Direct measurement of the energy gap of superfluid He-3-B in the low-temperature limit

The zero-temperature limit of the energy gap, Delta(P, T -> 0), of superflu id He-3-B has been measured at T/T-c less than or similar to 0.25, near 0.1 and 4.8 bars, and in zero magnetic field. The energy gap was determined fr om the 2 Delta pair-breaking edge of an acoustic signal obtained by novel, pulsed Fourier-Transform ultrasonic spectroscopy. Out results are independe nt of the temperature scale and the theoretical model of the gap. The value s for Delta(P, T -> 0) are lower than predicted by the weak-coupling-plus t heory, and the Delta(P approximate to 0.1 bars, T -> 0) values are lower th an predicted by BCS theory. The data indicate that Delta(P, T -> D) of supe rfluid He-3-B is not well modeled at the lowest pressures.