Abstract
We study a series of compounds with different Co concentrations by transport, optical spectroscopy, angle-resolved photoemission spectroscopy, and nuclear magnetic resonance. We observe a Fermi-liquid to non-Fermi-liquid to Fermi-liquid (FL-NFL-FL) crossover alongside a monotonic suppression of the superconductivity with increasing Co content. In parallel to the FL-NFL-FL crossover, we find that both the low-energy spin fluctuations and Fermi surface nesting are enhanced and then diminished, strongly suggesting that the NFL behavior in is induced by low-energy spin fluctuations that are very likely tuned by Fermi surface nesting. Our study reveals a unique phase diagram of where the region of NFL is moved to the boundary of the superconducting phase, implying that they are probably governed by different mechanisms.
4 More- Received 5 February 2015
DOI:https://doi.org/10.1103/PhysRevX.5.031035
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Published by the American Physical Society
Popular Summary
A well-known, intriguing, and long-standing question in condensed-matter physics pertains to the origin of anomalous non-Fermi-liquid behavior and its relationship to high-temperature superconductivity. Here, we study single crystals of with different Co concentrations to investigate the crossover from Fermi liquid to non-Fermi liquid and back. We find that this crossover is mediated by low-energy spin fluctuations in the material.
, which is characterized by in its stoichiometric form, exhibits a decreased transition temperature when Fe is substituted with Co (i.e., superconductivity is suppressed with increasing Co concentration). We study the transport and spectroscopic properties of the compounds to reveal a Fermi-liquid to non-Fermi-liquid to Fermi-liquid crossover induced by low-energy spin fluctuations that evolve in parallel with Fermi-surface nesting. We note that this crossover occurs irrespective of impurities in the material.
We produce the temperature-doping phase diagram of , and we demonstrate that the material is superconductive for parameters falling in the lower-left-hand corner of the phase diagram (which also happens to be free of long-range magnetic order). Our results provide a thorough understanding of the origin of the non-Fermi-liquid behavior in . Our findings reveal, for the first time, an unusual phase diagram where the region of non-Fermi-liquid behavior and enhanced low-energy spin fluctuations is located at the boundary of the superconducting phase. This result, which is completely novel and unexpected, challenges the popular notion of Fermi-surface-driven pairing mediated by low-energy spin fluctuations.
We expect that our results will motivate additional studies of high-temperature superconductivity and its relationship to Fermi-liquid behavior.