UNIAXIAL PRESSURE-DEPENDENCE OF THE CRITICAL-FIELD ANISOTROPY IN UPT3

UPt3 is known to possess a unique phase diagram. Close to T(c), three superconducting phases A, B and C coexist with the normal phase N givi ng rise to a tetracritical point. Furthermore, the anisotropy in the u pper critical field H(c2) near T(c) is reversed at low temperatures. I n this letter we show that the behaviour under uniaxial pressure is un usual as well. For pressure, P(c) parallel to the c axis, H(c2) is enh anced at low temperatures for H parallel to the a axis but is depresse d for H parallel to c. In contrast the B-C transition is always depres sed with increasing P(c) for both the field orientations. Order-parame ter candidates selected to explain the tetracritical point in the phas e diagram of UPt3 near T(c) must also yield results consistent with th e behaviour of the phase lines observed by us at low temperatures.