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

The zero-temperature limit of the energy gap, Delta(P,T-->0), of superfluid 3He-B has been measured at T/T(c) less, similar0.25, near 0.1 and 4.8 bars, and in zero magnetic field. The energy gap was determined from the 2Delta pair-breaking edge of an acoustic signal obtained by novel, pulsed Fourier-Transform ultrasonic spectroscopy. Our results are independent of the temperature scale and the theoretical model of the gap. The values for Delta(P,T-->0) are lower than predicted by the weak-coupling-plus theory, and the Delta(P approximately 0.1 bars,T-->0) values are lower than predicted by BCS theory. The data indicate that Delta(P,T-->0) of superfluid 3He-B is not well modeled at the lowest pressures.