The transition between real and complex superconducting order parameter phases in UPt3.

Order parameter symmetry is one of the basic characteristics of a superconductor. The heavy fermion compound UPt3 provides a rich system for studying the competition between superconductivity and other forms of electronic order and exhibits two distinct superconducting phases that are characterized by different symmetries. We fabricated a series of Josephson tunnel junctions on the as-grown surfaces of UPt3 single crystals spanning the a-b plane. By measuring their critical current, we mapped out the magnitude of the superconducting order parameter as a function of the momentum-space direction and temperature. In the high-temperature phase, we observed a sharp node in the superconducting gap at 45 degrees with respect to the a axis; an out-of-phase component appeared in the low-temperature phase, creating a complex order parameter.

Evidence for complex superconducting order parameter symmetry in the low-temperature phase of UPt3 from Josephson interferometry.

We present data on the modulation of the critical current with applied magnetic field in UPt3-Cu-Pb Josephson junctions and SQUIDs. The junctions were fabricated on polished surfaces of UPt3 single crystals. The shape of the resulting diffraction patterns provides phase-sensitive information on the superconducting order parameter. Our corner junction data show asymmetric patterns with respect to magnetic field, indicating a complex order parameter, and both our junction and SQUID measurements point to a phase shift of pi, supporting the E(2u) representation of the order parameter.

Dynamical superconducting order parameter domains in Sr2RuO4.

We present direct evidence for complex p-wave order parameter symmetry and the presence of dynamical chiral order parameter domains of the form px +/- ipy in the ruthenate superconductor Sr2RuO4. The domain structure creates differences in the magnetic field modulation of the critical current of Josephson junctions fabricated on orthogonal faces of Sr2RuO4 single crystals. Transitions between the chiral states of a domain or the motion of domain walls separating them generates telegraph noise in the critical current as a function of magnetic field or time and is responsible for hysteresis observed in field sweeps of the critical current. The presence of such domains confirms the p-wave triplet spin and complex (broken time-reversal symmetry) nature of the superconducting pairing state in Sr2RuO4.