Ultrasonic Studies of the Jahn-Teller Effect in a Model System
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The Jahn-Teller effect occurs when degenerate atomic levels lower their energy by distorting and therefore lowering the symmetry of the system. This effect occurs in a range of molecular and crystalline systems, but here we focus on one in particular, the crystal series TmₓY₁−ₓVO₄. The x = 1 member of this series was studied extensively in the 1970’s and 1980’s as a paradigmatic example of the cooperative version of the Jahn-Teller effect. In this material, Jahn-Teller- susceptible energy levels on the Tm3+ ions interact with each other to drive a phase transition at T = 2.15 K. On the dilute end of the series, for x « 1, Tm3+ ions replace yttrium at such a low density that they are unable to interact with each other and therefore act as though they are isolated. In this thesis, we examine both the cooperative and isolated Jahn-Teller effect as well as the regions in between where neither paradigm is completely true. We first use ultrasound-driven transitions between localized 4f levels to study the microscopic environment of isolated Jahn-Teller ions. We then present ultra- sonic measurements of the speed of sound across this material series and show how we can use this data to extract an effective interaction distance between Jahn-Teller ions. Finally, we examine the formation of Jahn-Teller strains at both the isolated and the cooperative ends of the series.
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