Fractional ac Josephson effect in unconventional superconductors

For certain orientations of Josephson junctions between two p_x-wave or twod-wave superconductors, the subgap Andreev bound states produce a 4pi-periodicrelation between the Josephson current I and the phase difference phi: I ~sin(phi/2). Consequently, the ac Josephson current has the fractional frequencyeV/h, where V is the dc voltage. In the tunneling limit, the Josephson currentis proportional to the first power (not square) of the electron tunnelingamplitude. Thus, the Josephson current between unconventional superconductorsis carried by single electrons, rather than by Cooper pairs. The fractional acJosephson effect can be observed experimentally by measuring frequency spectrumof microwave radiation from the junction.Comment: 8 pages, 3 figures, RevTEX 4; v2. - minor typos corrected in proof