Dynamic JT effect

H Oh = 0.5 mT, 4H) suggests an intermediate with Civ symmetry (Fig. 6.12, type At temperatures >77 K, anearly isotropic nine-line spectrum (oh = 1-33 mT) was observed, supporting a dynamic JT effect averaging all eight nuclei. 

The importance of the JT effect in the spectroscopy of s2 ions can be easily observed from the splitting of the 1S0->3f,1, 1P1 absorption (excitation) transitions [6]. Figure 7 gives as an example the spectra of Sb3 + (5s2) in Cs2NaScCl6 at 4.2 and 300 K. The SbClg" octahedron is cubic. The 1S0-3P1 transition at about 30000 cm -1 splits into two components, the 1S0-1/>1 transition at about 40000 cm-1 into three components [24]. This, together with their temperature dependence, is proof of the dynamic JT effect in the 3P state. 

The assumption of cooperative dynamical JT effect in rhombohedral LaMn03 can... 

The temperature of transition is small relative to the value of splitting of the Mn3+ 5E ground state. That is why we suppose that the transition from monoclinic to rhombohedral phase is a transition from static to dynamic JT effect. Because of the strong correlation in motion of different [Mn06] octahedra we suppose that there is a cooperative dynamical effect. 

Let us mention that there is a more subtle effect introduced by a dynamical JT effect in the form of a new degree of freedom, the berry phase . This can lead to interference effects, which would constrain the tunneling of charge carriers in a lattice of molecules. This can renormalize the electron-electron interaction and it has been proposed that it could favor electronic pairing and possibly superconductivity [17]. 

ESR was the experimental technique, which first gave clear evidence for the JT effect and also showed a static or dynamic behavior of the same system at different temperatures In fact the rather long characteristic time of measurement is of the right order of magnitude to reveal both static and dynamic JT effects in dependence on temperature. 

Fig. 3 a, b. Static or dynamic JT effect as a function of temperature or spectroscopic technique. ESR (a) and ligand field (b) spectra of elpasolite-type mixed crystals Ba2Zni, Cu, WO at different temperatures (adapted from Ref. 42)... 

Fig. 4a, b. Dynamic JT effect in KMgF3 V. a Zero-phonon energy levels CAj,— as a... 

Finally the dynamic JT effect has been included to explain the low-temperature MCD of NO-Fe(II) hemoproteins In 1956 Griffith had already discussed the possible presence of a JT effect in NO-Hb but did not proceed further because the NO molecule... 

We point out that similar analyses and results have been performed and obtained also by other authors [33, 35, 38 0]. The spectral lines at 86meV and 123 meV excitation energy in the theoretical spectrum correspond to excitation of the modes V6 and vi, respectively. The first spacing deviates from the harmonic frequency of mode V6 in Table 3 because of the JT effect, while the second coincides with that of mode vi because of the linear coupling scheme adopted. For higher excitation energies the lines represent an intricate mixture of the various modes because of the well-know nonseparability of modes in the multi-mode dynamical JT effect. Overall, the excitation of the various modes can be characterized as moderately weak. The total JT stabilization energy amounts to 930 cm and is dominated by the contribution of mode ve- The barrier to pseudorotation is of the order of 10 cm only, consistent with the fact that the theoretical spectrum of Fig. 3 is obtained within the LVC scheme (see Sect. 2.1 above). 

Two different kinds of JT effect are distinguished in the literature the static and dynamic JT effects. In the static JT effect the molecule or complex remains distorted in a particular way long enough for the distortion to be detected experimentally. In the dynamic JT effect, the molecule or complex resonates between two or more equivalent modes of distortion, and the distortion is not directly observable [9]. 

In the next sections we describe briefly the main interactions, which are in charge of splitting of the 3d ions energy levels in crystals. These interactions include the Coulomb interaction, the crystal field interaction, the spin-orbit interaction and the JT interaction. As it was pointed out by Ham [13], the observed spin-orbit and trigonal field splittings of the orbital triplet states are significantly affected by the dynamic JT effect. 

The present chapter was devoted to the detailed consideration of the dynamic JT effect in the orbital triplet states for the 3d ions in a cubic crystal field, which included analysis of the spin-orbit splitting quenching (Ham effect) and geometry of the excited states (deformation of the equilibrium ligands configuration and cross-section of the potential energy surfaces). All necessary equations involved into such an analysis were given and explained. Theoretical description has been supported by... 

Fig. 21 STM simulations for systems subject to a static vs. dynamic JT effect. The top row corresponds to the excited state and the bottom to the ground state. In (a), infinitely strong coupling locks the molecule into one particular well. Finite but strong coupling (so that the system jumps between three wells) is shown in (b). Further reduction in localisation leads to essentially free pseudorotation, producing the time-averaged images in (c)...

We have tried to distinguish between static and dynamic JT effects. However, the difference between these two regimes is really the time-scale with which the molecule is observed. Data capture in STM is undoubtedly slow and this must be seen as one drawback of this method of study. For a JT-active molecule, there is usually a set of distorted configurations that are isoenergetic (or, perhaps, nearly isoenergetic if the host surface has a weak effect on them) and interconversion between them is to be expected. The interconversion rate is expected to be rapid on the STM time-scale and so its effect on the recorded STM image needs to be addressed. 

That a dynamic Jahn-Teller effect indeed may be of importance in SmBe has also been suggested in a paper by Uemura et al. (1986a,b). They clearly observe unusual AM - 2 and AM = 3 transitions in the ESR spectra of Eu + in SmBe, although the mixing matrix elements in cubic symmetry should be negligible. Reasons for the appearance of the forbidden transitions could be a dynamical JT-effect and/or fluctuations between... 

---