Superconducting critical temperature $T_c$ and electronic structure of pseudoternary Y(${\mathrm{Rh}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ru}}_{\mathrm{x}}$${)}_4$${\mathrm{B}}_4$ studied by high-resolution photoelectron spectroscopy

High-resolution photoemission measurements using synchrotron radiation are presented for the body-centered-tetragonal superconducting pseudoternary system Y(${\mathrm{Rh}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ru}}_{\mathrm{x}}$${)}_4$${\mathrm{B}}_4$ covering the range of composition 0.10≤x≤0.90. The superconducting phase diagram determined down to 4 mK reproduces the sharp drop in the superconducting critical temperature $T_c$ for x>$x_{\mathrm{cr}=0.35\mathrm{}}$, which is characteristic for this class of compound. No superconductivity was observed for the sample with x=0.70, whereas the sample with x=0.90 became superconducting again at $T_c$=140 mK. From a comparison between the valence-band spectra near the Fermi level $E_F$ and band-structure calculations by Jarlborg et al. for the related compounds M${\mathrm{Rh}}_4$${\mathrm{B}}_4$ with M=Y, Gd, Ho, Er, and Lu, we conclude that a shift of $E_F$ to lower energies with increasing x is the underlying mechanism for the $T_c$(x) behavior. In our previous study on Ho(${\mathrm{Rh}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Ru}}_{\mathrm{x}}$${)}_4$${\mathrm{B}}_4$, for which supplementary data are also presented, the same features were observed. Our measurements indicate that hybridization effects upon alloying Ru for Rh cannot explain the observed behavior of $T_c$.