Evidence for unconventional superconductivity in from magnetic torque studies

We studied the anisotropic magnetic response of the internal superconducting phases of (Formula presented) and its anisotropic magnetic susceptibility with a capacitive torque meter which is very sensitive in high fields. Experiments were performed at temperatures down to 20 mK and at various angles between the c axis (hexagonal structure) and (Formula presented) ranging from 16° to 82°. The samples were four single crystals grown with different methods and subjected to different annealing procedures. The normal state susceptibility has a maximum around 20 K for (Formula presented) in the (Formula presented) plane which we followed up to 14 T. It may arise from hybridized uranium ion states split by the hexagonal crystal field. The magnetization curves in the superconducting (SC) regime show strong irreversibilities which are highly sample dependent. They are not correlated with the internal SC phase lines but continue up to a line of fields that lies parallel to the (Formula presented) curve and even follows its kink at the tetracritical point (Formula presented) In the cleanest sample this line is shifted to fields well below the (Formula presented) internal phase line which then manifests itself in a pronounced kink of the magnetization curve indicating an enhanced Ginzburg-Landau parameter (Formula presented) In another sample the (Formula presented) phase line between two of the three internal SC states could be detected even in the hysteretic region. The enhanced Ginzburg-Landau parameter (Formula presented) means a larger penetration depth and/or a shorter coherence length, clear evidence for the unconventional character of the (Formula presented) phase transition. With our cleanest sample we also observe an anomalous peak effect, a region of enhanced flux pinning near (Formula presented) which is probably related to the Fulde-Ferrell-Larkin-Ovchinnikov state. In yet another sample we find a crossing of the up-down magnetization curves, also near (Formula presented) but with reversed orientation of the magnetization loops. We interpret this in terms of different flux pinning in the two main crystal directions, possibly in relation to the peak effect which is, however, masked in this sample by strong irreversibilities.