Non-Fermi liquid behavior and changing coherence in Ce1−xYbxCoIn5

Carmen C. Almasan, Kent State University, DMR 1006606

Evolution of the coherence temperature Tcoh, temperature of resistivity minimum Tmin and normal state Sommerfeld coefficient for Ce1-xYbxCoIn5 as a function of doping

How unconventional superconductivity emerges in the f-orbital materials still remains one of the most fundamental problems. Recently, the idea of unconventional superconductivity induced by the proximity to a quantum critical point in CeCoIn5 has been challenged by the study of Ce1−xYbxCoIn5 alloys.1 The effect of small Yb substitution (x ≤ 0.2) is very dramatic and a number of unconventional physical properties are consequently observed. In particular, the critical field (HQCP) at which the quantum phase transition between the antiferromagnetic and paramagnetic phases takes place is significantly reduced as a function of Yb concentration for x > 0.1 and vanishes as x → 0.2. At the same time, the value of the superconducting critical temperature Tc is reduced by only approximately 15% at x = 0.2 and by half at x = 0.5; this result is completely unexpected given the fact that Yb ions are in a mixed valence state with the average valence of 2.3 so that the magnetic valence state of Yb3+ together with essentially unscreened Ce moments should provide enough scattering to completely suppress superconductivity. Thus, the robustness of the unconventional superconductivity in Ce1-xYbxCoIn5 for the intermediate values of Yb concentrations still presents a major challenge to our understanding of the physics of this compound. To address this issue we perform transport and thermodynamic studies of Ce1-xYbxCoIn5 alloys focusing on the intermediate range of concentrations (0.5 ≤ x ≤ 0.775). We observe the gradual crossover from the coherent Kondo lattice behavior to the coherent many-body state formed by strong hybridization between conduction electrons and Yb f-moments (notice the increase in coherence temperature Tcoh and Sommerfeld coefficient , and decrease in residual resistivity in the figure for x > 0.6), while Tc decreases monotonically over the whole Yb doping, We also observe the single impurity Kondo behavior of Ce ions in magneto-transport for x > 0.6. 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 an intriguing possibility that unconventional superconductivity has purely local origin and is driven by the presence of Ce f-moments. The sub-linear temperature dependence of resistivity across the whole range of Yb concentrations suggest the presence of a nontrivial scattering mechanism for the conduction electrons.

1T. Hu, Y. P. Singh, L. Shu, M. Janoschek, M. Dzero, M. B. Mapleb, and C. C. Almasan, “Non-Fermi liquid regimes with and without quantum criticality in Ce1−xYbxCoIn5”, PNAS 110, 187160 (2013).

Non-Fermi liquid behavior and changing coherence in Ce1−xYbxCoIn5

Carmen C. Almasan, Kent State University, DMR 1006606

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 may include equipment and electronics design and construction, experiment “debugging”, data acquisition and analysis, and comprehension of and comparison with the relevant theoretical results. The postdoc Tao Hu is presently an Associate Professor of Physics at the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China, while 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.

Graduate student Derek Haney describes to high school students the demos he is about to show them.