Regensburg 2016 – scientific programme

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DF: Fachverband Dielektrische Festkörper

DF 11: Poster

DF 11.15: Poster

Wednesday, March 9, 2016, 18:00–20:00, Poster E

Spin-phonon coupling in ACrO₂ (A=Cu, Ag, Pd) studied by Raman spectroscopy — •Sebastian Elsässer¹, Anna Pimenov², and Jean Geurts¹ — ¹Universität Würzburg, Exp.Physik 3, Würzburg, Germany — ²Institut für Festkörperphysik, TU Wien, Wien, Austria

The magnetic moments of Cr³⁺ ions in the delafossite ACrO₂ systems are coordinated in planes of triangular lattices, which leads to geometric frustration of the spins with an incommensurate proper-screw spin arrangement in the ground state. While multiferroic behaviour was shown, the underlying mechanism is still under debate: The incommensurate order rules out conventional magnetostriction, while inverse Dzyaloshinskii-Moriya interaction does not produce a polarization for the proper-screw pattern. A variation of p-d hybridization was proposed to cause the imbalance in charge transfer between the Cr and O ions on different sites, leading to an electric polarization that depends on the modulation vector Q. We study the three compounds with A=Cu⁺, Ag⁺, and Pd⁺ with T-dependent Raman spectroscopy down to 6K. As predicted by group theory (space group R3m), both Raman-active modes with E_g and A_g symmetry are observed. Especially the E_g mode modulates the Cr-O bond, which mediates the exchange between adjacent Cr ions. Below T=100 K, the E_g mode shows a frequency softening which is therefore ascribed to spin-phonon coupling. In clear contrast, the A_g mode shows no such softening. In CuCrO₂ at the high-energy side of the E_g mode an unexplained shoulder peak is observed, which at 300 K nearly coincides with this mode.