Resolution of the discrepancy between the variation of the physical properties of Ce_1-<i>x</i>Yb_<i>x</i>CoIn₅single crystals and thin films with Yb composition

Abstract

Extraordinary electronic phenomena including an Yb valence transition, a change in Fermi surface topology, and suppression of the heavy fermion quantum critical field at a nominal concentration 0.2 have been found in the system. These phenomena have no discernable effect on the unconventional superconductivity and normal-state non-Fermi liquid behaviour that occur over a broad range of up to 0.8. However, the variation of the coherence temperature and the superconducting critical temperature with nominal Yb concentration for bulk single crystals is much weaker than that of thin films. To determine whether differences in the actual Yb concentration of bulk single crystals and thin film samples might be responsible for these discrepancies, we employed Vegard’s law and the spectroscopically determined values of the valences of Ce and Yb as a function of to determine the actual composition of bulk single crystals. This analysis is supported by energy-dispersive X-ray spectroscopy, wavelength-dispersive X-ray spectroscopy, and transmission X-ray absorption edge spectroscopy measurements. The actual composition is found to be about one-third of the nominal concentration up to 0.5, and resolves the discrepancy between the variation of the physical properties of single crystals and thin films with Yb concentration.