D. J. Bahr’s research while affiliated with University of Illinois, Urbana-Champaign and other places

There is substantial evidence that the ruthenate superconductor Sr2RuO4 has a chiral order parameter of the form px±ipy and forms chiral domains. In order to verify this picture and determine the size of the domains, we have fabricated Josephson junctions on the order of and smaller than the domain width of ˜1mum implied by Josephson interferometer experiments. Using Focused Ion Beam milling, we have patterned Sr2RuO4-Cu-PbIn junctions ranging in size from 0.5mum x 0.5mum to 4mum x 4mum on the edge face of a Sr2RuO4 crystal. By measuring the magnetic field modulation of the Josephson critical current, we can probe the phase anisotropy across the junction and determine the size and dynamics of chiral domains. Our data is consistent with the predicted domain width but also exhibits signatures that suggest the formation of chiral domain structure along the c-axis. The theoretical model of domain wall movement proposed by Bouhon and Sigrist is supported by our data.

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° with respect to the a axis; an out-of-phase component appeared in the low-temperature phase, creating a complex order parameter.

The ruthenate superconductor Sr2RuO4 may have a chiral order parameter of the form px±ipy,, making it a candidate for nucleation of excitations with non-Abelian statistics that could enable topologically-protected quantum computing. To test this scenario, we have measured the magnetic response of Sr2RuO4 nanocrystals to search for spontaneous chiral currents and half-integer vortices. Each nanocrystal (2mum x 2mum x 0.5mum) was extracted from a large single crystal with bulk transition temperature in the range 1.2K-1.4K. It was then glued into the pickup loop of a flux transformer or a gradiometer that is inductively-coupled to a dc-SQUID magnetometer. We monitor the diamagnetic screening of the crystal and the entry of discrete vortices in perpendicular and parallel applied magnetic fields and as a function of temperature to search for the existence of half-quantum vortices in this system. We are also developing other techniques such as nanoscale Scanning SQUID Microscopy for probing the vortex dynamics in Sr2RuO4.

We present direct measurements of the current-phase relation (CPR) of SFS Josephson junctions in an rf-SQUID geometry. The junctions are fabricated from Nb-Cu47Ni53-Nb trilayers with a junction area of 2x2 mum^2 and a CuNi thickness of 7 nm. By measuring the magnetic flux through the rf-SQUID as a function of applied current, we observe transitions between an ordinary 0-Josephson junction state and a pi-junction state characterized by a phase difference of pi in the ground state occurring at temperatures between 1.5 K and 3.5 K. Near this temperature crossover, we observe period-doubling of the CPR indicating the existence of a term proportional to sin(2phi). Work is underway to determine if this signifies an intrinsic second-order tunneling mechanism or is the result of junction inhomogeneities.