By Michael D. Kaplan
This publication by means of Kaplan and Vekhter brings jointly the molecular international of the chemist with the condensed topic international of the physicist. ahead of the cave in of the Soviet Union, chemists within the West dedicated lit to relationships among molecular digital constitution and tle awareness solid-state vibronic phenomena. Treating quantum mechanical difficulties in which the adiabatic Born-Oppenheimer approximation fails used to be performed by means of "brute strength. " With larger and higher pcs to be had within the West, molecular orbital calculations have been performed on saw and conceived static buildings with little obstacle for any cooperativity of vibrational habit that may attach those states. whereas it had lengthy been understood within the West that occasions do ensue during which assorted static constructions are came across for molecules that experience exact or approximately exact digital constructions, little consciousness have been paid to knowing the vibrational states that can attach such buildings. It was once more straightforward to calculate the digital constitution saw with numerous attainable distortions than to target how one can couple digital and vibrational habit. within the former Soviet Union, computational strength used to be no longer as acces sible as within the West. a lot better cognizance, as a result, was once dedicated to holding computational time through contemplating primary how you can han dle the vibrational connectivity among degenerate or approximately degenerate digital states.
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Extra info for Cooperative Phenomena in Jahn—Teller Crystals
The point is that the physical concept underlying the Jahn-Teller effect, namely the instability of a high-symmetry system as a result of the interaction of degenerate electronic states with another subsystem and the consequent formation of several degenerate low-symmetry configurations of the total system, has a very general character and shows up in a multitude of physical situations. As an example, we discuss a magnetic analog of the Jahn-Teller effect. In this case the problem is the magnetic configuration of the crystal, which can change as a result of the interaction of host lattice ions with degenerate electronic states of Kramers or non-Kramers magnetic impurity ions.
We now consider in closer detail the consequences of quadratic interaction. The Jahn-Teller effect is associated with a nonzero coefficient of the term, linear in the displacements Q, of the electron energy. It is this linear term that determines the distortion configuration. It is clear that terms quadratic in Q (and higher terms) of the expansion of the electron-vibration The Jahn-Teller Effect 25 coupling scarcely alter the behavior of the adiabatic potential sheets near Q = O. Meanwhile, far from this point the contribution of such terms may be substantial.
11. Charge density wave. this event is equivalent to b1 distortion, because it lowers the symmetry to orthorhombic. By changing the electron density, we can simulate not only the JahnTeller situation in molecules and impurity centers, but also Peierls instability of the crystal lattice. 11), this situation corresponds to a symmetry reduction similar to that associated with the configurational distortion of a chain by the phonon with q = 7r /2a. Such a circumstance, when the state of the many-electron system with an asymmetric electron density distribution is more stable, is usually called charge instability and corresponds to a charge-density wave.
Cooperative Phenomena in Jahn—Teller Crystals by Michael D. Kaplan
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