Ultrasonic investigation ofZnSe:V2+andZnSe:Mn2+: Lattice softening and low-temperature relaxation in crystals with orbitally degenerate states

Abstract

Temperature dependences of elastic moduli and ultrasonic attenuation were investigated in $\mathrm{Zn}\mathrm{Se}:{\mathrm{V}}^{2+}$ and $\mathrm{Zn}\mathrm{Se}:{\mathrm{Mn}}^{2+}$ crystals as functions of temperature in the interval of $1.4--100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for frequencies of $52--270\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. A peak in the attenuation was found in $\mathrm{Zn}\mathrm{Se}:{\mathrm{V}}^{2+}$ below $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for fast shear and longitudinal waves propagating along the [110] crystallographic axis. We interpret it to be a manifestation of a relaxation process related to the Jahn--Teller effect in the $3d$ electron system of the impurity. Softening of the elastic moduli ${C}_{44}$ and ${C}_{\ensuremath{\ell}}=({C}_{11}+{C}_{12}+2{C}_{44})∕2$ were observed below $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Temperature dependences were deduced for relaxation time and relaxed and unrelaxed ${C}_{44}$ and ${C}_{\ensuremath{\ell}}$ moduli. The temperature dependence of relaxation time shows that $\mathrm{Zn}\mathrm{Se}:{\mathrm{V}}^{2+}$ has potential barrier ${V}_{0}=5.6\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Softening of the ${C}_{44}$ modulus indicates that the local distortions have a trigonal character. Neither attenuation anomalies nor softening of the elastic moduli was observed in $\mathrm{Zn}\mathrm{Se}:{\mathrm{Mn}}^{2+}$, which has a singlet orbital ground state of the $3d$ impurity.