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Suppression of Antiferromagnetic Spin Fluctuations in Superconducting Cr0.8Ru0.2



M. Ramazanoglu, B. G. Ueland, D. K. Pratt, Leland Weldon Harriger, Jeffrey W. Lynn, G. Ehlers, G. E. Granroth, S. L. Bud'Ko, P. C. Canfield, D. Schlagel, A. I. Goldman, T. A. Lograsso, R. J. McQueeney


Although the precise mechanism is under, unconventional superconductivity (SC) is linked to the pairing of electrons by itinerant antiferromagnetic (AFM) spin fluctuations. Thus, unconventional SC often develops in magnetic metals on the cusp of static AFM order where fluctuations are strong. This association is so compelling that many SC materials are identified as unconventional by their proximity to an ordered AFM state. The Cr-Ru alloy system is based on Cr metal, the canonical itinerant AFM, and possesses such a phase diagram [see Fig 1(a)]. Here we use inelastic neutron scattering to show that itinerant AFM spin fluctuations are present in a superconducting Cr0.8Rud0.2^ alloy (Tc = 1.35 K). However, the neutron spin resonance, a signature of unconventional SC, is not observed due to vanishingly small magnetic spectral weight at low energies. Instead, the data indicate a spin gap of order 2{Δ} (the superconducting gap) and a suppression of magnetic spectral weight at energies well above 2{Δ}. We show that the suppression decreases the magnetic exchange energy, suggesting that spin fluctuations oppose the formation of SC. Thus, despite indications that Cr-Ru is an unconventional SC, we conclude that conventional SC sits on the cusp of AFM order in Cr-Ru alloys, where both static AFM order and fluctuations oppose the formation.
Physical Review B


Superconductivity, Spin Fluctuations, Cooper Pairing, Antiferromagnetic Order, Spin Gap
Created October 25, 2018, Updated February 26, 2020