Uniaxial pressure dependence of the critical field anisotropy in UPt3

UPt3 is known to possess a unique phase diagram. Close to Tc, three superconducting phases A, B and C coexist with the normal phase N giving rise to a tetracritical point. Furthermore, the anisotropy in the upper critical field Hc2 near Tc is reversed at low temperatures. In this letter we show that the behaviour under uniaxial pressure is unusual as well. For pressure, Pc parallel to the c axis, Hc2 is enhanced at low temperatures for H parallel to the a axis but is depressed for H parallel to c. In contrast the B-C transition is always depressed with increasing Pc for both the field orientations. Order-parameter candidates selected to explain the tetracritical point in the phase diagram of UPt3 near Tc must also yield results consistent with the behaviour of the phase lines observed by us at low temperatures. (Author)