Field and excitation dependent ultrasonic attenuation in³He-B

Abstract

Sound absorption of³He-B at 0 bar was studied at a frequency of 10 MHz. We performed pulsed experiments with different pulse lengths (4 to 15 μs) and pulse powers (0.1 to 160 μW) in the temperature range 0.2 < T/T_C < 2. For pulse power less than pth ∼ 50 μW, and in the temperature range 0.4 to 0.8 T_c and zero field, the attenuation coefficient a decreases monotonically at about 0.2 cm⁻¹ per decade of power. Above pth. α increases up to values of 3.5 cm⁻¹ and 1.9 cm⁻¹ at highest power for temperatures of 0.8 T_c and 0.4 T_c, respectively. At pulse power around 100 μW and at the lowest temperatures, the attenuation changes within the first 200 μs after the application of the pulse; α also increases with increasing field. In the normal fluid α decreases with increasing power. All of our observations are lacking a rigorous theoretical understanding.