investigate the influence of disorder on superfluid 3He, one of an important class of quantum fluids that includes high temperature superconductors. These systems exhibit a high degree of fragility, that is, they have extreme sensitivity to all forms of imperfection and impurities. Investigation of disorder is crucial to their basic understanding and to a realization of their potential for applications. 3He is an ideal choice to study such effects since its pure state is well understood. This is in contrast to many superconductors which have physical and chemical complexity that is difficult to control. The proposed research includes, thermodynamic and acoustic measurements while systematically controlling disorder introduced with highly porous silica aerogel. The work is relevant to theoretical models for neutron star matter, quark Cooper pairs, and exotic vortex states for which 3He is often a reference point. Undergraduate students will participate in the proposed research using phase modulation acoustics and second sound propagation. Training of graduate students in the technically sophisticated areas of magnetic resonance, low-temperature methods, and high frequency acoustics will prepare them for a range of professional career opportunities with a significant potential impact to our society.