Abstract
We report on results of ultrasonic investigations of the two-dimensional dimer spin system YbAl3C3 in magnetic fields (H) up to 10 T and for temperatures (T) down to 0.5 K A magnetic field induces a pronounced minimum in the elastic constants far below the transition temperature of 80 K The minimum disappears immediately on increasing T and becomes undetectable above around 2 K We propose the two-dimensional dimer spin model with the energy difference of Δ=15 K between the singlet ground state and the triplet excited one. We discuss the mechanism how to form the two-dimensional dimer spin system in YbAl3C3 where Yb ions have a high degree of geometrical frustration. Our observations show an important role of an exchange striction coupling in YbAl3C3 in the ordered state below 80 K.
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