Jahn-Teller domains

In addition, a reorientation of the Jahn-Teller domains occurs in the magnetic field, and at some critical mean field the sample reaches a single domain state (Cooke et al. 1972, Kazey and Sokolov 1986, Melcher et al. 1973). in TmVO is, apparently, equal to 70 kOe, because at this mean field the following condition is fulfilled phonons are scattered on the sample grain boundaries but not on the boundaries which separate the domains (fig. 100, dashed line). 

The 3d ab initio simulations [4] for Na3 are based, in a similar way, on three ab initio potential-energy surfaces for Na3(X), Na3(B), and Na3(X), with 3d ab initio dipole coupling between Na3(X) and Na3(B) evaluated by V. Bonacic-Koutecky et al. [5] plus Condon-type coupling between Na3(B) and Na3(X). Additional potential-energy surfaces interfere at the conical intersections of the pseudo-Jahn-Teller distorted Na3(B) state (see Ref. 6), but we have tested carefully [4] that these interferences are negligible in the frequency domains of the experimental femtosecond/picosecond laser pulse experiments [7] as well as in the continuous-wave experiments [8]. 

As discussed in the preceding chapter, ferromagnetism may be induced by an ordering of the orbitals of Jahn-Teller ions. When the structural arrangement has 3-D character, the ferromagnetic layers are coupled antiparallel and antiferromagnetism is observed in the 3-D domain. 

In an NMR investigation of the spectrum in TmV04, Bleaney and Wells (1980) found that the frequency of oscillation of the RF circuit containing the crystal was a sensitive monitor of variations in the RF (adiabatic) magnetic susceptibility Zs-Below To, where the ground doublet is split by the distortion, is independent of a field applied along the c-axis, until this reaches the critical field B at which the Jahn-Teller distortion is reduced to zero. At this field Zs falls sharply, and the effect can be used to give a precise measurement of B. The results were fitted accurately to the relation b = B /Bo = tanh(b/f), where t - T/T ) = T/2.156 is the reduced temperature. This is the result expected from mean-field theory. In the distorted state below T, the presence of domains with principal axes [110] and [110] was confirmed by observation of two sets of anisotropic resonance curves (see fig. 13c), one set for (/ —i) and the other for the enhanced NMR of Tm (/ = i). 

In the previous sections on the Jahn-Teller effect we have been concerned with uniform strains, the same at all lattice sites, at any rate within a domain. Such uniform strains are important below the distortion temperature Tq. Above Tq dynamic random strains are present, varying from site to site, for which the fluctuations slow down as the temperature is reduced. The presence of such effects has been revealed by observation of the EPR spectrum of Gd in substances such as TmV04 and TmAs04 and is described in the review by Mehran and Stevens... 

There is a relationship here to the Peierls distortion for one-dimensional materials discussed in Section 13.2 and the first-order Jahn-Teller distortion in the molecular domain (Section 7.4.B). We shall not cover the details of this phase distortion or the formation of charge density waves or spin density waves here, but direct the reader to a comprehensive treatment for chemists [22]. All of these are potential factors that may drive metallic states as represented in 13.3 into insulators. Predicting this behavior a priori is very difficult indeed. 

The condition expressed by Eq. (3.19) is no longer valid as type 11 electrode materials have narrow nonstoichiometric domains (Fig. 3.7). When the host lattice contains a transition-metal element M, the electrons injected in the insertion process are distributed in the empty d orbitals. The decrease in the formal oxidation state results in a change of either the ionic radius of the coordination shell symmetry (Jahn-Teller effect), inducing strains on the (77) framework. This situation is expressed as a strong positive interaction term proportional to the number of intercalated species (Eq. 3.19). It appears that the existence of a maximum of potential V(x) implies some instability in the [e-2] domain. As a consequence, the voltage composition curve shows a plateau in the forbidden composition range due to the equilibrium of the two pseudo-phases. 

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