Existence of the LOFF State in Superconducting UPd₂Al₃

In a recent Letter, Gloos et al have suggested that the heavy fermion superconductor, UPd_2Al_3, has a transition in magnetic field from the normal state to an inhomogeneous superconducting state, known as the LOFF state, followed at lower fields by a first order phase transition to the usual flux lattice state. To investigate this, the author derives a formula for the upper critical field for the LOFF state valid at all temperatures in the clean limit (the previous work of Gruenberg and Gunther was restricted to zero temperature), and shows that the LOFF state cannot quantitatively explain the phase diagram for UPd_2Al_3. This indicates that a more general solution which breaks cylindrical symmetry is necessary to explain the data.