Measurement of the Ultrasonic Attenuation in the Mixed State of Superconducting Niobium

A method for measuring quantitatively both the attenuation and the magnetization in a transverse magnetic field has been developed. The sample had the shape of an oblong platelet of single crystal niobium. A quartz rod bonded to the platelet constituted transducer and delay line. At 518 MHz and at 1,6 °K through 10 °K the attenuation and the magnetization were measured as functions of the transverse magnetic field. The attenuation curve increased monotonically with a horizontal tangent at H _c1 (lower critical field) and a vertical tangent at H _c2 (upper critical field). The combined measurements were compared with a recent theory of Maki. Full agreement was obtained adjusting the density of states: N(0) = (2,5 ± 0,3)·10³⁴/cm³ erg.