The Pseudo Jahn-Teller Effect in the Excited State ²E(²D) of Tetragonal Crystals Cs₃CoCl₅ and Cs₃CoBr₅ Studied by the Effect of Uniaxial Stress and the Zeeman Effect on Absorption Spectra

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The effect of uniaxial stress and the Zeeman effect on spectra ⁴A₂→²E(²D) are investigated at 4.2 K by using applied uniaxial stress P up to 2.4 kbar and applied magnetic field up to 79 kOe. (i) The lowest two excited vibronic states have the same g_⁄⁄-values with each other: +0.54 for Cs₃CoCl₅ and +0.73 for Cs₃CoBr₅. (ii) The energy difference between the above two states of Cs₃CoCl₅ decreases, while that of Cs₃CoBr₅ increases, with the increase of |P| in P⊥(001). (iii) Intensities of some absorption-lines of Cs₃CoCl₅ changed drastically with P in P⊥(001). It is shown that these three experimental facts are typical results brought about by the Jahn-Teller coupling with the ν₂-mode. From these, parameters E_JT⁄hν₂=1.1∼1.6 [0.5∼1.0] and 2Δ₀⁄hν₂=1.3∼1.7 [2.4∼2.8] are obtained for Cs₃CoCl₅[Cs₃CoBr₅], where E_JT is the Jahn-Teller stabilization energy, and 2Δ₀ is the splitting of ²E(²D) in E_JT=0 and P=0