Phase-Sensitive Determination of the Pairing Symmetry in Sr<SUB>2</sub>RuO<SUB>4</sub>

Open Access
- Author:
- Nelson, Karl D.
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 30, 2004
- Committee Members:
- Ying Liu, Committee Chair/Co-Chair
Nitin Samarth, Committee Member
Vincent Henry Crespi, Committee Member
Darrell Schlom, Committee Member - Keywords:
- odd parity
spin triplet
Sr2RuO4
p-wave
phase-sensitive
unconventional superconductivity - Abstract:
- Sr<SUB>2</SUB>RuO<SUB>4</SUB>, an unconventional superconducting material with T<SUB>c</SUB>=1.5 K, was predicted to possess a superconducting state with an order parameter of odd-parity symmetry. Numerous experiments have given support to the prediction, but none of these experiments has been sufficiently definitive to allow certainty about the symmetry of the order parameter. Direct evidence of odd-parity symmetry can be obtained from a phase-sensitive experiment which measures interference in the critical current between two coupled Josephson junctions. <P> Odd-parity symmetry in Sr<SUB>2</SUB>RuO<SUB>4</SUB> is established here by a phase-sensitive measurement using Josephson junctions on single crystals of Sr<SUB>2</SUB>RuO<SUB>4</SUB>, with the junctions prepared on opposite sides of the crystal. Measurements of the critical current versus external magnetic flux for these samples show quantum interference between the junctions, with the critical current at a minimum when zero external flux is applied. For junctions on opposite sides, this result is possible only for a material with odd-parity symmetry. <P> The phase-sensitive measurements are difficult because of a suppression of superconductivity at the surface of Sr<SUB>2</SUB>RuO<SUB>4</SUB> crystal which is demonstrated by single-particle tunneling measurements. Tunneling spectra for Au–Sr<SUB>2</SUB>RuO<SUB>4</SUB> junctions show no features due to superconductivity in Sr<SUB>2</SUB>RuO<SUB>4</SUB>. However, metal alloys such as Hg-In and Au-In which wet the Sr<SUB>2</SUB>RuO<SUB>4</SUB> surface are shown to form Josephson junctions with finite supercurrent despite the normal surface layer, making possible the phase-sensitive experiment. <P> In addition, previous analysis of experiments on Sr<SUB>2</SUB>RuO<SUB>4</SUB> has assumed weak spin–orbit coupling in the material. A large product of critical current and normal state resistance (I<SUB>c</SUB>R<SUB>n</SUB>) for Au-In–Sr<SUB>2</SUB>RuO<SUB>4</SUB> junctions has been measured and is shown to be an indication of strong spin–orbit coupling near the Sr<SUB>2</SUB>RuO<SUB>4</SUB> surface. <P> The full text is always available for free at http://etda.libraries.psu.edu/.