M. N. Sarychev

Ultrasonic investigation of the Jahn-Teller effect in GaAs semiconductors doped by transition metals

Transition-metal-doped semiconductor GaAs crystals are used as model systems for spintronic research, as well as in other applications. To explore the structure and properties of such impurities, we extended the methodology of ultrasonic investigation of the Jahn-Teller effect in dielectric impurity-centers to the study of significantly different semiconductor impurities using the GaAs:Cu crystal as an example. Phase velocity and attenuation of ultrasound in this system were measured in the temperature interval of 1.9–80u2009K at 52u2009MHz and 156u2009MHz. The anomaly in the velocity and a peak of attenuation found for the longitudinal and slow shear waves indicate the presence of the Jahn-Teller effect with the e-type local distortions of the CuGa4As impurity complex. The temperature dependence of the elastic modulus and relaxation time shows that below 5u2009K, the thermal activation mechanism of relaxation is possibly replaced by resonance type transitions. The main parameters of the Jahn-Teller effect, stabilization...

Numerical adiabatic potentials of orthorhombic Jahn-Teller effects retrieved from ultrasound attenuation experiments. Application to the SrF2:Cr crystal

A methodology is worked out to retrieve the numerical values of all the main parameters of the six-dimensional adiabatic potential energy surface (APES) of a polyatomic system with a quadratic T-term Jahn-Teller effect (JTE) from ultrasound experiments. The method is based on a verified assumption that ultrasound attenuation and speed encounter anomalies when the direction of propa- gation and polarization of its wave of strain coincides with the characteristic directions of symmetry breaking in the JTE. For the SrF2:Cr crystal, employed as a basic example, we observed anomaly peaks in the temperature dependence of attenuation of ultrasound at frequencies of 50-160 MHz in the temperature interval of 40-60 K for the wave propagating along the [110] direction, for both the longitudinal and shear modes, the latter with two polarizations along the [001] and [110] axes, respectively. We show that these anomalies are due to the ultrasound relaxation by the system of non-interacting Cr2+ JT centers with orthorhombic local distortions. The interpretation of the ex- perimental findings is based on the T2g (eg +t2g) JTE problem including the linear and quadratic terms of vibronic interactions in the Hamiltonian and the same-symmetry modes reduced to one interaction mode. Combining the experimental results with a theoretical analysis we show that on the complicated six-dimensional APES of this system with three tetragonal, four trigonal, and six orthorhombic extrema points, the latter are global minima, while the former are saddle points, and we estimate numerically all the main parameters of this surface, including the linear and quadratic vibronic coupling constants, the primary force constants, the coordinates of all the extrema points and their energies, the energy barrier between the orthorhombic minima, and the tunneling splitting of the ground vibrational states.

Acoustic Properties of Crystals with Jahn–Teller Impurities: Elastic Moduli and Relaxation Time. Application to SrF2:Cr2+

A new approach to evaluate the relaxation contribution to the total elastic moduli for crystals with Jahn–Teller (JT) impurities is worked out and applied to the analysis of the experimentally measured ultrasound velocity and attenuation in SrF2:Cr2+. Distinguished from previous work, the background adiabatic contribution to the moduli, important for revealing the impurity relaxation contribution, is taken into account. The temperature dependence of the relaxation time for transitions between the equivalent configurations of the JT centers has been obtained, and the activation energy for the latter in SrF2:Cr2+, as well as the linear vibronic coupling constant have been evaluated.

Temperature Anomaly of the Coefficient of Ultrasonic Absorption by Electrons of Hybridized States of Cobalt Impurities in Mercury Selenide

The effects of the interaction of ultrasound with donor d electrons of cobalt impurity atoms at low concentrations in mercury selenide crystals have been investigated. The temperature dependences of the electronic contribution to the absorption coefficient at a frequency of 53 MHz in crystals with cobalt concentrations from 1018 to 1020 cm−3 and in the undoped crystal have been observed experimentally. It has been found that crystals with impurities are characterized by an anomalous nonmonotonic temperature dependence of the absorption coefficient of the slow transverse wave in a narrow temperature range near 10 K. A smooth monotonic temperature dependence has been observed for longitudinal and fast transverse waves. Based on the developed theoretical interpretation, it has been established that the anomaly in the temperature dependence of the absorption coefficient of a slow transverse wave is associated with the hybridization of impurity d states in the conduction band of the crystal. A comparison of the theoretical and experimental dependences has made it possible to determine the parameters characterizing the hybridized electronic states.

Magnetoacoustic Investigation of the Jahn-Teller Effect in Chromium Doped ZnSe Crystal

Ultrasonic investigation of ZnSe:Cr2+ crystal was carried out at low temperatures in magnetic field applied along the wave vector. The observed anomalies in magnetic field dependences of attenuation and wave number evidence for existence of resonant transitions between the energy levels of the Jahn-Teller tetrahedral complex CrSe4.

Relaxation Attenuation of Ultrasound by the Jahn-Teller Centers in ZnSe:Cr in High Magnetic Fields

The magnetic field dependence of ultrasonic attenuation α (B) of slow shear waves in the ZnSe:Cr2+ crystal at a number of fixed temperatures from T=1.4 K to 20 K in magnetic fields of up to B=14 T were investigated. For magnetic fields B above 5 T we found that the attenuation increases with B monotonically, and at a given temperature it is proportional to the magnitude of relaxation attenuation at B=0. We show that the magnetic field dependent attenuation is due to the change in populations of the lowest energy levels of the impurity centers CrZn4Se, produced by the Jahn-Teller effect and split by the spin-orbital interaction and the magnetic field. The calculations carried out without fitting parameters are in good agreement with the experimental data.

Manifestation of the Jahn-Teller effect in elastic moduli of strontium fluorite crystals doped with chromium ions

Attenuation and phase velocity of ultrasound have been measured in strontium fluoride single crystal doped with chromium in the temperature range of 4 – 185 K at 56 -162 MHz. Anomalies have been found for all the normal modes corresponding to the non-vanishing elastic moduli of a cubic crystal. Interpretation of the observed anomalies has been done in the framework of relaxation in the system of Jahn-Teller (JT) complexes CrF8 subject to full T2g ⊗(eg + t2g) JT problem. Relaxation time has been calculated from the experimental data on ultrasonic attenuation and adiabatic and isothermal contributions of the impurity subsystem to the total elastic moduli have been obtained.