From Local Moment to Mixed-Valence Regime in Ce 1−x Yb x CoIn 5 alloys Carmen Almasan, Kent State University, DMR Ce 1−x Yb x CoIn 5 alloys have emerged as extraordinary members of the Ce-115 family of heavy fermion superconductors. Tuning the parent compound CeCoIn 5 through Yb- substitution has consistently revealed physical properties that are unusual to the other members of this family. Some of these properties include: the robust nature of superconducting transition temperature (T c ), the non-monotonic coherence temperature (T coh ), and the presence of a quantum phase transition as x → 0.2. Since the unusual behavior is exclusive to Yb-substitution, which is in a mixed valence state, it is extremely important to understand the role played by the Yb mixed valence on its physical properties. A major part of the research efforts in our lab has being devoted to this study. Our transport and thermodynamic studies of Ce 1-x Yb x CoIn 5 alloys in the intermediate doping range (0.40 ≤ x ≤ 0.775) have generated a number of important results: 1 observation of a crossover from the predominantly localized Ce moment regime to the predominantly itinerant Yb f -electronic states regime for x = 0.65 (Fig. 1); observation of scaling between T coh and T c, indicating that the onset of many-body coherence in the Kondo lattice and emergence of superconductivity have same physical origin, i.e., hybridization between conduction and localized Ce f-electron states; unveiling of presence of correlations between Yb ions at low Yb doping (Fig. 2(a)); presence of √T dependence of resistivity across the whole range of Yb concentrations (Fig. 2(b)), suggesting a non-trivial scattering mechanism of the conduction electron, result in conformity to the recently proposed composite pairing mechanism; unveiling the single impurity Kondo behavior of Ce ions from magneto-transport for 0.40 ≤ x ≤ 0.70 (Fig. 3), which points toward an intriguing possibility that unconventional superconductivity has purely local origin and is driven by the presence of Ce f- moments. All these results show that the onset of the many-body coherence in the Yb lattice diluted with Ce f-moments and unconventional superconductivity are decoupled from each other and suggest that unconventional superconductivity does not have magnetic origin, but it has purely local origin and is driven by the presence of Ce f-moments. 1 Y. P. Singh, D. J. Haney, X. Y. Huang, I. K. Lum, B. D. White, M. Dzero, M. B. Maple, and C. C. Almasan, “From local moment to mixed-valence regime in Ce 1−x YbxCoIn 5 alloys”, Phys. Rev. B 89, (2014). Fig. 1 Magnetoresistivity (MR) as a function of magnetic field squared H 2 for different Yb doping. The positive MR at low fields in the first and third panels reveal the presence of coherent states for these Yb doping, while the negative MR at higher H and in the second panel reveal incoherent states. Fig. 2 (a) Residual resistivity as a function of Yb doping x. The solid line is a quadratic polynomial fit of the residual resistivity data at low x values, indicating that Yb ions are correlated, while the dotted curve is a guide to the eye. (b) Temperature-dependent resistivity fitted with expression shown. Fig. 3 Plot of normalized magnetoresistivity vs H/H * for x = 0.65, measured at different temperatures. The solid line is the Schlottmann’s theoretical MR curve. The scaling shows that single impurity Kondo behavior dominates for this Yb doping. Inset: scaling parameter H* vs T.

Education An essential activity performed has been the education of three graduate students and one postdoc. They did not only pursue cutting edge science, but they also became knowledgeable in a highly varied array of skills, technologies, and forefront topics in condensed matter physics by being involved in all phases of their project. These phases included equipment and electronics design and construction, experiment “debugging”, data acquisition and analysis, and comprehension of and comparison with the relevant theoretical results. The continuing Ph. D. students are Derek Haney, Xinyi Huang, and Yogesh Singh. Outreach As part of the outreach activities of the PI, underrepresented Honors Physics students from nearby high schools visited her laboratory and learned about physics in general and superconductivity and magnetism in particular, through demonstrations and short lectures given by the PI and her graduate students. These inspirational activities exposed them to a research environment and gave them a flavor of the science done in the lab and the technology involved in doing it. In addition, the PI continued to be involved in the Young Women’s Summer Institute, which is a competitive program across Ohio for middle school girls, held on the Kent State University Campus. During this program, middle school girls are exposed to science experiments and interact with female scientists at different stages in their career. From Local Moment to Mixed-Valence Regime in Ce 1−x Yb x CoIn 5 alloys Carmen Almasan, Kent State University, DMR Graduate student Derek Haney describes to high school students the demos he is about to show them.