We report on detailed measurements of the specific heat of UPt3, performed on a high-quality single crystal in a magnetic field perpendicular to the c axis, at temperatures down to 100 mK. Two distinct phase transitions at zero field are seen to converge at a critical point, near H = 5 kOe, which coincides with the sharp break in the H c i curve. Beyond that point, there is evidence for only one phase. Combining these thermodynamic results with ultrasonic attenuation and H c i data, an H-T diagram is constructed which consists of a "polycritical" point and several superconducting phases.PACS numbers: 74.70.Tx, 74.30.Ek Much of the wide interest shown in the heavy-electron compound UPt3 since it was discovered x to exhibit superconducting properties below r=0.5 K has been fueled by the possibility that the superconducting instability in this metal is mostly the result of direct electron-electron interactions. Our current knowledge of the normal state in UPt3 is perhaps the strongest support for this possibility. At low temperatures, the correlated electrons behave much like a normal Fermi liquid: The resistivity shows a quadratic dependence 2,3 and the specific heat a linear dependence 1,4 on temperature (up to T = 3T C ), and the NMR relaxation time shows a Korringa law. 5 There exists a well-defined Fermi surface, 6 but the cyclotron masses are extraordinarily high, enhanced on average by a factor of 20 over and above the corresponding band masses. Such an enormous mass enhancement cannot be explained by a simple coupling to phonons, and the electron-electron interactions must play a predominant role. The residual interactions between the normalized quasiparticles, responsible for the formation of a superconducting state, may well also share this predominance. To test this idea, much direct information about the superconducting state has been accumulated, so far mainly in the form of temperature dependences of thermodynamic and transport properties. Although this information certainly points to an unusual behavior, it does not yet allow for a firm conclusion that the superconductivity in UPt3 is truly unconventional. The ambiguity arises partly because of the range of temperatures to which most measurements so far have been restricted; the relevant low-T regime (0
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We report neutron diffraction measurements on U(Ru(0.96)Rh(0.04))(2)Si(2) single crystal under pulsed high magnetic fields up to 30 T applied along the tetragonal c axis. The high-field experiments revealed that the field-induced phase II above 26 T corresponds to a commensurate up-up-down ferrimagnetic structure characterized by the wave vector q=(2/3,0,0) with the magnetic moments parallel to the c axis, which naturally explains the one-third magnetization plateau and the substantially changed Fermi surface in phase II. This a-axis modulated magnetic structure indicates that the phase II near the hidden order phase is closely related to the characteristic incommensurate magnetic fluctuations at Q(1)=(0.6,0,0) in the pure system URu(2)Si(2), in contrast to the pressure-induced antiferromagnetic order at Q(0)=(1,0,0).
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