Phase diagram and high-resolution photoemission study of the superconducting and magnetic pseudoternary body-centered-tetragonal Ho(Rh1−xRux)4B4 system

Abstract

The phase diagram for the body-centered-tetragonal superconducting pseudoternary system $\mathrm{Ho}{\left({\mathrm{Rh}}_{1-x}{\mathrm{Ru}}_x\right)}_4{\mathrm{B}}_4 \left(0.07\mathrm{}\le x\le 1\right)$ is reported for temperatures above 1.3 K, revealing an abrupt drop in the superconducting critical temperature $T_c$ near the critical concentration $x_{\mathrm{cr}}\approx 0.40$, which is characteristic for this class of compounds. In order to clarify the origin of this phenomenon, synchrotron radiation from the DORIS storage ring (Hamburg, Germany) has been used to measure high-resolution photoemission spectra for samples with $x=0.07,0.37, \mathrm{and} 0.90$. The analysis of the valence-band data near the Fermi energy $E_F$ shows a shift of $E_F$ to lower energies with increasing $x$ relative to the transition-metal $d$-electron structure. The results are consistent with existing band-structure calculations for the primitive-tetragonal modification of this structure type and provide a straightforward explanation of the observed $T_c$ behavior.