Dephasing Relaxation of the Electron Spin Echo of the Vibronic Cu(H2O)6 Complexes in Tutton Salt Crystals at Low Temperatures
✍ Scribed by S.K. Hoffmann; J. Goslar; W. Hilczer; M.A. Augustyniak-Jabłokow; S. Kiczka
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 181 KB
- Volume
- 153
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
Two-pulse electron spin echo (ESE) measurements of the phase relaxation (phase memory time T M ) were performed in a series of Tutton salt crystals M I 2 M II (SO 4 ) 2 • 6X 2 O (M I = NH 4 , K, Cs; M II = Zn, Mg; X = H, D) weakly doped with Cu 2+ ions (c ≈ 10 18 ions/cm 3 ) in temperature range 4-60 K where ESE signals were detectable. The ESE decay was strongly modulated with proton (or deuteron) frequencies and described by the decay function V(2τ ) = V 0 exp(-bτmτ 2 ) with the mτ 2 term being temperature independent and negligible above 20 K. Various mechanisms leading to the τ -or τ 2 -type ESE decay are reviewed. The m and b coefficients for nuclear spectral diffusion (NSD), electron spectral diffusion (SD), and instantaneous diffusion (ID) were calculated in terms of existing theories and the resulting rigid lattice T 0 M times were found to be close one to another within the crystal family with average values: 17.5 µs (NSD protons), 200 µs (NSD deuterons), 8 µs (SD), and 5 µs (ID). The ID dominates but the calculated effective T 0 M is longer than the experimental T 0 M = 2 µs. This is due to a nonuniform distribution of the Cu 2+ ions with a various degree of the disorder in the studied crystals. The acceleration of the dephasing rate 1/T M with temperature is due to the mechanisms producing exp(-bτ ) decay. They are reviewed and two of them were found to be operative in Tutton salt crystals: (a) Excitations to the vibronic levels of energy ∆ leading to the temperature dependence 1/T M = B exp(-∆/kT), with the vibronic levels produced by strong Jahn-Teller effect, and (b) spin-lattice relaxation processes being effective above 50 K. Based on the ∆ values being on the order of 100 cm -1 , the scheme of vibronic levels in the Tutton salts is presented, and the independence of the ∆ on temperature proves that the adiabatic potential surface shape of Jahn-Teller active Cu(H 2 O) 6 complexes is not affected by temperature below 65 K.