London penetration depth in Ce$_{1-x}$Yb$_x$CoIn$_5$ ($0 \le x \le 0.4$)

The London penetration depth was measured in single crystals of superconducting Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ ($0\leq x\leq0.4$) by means of a tunnel diode resonator technique operating at 15 MHz in a dilution refrigerator down to 100 mK. Judging from the suppression of the superconducting transition temperature, the superconductivity in CeCoIn$_{5}$ is relatively robust to Yb - substitution on Ce site unlike the substitution with other rare earth elements. On the other hand, the Yb substitution induces a drastic change of the Fermi surface near $x=0.2$, from which one may expect a significant change in properties of superconducting pairing upon doping. Possible evolution of superconducting order parameter in Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ with increasing $x$ will be discussed based on the results from penetration depth measurements. \\ \\ Work in Ames was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358. Work in San Diego was supported by the Department of Energy Office of Science (Grant DE-FG02-04-ER46105).