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METALS, SUPERCONDUCTORS... 2009High field resistivity measurements on single crystalline UPt 2 Si 2Tetragonal UPt 2 Si 2 has recently been characterized as amoderately mass enhanced antiferromagnet (T N = 32 K),which in various physical properties closely resembles thehidden order material URu 2 Si 2 [Süllow et al., J. Phys. Soc.Jpn. 77, 024708 (2008); Johannsen et al., Phys. Rev. B 78,121103 (2008) ; Kim et al., Phys. Rev. Lett. 91, 256401(2003); Amitsuka et al., Physica B 177, 173 (1992) ].are clearly to be seen, supporting the notion of the similaritybetween the two systems. Presently, high field magnetizationmeasurements at 3 He temperatures are underway toextend the determination of these phase boundaries to verylow temperatures.Similar to URu 2 Si 2 , high field phase transitions at low temperatureshave been observed in UPt 2 Si 2 , that is in fieldsof 30 T to 45 T along the a direction and 20 T to 33 Talong c [Amitsuka]. Recent high field measurements revealadditional structure in the magnetic phase diagram, emphasizingthe close resemblance to URu 2 Si 2 .Based on these findings, the goal of the present set of experimentswas to verify and more accurately define the highfield phase boundaries observed in previous resistivity andmagnetization measurements. Therefore, high field resistivityexperiments at low temperatures along the crystallographica and c axes of UPt 2 Si 2 were initiated and carriedout.We have measured the high field resistivity along the crystallographica and c axes in field and temperature sweeps upto 28 T and 40 K. From our data we calculated the differentialresistivities ∂R/∂B and ∂R/∂T , in which the transitionsappear as local maxima (∂R/∂B) or minima (∂R/∂T ).In the temperature sweeps the transition to the antiferromagnetphase is clearly visible for both directions. In fieldsweeps with B applied along the a axis the high field phaseis not accessible but for B//c the phase boundary to thelowest high field phase occurring at ∼ 25 T below 18 K isvisible.Together with the data taken previously, we construct themagnetic phase diagram of UPt 2 Si 2 , as shown in figure 86.In these diagrams, field induced phases similar to URu 2 Si 2Figure 86: Proposed magnetic phase diagrams for the crystallographica and c directions derived from previous and actual magnetizationand resistivity measurements.I. SheikinD. Schulze Grachtrup, S. Süllow (TU Braunschweig, Institute for Physics of Condensed Matter, Braunschweig, Germany)64