Ultrasonic excitations in ${\mathrm{He}}^3$‐B

Of the three superfluid phases of ${\mathrm{He}}^3,$ the one labeled ${\mathrm{He}}^3$‐B has often been considered the “dull” superfluid because it lacked the interesting anisotropies of the A and A1 phases. The B phase has now become more intriguing: Over the past few years, ultrasonic spectroscopy has uncovered ever more details of the excited states of ${\mathrm{He}}^3$‐B, and has revealed that the collective excitations of this many‐body system have a form reminiscent of simple atomic spectra. As Peter Wolfle from the Technical University of Munich told us, the superfluid ${\mathrm{He}}^3$‐B acts like “a giant molecule in momentum space.” The excited states, observed through strong absorption peaks in the transmission of ultrasound, have essentially the structure one would expect from a triplet p‐wave pairing of the helium atoms. Measurements have uncovered effects that are the equivalent of Zeeman splitting and the Paschen‐Back effect. Some of these measurements were described at the International Symposium on Quantum Fluids and Solids held at Sanibel Island, Florida, in April. (The story on page 23 describes other work on ${\mathrm{He}}^3$‐B presented at the same meeting.)