Jahn-Teller effect problems via ultrasonic experiments. Application to the impurity crystal CdSe:Cr

Based on the data analysis of ultrasonic experiments, a novel approach has been developed to explore Jahn-Teller effect (JTE) problems in non-cubic crystals with JT centers without involving additional experimental data beyond the information about the electronic term and crystal symmetry. Distinguished from cubic crystals, the axis of symmetry of the bulk non-cubic crystal do not necessarily coincide with those of the local impurity center, thus complicating the relation between the distortions produced by the ultrasound wave and the JTE active modes. We analysed the problem with corresponding calculations for the wurtzite-type hexagonal crystal CdSe:Cr 2+ , in which the chromium ion substitutes the cadmium one in the tetrahedral environment, resulting in its electronic ground state 5 T 2 (e 2 t 2 ). Experimental investigation of this system by ultrasound at frequencies of 28-105 MHz in the temperature range of 4-180 K, yields a peak in the attenuation of the ultrasound below 40 K for the normal modes related to the c 11 , c 44 , c 55 , c 55 , and c 66 elastic moduli. The peak has been interpreted as the manifestation of the JTE, similar to the one, observed in cubic crystals doped with 3d ions. However, no anomalies of attenuation have been detected for the mode related to the c 33 elastic modulus, in contradiction to the theoretical predictions based on the previous method, worked out for cubic crystals. In the new method we obtained direct relations between the deformations, related to the crystal moduli, and the local JT modes, calculated the partition functions for each of the three possible JTE problems for systems with an electronic T term, T⊗e, T⊗t 2 and T⊗(e + t 2 ) revealed how these deformations alter the vibronic energy levels responsible for the relaxations in the JT centers. It emerged that in the wurtzite crystal under consideration, only in the T⊗e problem the deformation related to the elastic moduli c 33 displaces all the vibronic energy level uniformly, without relaxation possibilities, thus supporting the new method and explaining the experimental observations. © Published under licence by IOP Publishing Ltd.This research work was carried out within the Russian state assignment No.AAAA-A18-118020190098-5. We acknowledge the support from the HLD at HZDR, member of the European Magnetic Field Laboratory (EMFL), from Russian Foundation for Basic Research (project 18-02-00332 a), and from UrFU Center of Excellence "Radiation and Nuclear Technologies" (Competitiveness Enhancement Program). N.S.A. thanks the Foundation for the Advancement of Theoretical Physics and Mathematics "BASIS" (Russia)

Experimental observation of spontaneous spin polarization of electrons in hybridized states of transition element impurities in semiconductors

Experimental evidence of the possible existence of spontaneous spin polarization of the electron system in hybridized states formed by transition element impurity atoms in the conduction band of semiconducting crystals is examined. The details of a quantitative interpretation of experiments on the temperature dependence of the specific heat and elastic moduli of mercury selenide crystals with iron impurities confirm the feasibility of establishing the presence of electron spin polarization in this type of experiment, as well as the possible existence of polarization in the crystals studied here. Theoretical arguments support the observation of a thermodynamic anomalous Hall effect owing to spontaneously polarized donor electrons from low-concentration impurities. © 2013 AIP Publishing LLC

Sub-lattice of Jahn-Teller centers in hexaferrite crystal

A novel type of sub-lattice of the Jahn-Teller (JT) centers was arranged in Ti-doped barium hexaferrite BaFe12O19. In the un-doped crystal all iron ions, sitting in five different crystallographic positions, are Fe3+ in the high-spin configuration (S = 5/2) and have a non-degenerate ground state. We show that the electron-donor Ti substitution converts the ions to Fe2+ predominantly in tetrahedral coordination, resulting in doubly-degenerate states subject to the E⊗ e problem of the JT effect. The arranged JT complexes, Fe2+O4, their adiabatic potential energy, non-linear and quantum dynamics, have been studied by means of ultrasound and terahertz-infrared spectroscopies. The JT complexes are sensitive to external stress and applied magnetic field. For that reason, the properties of the doped crystal can be controlled by the amount and state of the JT complexes. © 2020, The Author(s).Deutscher Akademischer Austauschdienst, DAADRussian Foundation for Basic Research, RFBR: 18–02–00332 aDeutscher Akademischer Austauschdienst, DAAD: 91728513Ministry of Education and Science of the Russian Federation, Minobrnauka: 19–53–0401019–72–00055Ministry of Education and Science of the Russian Federation, MinobrnaukaThe authors acknowledge fruitful discussions with A.S. Prokhorov. We acknowledge support of the HLD at HZDR, member of the European Magnetic Field Laboratory (EMFL). At Ural Federal University, the research was supported by the Russian Foundation for Basic Research (18–02–00332 a), UrFU Center of Excellence “Radiation and Nuclear Technologies” (Competitiveness Enhancement Program), the Ministry of Education and Science of the Russian Federation (Program 5–100). In M.N. Miheev Institute of Metal Physics, the research was carried out within the state assignment of the Ministry of Education and Science of the Russian Federation (theme “Electron” No. AAAA-A18–118020190098–5. At South Ural State University, the authors were generally supported by Act 211 Government of the Russian Federation, contract № 02.A03.21.0011. The single crystal growth part was supported by Russian Foundation for Basic Research (19–53–04010). At Moscow Institute of Physics and Technology, the work was supported by the Russian Ministry of Education and Science (Program 5–100) and by the German Academic Exchange Service (DAAD) Michael Lomonosov Programm Linie B, 91728513. Time-domain low temperature spectroscopic experiments were financially supported by the Russian Scientific Foundation (19–72–00055)

Activation Energy of the Jahn-Teller Complexes in CaF2:Cu2+ Crystal

In CaF2 crystal doped with Cu^2+ ions, attenuation of all the normal ultrasonic modes with the wave vector k // were investigated at 22 -163 MHz in the temperature region of 4 - 200 K

SHIFT OF OPTICAL BANDGAP IN CH3NH3PbBr3 HYBRID PEROVSKITE SINGLE CRYSTALS

We report the results of photoluminescence (PL) and cathodoluminescence (CL) in organic-inorganic hybrid perovskite CH3NH3PbBr3. The PL spectra were registered with the 455 nm laser over temperatures from 10 to 320 K. The CL spectra were taken at accelerating voltages of 1.1 kV and 1.3 kV

STABILIZATION ENERGIES OF THE JAHN-TELLER COMPLEXES IN CaF2:Cr2+ CRYSTAL

In CaF2 crystal doped with Cr2+ ions, attenuation of all the normal ultrasonic modes with the wave vector k were investigated at 26 -158 MHz in the temperature region of 4 - 170 K. The observed peaks of relaxation origin were interpreted as manifestation of the Jahn-Teller effect

RELAXATION OF THE SYSTEM OF JAHN-TELLER COMPLEXES IN CdF2:Cr CRYSTAL

Anomalies in temperature dependence of ultrasound attenuation and velocity that are typical for the Jahn-Teller effect were observed in CdF2:Cr crystal. Parameters characterizing relaxation mechanism of the Jahn-Teller subsystem were determined.Финансирование. Работа выполнена при финансовой поддержке Российского научного фонда (проект № 22-22-00735)