Temperature dependence of AC losses in CVD-prepared Nb3Ge

Hysteretic losses have been measured as a function of temperature from 4 to 20 K in bulk samples of CVD prepared Nb
3
Ge. The measurements have been performed on samples which contained differing amounts of second phase Nb
5
Ge
3
and which have been subjected to various surface treatments. Losses at 50 Hz and 500 rms A/cm have been determined to be as low as 0.2 μW/cm
²
at 4 K and 12.7 μW/cm
²
at 12 K in samples containing a small per cent of second phase and whose surfaces have been both mechanically polished and chemically etched. From measurements of the hysteretic power loss, the temperature dependence of the critical current density J
c
(T) can be determined by assuming a Bean-London type critical state model in the high current regime. It is found that J
c
(T) = J
c
(0)[1-(T/T*
c
)
²
] best describes the data over the range 4 K to T
c
*. At low values of induced current, substantial surface shielding Δσ is observed in samples which have been polished and etched. Δσ has been measured to be as large as 450 rms A/cm at 4.0 K.