Experimental Investigation of the Cooperative Jahn-Teller Effect in TmCd

TmCd has the CsCl structure and shows a Jahn-Teller distortion to a tetragonal structure at $T_a=3.16\mathrm{}$ K. Specific-heat, electrical-resistance, and elastic measurements show that the ground state of the ${\mathrm{Tm}}^{3+}$ ion is a nonmagnetic ${\Gamma }_3$ doublet. The next-higher excited level is at approximately 20 K. We do not find evidence for magnetic ordering down to 40 mK. The susceptibility and high-field magnetization can be interpreted with these crystal-field levels without taking exchange into account. The elastic $c_{44}$ mode shows a small anomaly of 0.9% at $T_a$, whereas the $c_{11}-c_{12}$ mode shows strong softening below 100 K of 33% down to $T_a$. We can interpret the temperature dependence of this mode quantitatively. High magnetic fields have a profound influence on these elastic constants and electrical resistivity. We find an apparent increase of $T_a$ for magnetic fields applied in the [100] direction and somewhat smaller effects for other directions. We explain this effect tentatively by showing that the first-order transition is shifted in high fields to higher temperatures and that the discontinuity in the order parameter diminishes continuously with increasing field.