From vibronic to itinerant electronic behavior

The FM phase is also stabilized by pressure. In an x = 0.15 sample, we [199] found a transition from vibronic to itinerant electronic behavior in the pressure range 5 < P < 6 kbar and the temperature range Tco < T < Tc it marks the FV-FM transition. A neutron-diffraction study [220] of an x = 0.165 sample showed that pressure stabilizes the R-rhombohedral phase relative to the O phase. We have also found that Tc jumps discontinuously by about 15-20 °C not only at the first-order O —O transition, but also at the FV-FM and the 0 -R transitions [199, 220a]. In addition, the slope dTc/dP drops discontinuously on crossing each transition. 

An alternative view is to consider pairing of itinerant vibronic states. The periodicity of the travelling CDW to which the electrons are coupled would introduce a pairing of electrons of momentum k -I- q and -(k -I- q) to open a gap at the Fermi surface that has d-wave symmetry with a maximum in the ( 71, 0) and (0, 7r) directions. This approach, which has been considered by Seibold and Varlamov [320] is more consistent with the massive evidence now available for strong electron-lattice interactions in the copper oxides, as in the perovskites, at the cross-over from localized to itinerant electronic behavior. 

Okuda et al. [221] have shown that the Dehye temperature increases with x in the range 0.12 < x < 0.30, saturating at a normal value for the perovskite structure above x = 0.30, which indicates that strong electron-lattice interactions persist in the FM phase out to x = 0.30. The electronic specific heat is only slightly enhanced as x decreases to x = 0.17, but it drops sharply to zero with decreasing x in the interval 0.15 < x < 0.17 where there is a transition from itinerant to vibronic electronic behavior. 

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