Penetration depth and torque of superconducting UPt 3

Abstract

In order to identify the properties of the internal superconducting phases of UPt 3 we studied the DC penetration depth in a SQUID system and the anisotropic s.c. response in a magnetic torque meter on three high-quality single crystals down to 5 mK. Near T C and with B in the c-direction we find a distinct s.c. double transition. With B along the a-axis there is only a single transition. These results seem to be in favor of a two-dimensional s.c. order parameter. At lower temperatures a power law λ∼ T 2 is found for B||c , whereas in the a-direction a T 1 dependence occurs. However, both power laws do not continue to very low temperatures, but are followed by a slight `paramagnetic’ increase below 30 mK and a sharp `diamagnetic’ drop below 20 mK which points to long-range antiferromagnetic order at low temperatures. The s.c. magnetization curves studied in the capacitive torque meter do not show any sign of the internal phase lines but a crossing line parallel to B c2( T), similar to the peak effect. This behavior is interpreted in terms of anisotropic flux pinning.