Physical properties of Lu_1−xYb_xNi₂B₂C

Abstract

The Lu_1−x Yb _x Ni₂B₂ system exhibits superconductivity at low Yb concentrations (0≤x≤ 0.1), Kondo behaviour at low and moderate Yb concentrations (0≤x≤ 0.34), and heavy fermion behaviour with a non-magnetic ground state at high Yb concentrations. In order to characterize the evolution of these phenomena with x, we have performed dc magnetic susceptibility, electrical resistivity, thermoelectric power, and specific heat measurements on single crystals of Lu_1−x Yb _x Ni₂B₂ with various values of x between 0 and 1. The enhanced suppression of T _c by Yb substitution, compared to other rare earth substitutions, is consistent with the behaviour of superconducting Kondo systems with T _K≫T _c when T _K decreases with increasing x. The electronic specific heat coefficient γ increases from ∼11 mJ/mol K² for x=0 to ∼530 mJ/mol K² for x=1. The curve of the reduced specific heat jump ΔC/ΔC ₀ versus T _c/T _c0, where ΔC ₀ and T _c0 refer to the LuNi₂B₂C matrix, does not conform to the BCS theory, but is consistent with the magnetic pair breaking theory of Abrikosov and Gor'kov or the theory of Müller-Hartmann and Zittartz for superconductivity in the presence of the Kondo effect for . This is in agreement with the evolution of the upper critical field H _c2 with x, where the slope decreases with increasing x.

Acknowledgements

We would like to thank E. D. Bauer for valuable discussions. This research was supported by US Department of Energy at UCSD under Grant No. DE FG03−86ER-45230, and at Texas A&M by the National Science Foundation under Grant Nos. DMR-0103455 and -0422949, the Robert A. Welch Foundation (A-0514, Houston, Texas) and the Texas Center for Superconductivity at the University of Houston (TCSUH). Ames Laboratory is operated by the US Department of Energy under Grant No. W-7405-ENG-82. C. S. acknowledges support from the Spanish MEC.