Pressure, substitution, and magnetic-field dependence of the valence phase transition in ${\mathrm{Yb}}_{0.4}$${\mathrm{In}}_{0.6}$${\mathrm{Cu}}_2$

Magnetization, resistivity, thermal expansion coefficient, and x-ray unit-cell measurements of ${\mathrm{Yb}}_{0.4}$ ${\mathrm{In}}_{0.6}$ ${\mathrm{Cu}}_2$ under pressure (P up to 25 kbar) and in magnetic fields ($H_{\mathrm{ext}}$ up to 180 kOe) at temperatures 4.2–80 K have been performed. The valence-phase-transition temperature, $T_v$, is pressure dependent and surprisingly enough also field dependent. One obtains ${\mathrm{dT}}_v$/dP=-2.1 K/kbar for P<6 kbar. Negative chemical pressure obtained by substituting Yb by La or Cu by Ag increases $T_v$. A theoretical analysis within a previously published model shows that the interconfigurational excitation energy below $T_v$ changes with pressure at a rate ${\mathrm{dE}}_0$/dP=-11 K/kbar. The external magnetic field does not affect $E_0$, but it increases the ${\mathrm{Yb}}^{3+}$ excited-state population and thus drives the phase transition to a lower temperature at a rate ${\mathrm{dT}}_v$/${\mathrm{dH}}_{\mathrm{ext}=\mathrm{-}}$0.022 K/kOe.