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We investigate the current-phase relation of S/F/S junctions near the crossover between the 0 and the π ground states. We use Nb/CuNi/Nb junctions where this crossover is driven both by thickness and temperature. For a certain thickness a non-zero minimum of critical current is observed at the crossover temperature. We analyze this residual supercurrent by applying a high frequency excitation and observe the formation of half-integer Shapiro steps. We attribute these fractional steps to a doubling of the Josephson frequency due to a sin(2φ) current-phase relation. This phase dependence is explained by the splitting of the energy levels in the ferromagnetic exchange field.PACS numbers: 74.50.+r,74.45.+cThe current-phase relation of ballistic and diffusive S/N/S junctions is predicted to be strongly nonsinusoidal at zero temperature in contrast to that of tunnel junctions. This is due to a different conductivity mechanism involving Andreev bound states created in the normal metal (N) by the superconductors (S). These states are sensitive to the superconducting phase difference φ and carry the supercurrent I S . In S/F/S junctions the current-phase relation is strongly distorted by the exchange field of the ferromagnet (F) and can even be reversed leading to the famous π state [1]. The microscopic mechanism responsible for this negative supercurrent can be intuitively explained in the clean limit where the Andreev spectrum is discrete [2,3,4]. The two spin configurations of each bound state are indeed split by the ferromagnetic exchange energy. When the first bound state is shifted from finite energy to zero energy (at φ = 0), the direction of the total supercurrent given by the lowest level (for φ > 0) is negative (instead of positive). In this case the ground state is at φ = π.In this article we consider the situation where the exchange energy E ex is half that of the π junction described above for the same thickness d. In this case the Andreev spectrum of a ballistic junction contains equidistant states twice closer than usual (Fig. 1a). As a result the supercurrent is π periodic in phase with a saw-tooth shape at zero temperature (Fig. 1b) and the ground states φ = 0 and φ = π are degenerate [5]. The current-phase relation becomes more rounded in the diffusive regime (Fig. 1c) where the discrete spectrum is replaced by a continuous density of Andreev states [6]. At this 0 -π crossover the current-phase relation contains a dominant sin(2φ) component and the critical current presents a non-zero minimum with respect to thickness or exchange energy variations. Experimentally the critical current of S/F/S junctions at the crossover is however so small that it was always assumed to vanish completely. In Nb/CuNi/Nb junctions [4,7] this behavior could be related to the strong decoherence of the magnetic alloy and in Nb/PdNi/Al 2 O 3 /Nb junctions [8] to the presence of the tunnel barrier.In this Letter we report the first observation of a small non-zero critical current at the 0 -π crossover of a Nb/CuNi/Nb junction an...

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