Pressure, substitution, and magnetic-field dependence of the valence phase transition in Yb0.4In0.6Cu2

Magnetization, resistivity, thermal expansion coefficient, and x-ray unit-cell measurements of Yb0.4In0.6Cu2 under pressure (P up to 25 kbar) and in magnetic fields (Hext up to 180 kOe) at temperatures 4.2-80 K have been performed. The valence-phase-transition temperature, Tv, is pressure dependent and surprisingly enough also field dependent. One obtains dTv/dP=-2.1 K/kbar for P<6 kbar. Negative chemical pressure obtained by substituting Yb by La or Cu by Ag increases Tv. A theoretical analysis within a previously published model shows that the interconfigurational excitation energy below Tv changes with pressure at a rate dE0/dP=-11 K/kbar. The external magnetic field does not affect E0, but it increases the Yb3+ excited-state population and thus drives the phase transition to a lower temperature at a rate dTv/dHext=-0.022 K/kOe.