Collective Modes of a Superfluid William Halperin , Northwestern University DMR-0244099

1. Collective Modes of a SuperfluidWilliam Halperin, Northwestern University DMR-0244099 One of the fundamental notions of modern condensed matter physics is that of spontaneous symmetry breaking at a phase transition. A by-product of broken symmetry is the appearance of order parameter collective modes, which are a fingerprint of the underlying structure of the condensed state[1-3]. We can determine a wealth of information about the system through measurement of these collective modes. Early observations of order parameter collective modes in superfluid 3He provided conclusive confirmation of its pairing state. In addition to better understanding the superfluid we can also learn about the normal state particle interactions[2]. Our work was published in Physical Review Letters this month[5]. Propagation of transverse sound waves in a fluid was first performed with NSF support by our group[4]. The wiggles correspond to cavity resonances for transverse sound in superfluid 3He. The arrow marks the extinction where the collective mode and sound frequencies cross.

2. Collective Modes of a Superfluid William Halperin, Northwestern University DMR-0244099 Education: Northwestern University undergraduates, Sam Blinstein and Tom Lippman together with, Northwestern graduate students, Johannes Pollanen, Hyongsoon Choi, and John Davis are collaborating on physics projects in the Halperin laboratory to explore the properties of anisotropic superfluids. One of the techniques we have developed requires imbibing superfluid into a highly porous silica gel. The above students (shown also at right) have discovered how to grow these gels with intrinsic anisotropy, a key to exploring anisotropic superfluidity Outreach: We have started collaborations with four other laboratories in France, Japan and England, supplying them with samples of silica aerogels made by the Halperin group shown below.