Fig. 1.

Effect of strains of different symmetries on the CEF eigenstates of ${\mathrm{Y}\mathrm{b}\mathrm{R}\mathrm{u}}_2{\mathrm{G}\mathrm{e}}_2$ and measurement of the associated elastoresistivity coefficients. Strains corresponding to distinct irreducible representations, induced by external stresses, are indicated by thick black arrows. These strains admix and split the CEF eigenstates; the effect on the low-energy CEF quasi-quartet and the charge density of the ground-state orbitals is illustrated. For simplicity, the ${\mathrm{\Gamma }}_6$ and ${\mathrm{\Gamma }}_7$ eigenstates are illustrated for pure $|\pm 1/2⟩$ and $|\pm 3/2⟩$ character respectively (see main text). Admixture of other ${\mathrm{m}}_J$ states gives the ${\mathrm{\Gamma }}_6$ and ${\mathrm{\Gamma }}_7$ eigenstates a discrete fourfold symmetry, but does not otherwise affect the functional form of the quadrupole-strain susceptibility. The elastoresistance column shows the required orientation of the normal strain (thick black arrows), crystal axes (insets to gray schematic crystals), and voltage contacts (yellow pads, with measured voltages indicated for longitudinal and transverse pairs) for measurement of the associated elastoresistivity coefficients in each symmetry channel.