L. Shu et al ., Phys. Rev. Lett. 106 , 156403 (2011)

1. L. Shu et al., Phys. Rev. Lett. 106, 156403 (2011) Correlated Electron State in Ce1-xYbxCoIn5 Stabilized by Cooperative Valence FluctuationsBrian M. Maple, University of California, San Diego, DMR 0802478 X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce1-xYbxCoIn5 (0 ≤ x ≤1) reveal that many of the characteristic features of the x = 0 correlated electron state are stable all the way up to x = 0.775 and that phase separation occurs for x > 0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. It is generally thought that critical fluctuations associated with a magnetic quantum critical point (QCP), where a second-order magnetic phase transition is suppressed to 0 K by an external control parameter, can provide a mechanism for non-Fermi liquid (NFL) behavior and unconventional superconductivity. Low-temperature NFL behavior is observed in Ce1-xYbxCoIn5 and varies with x, even though there is no readily identifiable QCP. The NFL state is tuned by valence fluctuations. The superconducting critical temperature Tc decreases linearly with x towards 0 K as x→1, while the coherence temperature Tcoh is nearly independent of x, in contrast with other heavy fermion superconductors where Tc scales with Tcoh. (a) Coherence temperature Tcoh, where ρ(T) exhibits a maximum, vs x. (b) Tc vs x. The solid line shows the suppression of Tc as reported for other rare earth substitutions. (c) Fit parameters nρ, extracted from , vs x. (d) Fit parameters nχ, extracted from , vs x. The light grey shading represents the region of phase separation.

2. Correlated Electron State in Ce1-xYbxCoIn5 Stabilized by Cooperative Valence Fluctuations Brian M. Maple, University of California, San Diego, DMR 0802478 Education Individuals at different stages of their careers (two Assistant Professors, four postdocs, three graduate students, four undergraduates – including one who participated in the research experience for undergraduate [REU] program, and a Quantum Design, Inc. research scientist) have been involved in this ongoing research. Lei Shu and Marc Janoschek, continue to work on the project (currently studying the electronic structure of Ce1-xYbxCoIn5 from spectroscopy and bulk properties), several of the undergraduate students are still with us in the lab. J. Paglione, currently an Assistant Professor at University of Maryland, College Park, is pursuing related research projects. Outreach Our lab has a strong commitment to increasing awareness and understanding of the importance of experimental physics to a variety of communities. We currently teach an advanced physics lab (Physics 133) at the University of California, San Diego, where students are required to plan and carry out a research project, and then present the results to their peers in the class. Additionally, each year we perform regularly scheduled demonstrations at nearby elementary schools displaying some of the interesting aspects of low temperature physics including superconductivity and the behavior of various materials when they are cooled to 77 K with liquid nitrogen. Top: A postdoc from our lab demonstrates the characteristics of liquid nitrogen to high school students. Bottom: Using liquid nitrogen, we discuss and demonstrate the characteristics of a high temperature cuprate superconductor.