CNAM COLLOQUIUM: Brad Ramshaw, Cornell

Thu, Oct 10, 2019 2:00 pm - 3:00 pm


Title: Using resonant ultrasound spectroscopy to identify multi-component superconductors
Speaker: Brad Ramshaw, Cornell
Sr2RuO4 has stood for over 25 years as the most likely candidate for spin-triplet superconductivity, where Cooper pairs are formed from electrons of the same spin type, rather than the opposite-spin pairing found in nearly all other superconductors. One of the best pieces of experimental evidence for spin-triplet superconductivity---a lack of Knight shift through Tc---was recently found to have been measured incorrectly, and subsequent measurements show a conventional Knight shift. This has cast a great amount of doubt over whether Sr2RuO4 is spin triplet, although there are still many other puzzling experimental results that must be reconciled. One such result is an unpublished measurement of a discontinuity in the the c66 shear elastic modulus through Tc. Such a discontinuity can only result from a two-component superconducting order parameter, and the spin-triplet px+ipy is the best candidate. This result, however, was unpublished due to systematic uncertainty in the measurement and an inability to reproduce it. We have performed the first measurement of all six elastic moduli through Tc Sr2RuO4, and found that the shear modulus c66 shows a clear discontinuity at the phase transition, identifying the superconducting order parameter as two-component. While px+ipy remains the most likely candidate, dxz,yz remains possible, along with other p-wave states which may be ruled out by other measurements. We also obtain dynamic information about the order parameter through the ultrasonic attenuation, which exhibits a peak below Tc in the compressional moduli and suggests the formation of domains of the superconducting order parameter - more evidence for a two-component order parameter. Taken together this information strongly constrains the order parameter symmetry in Sr2RuO4, and makes the recent discovery of a Knight-shift below Tc in Sr2RuO4 even more puzzling.

Host: Johnpierre Paglione

Refreshments 1:30pm John S Toll Physics Bldg Room 1117

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